Indoor Air 2015; 25: 662–671 wileyonlinelibrary.com/journal/ina Printed in Singapore. All rights reserved
© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd INDOOR AIR doi:10.1111/ina.12188
Experimental study of airflow characteristics of stratum ventilation in a multi-occupant room with comparison to mixing ventilation and displacement ventilation Abstract The motivation of this study is stimulated by a lack of knowledge about the difference of airflow characteristics between a novel air distribution method [i.e., stratum ventilation (SV)] and conventional air distribution methods [i.e., mixing ventilation (MV) and displacement ventilation (DV)]. Detailed air velocity and temperature measurements were conducted in the occupied zone of a classroom with dimensions of 8.8 m (L) 9 6.1 m (W) 9 2.4 m (H). Turbulence intensity and power spectrum of velocity fluctuation were calculated using the measured data. Thermal comfort and cooling efficiency were also compared. The results show that in the occupied zone, the airflow characteristics among MV, DV, and SV are different. The turbulent airflow fluctuation is enhanced in this classroom with multiple thermal manikins due to thermal buoyancy and airflow mixing effect. Thermal comfort evaluations indicate that in comparison with MV and DV, a higher supply air temperature should be adopted for SV to achieve general thermal comfort with low draft risk. Comparison of the mean air temperatures in the occupied zone reveals that SV is of highest cooling efficiency, followed by DV and then MV.
Y. Cheng, Z. Lin Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong S.A.R. Key words: Air distribution; Stratum ventilation; Airflow; Turbulence intensity; Power spectrum; Thermal comfort; Energy saving.
Z. Lin Building Energy & Environmental Technology Research Unit, Division of Building Science and Technology, City University of Hong Kong, Kowloon, Hong Kong S.A.R Tel.: +852 3442 9805 Fax: +852 3442 9716 e-mail: [email protected] Received for review 20 July 2014. Accepted for publication 19 January 2015.
Practical Implications
This study reports the unique profiles of flow, temperature, turbulence intensity, and power spectrum of stratum ventilation, which can have a number of implications for both knowledge and understanding of the flow characteristics in a stratum-ventilated room. With respect to the former, it expounds the fundamental characteristics of this air distribution method; and with respect to the latter, it reveals the mechanism of thermal comfort and energy saving under stratum ventilation.
Introduction
Room air distribution plays an important role in creating a thermally comfort, healthy, and energy-efficient indoor environment (Awbi, 1998; Chung and Hsu, 2001). Resulting from poor air distribution, thermal comfort problems such as draft (Melikov et al., 2005) and local discomfort from cold feet and/or warm head due to a large vertical temperature gradient (Pitchurov et al., 2002), and indoor air quality (IAQ) problems (Cheong et al., 2006; Lin et al., 2005a) are often encountered indoors. These have stimulated interest in how to improve room air distribution. Additionally, proper air distribution could reduce ventilation rate necessary for maintaining an acceptable level of 662
thermal comfort and air quality, and thus help to save energy. Therefore, understanding room airflow distribution characteristics is essential for selecting and designing an appropriate air distribution system. Usually, three methods of air distribution are available for total-volume ventilation of rooms: mixing ventilation (MV) (Figure 1), displacement ventilation (DV) (Figure 2), and stratum ventilation (SV) (Figure 3). For MV, a uniform environment is typically created because of a strong mixing of the supply jets with room air. For DV, owing to the gravity, a stratified environment is formed. However, for SV, as the air is directly delivered to occupants’ head level, the air velocity and temperature profiles are in a sandwich shape with the highest velocity and the lowest
Airflow characteristics of a ventilated room
Fig. 1 Mixing ventilation with ceiling supply and ceiling return
Fig. 2 Displacement ventilation with antipodal supply at low level and ceiling return
Although several recent studies have been carried out to compare the thermal, ventilation, and energy performance of SV with those of MV and/or DV (Fong et al., 2011; Lin et al., 2011a, 2013; Tian et al., 2010), they put little attention on the difference of airflow characteristics among them, particularly for airflow dynamic characteristics like turbulence intensity and power spectrum of velocity fluctuation. As indoor heat and airborne contaminant transports are directly related to airflow diffusion, it is necessary to compare the airflow characteristics of these three air distribution methods. In addition, previous investigations on the airflow distribution of SV focused on an individual office environment (Lin et al., 2011b; Tian et al., 2011a,b). These results may be different than that in a classroom with multiple occupants and higher heat load because thermal buoyancy and indoor furniture may affect the airflow distribution. The main objective of this study was to investigate the effect of different air distribution methods on the airflow characteristics in the occupied zone of a multioccupant classroom. Air velocity and temperature in the occupied zone under MV, DV, and SV would be measured in details. These data would also be used to compare their thermal comfort and cooling efficiency. These benchmark data helped to improve our understanding on the nature of airflow for MV, DV, and SV and optimize the design of room ventilation system.
Methodology Environmental chamber and experimental setup
Fig. 3 Stratum ventilation with front wall supply at middle level and rear wall return at same level
temperature at the head level. Therefore, a strong cooling effect is imposed at the head level to cool occupants more efficiently (Tian et al., 2011a,b). Literature review found that there exist a number of investigations on air distribution characteristics of MV (Chow et al., 1994; Hanzawa et al., 1987; Jouini et al., 1994; Zhang et al., 1992) and DV (Magnier et al., 2012; Melikov et al., 1990; Rees and Haves, 2013). Some comparative studies have been also carried out to investigate the airflow characteristics and performance of MV and DV (Awbi, 1998; Cermak and Melikov, 2006; Jiang et al., 1992; Lin et al., 2005a,b; Yin et al., 2009). However, most of them were conducted in the empty rooms or the rooms with a few occupants. Furthermore, SV was not evaluated in these studies.
All experiments were conducted in the environmental chamber at the City University of Hong Kong. The chamber is constructed to resemble a classroom with dimensions of 8.8 m (L) 9 6.1 m (W) 9 2.4 m (H), as shown in Figure 4. It could be served by various air distribution strategies. The installed air-conditioning system consisted of a ceiling-mounted variable air volume-type air-handling unit, ceiling-mounted diffusers for MV, wall-mounted diffusers for DV and SV, and associated motorized dampers and ductwork. Two plenums installed in the front and rear wall were employed to assist the wall-mounted diffusers in DV and SV (Figure 4). The supply airflow rate could be varied by adjusting the operating frequency of the frequency conversion fan inverter. The supply air temperature could be controlled by adjusting the off-coil temperature. The room air temperature in the chamber could be maintained with a precision of 0.5°C. The indoor relative humidity could be controlled between 50% and 65%. The thermal sensation of Hong Kong people was found to be insensitive to humidity if the relative humidity is between 50% and 80% (Fong et al., 2010). 663
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Fig. 4 Setup of experiments and arrangement of measurement plumb lines (mm)
In this study, three air distribution methods (i.e., MV, DV, and SV) were used respectively to ventilate this chamber. For MV, supply air is circulated to the occupied zone via six ceiling supply square diffusers and three ceiling return air grills. For DV, to avoid the risk of draft discomfort, the cool air is delivered from the opposite sides with fourteen wall-mounted perforation diffusers (D1–D14) at 0.3 m height above floor and returns to the same three ceiling grilles as for MV. For SV, fresh and cool air is supplied horizontally from four wall-mounted perforation diffusers (S1–S4) installed on the front wall at the height of 1.3 m and returned via four double deflection grilles (R1–R4) on the rear wall at the same height (Figure 4). The layout of a typical classroom was arranged for this study. Sixteen rectangular thermal manikins with dimensions of 0.4 m (L) 9 0.3 m (W) 9 0.76 m(H) were located at the seats to represent the sedentary students, who were seated at a working desk in four columns and two rows. Each manikin was heated by a 100 W light bulb (Zukowska et al., 2007). As this classroom was surrounded by air-conditioned spaces, the walls, floor, and ceiling were assumed to be adiabatic, which was also demonstrated by the infrared testing of indoor environments (Figure S1). Therefore, 664
indoor cooling loads due to the thermal manikins, and a laboratory staff member, the ceiling fluorescent lamps, the workstation for system control, and the computers for data sampling were 17 9 100, 21 9 56, 300 and 2 9 150 W, respectively. The cooling load per unit floor area was 64.8 W/m2. Measurement instrumentation
SWEMA omnidirectional hot-wire anemometer systems were employed to measure the indoor air velocity and temperature distributions. The measurement range of air velocity is 0.05–3.0 m/s; the accuracy is 0.02 m/s for 0.07–0.50 m/s and 0.03 m/s for 0.50– 3.00 m/s; the dynamic response time was 0.2 s. For air temperature, the measurement range is 10–40°C; the measurement error is 0.2°C. A hand-held anemometer LITE hot-wire anemometer was used to measure the supply face velocity and temperature. The air velocity measurement range is 0.01–20.0 m/s; the accuracy is the greater one of 5% of readings and 0.015 m/s. The air temperature measurement range is 20 to 70°C; the measuring accuracy is 1°C. An infrared thermal tracer was used to check the room surface temperatures against the room air temperatures.
Airflow characteristics of a ventilated room Table 1 Parameter combination of the cases studied Supply airflow rate 10 ACH
15 ACH
Mode
Nominal supply temperature (°C)
Measured supply temperature (ts) (°C)
MV-18 DV-18 SV-18 SV-21 SV-23 MV-18 DV-18 SV-18 SV-21 SV-23
18 18 18 21 23 18 18 18 21 23
18.3 18.3 18.3 20.8 22.5 18.3 18.3 18.3 20.5 22.6
0.2a 0.2 0.2 0.3 0.1 0.2 0.2 0.1 0.2 0.3
Supply face velocity (Us) (m/s) 1.85 0.34 2.82
2.32 0.44 4.71
ACH, air change per hour; DV, displacement ventilation; MV, mixing ventilation; SV, stratum ventilation. a Mean value standard deviation.
Studied cases
Ten experimental runs were conducted to capture the air velocity and temperature distributions in the occupied zone of this classroom served by MV, DV, and SV, respectively. Table 1 summarizes the combinations of parameters for all experimental runs. Airflow characteristics of these three air distribution methods were compared and their performances were evaluated in terms of thermal comfort and cooling efficiency. The typical supply air temperature of 18°C was adopted for MV and DV (Cao et al., 2014). For SV, as supply air is delivered directly to the headchest level of occupants, the supply air temperature should logically be higher than that of MV and DV due to the requirement of thermal comfort. The supply air temperatures of 21 and 23°C were therefore attempted. To make a meaningful comparison for cooling efficiency, the supply air temperature of 18°C was also tested for SV. For a given indoor heat gain and supply air temperature, the range of feasible ventilation rate is limited because of the thermal comfort requirement. Two supply airflow rates, that is, 10 air change per hour (ACH) and 15 ACH, were adopted to study the influence of ventilation rate on airflow distribution as well as thermal comfort and cooling efficiency. The supply air temperature and face velocity of supply terminals were also measured (Table 1). The measured supply air temperatures were close to the corresponding nominal values, indicating that the actual experimental conditions were well controlled. Experimental procedure and measurements
All measurements were taken under steady states. When the monitoring air temperature and velocity in the chamber were statistically steady, a steady state condition was assumed to be achieved. To sufficiently
characterize the airflow characteristics, 10 measuring plumb lines (L1–L10) were arranged in the occupied zone (Figure 4). Instantaneous air velocity and temperature were measured at 0.1, 0.6, 1.1, and 1.2 m above the floor using SWEMA omnidirectional anemometers, respectively. Due to the limited quantity of instruments, 10 anemometers were employed to conduct the measurements one plane (horizontal section) by one plane. By elevating the anemometers, air parameters along the 10 plumb lines were obtained for four planes. For two adjacent measurement planes, at least 30 min was kept to ensure air distribution to reach statistically steady state again after elevating. For each plane, the measuring duration was 10 min. The sampling frequency was 2 Hz, which implied that the anemometers recorded two instantaneous data of air velocity and temperature every second. Although the sampling frequency was relatively low for the turbulent airflow measurement, it was sufficient to capture the information in the frequency band concerned because the occupants were sensitive to the airflow fluctuations with frequency lower than 1 Hz (Ouyang et al., 2006). All experiments were repeated twice to ensure the repeatability of the laboratory runs. To qualitatively observe the flow patterns, a flow visualization system was adopted. When the smoke generated by a fog generator is injected into the supply air plenum, a camera is employed to record the flow pattern.
Results and discussion Airflow characteristics
The airflow pattern and temperature profile under 15 ACH are similar to those under 10 ACH, but with different magnitudes. Limited by the length, only the results under 10 ACH are presented. Airflow characteristics in the occupied zone for MV, DV, and SV are compared in terms of air velocity, air temperature, turbulence intensity, and power spectrum of velocity fluctuation. Air velocity distribution. Figure 5 shows the typical profiles of mean air velocity in the occupied zone under supply airflow rate of 10 ACH. The velocity is normalized by the corresponding supply face velocity. Different air distribution methods form different airflow patterns. For MV, due to the strong mixing between the supply air jets and room air, air velocity remains almost constant through the entire occupied zone. For DV, the highest air velocity occurs in the lower part of the occupied zone because of the momentum of the supply air, whereas the fairly low air velocity prevails in the upper part of the occupied zone. For SV, air velocity generally increases along the height in the occupied zone, wherein the highest air velocity is observed at the head level. This characteristic is more
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Fig. 5 Typical distribution of mean air velocity in occupied zone under 10 air change per hour (ACH) (sampling lines L2, L4, L6, and L10)
prominent for the locations within the supply jets (e.g., L4). It is because supply air jets are at this level. As a result, the significant horizontal variation of air velocity occurs, which does not exist in cases with MV and DV. The air velocity distribution pattern observed is in agreement with that in an individual office served by SV (Tian et al., 2011a,b). The profiles of mean air velocity at the other sampling lines in the occupied zone are given in Figure S2. Figure 5 indicates that under identical ventilation rate, the air velocity in the occupied zone of the SV case is highest, followed by the MV case and then the DV case. It is partly because of the high jet momentum, and partly because the supply air is delivered directly to the occupied zone. According to ASHRAE 55-2013, to avoid thermal discomfort caused by the elevated air movement, a higher room temperature and thus supply air temperature is necessary, which will be seen from the subsequent thermal comfort assessment. Figures 5 indicates that under SV when elevating supply air temperature from 18 to 21°C, air velocity distribution changes slightly though other boundary conditions keep the same. This is mainly because of the buoyancy effect. The Archimedes number (Ar), a parameter represents the ratio of buoyancy force due to air density difference and inertial force of supply jet, is defined as follows: Ar ¼
glðtroom ts Þ ; U2s troom
Air temperature distribution. Similar to the velocity pattern, the air temperature profile in the occupied zone is closely associated with air distribution method (Figure 6). The temperature scale is the difference between the local air temperature and the supply air temperature. Under MV, air temperature is uniformly distributed. For DV, similar to the previous investigations (Jiang et al., 1992; Yin et al., 2009), temperature stratification is formed. Air temperature is almost linearly elevated with the height. However, as shown in Figure 6, a slightly large vertical temperature difference between the head level (1.1 m) and the ankle level (0.1 m) exists. It might be attributed to the low headroom of this chamber which causes some hot air in the upper zone of the room to recirculate back into the occupied zone and thus elevates the temperature gradient in the occupied zone. For SV, the air temperature distribution in the occupied zone is different from the straight line temperature distribution of MV and the slope line temperature distribution of DV. The cool air is directly delivered to the head level, so a reverse temperature gradient is generally formed in the occupied
ð1Þ
where g is the gravitational acceleration, l is length scale of the supply terminal, Us is the supply air velocity, ts and troom are the supply air temperature and the room air temperature, respectively. The Archimedes number indicates that the cooler the supply air is, the stronger the buoyancy effect. Thus, the supply air jets drop faster with shorter throw, which increases the air velocity in lower zone far away from the supply terminals. The measured air velocities 666
in this study also show this trend (L4 and L10 in Figure 5). Qualitatively, this is also demonstrated with the airflow patterns visualized with smoke (Figure S3). However, the Archimedes number only states the ratio between buoyancy and momentum at the supply point. To quantify the relative strengths of buoyancy and momentum in the occupied zone, thermal length is calculated (Elvsen and Sandberg, 2009). Even under the condition of 10 ACH and the supply air temperature of 18°C, which represents the worst scenario for SV due to its low air supply velocity and high temperature difference, the thermal length is about 3.5 m. The net room length is 5.1 m in the flow direction. Thus, under SV, the flow in the occupied zone is typically dominated by the momentum of the supply air jets.
Fig. 6 Typical distribution of mean air temperature in the occupied zone under 10 air change per hour (ACH) (Sampling lines L2, L4, L6, and L10)
Airflow characteristics of a ventilated room zone with the lowest values at the head level. This feature is clearer for the positions in the jets (e.g., L4), where a quite small temperature gradient exists in lower part of the occupied zone and a high temperature gradient occurs around the head-chest level. Due to the entrainment of room air by the supply airflows, the temperature gradient at the head-chest level of the other plumb sampling lines is reduced. The temperature profile in this classroom is similar to that in the stratum-ventilated office reported by Tian et al. (2011a,b). It is because airflow pattern of SV is dominated by the sources, that is, the supply air jets. The profiles of mean air temperature for the other sampling lines in the occupied zone are enclosed in the ‘Supporting Information’ (Figure S4). Turbulence intensity. The turbulence intensity (TI) represents the relative level of velocity fluctuation, which is defined as follows:
TI ¼
V0 100%; V
ð2Þ
where V0 is the standard deviation of velocity fluctuation and V is the mean velocity at a local point. Figure 7 presents the turbulence intensity at different heights in case of 10 ACH. The mean values of turbulence intensity are indicated by the bars, and the vertical lines within the bars illustrate the upper and lower limits. For the three air distribution methods, the turbulence intensity in the occupied zone varies largely. Under the ventilation rate of 10 ACH, the typical turbulence intensity levels of 40–80%, 30–80%, and 30–70% are found for MV, DV, and SV, respectively. Compared with the previous results that the typical levels of 10–50% were found for MV (Hanzawa et al., 1987) and the typical levels of 10–40% were found for DV (Melikov et al., 1990), the turbulence intensity levels in present cases of MV and DV are higher. It might be attributed to the effect of multiple thermal manikins in this classroom (Zhang et al., 1992). For MV and DV, the mean turbulence intensity generally increases
Fig. 7 Distribution of turbulence intensity in the occupied zone under 10 air change per hour (ACH)
with the height, while for SV, it slightly decreases with the height because of the elevated air velocity with the height (Figure 5). As a result, at the head level, the mean turbulence intensity under SV is relatively lower than that under MV and DV, but still maintains at a high level ranging between about 45% and 55%, which is close to the high turbulence intensity level found in the experiments by Fanger et al. (1988). This may be ascribed partly to the interaction between the inertial force of the supply jets and the thermal buoyancy caused by the thermal manikins and partly to the enhanced mixing effect caused by the separated flows around the thermal manikins (Lin et al., 2011b). The previous studies showed that airflow fluctuation would result in a stronger cooling effect of air movement and thus improve thermal comfort in warm environments (Xia et al., 2000). It could be therefore inferred that the high turbulence intensity may be beneficial to thermal comfort under SV. Power spectrum analysis. To fully characterize the turbulent airflows, spectrum analysis of the airflow fluctuation is often conducted (Hanzawa et al., 1987; Kovanen et al., 1989). Power spectrum of the velocity fluctuation illustrates power density distribution over the frequency (f), which is defined as follows:
Z
1
EðfÞdf ¼ V02 ;
ð3Þ
0
where E(f) is power spectrum density function. The area surrounding by E(f) and the frequency axis represents the total power of turbulent fluctuation. The spectral distribution of the velocity fluctuation under MV and DV exhibits similar characteristics, whereas for SV, it depends on the horizontal distance from the supply terminal and height above the floor. Figures 8 and 9 compare the power spectrum density measured at the ankle level (0.1 m) and the head level (1.1 m) for two positions (L4 and L10), respectively. These plots show that for the ankle level, similar distributions of power spectrum density function are obtained for all three air distributions. The turbulent energy concentrates at lower frequencies and largescale eddies, and converge at higher frequencies in the dissipation region. And these curves show the existence of an inertial subrange where the power spectrum density function follows that E(f) / f(5/3), indicating that therein the turbulent flows are fully developed. However, the obvious difference of the spectral distribution of the turbulent energy is observed at the head level (Figures 8b and 9b). The turbulence under SV is the highest, followed by MV and then DV because the jets are at this level under SV. For SV, a completely different energy spectrum is found at head level of L4 near the supply terminal (S4) (Figure 8b). The turbulence at this location is not yet fully developed. The turbulent 667
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shifts to fully developed turbulence (Figure 9b). This reveals that under SV, the turbulence structure at the head level close to a supply terminal is dominated by the diffuser characteristic length scale, but this impact is negligible for the region further away from the supply diffusers.
-1
Power spectrum density(m2/s)
(a) 10
10
10
-2
-3
MV DV SV
-4
Thermal comfort evaluation
-5
10 -3 10
10
-2
10
-1
0
10
f(Hz) 0
(b)
2
Power spectrum density(m /s)
10
10
10
10
-2
-4
MV DV SV
-6
-8
10 -3 10
-2
10
10
-1
0
10
f(Hz)
Fig. 8 Power spectra E(f) under 10 air change per hour (ACH), (a) L4 at 0.1 m; (b) L4 at 1.1 m
10
-1
Power spectrum density(m2/s)
(a) 10
10
10
-2
-3
MV DV SV
-4
-5
10 -3 10
-2
10
10
-1
10
0
f(Hz) 10
-1
Similar to our previous studies (Cheng et al., 2014; Tian et al., 2011a), in this study, thermal comfort performance of these three air distribution methods is evaluated by several indices, including air diffusion performance index (ADPI), predicted mean vote (PMV) and draft rating (DR). The detailed formulas of these indices are available in the literatures (Cheng et al., 2014; International Standard EN ISO 7730, 2005). Table 2 shows the ADPI values for all cases studied. In the cases with MV and SV under 10 ACH, the ADPI values are more than 80%, which indicates that for MV and SV, these thermal environments are uniform. However, for DV, the low ADPI values show that the thermal environments are non-uniformity. Excessive supply air velocity could degrade the uniformity of stratum-ventilated thermal environments such as in case of 15 ACH. This is because the airflow pattern of SV is dominated by supply air jet(s). In this case, the number of supply terminals should be increased to keep the ADPI values above 80%. The subjective thermal sensations could be represented by the PMV at a specific height. For MV, DV and SV, this height should be both 1.1 and 0.6 m (Cheong et al., 2007), or just 1.1 m (Cheng et al., 2014). For 10 ACH, the PMV values at these heights are presented in Figure 10. The mean PMV values at these heights are summarized in Table 2. With the proper combination of supply airflow rate and temperature, three air distribution methods can achieve satisfactory thermal comfort. However, compared with MV and DV, higher supply air temperatures, for example, 23°C, are neces-
Power spectrum density(m2/s)
(b) 10
-2
Table 2 Summary of thermal environment evaluations Supply airflow rate
10
-3
10 ACH 10
MV DV SV
-4
15 ACH Draft (%)
Mode
ADPI (%)
Mean PMV
MV-18 DV-18 SV-18 SV-21 SV-23
97.5 25.0 90.0 87.5 82.5
0.06 0.32 1.17 0.50 +0.11
Draft (%)
0.1 m
1.1 m
ADPI (%)
Mean PMV
0.1 m
1.1 m
0a 0 0 0 0
0 0 20 30 0
97.5 25.0 72.5 75.0 65.0
0.41 0.70 1.62 1.04 0.37
0 0 0 0 0
0 0 50 60 0
-5
10 -3 10
10
-2
10
-1
10
0
f(Hz)
Fig. 9 Power spectra E(f) under 10 air change per hour (ACH), (a) L10 at 0.1 m; (b) L10 at 1.1 m
energy is almost uniformly distributed over the whole analyzed frequency range. Nonetheless, with airflow diffusion, the energy spectrum at head level of L10 668
ACH, air change per hour; ADPI, air diffusion performance index; DV, displacement ventilation; MV, mixing ventilation; PMV, predicted mean vote; SV, stratum ventilation. a Percentage of the measuring points with ‘draft’.
Airflow characteristics of a ventilated room
Fig. 10 Comparison of predicted mean vote (PMV) under 10 air change per hour (ACH)
Fig. 11 Comparison of mean air temperature in the occupied zone for three air distribution methods
sary for SV to ensure thermal comfort, especially for the locations within supply air jets (e.g., L1 and L4) where the air temperature is relatively lower and the air velocity is relatively higher. Table 2 shows the percentage draft feeling at the ankle (0.1 m) and head (1.1 m) levels under 10 ACH and 15 ACH, respectively. In this study, if the percentage draft feeling at a position is less than 30% corresponding to Category C of ISO 7730, this position is considered to be ‘draft free’; otherwise it is considered to be ‘draft’. The results show that under an appropriate combination of supply airflow rate and temperature, no draft discomfort occurs under the three air distribution methods. In particular for DV, this may benefit from the sufficiently large supply diffuser area used in this study. The number of diffusers used in the DV system is almost four times of that used in the SV system. For SV, similar to thermal sensation discomfort, draft discomfort also comes at the positions within the supply jets particularly for the cases with the low supply temperatures. Excessive air movement (15 ACH) further deteriorates the DR value for the cases with low supply temperature (e.g., 18°C). By elevating the supply air temperature to 23°C, no predicted draft discomfort is found for both supply airflow rates (Table 2). Moreover, the DR model does not consider the airflow direction. In reality, it probably over predict the draft effect under SV because horizontal airflows from the front or side direction are preferred in warm conditions (Mayer, 1992; Toftum, 1997). In summary, under proper supply boundary conditions, SV can provide general comfortable environments with low draft risk.
the occupants, which is also a more efficient way to exhaust the heat dissipated by the occupants. The air temperature in the occupied zone under DV is slightly lower than that under MV, suggesting that DV has a slightly higher cooling efficiency than that of MV. This finding is in line with the previous result (Jiang et al., 1992). Therefore, from the cooling efficiency point of view, SV is the most favorable, followed by DV and then MV.
Comparison of cooling efficiency
Figure 11 compares the mean air temperatures in the occupied zone of the three air distribution methods under 10 ACH and 15 ACH. To make a meaningful comparison, the supply air temperatures are set the same at 18°C. Compared to MV or DV, the air temperature in the occupied zone under SV is lower, which indicates a higher cooling efficiency. Under SV, the cool air is directly supplied toward the upper torsos of
Uncertainty analysis
The accuracy of the statistical parameters such as the mean air velocity and the turbulence intensity depends on a combination of the sampling rate and the measuring duration (the number of samples) (Melikov et al., 1998). Increasing the number of samples could improve the accuracy. But, beyond a threshold, the improvement is minor. In this study, the sets of the sampling rate and the measurement duration comply with the requirements set by Melikov et al. (2007). To examine the effect of the number of samples on the mean velocity and the turbulence intensity, some additional measurements were conducted. The results show that the number of samples used in present study is sufficient to provide the satisfactory accuracy for the mean air velocity and the turbulence intensity. Based on the instruments applied in the current study, the maximum uncertainty of velocity measurements with the anemometer is in the order of 0.03 m/s. The random error due to instability of experiments is often of the order of 0.01 m/s (Magnier et al., 2012). The total uncertainty of velocity measurements is therefore estimated in the order of 0.04 m/s. Likewise, the total uncertainty of temperature measurements due to both the instrument error and instability of the experiments is evaluated in the order of 0.2°C. The absolute uncertainty for the turbulence intensity, TI, is thus between 5% and 25% (Melikov et al., 2007). The uncertainty for the effective draft temperature (EDT) is estimated to be in the order of 0.38 K. The uncertainty of the predicted DR is between 5% and 30%. 669
Cheng & Lin Summary and conclusions
The airflow characteristics in the occupied zone of a classroom with sixteen thermal manikins are investigated experimentally under three air distribution methods: namely, MV, DV, and SV. Based on the measured air velocity and temperature data, their thermal comfort performances and cooling efficiencies are also compared. Several conclusions can be drawn as the follows. In the occupied zone, different air velocity and temperature profiles are obtained for these three air distribution methods. In comparison with the results found in the literatures, it is found that MV maintains the uniform distribution and DV maintains the vertical gradient distributions of air velocity and temperature, but the airflow fluctuation is higher in this classroom with the multiple thermal manikins. For SV, the profiles of the air velocity and temperature in this classroom are similar to that in an individual office reported by Tian et al. (2011a,b). At the head level (1.1 m), the typical turbulence intensity of 30–70% with 50% on average is measured. Under SV, a power spectrum of velocity fluctuation different from that under MV or DV is found at the head level. For positions closer to the supply terminals, the turbulence structure is influenced by the characteristic length scale of the supply diffusers. With airflow diffusion, it shifts to the similar power spectrum as that of a fully developed turbulent flow. With regard to the cases under investigation, for all three air distribution methods, thermal comfort under the supply airflow rate of 10 ACH is better than that under 15 ACH quantified by ADPI, PMV, and DR. Compared to MV and DV, the supply air temperature of SV is necessarily higher (e.g., 23°C in this study) to
provide a comfortable thermal environment, which implies a potential of energy saving. At the same supply air temperature, the mean air temperature in the occupied zone under SV is lower than that under DV or MV, indicating a higher cooling efficiency. Therefore, under proper supply parameters, SV is able to provide general satisfactory thermal comfort with higher room air temperature and lower energy consumption as compared with MV and DV. Acknowledgements
The work described in this article is supported by a General Research Grant from the National Natural Science Foundation of China (Project No. 51178407). The authors would also like to express thanks to Miss Weiqin Wu and Dr. Ting Yao for their assistants in experimental setup and Mr. Zhengtao Ai for his valuable advices. Supporting Information
Additional Supporting Information may be found in the online version of this article: Figure S1. Infrared photo of thermal manikins (°C). Figure S2. Distribution of mean air velocity for the other sampling lines in the occupied zone under 10 ACH. Figure S3. Visualization of stratum-ventilated flow pattern under 10 ACH, (a) supply temperature = 18°C; (b) supply temperature = 21°C. Figure S4. Distribution of mean air temperature for the other sampling lines in the occupied zone under 10 ACH.
References ASHRAE (2013) ANSI/ASHRAE Standard 55–2013: Thermal Environmental Conditions for Human Occupancy, Atlanta, GA, American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc. Awbi, H.B. (1998) Energy efficient room air distribution, Renew. Energy, 15, 293–299. Cao, G.Y., Awbi, H.B., Yao, R.M., Fan, Y.Q., Siren, K., Kosonen, R. and Zhang, J.S. (2014) A review of the performance of different ventilation and airflow distribution systems in buildings, Build. Environ., 73, 171–186. Cermak, R. and Melikov, A.K. (2006) Air quality and thermal comfort in an office with underfloor, mixing and displacement ventilation, Int. J. Vent., 5, 323–332. Cheng, Y., Fong, M.L., Yao, T., Lin, Z. and Fong, K.F. (2014) Uniformity of stratum-
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ventilated thermal environment and thermal sensation, Indoor Air, 24, 521–532. Cheong, K.W.D., Yu, W.J., Tham, K.W., Sekhar, S.C. and Kosonen, R. (2006) A study of perceived air quality and sick building syndrome in a field environment chamber served by displacement ventilation system in the tropics, Build. Environ., 41, 1530–1539. Cheong, K.W.D., Yu, W.J., Sekhar, S.C., Tham, K.W. and Kosonen, R. (2007) Local thermal sensation and comfort study in a field environment chamber served by displacement ventilation system in the tropics, Build. Environ., 42, 525– 533. Chow, W.K., Wong, L.T., Chan, K.T. and Yiu, J.M.K. (1994) Experimental studies on the airflow characteristics of air-conditioned spaces, ASHRAE Trans., 100, 256–263.
Chung, K.C. and Hsu, S.P. (2001) Effect of ventilation pattern on room air and contaminant distribution, Build. Environ., 36, 989–998. Elvsen, P. and Sandberg, M. (2009) Buoyant jet in a ventilated room: velocity field, temperature field and airflow patterns analysed with three different whole-field methods, Build. Environ., 44, 137–145. European Standard EN ISO 7730 (2005) Ergonomics of the thermal environmentanalytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria. Fanger, P.O., Melikov, A.K., Hanzawa, H. and Ring, J. (1988) Air turbulence and sensation of draught, Energy Buildings, 12, 21–39. Fong, K.F., Chow, T.T. and Li, C.Y. (2010) Comfort zone of air speeds and tempera-
Airflow characteristics of a ventilated room tures for air-conditioned environment in the subtropics Hong Kong, Indoor Built Environ., 19, 375–381. Fong, M.L., Lin, Z., Fong, K.F., Chow, T.T. and Yao, T. (2011) Evaluation of thermal comfort conditions in a classroom with three ventilation methods, Indoor Air, 21, 231–239. Hanzawa, H., Melikov, A.K. and Fanger, P.O. (1987) Airflow characteristics in the occupied zone of ventilated spaces, ASHRAE Trans., 93, 524–539. Jiang, Z., Chen, Q.Y. and Moser, A. (1992) Comparison of displacement and mixing diffusers, Indoor Air, 2, 168–179. Jouini, D.B., Said, M.N. and Plett, E.G. (1994) Measurements of room air distribution in a ventilated office environment, Build. Environ., 29, 511–521. Kovanen, K., Sepp€anen, O., Siren, K. and Majanen, A. (1989) Turbulent air flow measurements in ventilated spaces, Environ. Int., 15, 621–626. Lin, Z., Chow, T.T., Fong, K.F., Tsang, C.F. and Wang, Q.W. (2005a) Comparison of performances of displacement and mixing ventilations, Part II: indoor air quality, Int. J. Refrig, 28, 288–305. Lin, Z., Chow, T.T., Fong, K.F., Wang, Q.W. and Li, Y. (2005b) Comparison of performances of displacement and mixing ventilations, Part I: thermal comfort, Int. J. Refrig, 28, 276–287. Lin, Z., Lee, C.K., Fong, K.F., Chow, T.T., Yao, T. and Chan, L.S. (2011a) Comparison of annual energy performances with different ventilation methods for cooling, Energy Buildings, 43, 30–136. Lin, Z., Tian, L., Yao, T., Wang, Q.W. and Chow, T.T. (2011b) Experimental and numerical study of room airflow under stratum ventilation, Build. Environ., 46, 235–244. Lin, Z., Wang, J.L., Yao, T., Chow, T.T. and Fong, K.F. (2013) Numerical comparison of dispersion of human exhaled droplets under different ventilation meth-
ods, Word Rev Sci Technol Sust Dev, 10, 142–161. Magnier, L., Zmeureanu, R. and Derome, D. (2012) Experimental assessment of the velocity and temperature distribution in an indoor displacement ventilation jet, Build. Environ., 47, 150–160. Mayer, E.. (1992) New measurements of the convective heat transfer coefficients: influences of turbulence, mean air velocity and geometry of human body. Proceedings of the ROOMVENT’92 vol.3, Lyngby, Danish Association of HVAC Engineers (DANVAK), 263– 276. Melikov, A.K., Langkilde, G. and Derbiszewski, B. (1990) Airflow characteristics in the occupied zone of rooms with displacement ventilation, ASHRAE Trans., 96, 555–563. Melikov, A.K., Popiolek, Z., Finkelstein, W., Sefker, T. and Care, I. (1998) Requirements and guidelines for lowvelocity measurements, ASHRAE Trans., 104, 1529–1539. Melikov, A.K., Pitchurov, G., Naidenov, K. and Langkilde, G. (2005) Field study of occupants thermal comfort in rooms with displacement ventilation, Indoor Air, 15, 205–214. Melikov, A.K., Popiolek, Z. and Silva, M.C.G. (2007) Accuracy limitations for low-velocity measurements and draft assessment in rooms, HVAC&R Res., 13, 971–986. Ouyang, Q., Dai, W., Li, H.J. and Zhu, Y.X. (2006) Study on dynamics characteristics of natural and mechanical wind in built environment using spectral analysis, Build. Environ., 41, 418–426. Pitchurov, G., Naidenov, K., Melikov, A. and Langkilde, G.. (2002) Field survey of occupants thermal comfort in rooms with displacement ventilation, Proceedings of Roomvent’2002, Copenhagen, Demark, 479–482.
Rees, S.J. and Haves, P. (2013) An experimental study of air flow and temperature distribution in a room with displacement ventilation and a chilled ceiling, Build. Environ., 59, 358–368. Tian, L., Lin, Z. and Wang, Q.W. (2010) Comparison of gaseous contaminant diffusion under stratum ventilation and under displacement ventilation, Build. Environ., 45, 2035–2046. Tian, L., Lin, Z. and Wang, Q.W. (2011a) Experimental investigation of thermal and ventilation performances of stratum ventilation, Build. Environ., 46, 1309– 1320. Tian, L., Lin, Z., Liu, J., Yao, T. and Wang, Q.W. (2011b) The impact of temperature on mean local air age and thermal comfort in a stratum ventilated office, Build. Environ., 46, 501–510. Toftum, J. (1997) Effect of airflow direction on human perception of draught, Proceedings of CLIMA 2000, 11. Xia, Y.Z., Niu, J.L., Zhao, R.Y. and Burnett, J. (2000) Effects of turbulent air on human thermal sensations in a warm isothermal environment, Indoor Air, 10, 289–296. Yin, Y.G., Xu, W.R., Gupta, J.K., Guity, A., Marmion, P., Manning, A., Gulick, B., Zhang, X.S., Chen, Q.Y. (2009) Experimental study on displacement and mixing ventilation systems for a patient ward, HVAC&R Res., 15, 1175–1191. Zhang, J.S., Wu, G.J. and Christianson, L.L. (1992) Full-scale experimental results on the mean and turbulent behavior of room ventilation flows, ASHRAE Trans., 98, 307–318. Zukowska, D., Melikov, A. and Popiolek, Z. (2007) Thermal plume above a simulated sitting person with different complexity of body geometry, Proceedings of the 10th international conference on air distribution in rooms-Roomvent 2007, Helsinki, Finland, Vol. 3, 191–198.
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© 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd INDOOR AIR doi:10.1111/ina.12188
Experimental study of airflow characteristics of stratum ventilation in a multi-occupant room with comparison to mixing ventilation and displacement ventilation Abstract The motivation of this study is stimulated by a lack of knowledge about the difference of airflow characteristics between a novel air distribution method [i.e., stratum ventilation (SV)] and conventional air distribution methods [i.e., mixing ventilation (MV) and displacement ventilation (DV)]. Detailed air velocity and temperature measurements were conducted in the occupied zone of a classroom with dimensions of 8.8 m (L) 9 6.1 m (W) 9 2.4 m (H). Turbulence intensity and power spectrum of velocity fluctuation were calculated using the measured data. Thermal comfort and cooling efficiency were also compared. The results show that in the occupied zone, the airflow characteristics among MV, DV, and SV are different. The turbulent airflow fluctuation is enhanced in this classroom with multiple thermal manikins due to thermal buoyancy and airflow mixing effect. Thermal comfort evaluations indicate that in comparison with MV and DV, a higher supply air temperature should be adopted for SV to achieve general thermal comfort with low draft risk. Comparison of the mean air temperatures in the occupied zone reveals that SV is of highest cooling efficiency, followed by DV and then MV.
Y. Cheng, Z. Lin Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong S.A.R. Key words: Air distribution; Stratum ventilation; Airflow; Turbulence intensity; Power spectrum; Thermal comfort; Energy saving.
Z. Lin Building Energy & Environmental Technology Research Unit, Division of Building Science and Technology, City University of Hong Kong, Kowloon, Hong Kong S.A.R Tel.: +852 3442 9805 Fax: +852 3442 9716 e-mail: [email protected] Received for review 20 July 2014. Accepted for publication 19 January 2015.
Practical Implications
This study reports the unique profiles of flow, temperature, turbulence intensity, and power spectrum of stratum ventilation, which can have a number of implications for both knowledge and understanding of the flow characteristics in a stratum-ventilated room. With respect to the former, it expounds the fundamental characteristics of this air distribution method; and with respect to the latter, it reveals the mechanism of thermal comfort and energy saving under stratum ventilation.
Introduction
Room air distribution plays an important role in creating a thermally comfort, healthy, and energy-efficient indoor environment (Awbi, 1998; Chung and Hsu, 2001). Resulting from poor air distribution, thermal comfort problems such as draft (Melikov et al., 2005) and local discomfort from cold feet and/or warm head due to a large vertical temperature gradient (Pitchurov et al., 2002), and indoor air quality (IAQ) problems (Cheong et al., 2006; Lin et al., 2005a) are often encountered indoors. These have stimulated interest in how to improve room air distribution. Additionally, proper air distribution could reduce ventilation rate necessary for maintaining an acceptable level of 662
thermal comfort and air quality, and thus help to save energy. Therefore, understanding room airflow distribution characteristics is essential for selecting and designing an appropriate air distribution system. Usually, three methods of air distribution are available for total-volume ventilation of rooms: mixing ventilation (MV) (Figure 1), displacement ventilation (DV) (Figure 2), and stratum ventilation (SV) (Figure 3). For MV, a uniform environment is typically created because of a strong mixing of the supply jets with room air. For DV, owing to the gravity, a stratified environment is formed. However, for SV, as the air is directly delivered to occupants’ head level, the air velocity and temperature profiles are in a sandwich shape with the highest velocity and the lowest
Airflow characteristics of a ventilated room
Fig. 1 Mixing ventilation with ceiling supply and ceiling return
Fig. 2 Displacement ventilation with antipodal supply at low level and ceiling return
Although several recent studies have been carried out to compare the thermal, ventilation, and energy performance of SV with those of MV and/or DV (Fong et al., 2011; Lin et al., 2011a, 2013; Tian et al., 2010), they put little attention on the difference of airflow characteristics among them, particularly for airflow dynamic characteristics like turbulence intensity and power spectrum of velocity fluctuation. As indoor heat and airborne contaminant transports are directly related to airflow diffusion, it is necessary to compare the airflow characteristics of these three air distribution methods. In addition, previous investigations on the airflow distribution of SV focused on an individual office environment (Lin et al., 2011b; Tian et al., 2011a,b). These results may be different than that in a classroom with multiple occupants and higher heat load because thermal buoyancy and indoor furniture may affect the airflow distribution. The main objective of this study was to investigate the effect of different air distribution methods on the airflow characteristics in the occupied zone of a multioccupant classroom. Air velocity and temperature in the occupied zone under MV, DV, and SV would be measured in details. These data would also be used to compare their thermal comfort and cooling efficiency. These benchmark data helped to improve our understanding on the nature of airflow for MV, DV, and SV and optimize the design of room ventilation system.
Methodology Environmental chamber and experimental setup
Fig. 3 Stratum ventilation with front wall supply at middle level and rear wall return at same level
temperature at the head level. Therefore, a strong cooling effect is imposed at the head level to cool occupants more efficiently (Tian et al., 2011a,b). Literature review found that there exist a number of investigations on air distribution characteristics of MV (Chow et al., 1994; Hanzawa et al., 1987; Jouini et al., 1994; Zhang et al., 1992) and DV (Magnier et al., 2012; Melikov et al., 1990; Rees and Haves, 2013). Some comparative studies have been also carried out to investigate the airflow characteristics and performance of MV and DV (Awbi, 1998; Cermak and Melikov, 2006; Jiang et al., 1992; Lin et al., 2005a,b; Yin et al., 2009). However, most of them were conducted in the empty rooms or the rooms with a few occupants. Furthermore, SV was not evaluated in these studies.
All experiments were conducted in the environmental chamber at the City University of Hong Kong. The chamber is constructed to resemble a classroom with dimensions of 8.8 m (L) 9 6.1 m (W) 9 2.4 m (H), as shown in Figure 4. It could be served by various air distribution strategies. The installed air-conditioning system consisted of a ceiling-mounted variable air volume-type air-handling unit, ceiling-mounted diffusers for MV, wall-mounted diffusers for DV and SV, and associated motorized dampers and ductwork. Two plenums installed in the front and rear wall were employed to assist the wall-mounted diffusers in DV and SV (Figure 4). The supply airflow rate could be varied by adjusting the operating frequency of the frequency conversion fan inverter. The supply air temperature could be controlled by adjusting the off-coil temperature. The room air temperature in the chamber could be maintained with a precision of 0.5°C. The indoor relative humidity could be controlled between 50% and 65%. The thermal sensation of Hong Kong people was found to be insensitive to humidity if the relative humidity is between 50% and 80% (Fong et al., 2010). 663
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Fig. 4 Setup of experiments and arrangement of measurement plumb lines (mm)
In this study, three air distribution methods (i.e., MV, DV, and SV) were used respectively to ventilate this chamber. For MV, supply air is circulated to the occupied zone via six ceiling supply square diffusers and three ceiling return air grills. For DV, to avoid the risk of draft discomfort, the cool air is delivered from the opposite sides with fourteen wall-mounted perforation diffusers (D1–D14) at 0.3 m height above floor and returns to the same three ceiling grilles as for MV. For SV, fresh and cool air is supplied horizontally from four wall-mounted perforation diffusers (S1–S4) installed on the front wall at the height of 1.3 m and returned via four double deflection grilles (R1–R4) on the rear wall at the same height (Figure 4). The layout of a typical classroom was arranged for this study. Sixteen rectangular thermal manikins with dimensions of 0.4 m (L) 9 0.3 m (W) 9 0.76 m(H) were located at the seats to represent the sedentary students, who were seated at a working desk in four columns and two rows. Each manikin was heated by a 100 W light bulb (Zukowska et al., 2007). As this classroom was surrounded by air-conditioned spaces, the walls, floor, and ceiling were assumed to be adiabatic, which was also demonstrated by the infrared testing of indoor environments (Figure S1). Therefore, 664
indoor cooling loads due to the thermal manikins, and a laboratory staff member, the ceiling fluorescent lamps, the workstation for system control, and the computers for data sampling were 17 9 100, 21 9 56, 300 and 2 9 150 W, respectively. The cooling load per unit floor area was 64.8 W/m2. Measurement instrumentation
SWEMA omnidirectional hot-wire anemometer systems were employed to measure the indoor air velocity and temperature distributions. The measurement range of air velocity is 0.05–3.0 m/s; the accuracy is 0.02 m/s for 0.07–0.50 m/s and 0.03 m/s for 0.50– 3.00 m/s; the dynamic response time was 0.2 s. For air temperature, the measurement range is 10–40°C; the measurement error is 0.2°C. A hand-held anemometer LITE hot-wire anemometer was used to measure the supply face velocity and temperature. The air velocity measurement range is 0.01–20.0 m/s; the accuracy is the greater one of 5% of readings and 0.015 m/s. The air temperature measurement range is 20 to 70°C; the measuring accuracy is 1°C. An infrared thermal tracer was used to check the room surface temperatures against the room air temperatures.
Airflow characteristics of a ventilated room Table 1 Parameter combination of the cases studied Supply airflow rate 10 ACH
15 ACH
Mode
Nominal supply temperature (°C)
Measured supply temperature (ts) (°C)
MV-18 DV-18 SV-18 SV-21 SV-23 MV-18 DV-18 SV-18 SV-21 SV-23
18 18 18 21 23 18 18 18 21 23
18.3 18.3 18.3 20.8 22.5 18.3 18.3 18.3 20.5 22.6
0.2a 0.2 0.2 0.3 0.1 0.2 0.2 0.1 0.2 0.3
Supply face velocity (Us) (m/s) 1.85 0.34 2.82
2.32 0.44 4.71
ACH, air change per hour; DV, displacement ventilation; MV, mixing ventilation; SV, stratum ventilation. a Mean value standard deviation.
Studied cases
Ten experimental runs were conducted to capture the air velocity and temperature distributions in the occupied zone of this classroom served by MV, DV, and SV, respectively. Table 1 summarizes the combinations of parameters for all experimental runs. Airflow characteristics of these three air distribution methods were compared and their performances were evaluated in terms of thermal comfort and cooling efficiency. The typical supply air temperature of 18°C was adopted for MV and DV (Cao et al., 2014). For SV, as supply air is delivered directly to the headchest level of occupants, the supply air temperature should logically be higher than that of MV and DV due to the requirement of thermal comfort. The supply air temperatures of 21 and 23°C were therefore attempted. To make a meaningful comparison for cooling efficiency, the supply air temperature of 18°C was also tested for SV. For a given indoor heat gain and supply air temperature, the range of feasible ventilation rate is limited because of the thermal comfort requirement. Two supply airflow rates, that is, 10 air change per hour (ACH) and 15 ACH, were adopted to study the influence of ventilation rate on airflow distribution as well as thermal comfort and cooling efficiency. The supply air temperature and face velocity of supply terminals were also measured (Table 1). The measured supply air temperatures were close to the corresponding nominal values, indicating that the actual experimental conditions were well controlled. Experimental procedure and measurements
All measurements were taken under steady states. When the monitoring air temperature and velocity in the chamber were statistically steady, a steady state condition was assumed to be achieved. To sufficiently
characterize the airflow characteristics, 10 measuring plumb lines (L1–L10) were arranged in the occupied zone (Figure 4). Instantaneous air velocity and temperature were measured at 0.1, 0.6, 1.1, and 1.2 m above the floor using SWEMA omnidirectional anemometers, respectively. Due to the limited quantity of instruments, 10 anemometers were employed to conduct the measurements one plane (horizontal section) by one plane. By elevating the anemometers, air parameters along the 10 plumb lines were obtained for four planes. For two adjacent measurement planes, at least 30 min was kept to ensure air distribution to reach statistically steady state again after elevating. For each plane, the measuring duration was 10 min. The sampling frequency was 2 Hz, which implied that the anemometers recorded two instantaneous data of air velocity and temperature every second. Although the sampling frequency was relatively low for the turbulent airflow measurement, it was sufficient to capture the information in the frequency band concerned because the occupants were sensitive to the airflow fluctuations with frequency lower than 1 Hz (Ouyang et al., 2006). All experiments were repeated twice to ensure the repeatability of the laboratory runs. To qualitatively observe the flow patterns, a flow visualization system was adopted. When the smoke generated by a fog generator is injected into the supply air plenum, a camera is employed to record the flow pattern.
Results and discussion Airflow characteristics
The airflow pattern and temperature profile under 15 ACH are similar to those under 10 ACH, but with different magnitudes. Limited by the length, only the results under 10 ACH are presented. Airflow characteristics in the occupied zone for MV, DV, and SV are compared in terms of air velocity, air temperature, turbulence intensity, and power spectrum of velocity fluctuation. Air velocity distribution. Figure 5 shows the typical profiles of mean air velocity in the occupied zone under supply airflow rate of 10 ACH. The velocity is normalized by the corresponding supply face velocity. Different air distribution methods form different airflow patterns. For MV, due to the strong mixing between the supply air jets and room air, air velocity remains almost constant through the entire occupied zone. For DV, the highest air velocity occurs in the lower part of the occupied zone because of the momentum of the supply air, whereas the fairly low air velocity prevails in the upper part of the occupied zone. For SV, air velocity generally increases along the height in the occupied zone, wherein the highest air velocity is observed at the head level. This characteristic is more
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Fig. 5 Typical distribution of mean air velocity in occupied zone under 10 air change per hour (ACH) (sampling lines L2, L4, L6, and L10)
prominent for the locations within the supply jets (e.g., L4). It is because supply air jets are at this level. As a result, the significant horizontal variation of air velocity occurs, which does not exist in cases with MV and DV. The air velocity distribution pattern observed is in agreement with that in an individual office served by SV (Tian et al., 2011a,b). The profiles of mean air velocity at the other sampling lines in the occupied zone are given in Figure S2. Figure 5 indicates that under identical ventilation rate, the air velocity in the occupied zone of the SV case is highest, followed by the MV case and then the DV case. It is partly because of the high jet momentum, and partly because the supply air is delivered directly to the occupied zone. According to ASHRAE 55-2013, to avoid thermal discomfort caused by the elevated air movement, a higher room temperature and thus supply air temperature is necessary, which will be seen from the subsequent thermal comfort assessment. Figures 5 indicates that under SV when elevating supply air temperature from 18 to 21°C, air velocity distribution changes slightly though other boundary conditions keep the same. This is mainly because of the buoyancy effect. The Archimedes number (Ar), a parameter represents the ratio of buoyancy force due to air density difference and inertial force of supply jet, is defined as follows: Ar ¼
glðtroom ts Þ ; U2s troom
Air temperature distribution. Similar to the velocity pattern, the air temperature profile in the occupied zone is closely associated with air distribution method (Figure 6). The temperature scale is the difference between the local air temperature and the supply air temperature. Under MV, air temperature is uniformly distributed. For DV, similar to the previous investigations (Jiang et al., 1992; Yin et al., 2009), temperature stratification is formed. Air temperature is almost linearly elevated with the height. However, as shown in Figure 6, a slightly large vertical temperature difference between the head level (1.1 m) and the ankle level (0.1 m) exists. It might be attributed to the low headroom of this chamber which causes some hot air in the upper zone of the room to recirculate back into the occupied zone and thus elevates the temperature gradient in the occupied zone. For SV, the air temperature distribution in the occupied zone is different from the straight line temperature distribution of MV and the slope line temperature distribution of DV. The cool air is directly delivered to the head level, so a reverse temperature gradient is generally formed in the occupied
ð1Þ
where g is the gravitational acceleration, l is length scale of the supply terminal, Us is the supply air velocity, ts and troom are the supply air temperature and the room air temperature, respectively. The Archimedes number indicates that the cooler the supply air is, the stronger the buoyancy effect. Thus, the supply air jets drop faster with shorter throw, which increases the air velocity in lower zone far away from the supply terminals. The measured air velocities 666
in this study also show this trend (L4 and L10 in Figure 5). Qualitatively, this is also demonstrated with the airflow patterns visualized with smoke (Figure S3). However, the Archimedes number only states the ratio between buoyancy and momentum at the supply point. To quantify the relative strengths of buoyancy and momentum in the occupied zone, thermal length is calculated (Elvsen and Sandberg, 2009). Even under the condition of 10 ACH and the supply air temperature of 18°C, which represents the worst scenario for SV due to its low air supply velocity and high temperature difference, the thermal length is about 3.5 m. The net room length is 5.1 m in the flow direction. Thus, under SV, the flow in the occupied zone is typically dominated by the momentum of the supply air jets.
Fig. 6 Typical distribution of mean air temperature in the occupied zone under 10 air change per hour (ACH) (Sampling lines L2, L4, L6, and L10)
Airflow characteristics of a ventilated room zone with the lowest values at the head level. This feature is clearer for the positions in the jets (e.g., L4), where a quite small temperature gradient exists in lower part of the occupied zone and a high temperature gradient occurs around the head-chest level. Due to the entrainment of room air by the supply airflows, the temperature gradient at the head-chest level of the other plumb sampling lines is reduced. The temperature profile in this classroom is similar to that in the stratum-ventilated office reported by Tian et al. (2011a,b). It is because airflow pattern of SV is dominated by the sources, that is, the supply air jets. The profiles of mean air temperature for the other sampling lines in the occupied zone are enclosed in the ‘Supporting Information’ (Figure S4). Turbulence intensity. The turbulence intensity (TI) represents the relative level of velocity fluctuation, which is defined as follows:
TI ¼
V0 100%; V
ð2Þ
where V0 is the standard deviation of velocity fluctuation and V is the mean velocity at a local point. Figure 7 presents the turbulence intensity at different heights in case of 10 ACH. The mean values of turbulence intensity are indicated by the bars, and the vertical lines within the bars illustrate the upper and lower limits. For the three air distribution methods, the turbulence intensity in the occupied zone varies largely. Under the ventilation rate of 10 ACH, the typical turbulence intensity levels of 40–80%, 30–80%, and 30–70% are found for MV, DV, and SV, respectively. Compared with the previous results that the typical levels of 10–50% were found for MV (Hanzawa et al., 1987) and the typical levels of 10–40% were found for DV (Melikov et al., 1990), the turbulence intensity levels in present cases of MV and DV are higher. It might be attributed to the effect of multiple thermal manikins in this classroom (Zhang et al., 1992). For MV and DV, the mean turbulence intensity generally increases
Fig. 7 Distribution of turbulence intensity in the occupied zone under 10 air change per hour (ACH)
with the height, while for SV, it slightly decreases with the height because of the elevated air velocity with the height (Figure 5). As a result, at the head level, the mean turbulence intensity under SV is relatively lower than that under MV and DV, but still maintains at a high level ranging between about 45% and 55%, which is close to the high turbulence intensity level found in the experiments by Fanger et al. (1988). This may be ascribed partly to the interaction between the inertial force of the supply jets and the thermal buoyancy caused by the thermal manikins and partly to the enhanced mixing effect caused by the separated flows around the thermal manikins (Lin et al., 2011b). The previous studies showed that airflow fluctuation would result in a stronger cooling effect of air movement and thus improve thermal comfort in warm environments (Xia et al., 2000). It could be therefore inferred that the high turbulence intensity may be beneficial to thermal comfort under SV. Power spectrum analysis. To fully characterize the turbulent airflows, spectrum analysis of the airflow fluctuation is often conducted (Hanzawa et al., 1987; Kovanen et al., 1989). Power spectrum of the velocity fluctuation illustrates power density distribution over the frequency (f), which is defined as follows:
Z
1
EðfÞdf ¼ V02 ;
ð3Þ
0
where E(f) is power spectrum density function. The area surrounding by E(f) and the frequency axis represents the total power of turbulent fluctuation. The spectral distribution of the velocity fluctuation under MV and DV exhibits similar characteristics, whereas for SV, it depends on the horizontal distance from the supply terminal and height above the floor. Figures 8 and 9 compare the power spectrum density measured at the ankle level (0.1 m) and the head level (1.1 m) for two positions (L4 and L10), respectively. These plots show that for the ankle level, similar distributions of power spectrum density function are obtained for all three air distributions. The turbulent energy concentrates at lower frequencies and largescale eddies, and converge at higher frequencies in the dissipation region. And these curves show the existence of an inertial subrange where the power spectrum density function follows that E(f) / f(5/3), indicating that therein the turbulent flows are fully developed. However, the obvious difference of the spectral distribution of the turbulent energy is observed at the head level (Figures 8b and 9b). The turbulence under SV is the highest, followed by MV and then DV because the jets are at this level under SV. For SV, a completely different energy spectrum is found at head level of L4 near the supply terminal (S4) (Figure 8b). The turbulence at this location is not yet fully developed. The turbulent 667
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shifts to fully developed turbulence (Figure 9b). This reveals that under SV, the turbulence structure at the head level close to a supply terminal is dominated by the diffuser characteristic length scale, but this impact is negligible for the region further away from the supply diffusers.
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f(Hz) 10
-1
Similar to our previous studies (Cheng et al., 2014; Tian et al., 2011a), in this study, thermal comfort performance of these three air distribution methods is evaluated by several indices, including air diffusion performance index (ADPI), predicted mean vote (PMV) and draft rating (DR). The detailed formulas of these indices are available in the literatures (Cheng et al., 2014; International Standard EN ISO 7730, 2005). Table 2 shows the ADPI values for all cases studied. In the cases with MV and SV under 10 ACH, the ADPI values are more than 80%, which indicates that for MV and SV, these thermal environments are uniform. However, for DV, the low ADPI values show that the thermal environments are non-uniformity. Excessive supply air velocity could degrade the uniformity of stratum-ventilated thermal environments such as in case of 15 ACH. This is because the airflow pattern of SV is dominated by supply air jet(s). In this case, the number of supply terminals should be increased to keep the ADPI values above 80%. The subjective thermal sensations could be represented by the PMV at a specific height. For MV, DV and SV, this height should be both 1.1 and 0.6 m (Cheong et al., 2007), or just 1.1 m (Cheng et al., 2014). For 10 ACH, the PMV values at these heights are presented in Figure 10. The mean PMV values at these heights are summarized in Table 2. With the proper combination of supply airflow rate and temperature, three air distribution methods can achieve satisfactory thermal comfort. However, compared with MV and DV, higher supply air temperatures, for example, 23°C, are neces-
Power spectrum density(m2/s)
(b) 10
-2
Table 2 Summary of thermal environment evaluations Supply airflow rate
10
-3
10 ACH 10
MV DV SV
-4
15 ACH Draft (%)
Mode
ADPI (%)
Mean PMV
MV-18 DV-18 SV-18 SV-21 SV-23
97.5 25.0 90.0 87.5 82.5
0.06 0.32 1.17 0.50 +0.11
Draft (%)
0.1 m
1.1 m
ADPI (%)
Mean PMV
0.1 m
1.1 m
0a 0 0 0 0
0 0 20 30 0
97.5 25.0 72.5 75.0 65.0
0.41 0.70 1.62 1.04 0.37
0 0 0 0 0
0 0 50 60 0
-5
10 -3 10
10
-2
10
-1
10
0
f(Hz)
Fig. 9 Power spectra E(f) under 10 air change per hour (ACH), (a) L10 at 0.1 m; (b) L10 at 1.1 m
energy is almost uniformly distributed over the whole analyzed frequency range. Nonetheless, with airflow diffusion, the energy spectrum at head level of L10 668
ACH, air change per hour; ADPI, air diffusion performance index; DV, displacement ventilation; MV, mixing ventilation; PMV, predicted mean vote; SV, stratum ventilation. a Percentage of the measuring points with ‘draft’.
Airflow characteristics of a ventilated room
Fig. 10 Comparison of predicted mean vote (PMV) under 10 air change per hour (ACH)
Fig. 11 Comparison of mean air temperature in the occupied zone for three air distribution methods
sary for SV to ensure thermal comfort, especially for the locations within supply air jets (e.g., L1 and L4) where the air temperature is relatively lower and the air velocity is relatively higher. Table 2 shows the percentage draft feeling at the ankle (0.1 m) and head (1.1 m) levels under 10 ACH and 15 ACH, respectively. In this study, if the percentage draft feeling at a position is less than 30% corresponding to Category C of ISO 7730, this position is considered to be ‘draft free’; otherwise it is considered to be ‘draft’. The results show that under an appropriate combination of supply airflow rate and temperature, no draft discomfort occurs under the three air distribution methods. In particular for DV, this may benefit from the sufficiently large supply diffuser area used in this study. The number of diffusers used in the DV system is almost four times of that used in the SV system. For SV, similar to thermal sensation discomfort, draft discomfort also comes at the positions within the supply jets particularly for the cases with the low supply temperatures. Excessive air movement (15 ACH) further deteriorates the DR value for the cases with low supply temperature (e.g., 18°C). By elevating the supply air temperature to 23°C, no predicted draft discomfort is found for both supply airflow rates (Table 2). Moreover, the DR model does not consider the airflow direction. In reality, it probably over predict the draft effect under SV because horizontal airflows from the front or side direction are preferred in warm conditions (Mayer, 1992; Toftum, 1997). In summary, under proper supply boundary conditions, SV can provide general comfortable environments with low draft risk.
the occupants, which is also a more efficient way to exhaust the heat dissipated by the occupants. The air temperature in the occupied zone under DV is slightly lower than that under MV, suggesting that DV has a slightly higher cooling efficiency than that of MV. This finding is in line with the previous result (Jiang et al., 1992). Therefore, from the cooling efficiency point of view, SV is the most favorable, followed by DV and then MV.
Comparison of cooling efficiency
Figure 11 compares the mean air temperatures in the occupied zone of the three air distribution methods under 10 ACH and 15 ACH. To make a meaningful comparison, the supply air temperatures are set the same at 18°C. Compared to MV or DV, the air temperature in the occupied zone under SV is lower, which indicates a higher cooling efficiency. Under SV, the cool air is directly supplied toward the upper torsos of
Uncertainty analysis
The accuracy of the statistical parameters such as the mean air velocity and the turbulence intensity depends on a combination of the sampling rate and the measuring duration (the number of samples) (Melikov et al., 1998). Increasing the number of samples could improve the accuracy. But, beyond a threshold, the improvement is minor. In this study, the sets of the sampling rate and the measurement duration comply with the requirements set by Melikov et al. (2007). To examine the effect of the number of samples on the mean velocity and the turbulence intensity, some additional measurements were conducted. The results show that the number of samples used in present study is sufficient to provide the satisfactory accuracy for the mean air velocity and the turbulence intensity. Based on the instruments applied in the current study, the maximum uncertainty of velocity measurements with the anemometer is in the order of 0.03 m/s. The random error due to instability of experiments is often of the order of 0.01 m/s (Magnier et al., 2012). The total uncertainty of velocity measurements is therefore estimated in the order of 0.04 m/s. Likewise, the total uncertainty of temperature measurements due to both the instrument error and instability of the experiments is evaluated in the order of 0.2°C. The absolute uncertainty for the turbulence intensity, TI, is thus between 5% and 25% (Melikov et al., 2007). The uncertainty for the effective draft temperature (EDT) is estimated to be in the order of 0.38 K. The uncertainty of the predicted DR is between 5% and 30%. 669
Cheng & Lin Summary and conclusions
The airflow characteristics in the occupied zone of a classroom with sixteen thermal manikins are investigated experimentally under three air distribution methods: namely, MV, DV, and SV. Based on the measured air velocity and temperature data, their thermal comfort performances and cooling efficiencies are also compared. Several conclusions can be drawn as the follows. In the occupied zone, different air velocity and temperature profiles are obtained for these three air distribution methods. In comparison with the results found in the literatures, it is found that MV maintains the uniform distribution and DV maintains the vertical gradient distributions of air velocity and temperature, but the airflow fluctuation is higher in this classroom with the multiple thermal manikins. For SV, the profiles of the air velocity and temperature in this classroom are similar to that in an individual office reported by Tian et al. (2011a,b). At the head level (1.1 m), the typical turbulence intensity of 30–70% with 50% on average is measured. Under SV, a power spectrum of velocity fluctuation different from that under MV or DV is found at the head level. For positions closer to the supply terminals, the turbulence structure is influenced by the characteristic length scale of the supply diffusers. With airflow diffusion, it shifts to the similar power spectrum as that of a fully developed turbulent flow. With regard to the cases under investigation, for all three air distribution methods, thermal comfort under the supply airflow rate of 10 ACH is better than that under 15 ACH quantified by ADPI, PMV, and DR. Compared to MV and DV, the supply air temperature of SV is necessarily higher (e.g., 23°C in this study) to
provide a comfortable thermal environment, which implies a potential of energy saving. At the same supply air temperature, the mean air temperature in the occupied zone under SV is lower than that under DV or MV, indicating a higher cooling efficiency. Therefore, under proper supply parameters, SV is able to provide general satisfactory thermal comfort with higher room air temperature and lower energy consumption as compared with MV and DV. Acknowledgements
The work described in this article is supported by a General Research Grant from the National Natural Science Foundation of China (Project No. 51178407). The authors would also like to express thanks to Miss Weiqin Wu and Dr. Ting Yao for their assistants in experimental setup and Mr. Zhengtao Ai for his valuable advices. Supporting Information
Additional Supporting Information may be found in the online version of this article: Figure S1. Infrared photo of thermal manikins (°C). Figure S2. Distribution of mean air velocity for the other sampling lines in the occupied zone under 10 ACH. Figure S3. Visualization of stratum-ventilated flow pattern under 10 ACH, (a) supply temperature = 18°C; (b) supply temperature = 21°C. Figure S4. Distribution of mean air temperature for the other sampling lines in the occupied zone under 10 ACH.
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