Building and Environment 45 (2010) 1582–1593
Contents lists available at ScienceDirect
Building and Environment journal homepage: www.elsevier.com/locate/buildenv
A novel approach to enhance outdoor air quality: Pedestrian ventilation system Parham A. Mirzaei, Fariborz Haghighat* Department of Building, Civil and Environmental Engineering, Concordia University, Montreal, Quebec H3G 1M8, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 August 2009 Received in revised form 27 October 2009 Accepted 4 January 2010
Higher population density has altered the cities’ old landscape with dense areas consisting of high-rise buildings. As a result, detrimental phenomena appeared inside modern cities that threatened the inhabitant’s health and comfort. Among these phenomena, the Urban Heat Island (UHI) is known as the most harmful side effect of the urbanization which affects the Outdoor Air Quality (OAQ). In addition to the reduction of wind velocity within the urban canopies, the accumulated pollution decreases the OAQ and renders the pedestrian areas to hazardous level. According to earlier researches, the UHI generally shows more intensity in higher aspect ratio (the height of building to street breadth) canopies which mostly exist in high-density areas. These buildings’ canopies have typically higher pollution concentration than low-rise residential building canopies due to lower air exchange rate and heavy traffic load. The situation becomes worst under the stable atmospheric stratification condition when the canopy ground is colder than the ambient air. Many passive strategies have been proposed to enhance the OAQ. However, the variety of the UHI makes the passive mitigation strategies ineffective in some cases. In this paper a novel approach, the pedestrian ventilation system (PVS), is proposed to ventilate building canopy under various atmospheric stability conditions: stable, neutral, and unstable. The capability of this system to enhance the pedestrian level health and comfort parameters (i.e. velocity, temperature and air exchange rate) has been studied using Computational Fluid Dynamics (CFD) simulation. The results of the simulations confirm that the PVS can significantly improve the flow regime of the buildings’ canopy. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Urban heat island Mitigation techniques Comfort CFD Outdoor air quality Ventilation
1. Introduction Pedestrian’s health and thermal comfort become pivotal factors in design and planning of metropolitan areas. Temperature, humidity, solar radiation, wind velocity, and pollution concentration are known as outdoor comfort indices that are exacerbated by inappropriate design of the urban landscape. In addition to traffic, Urban Heat Island (UHI) is contributing to change the building canopies, major elements of an urban area, to hazardous places. Many studies have been conducted to understand the effect of urban feature and meteorological conditions on Outdoor Air Quality (OAQ) inside and over the building canopy; outdoor thermal comfort [5,19,22], pollution dispersion [2,4,26], effects of aspect ratio (the height of building to street breadth) and urban density on flow pattern [12,24,31], and thermal stratification [6,32,34]. Furthermore, research has been conducted to enhance the outdoor environment, especially at pedestrian level. The planting of trees and the greening of spaces [1,17], the material
* Corresponding author. E-mail address:
[email protected] (F. Haghighat). 0360-1323/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.01.001
alteration and shading [7,28], and urban design [21,25] are some attempts at improving the OAQ. Although it is possible to significantly improve the UHI and the pedestrian thermal comfort with widespread implementation of proposed strategies, it is not feasible to completely diminish the UHI and to significantly enhance the outdoor comfort within all the canopies spatially distributed in different points of a city. The countermeasures may affect the UHI and the pedestrian thermal comfort in many parts of a city, however, the range of comfort indices may not be acceptable in some building canopies. As an example, tree planting is normally a practical method to produce fresh air inside canopies by the natural transpiration effect of trees [17]. However, the trees themselves can become obstacles to the vertical or horizontal air movement which benefits the air pollution removal, thermal comfort, and the air conditioning of the building [20]. Moreover, they may shade the buildings in winter time and causes more energy consumption for heating purposes. All these aspects can be summarized as shortcomings of the passive strategies to control the OAQ. Nevertheless, it is obvious that the quality of outdoor environment has an important role on the quality of indoor environment [10,27]. Therefore, there is an urgent need to develop active strategy to improve the quality of outdoor air.
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
1583
on controlling pedestrian health and comfort parameters. For this purpose a three-dimensional building canopy with the aspect ratio of two is considered for this investigation. Moreover, the performance of the system is investigated under different atmospheric stratification conditions ranging from stable to unstable.
2. Pedestrian ventilation system 2.1. System configuration
Fig. 1. New design approach: Pedestrian Ventilation System (PVS).
In this article a novel mitigation technique called the Pedestrian Ventilation System (PVS) is proposed to actively control the pedestrian health and thermal comfort inside the canopies. Preliminary investigation is conducted to demonstrate the feasibility of the PVS
Three flow patterns are characterized based on geometry of a building canopy [24]: isolated roughness flow (IRF), wake interference flow (WIF), and skimming flow (SF). Many studies have been carried out to find corresponding threshold for aspect ratio of these flows [16,33]. It is found that the threshold from WIF to SF is around 0.7 (H/W). Moreover, after a certain aspect ratio (around 1.5) the main vortex in SF will be deformed to two vortices [16]. Again, the vortices number will be increased by raising aspect ratio of the building canopy. As most of available studies in literature, this research focuses on the skimming flow where consistent vortex/vortices that retain pollution and result weak air quality develop at the building canopy. In building canopies mean wind speed is not only an important parameter in air exchange, turbulence also plays a significant role on canopy ventilation, especially in skimming flow. Kim and Baik [11] demonstrated the importance of turbulence on the removal of pollutants. On the other hand, it is proven that buoyancy can increase or dominate turbulence inside some canopies by changing
Fig. 2. Different strategies of the PVS.
1584
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
Pedestrian Ventilation System
Prevailing Wind
A A
B
B
H
L
Pedestrian Ventilation Zone
Upward/ Downward Flow
Dampers
W
W
Pedestrian Walking Area
Lateral Flow
Fig. 3. (Left) Homogeneous array of buildings - (Right) Pedestrian ventilation system and pedestrian walking area.
the street and the buildings’ wall temperature [35]. This usually happens under different atmospheric stability conditions. Therefore, to control the air movement and pollutant dispersion inside building canopies, it is necessary to modify the air movement created by turbulence and buoyancy by imposing a controlled air movement. This air movement is different from unpredictable and stochastic vortex which is created by the top-canopy prevailing wind. It is postulated that the required air movement is obtainable with an active control system in the form of a pedestrian ventilation system. As shown in Fig. 1, the PVS induces air movement in a region near the ground, the Pedestrian Ventilation Zone (PVZ), using ventilation ducts. The PVZ volume is extended around the building up to 3 m in height (sidewalks or region in which most pedestrian activities occur). The mechanism for ventilation is based on guiding air through a designed vertical duct system from roof of the building to the surrounding street level. In stratified situation, the street temperature is lower than the prevailing wind temperature, and thus the pollutant is mostly accumulated in the PVZ. In this case, the PVS can replace the pedestrian level air with fresher air from the top-canopy level. On the other hand, this system is also useful to accelerate the movement of cooler air from the top-canopy to the PVZ where the weather is under unstable condition (when the prevailing wind temperature is colder than the canopy temperature). Therefore, the pedestrian air velocity, temperature and pollution concentration can be placed under control by changing the airflow rate within the building canopy: both natural and force convection can be used to provide the required pressure gradient for the system. Heating the duct can be used to provide the required air movement (stack flow). The required energy for heating can be provided by heat exhausted from condenser of the air conditioning systems, and/or solar energy which is mostly available during severe heat island episode. Alternately, force convection can be achieved using supply or exhaust fan. When the ambient air relative humidity is not within the thermal comfort range, the pedestrian ventilation system can Table 1 Proposed case studies based on thermal wind tunnel experiment by Uehara et al. [32].
Case I Case II
Stability condition
Bulk-Richardson number
Ta ¼ Wind temperature (K)
Tf ¼ Ground temperature (K)
Stable Unstable
0.89 0.18
351 293
294 352
humidify the PVZ with some water sprays (Fig. 1). Solar radiation can also be prevented by placing flexible pergolas (Fig. 1). In this research, however, only applicability of the PVS in air removal and changing air velocity and temperature within the building canopy has been studied using electrical fan. 2.2. Combined pedestrian ventilation system It is feasible to have various way of integrating the PVS inside a canopy by installing two systems on adjacent buildings (Fig. 2). These systems strengthen or weaken vortex/vortices of the building canopy. Strategy (A) uses two exhaust fans to intensify a downward flow. In strategy (B), an upward flow toward the topcanopy can be achieved using two supply fans. Strategies (C) and (D) are capable of establishing a washing flow through one sidewalk to another using a supply and an exhaust fan. It is noteworthy that closest vortex to the ground is either clockwise or counterclockwise depending on aspect ratio and number of vortices. Thus, always one of the strategies (C) or (D) is strengthening the flow and one is weakening that. Obviously, the required pedestrian comfort situation is an important factor in order to choose the effective strategy. This flexibility is investigated in the following sections under both stable and unstable conditions. 2.3. Air exchange concept The proposed PVS in this paper is a strategy to enhance pedestrian health and thermal comfort, during heat island and stratification period, using any available energy sources such as electricity which is used in this research. The air exchange rate (ACH) concept [3] is used to quantify air movement from the PVZ. The air exchange rate is defined as the total air that is entering
Table 2 Boundary conditions and solution schemes. Inflow boundary Outflow boundary Ground boundary Upper and side surface of domain Building surface boundary Turbulent scheme Momentum discretization Computational domain
Logarithmic flow from experiment [30] Zero gradient assumption Logarithmic law with roughness length (0.024 m) Free slip wall condition Logarithmic law for smooth wall Standard k 3 Second order Upwind 180 m(x) 280 m(y) 120 m(z)
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
Inflow
40
47
54
37
41
48
55
20 26 29 32
35 38
42
49
56 62
27 30 33
36 39
4
11
18
1
5
12
19
2
6
13
3
25 28
31 34
7
14
21
43
50
57
8
15
22
44
51
58
9
16
23
45
52
59
10
17
24
46
53 60
61
63
Fig. 4. Half-domain measured points by Tominaga et al. [30] located 2 mm above the ground (Top view).
(ACHþ) or leaving (ACH) from the lateral and top faces of the PVZ (Fig. 3 - Right). The magnitudes of ACHþ and ACH from PVS dampers and top and lateral surfaces of the PVZ are equal due to the mass conservation law:
ACH ¼ ðACHÞ þ ðACHþÞ ¼ ACH þ ACH0
(1)
ACHþ ¼ ðACHLateral þ Þ þ ACHTop þ þ ðACHPVS þÞ
(2)
ACH ¼ ðACHLateral Þ þ ACHTop þ ðACHPVS Þ
(3)
Without using PVS, ACHPVS is apparently zero in equations (2) and (3). Also, as shown in equation (1), ACH is made up of two parts: mean and fluctuation velocity. Vertical component (w) of these velocities has significant magnitude in the top-surface air exchange, while horizontal component (v) which is perpendicular to the flow is important in two lateral surfaces. Various twodimensional studies have been carried out to establish criteria for ACH which are only based on equation (1). Nonetheless, a well defined criterion is not available for three-dimensional ACH within the building canopies, and as presented later, a two-dimensional concept is adapted to investigate air movement in this study. This air movement, ACH, can be later used to understand the pollution exchange rate (PCH) of the canopy [18]. In this study, it is assumed that supply and exhaust fans control airflow rate of the PVS. 3. Methodology 3.1. Case study A simple case study is chosen to show the potential of the PVS in urban areas. An array of buildings with simple geometry has been selected with a PVZ volume of 1200 cubic meter (20 m(x) 20 m(y) 3 m(z)) for each building canopy (y is in the direction of prevailing wind). As demonstrated in Fig. 3 - Left, the
1585
urban landscape has been assumed as homogenous cuboid buildings with aspect ratio of two (H ¼ 40 m and L ¼ W ¼ 20 m). The PVS is also applied to a canopy between buildings (A) and (B) as depicted in Fig. 3. To include weather stability condition, simulations have been conducted separately for both stable and unstable conditions. Since there is a wide variation in wind speed and temperature values below the building height, Bulk-Richardson number ðRb ¼ gHðTH Tf Þ=fðTÞðUH Þ2 gÞ has been introduced in the building canopy literature as an appropriate dimensionless number to represent stability weather conditions [32]. In this equation g (m/ s2) is the acceleration due to gravity, TH (K) is temperature at the top of the building canopy, Tf (K) is temperature at the ground level, T (K) is the mean temperature, and UH (m/s) represents the mean wind speed at top of building canopy. As shown in Table 1, two Bulk-Richardson numbers, 0.89 and 0.18 have been chosen which respectively represent stable and unstable weather conditions. These numbers are in the range of thermal wind tunnel test (0.79 and 0.21) which was carried out by Uehara et al. [32]. The main concern in case (I), stable condition, is to remove the accumulated pollution from the PVZ that mostly occurs during nocturnal non-cloudy calm weather. In contrast, case (II) is related to unstable situations where the priority is to take advantage of the colder prevailing wind flowing over the canopy. Illustrated in Fig. 2, four strategies have been applied in each case study in this paper. In this study, the cross-section of the ducts is assumed to be rectangular for both cases installed on both buildings (A) and (B). Three dampers are also considered inside each sidewalk with an area of 1 m square (Fig. 3). Moreover, the duct surface is assumed to be well insulated to prevent any heat transfer. To provide the required airflow for ventilating the PVZ around once per minute (10–20 m3/s), adequate supply and/or exhaust fans with pressure differences of 100 pa are also assumed. Thus, the induced air velocity by PVS dampers remains between 1 and 2.3 m/s which is inside the light breeze norm [14]. Furthermore to simplify the calculation, radiation modeling and humidity calculation are neglected in this study. Many parameters contribute to the PVS performance, including building canopy aspect ratio and orientation, prevailing wind velocity and its direction, cloud cover, and the PVS design. Thus, various studies are necessary to understand the influence of the above mentioned parameters on the pedestrian comfort. 3.2. Solution scheme The PVS has been simulated using Computational Fluid Dynamics (CFD) approach: FLUENT software was used in this study
Fig. 5. Comparison between measurement and CFD with different mesh size.
1586
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
Fig. 6. Comparison between measurement and CFD with different domain fetch.
[8]. Around 320,000 structured meshes have been generated using a commercial package, GAMBIT [9]. Finer resolution has been applied to monitor the performance of the PVS within the target building canopy. Also, half of the domain has been computed due to symmetry of the cases. Steady scheme has been used with standard k 3 model for turbulent closure. Tominaga et al. [30] used different turbulence models and concluded that the standard k 3 model provides almost the same result as the other models except for the circulating flow region behind the building. Presented case studies in Table 1 equipped with the PVS, shown in Fig. 3 - Right, originated with the initial condition as tested by Uehara et al. [32]. The boundary conditions and solution schemes are also given in Table 2. 3.3. Mesh and domain size test The CFD model was firstly verified with wind tunnel experiment for case (C) of AIJ [30]. This test has been conducted to an array of building similar to Fig. 3 (L ¼ H ¼ W ¼ 0.2 m). An inflow velocity has been chosen with a recorded log-profile. The wind velocity has been measured at 63 points located 2 mm above the ground surface (Fig. 4). Several cases have been simulated to find the appropriate mesh size, fetch length and vertical height in order to optimize mesh numbers, and decrease the computational cost of the final study. Different mesh sizes have been tested to find the proper dimension to simulate the study domain, these included 0.2 H, 0.25 H, and 0.3 H. The result clearly demonstrates that a 0.25 H mesh size is good enough to model the case study (Fig. 5). A similar size is applied by [36]. They concluded that an appropriate mesh size is around H/10. The reason that 0.2 H for mesh size is not better than 0.25 H is related to the wall-function assumption for the walls
[13]. In the wall-function approach, semi-empirical formulas, the viscous sub-layer and the buffer layer is not resolved. This means that the wall-function is used to connect the viscosity-effected region between the wall and the fully-turbulent region. It is necessary for the horizontal computational domain to be maintained on a certain length extending outside of the urban block border. As illustrated in Fig. 6, three cases are compared with fetch sizes of 2 H, 4 H, and 10 H. It is obvious from this figure that the results do not change significantly when the fetch length is increased from 4 H to 10 H. Also, as demonstrated in Fig. 7, a height of 5 H provides almost the same result as the case where the height is 6 H (wind tunnel height). This conclusion is corroborated by Tominaga et al. [29]: they suggest a vertical domain height of 3 H or more. From results shown in Figs. 5–7, it can be concluded that a domain with a fetch size of 4 H, a height of 5 H, and a mesh size of 0.25 H may optimize the computational cost of the simulation. Therefore, the case (C) was again simulated with the obtained mesh, height and fetch size (around 150,000 meshes in total). As shown in Fig. 8, air velocities are in good agreement to wind tunnel measurements. 3.4. Model verification Choosing appropriate strategy of the PVS significantly depends on the air stability regime. Therefore, it is necessary to include this effect in the simulations. To verify the thermal stratification of the building canopy, the simulation was verified with thermal wind tunnel experiment by Uehara et al. [32]. The test was performed at the National Institute for environmental Studies of Japan [23]. The turbulence was modeled with an array of Styrofoam cubes. Also, stratification was produced by changing the ground and air inflow
Fig. 7. Comparison between measurement and CFD with different domain height.
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
1587
Fig. 8. Comparison between measurement and CFD with suggested mesh size, domain height and fetch length.
temperature similar to Table 1. The buildings consisted of 0.1 m cubes placed 0.1 m and 0.05 m apart along the length and width of the tunnel, respectively. The ground and air temperature were set in order to attain Bulk-Richardson number of 0.21 and 0.79 for unstable and stable stratification, respectively. Fetch length and mesh size of the domain were respectively chosen 4 H and 0.25 H based on last section test. Domain height, however, was selected 10 H to capture the thermal stratification effect. The number of meshes was around 350,000; k 3 model was used as turbulent model. As shown in Fig. 9, the simulation results agreed well with the measurements. Here U700 is attributed to air velocity at height of 0.7 m from bottom of the target building canopy in the wind tunnel experiment.
4. Results and discussion The result for various strategies of the PVS integration is illustrated in Table 3. Presented airflow rate in this table is attributed to air entering (positive number) or leaving (negative number) top pedestrian zone and two lateral faces (Fig. 3-Right). All numbers are normalized by PVZ volume per minute (Q ¼ PVZ/minute). In stable weather conditions (Rb ¼ 0.89), a balance exists between entering air from the top-surface (ACHTop ¼ 0.66) and leaving air from lateral surfaces (ACHLateral ¼ 0.66) of the pedestrian zone. This air circulation is much stronger in unstable condition where the entering and leaving airflow rates are 2.41 and 2.41, respectively. Apparently, the PVS is capable to produce a fair
Fig. 9. Comparison between measurement and CFD (Left) Velocity (Right) Temperature.
1588
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
Table 3 Air removal capability (normalized by Q) for the PVS strategies. Stable condition (Bulk-Richardson ¼ 0.89)
Unstable condition (Bulk-Richardson ¼ 0.18)
Strategy
No PVS
(A)
(B)
(C)
(D)
No PVS
(A)
(B)
(C)
(D)
ACHLateral ACHTop ACHPVS 0 ACHTop
0.66 0.66 0.00 1.20
0.44 1.12 0.68 1.37
0.85 0.11 0.74 1.83
0.68 0.63 0.05 1.55
0.73 0.68 0.05 1.98
2.41 2.41 0.00 3.39
1.21 1.81 0.60 2.57
2.79 2.18 0.61 3.19
1.49 1.45 0.04 2.43
2.69 2.69 0.00 3.24
1.2 0.098
0 .0 7 7
1 0 .0 77 77
0.6
0 .0
z /H
0.8
0.4
-0.5
-0.25
0
0 .0 3 2
0.0 20
0
07
0.050
0 .0 0 9
0.
0.2
0
0.25
x/H
0.5
Without PVS A
B
1.2
1
0.100 0.0 68
1.2
0.080
1
0.0 90
0.071
0 .0
0.080
z /H
z /H
68
0.068 0.6
0.0
.6
58
0.4
0 .0
58
0.4
0.2
0 .0 0
9
0.029
-0.25
0.02 9
0
0.25
x/H
0.5
1.2
1
0.080
0.1 00 0.060
D
-0.25
0
x/H
0 .1
0.1 00
00
0 .0
0.8
67
.6
0.25
0.5
1.2
1
z/H
0 .0
80
-0.5
0 .0
72
72
0.
0.6
06
6
0 .0
67
z/H
0.8
0 0.
0.014
0.037
0 .0 5
C
0
0.037
0
-0.5
7
71
0 .0 1 7
0
0 .0 3
0 .0 5 8
0 .0
0.2
71
0.8
0 .0
0.8
0 .0
0
x/H
0.25
0.5
0
0 .0
-0.5
-0.25
0 .0 66
0.03 9 0. 02 1
72
2
8
-0.25
02
-0.5
5 0.035
0.
0
0.2
0 .0 3
0 .0
0.0 37
0.2
0.4 0 .0
0.060 0 .0 5 4
0.0 17
0.4
0.050 0
x/H
0.25
0.5
Fig. 10. Spatial contours of K/U2H for various PVS strategies inside the building canopy under unstable condition (Rb ¼ 0.18).
7
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
1589
1.2 05
32
0
1
0.032
0.8
z /H
0 .0
0.
0.050
0 .0 2
0.6
5
0.4 0.016
0.012
0.2
0.008
0.006
0
-0.5
-0.25
0
0.25
x/H
0.5
without
B 1.2
1.2
0.06 1
6
0 .0 6 1
1
0. 0 04
0 .0
0.8
6
37
02
0 .0
0.
0.4
12
0.0 17
0.
02
0.4
0.6
31
0.032
0.008
0.01 0
-0.25
0.00 8
0.006
0
0.25
x/H
0.
0
0.25
x/H
0.5
z/H
0 .0 22
0.03 8
0.0 56
0.018
0 .0 1 7
-0.5
0 -0.25
22
0
x/H
0.2
0.008 .
1 03
0.25
0.5
0
0.035
0.026
4
0 .0
0 .0 2
0.2
5
.6
0.4 0.031
0 .0 3
0.05 6
.8
38
0.01 7
0.4
1
5
.6
03 8
03 0.
0 .0
0
-0.25
1 03
0.03 5
-0.5
-0.25
0 .0
33
0.026 0
x/H
0.006
0.050
0.8
z /H
-0.5
0.
D 1.2
1.2
1
0
0.5
0 .0 3 7 0.026
17
C
-0.5
0.034 0 .0
0
0.2
0 .0
0.2
37
0 .0
z/H
0.6
0
z /H
0.8
85
0 .0 2
0 .0
0.060
1
0.018
A
0.25
0.5
Fig. 11. Spatial contours of K/U2H for various PVS strategies inside the building canopy under stable condition (Rb ¼ 0.89).
air exchange within the PVZ where supply and exhaust fans are used. This means that these strategies can double the air exchange of the entire PVZ volume (1200 m3). For example, strategy (A) entrains more fresh air from top-PVZ (around 70%) to the PVZ under stable condition using exhaust fan. The air exchange ratios (ACH (No PVS)/ACH (Strategy A, B, C, or D)) are generally lower under unstable conditions due to the stronger circulation of the building canopy which partly opposes the induced flow by the PVS. Moreover, fan air exchange rate (ACHPVS) has the same order of magnitude with ACHLateral and ACHTop in stable case, however, the
ACHPVS is around four times smaller than ACHLateral and ACHTop in unstable case. This can be compensated using more powerful fan. Generally, it can be concluded that strategy (A) reduce horizontal air exchange inside the PVZ and therefore, entrains more air from top-PVZ. On the other hand, strategy (B) decreases the vertical air exchange. Also, strategies (C) and (D) change the magnitude of the initial balance between the vertical and horizontal air exchange rate imposing a washing flow from one sidewalk. As mentioned earlier, turbulence has significant effect on the air exchange of PVZ. Nonetheless, only few works have been carried
1590
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
out to analyze this effect and most of them were two-dimensional studies in which the influence of lateral surfaces is neglected. In this study, the approach proposed by Li et al. [15] is used to calculate the turbulence at top-PVZ surface. They assumed the transient air exchange is divided evenly into entering and leaving parts. There0 can be determined as follows: fore, ACHTop
1 ACHTop þ ¼ ACHTop ¼ 2 0
0
Z G
w00 w00 1=2
dG
(4)
roof
where w00
is the vertical velocity fluctuation. They also assumed the isotropic condition for turbulence because of the high-Reynolds number ðw00 w00 ¼ v00 v00 ¼ u00 u00 Þ:
k ¼ ðw00 w00 þ v00 v00 þ u00 u00 Þ=2 ¼ 3w00 w00 =2 0 1 ACHTop þ ¼ pffiffiffi 6
Z qffiffiffiffiffiffiffiffiffiffiffi kjroof dG
(5) (6)
G
Depicted in Table 3, however, in this study K is calculated on the PVZ top-surface which may cause some discrepancy. Obviously, turbulence fluctuation considerably changes in different strategies. For example, this number has been altered more than 50% in strategy (B) of stable case. Figs. 10 and 11 also demonstrate the normalized K by square of air inflow velocity (U2H) in midplane of the building canopy without and with PVS strategies. Again, it is evident that the turbulence kinetic energy roughly varies within the PVZ. The value inside the sidewalks is extremely weak in stable case; nonetheless it is remarkably increased using the PVS strategies.
In addition to the air removal ability, providing appropriate velocity is another task of the PVS. Fig. 12 illustrates the vertical profile of the velocity normalized by the inflow velocity (UH) in middle of the pedestrian sidewalks (1 m from walls), located at the center plane of the building canopy. The left and right graphs respectively depict the right-sidewalk and left sidewalk. The prevailing wind is from left to right as demonstrated in Fig. 3. In addition to applying different PVS strategies, the skimming flow air circulation inside the canopy initiates an asymmetry between these two sidewalks. Normally without the PVS, the vertical velocity profile tends to be the same under both stable and unstable conditions, even though the velocity magnitude is higher during unstable situations. This higher magnitude is related to the stronger initial air circulation (vortex). The maximum velocity in Fig. 12 is related to the place of dampers where air is supplied or exhausted. Similar to strategies (B) and (D), the vertical air velocity profile in the right-sidewalk is the same for strategies (A) and (C). This is due to the existence of a similar supply or exhaust fan in these cases. Correspondingly, in the left-sidewalk, the vertical velocity profile is almost the same for strategies (A)–(D) and (B)–(C). However, the velocity magnitude of strategy (D) is higher than strategy (A) in this case due to the direction of the horizontal washing flow produced by synchronizing of the supply and exhaust fans. This means that vortex flow (a secondary weak clock-wise vortex opposing with main counter clock-wise vortex of the building canopy) is intensified in the leftsidewalk for strategy (D). However, this effect is not seen for strategy (C) on the right-sidewalk (no considerable difference between strategy (A) and (C)). The reason is that vortex flow does
Fig. 12. Vertical Air velocity profile comparison for various PVS strategies through the pedestrian walking area (Left) Left-sidewalk of the canopy (Right) Right-sidewalk of the canopy.
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
1591
Fig. 13. Horizontal air velocity profile comparison for the PVS strategies through the pedestrian walking area (Left) Left-sidewalk of the canopy (Right) Right-sidewalk of the canopy.
not considerably affect the exhaust flow in this case. It can be generally concluded that the vertical velocity profile is slightly affected by the stability condition inside pedestrian sidewalks. On the contrary, velocity magnitude is highly dependent on the stability situation. Horizontal velocity profiles normalized by inflow velocity (UH) through a line located 1.5 m above the ground (parallel to the building canopy surface) in middle of right-sidewalk and leftsidewalk (1 m from walls) are demonstrated in Fig. 13. The graphs only show half of the canopy (due to the symmetry condition) where the zero in the (y) direction signifies the middle of the canopy. The peak values for air velocity in these graphs are assigned to the outlet dampers of the PVS. In this research three outlet dampers were used, and it is obvious that increase of the damper’s number will produce smoother velocities through the sidewalks. It seems that the effect of atmospheric stability is not significant on the horizontal velocity profile. However, vortex circulation within the building canopy has little influence on the velocity magnitude through the left-sidewalk. This effect is much weaker in rightsidewalk. Air supplying strategies on the right-sidewalk, (B) and (D), change the velocity ratio from approximately 0.1 to a maximum of 0.5. Strategy (B) has the same effect on the left-sidewalk. However, strategy (D) cannot produce a similar velocity because of the mixing of the supplied air with existed circulation regime of the building canopy. It can be seen that the mixing is reduced under stable conditions where the vortex circulation regime is weak.
Generally, the exhaust strategy, (A), provides smooth velocity in both right and left sidewalks, but normally with a lower velocity magnitude. The vertical air temperature profile in the middle of the building canopy is shown in Fig. 14. Obviously, the air temperature does not alter considerably above the pedestrian area. However, the air temperature fluctuation mostly occurs in pedestrian level as depicted in Fig. 15. Fig. 15 shows intense air temperature fluctuations inside sidewalks. These results are again related to the middle of the right-side and left-side pedestrian zones (1 m from walls) situated 1.5 m above the ground level. Also, the intensive increase and decrease of the air temperature again are related to place of the dampers. When atmospheric conditions are unstable, it means that the prevailing wind is cooler. Therefore, as demonstrated in Fig. 15, strategies (B) and (D) are appropriate to reduce sidewalk’s air temperature (greater number of (TTf)/(TaTf)). This temperature decrease is more predominant in the left-sidewalk than rightsidewalk. Strategy (C) is evidently not capable of making a significant change in the air temperature of the zones. Although using an exhaust fan in strategy (A) increases the air temperature of the leftsidewalk pedestrian zone, it reduces that of on the right-sidewalk. Using a warmer air temperature increases the pedestrian zone temperature in stable cases. Therefore, strategy (A) is the best technique to avoid the warmer prevailing wind temperature (smaller number of (TTf)/(TaTf)). Generally, it can be concluded that strategy (A) provides a fair air removal from the canopy. Although the velocity comfort parameter
1592
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
Fig. 14. Air temperature profile comparison of the PVS strategies in middle of the building canopy (Left) Unstable case (Right) Stable case.
is poor in this strategy, it is more practical than strategy (B) in increasing air exchange. The reason is that some part of the supplied air returns to the canopy by using strategy (B). On the other hand, strategies (C) and (D) are appropriate options when there is a high source of pollution (e.g. vehicles and pedestrians). With these options a proper air movement in addition to cooler temperature within the pedestrian walking area can be obtained, especially when the focus is on one side of the street. Although air exchange rate is weak in these techniques, a strong washing flow can be produced from one sidewalk to another. Under the above mentioned assumption of this research, it can be concluded that strategy (B), air supply mechanisms, produce
proper conditions for air velocity, temperature and air removal indices. Both mean and fluctuation velocities are considerable in this case. The air temperature decreases under unstable atmospheric conditions and remains almost constant under stable condition. Also, the air velocity advances significantly in both pedestrian sidewalks from a light-air situation to a light breeze norm [14]. Although two cases, under stable and unstable weather situation, have been investigated, the presented results are not unique answers of this system. As mentioned previously, the benefit of this system is its adaptability under various flow regimes. For example, the vortex circulation regime changes majorly in higher building
Fig. 15. Air temperature profile comparison of various PVS strategies on pedestrian walking area (Left) Left-sidewalk of the canopy (Right) Right-sidewalk of the canopy.
P.A. Mirzaei, F. Haghighat / Building and Environment 45 (2010) 1582–1593
aspect ratios, and this may cause different air movement than discussed in this study. Future research will focus on a parametric study of the pedestrian ventilation system to propose in situ control strategies for the pedestrian ventilation zone. 5. Conclusion A novel building canopy ventilation system, pedestrian ventilation system, has been introduced to improve pedestrian level air quality, especially within high-rise building areas. An active control technique is used in this system to enhance human comfort parameters including, the air exchange, temperature and velocity inside the pedestrian sidewalks. As its energy source, this system is capable of using electricity and solar energy, and even waste energy from the building’s condenser units. Effective system performance parameters are categorized as street design parameters (i.e. ducting design, energy source used, aspect ratio, etc.) and meteorological conditions (i.e. weather stratification regime, wind velocity, etc.). To obtain basic knowledge about the PVS and understand the effectiveness of the above mentioned parameters, firstly the model has been verified with two wind tunnel experiments. To show the applicability of the proposed strategies two case studies have been carried out using CFD simulation. The results fairly show the ability of this system to provide air movement inside the building canopy under stable and unstable weather condition considering the air exchange rate criteria. An increase in this parameter can improve pedestrian comfort, particularly during severe heat island episode. Moreover, the simulation demonstrated that this system can bring top-canopy air to the PVZ when the atmosphere is unstable. To propose the PVS as a practical ventilation system, more experimental and simulation based on influential parameters are needed. Also, to achieve more realistic results, coupling of heat storage, humidity and radiation models with CFD simulation is strongly recommended. Acknowledgements The authors would like to express their gratitude to the Natural Science and Engineering Research Council Canada (NSERC), and Concordia University for their financial support. References [1] Alexandria E, Jones P. Temperature decreases in an urban canyon due to green walls and green roofs in diverse climates. Building and Environment 2008;43:480–93. [2] Bady M, Kato S, Huang H. Towards the application of indoor ventilation efficiency indices to evaluate the air quality of urban areas. Building and Environment 2008;43:1991–2004. [3] Bentham T, Britter R. Spatially averaged flow within obstacle arrays. Atmospheric Environment 2003;37:2037–43. [4] Cermak JE. Thermal effects on flow and dispersion over urban areas: capabilities for prediction by physical modeling. Atmospheric Environment 1996;30:393–401. [5] Chen H, Ooka R, Harayama K, Kato S, Li X. Study on outdoor thermal environment of apartment block in Shenzhen, China with coupled simulation of convection, radiation and conduction. Energy and Buildings 2004;36:1247–58. [6] Cheng WC, Liu C-H, Leung DYC. On the correlation of air and pollutant exchange for street canyons in combined wind-buoyancy-driven flow. Atmospheric Environment 2009;43:3682–90. [7] Doulos L, Santamouris M, Livada I. Passive cooling of outdoor urban spaces. The role of materials. Solar Energy 2004;77:231–49. [8] Fluent, http://fluent.com/; 2008. [9] GAMBIT, http://fluent.com/software/gambit/; 2008. [10] Hoppe P. Different aspects of assessing indoor and outdoor thermal comfort. Energy and Buildings 2002;34:661–5.
1593
[11] Kim J-J, Baik J-J. Effects of inflow turbulence intensity on flow and pollutant dispersion in an urban street canyon. Journal of Wind Engineering and Industrial Aerodynamics 2003;91:309–29. [12] Kubota T, Miura M, Tominaga Y, Mochida A. Wind tunnel tests on the relationship between building density and pedestrian-level wind velocity: development of guidelines for realizing acceptable wind environment in residential neighborhoods. Building and Environment 2008;43:1699–708. [13] Launder BE, Spalding DB. Numerical computation of turbulent flows. Computer Methods in Applied Mechanics and Engineering 1974;3:269–89. [14] Lawson TV, Penwarden AD. The effects of wind on people in the vicinity of buildings. 4th International Conference on wind effects on buildings and structures. Cambridge University Press; 1975. pp. 605–622. [15] Li X-X, Liu C-H, Leung DYC. Development of a k-[epsilon] model for the determination of air exchange rates for street canyons. Atmospheric Environment 2005;39:7285–96. [16] Li X-X, Liu C-H, Leung DYC, Lam KM. Recent progress in CFD modelling of wind field and pollutant transport in street canyons. Atmospheric Environment 2006;40:5640–58. [17] Lin B, Xiaofeng Zhu L, Qin Y. Numerical simulation studies of the different vegetation patterns’ effects on outdoor pedestrian thermal comfort. Journal of Wind Engineering and Industrial Aerodynamics 2008;96:1707–18. [18] Liu C-H, Leung DYC, Barth MC. On the prediction of air and pollutant exchange rates in street canyons of different aspect ratios using large-eddy simulation. Atmospheric Environment 2005;39:1567–74. [19] Mochida A, Lun IYF. Prediction of wind environment and thermal comfort at pedestrian level in urban area. Journal of Wind Engineering and Industrial Aerodynamics 2008;96:1498–527. [20] Mochida A, Tabata Y, Iwata T, Yoshino H. Examining tree canopy models for CFD prediction of wind environment at pedestrian level. Journal of Wind Engineering and Industrial Aerodynamics 2008;96:1667–77. [21] Murakami S. Environmental design of outdoor climate based on CFD. Fluid Dynamics Research 2006;38:108–26. [22] Nikolopoulou M, Lykoudis S. Thermal comfort in outdoor urban spaces: analysis across different European countries. Building and Environment 2006;41:1455–70. [23] Ogawa Y, Diosey PG, Uehara K, Ueda H. A wind tunnel for studying the effects of thermal stratification in the atmosphere. Atmospheric Environment 1981;15:807–21. [24] Oke TR. Street design and urban canopy layer climate. Energy and Buildings 1988;11:103–13. [25] Priyadarsini R, Hien WN, Wai David CK. Microclimatic modeling of the urban thermal environment of Singapore to mitigate urban heat island. Solar Energy 2008;82:727–45. [26] Sarrat C, Lemonsu A, Masson V, Guedalia D. Impact of urban heat island on regional atmospheric pollution. Atmospheric Environment 2006;40:1743–58. [27] Stathopoulos T, Wu H, Zacharias J. Outdoor human comfort in an urban climate. Building and Environment 2004;39:297–305. [28] Synnefa A, Santamouris M, Apostolakis K. On the development, optical properties and thermal performance of cool colored coatings for the urban environment. Solar Energy 2007;81:488–97. [29] Tominaga Y, Yoshie R, Mochida A, Kataoka H, Harimoto K, Nozu T. Cross comparisons of CFD prediction for wind environment at pedestrian level around buildings. part 2: comparison of results for flowfield around building complex in actual urban area, The Sixth Asia-Pacific Conference on Wind Engineering, Korea; 2005. [30] Tominaga Y, Mochida A, Yoshie R, Kataoka H, Nozu T, Yoshikawa M, et al. AIJ guidelines for practical applications of CFD to pedestrian wind environment around buildings. Journal of Wind Engineering and Industrial Aerodynamics 2008;96:1749–61. [31] Uehara K, Wakamatsu S, Ooka R. Studies on critical reynolds number indices for wind-tunnel experiments on flow within urban areas. Boundary-Layer Meteorology 2003;107:353–70. [32] Uehara K, Murakami S, Oikawa S, Wakamatsu S. Wind tunnel experiments on how thermal stratification affects flow in and above urban street canyons. Atmospheric Environment 2000;34:1553–62. [33] Vardoulakis S, Fisher BEA, Pericleous K, Gonzalez-Flesca N. Modelling air quality in street canyons: a review. Atmospheric Environment 2003;37: 155–82. [34] Xie X, Liu C-H, Leung DYC. Impact of building facades and ground heating on wind flow and pollutant transport in street canyons. Atmospheric Environment 2007;41:9030–49. [35] Xie X, Huang Z, Wang J, Xie Z. The impact of solar radiation and street layout on pollutant dispersion in street canyon. Building and Environment 2005;40:201–12. [36] Yoshie R, Mochida A, Tominaga Y, Kataoka H, Yoshikawa M. Cross comparisons of CFD prediction for wind environmentat pedestrian level around buildings. comparison of results for flow-field around a high-rise building located in surrounding city blocks, The sixth Asia-Pacific conference on wind engineering; 2005.
