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Comparison of gaseous contaminant diffusion under stratum ventilation and under displacement ventilation
Building and Environment 45 (2010) 2035–2046
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Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Comparison of gaseous contaminant diffusion under stratum ventilation and under displacement ventilation Lin Tian a, Zhang Lin b, Qiuwang Wang a, * a b
State Key Lab of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China Building Energy & Environmental Technology Research Unit, Division of Building Science and Technology, City University of Hong Kong, Hong Kong SAR, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2009 Received in revised form 3 January 2010 Accepted 5 January 2010
The gaseous contaminant diffusion under stratum ventilation is investigated by numerical method which is validated by experiments carried out. The concentration of gaseous contaminants along the supply air jet is found to be lower than the other parts of the room. Compared with displacement ventilation, the formaldehyde concentration in breathing zone is lower when a contaminant source locates close to the occupant. The concentration is at the same level when the contaminant source locates up-steam to the occupant. The concentration in the occupied zone (<1.9 m from the floor) is also lower when the contaminant source locates on the floor. At supply air temperature optimized for displacement ventilation, the toluene concentration in breathing zone for stratum ventilation is higher than that for displacement ventilation when the area source locates on the four surrounding walls of the room. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Stratum ventilation Gaseous contaminant concentration Air quality Comparison
1. Introduction The earth is already showing many signs of worldwide climate change. The Intergovernmental Panel on Climate Change (IPCC) has used its strongest language to link human activities to the current planetary warming [1]. There is international consensus to reduce CO2 emission (Kyoto Protocol, etc.). Minimizing the energy consumption by air conditioning systems would help to reduce CO2 emission. Arens et al. 2009 reported that when the room air temperature above 22.5 C, there is a small risk of draft and a strong preference for more air movement [2]. ANSI/ASHRAE Standard 552004 has been updated with new provisions that allow elevated air movement to broadly offset the need to cool the air in warm conditions [3]. In revising EN ISO 7730, Olesen adopted Fountain and Arens’ (1993) theory that higher air speed was required to offset increased indoor temperature [4,5]. Proactive actions in this regard have been taken by several governments in East Asia. The Electrical and Mechanical Services Department (EMSD) of the Hong Kong S.A.R. government issues guidelines to ensure that normally room temperature is adjusted to 25.5 C in summer [6]. The National Development and Reform Commission (NDRC) of the Chinese State Council issued a similar guideline to set the indoor temperature to 26 C in the cooling season [7]. The room temperature in the ‘‘Office of President’’ in Taipei has been set to 27 C after
* Corresponding author. E-mail address: [email protected] (Q. Wang). 0360-1323/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.01.002
Mr. Ma Ying-jeou’s inauguration in May 2008 [8]. Similarly, in the Standard of Energy Management issued in last December, the Ministry of Knowledge and Economy of the Republic of Korea recommend that room temperature of a building should be ranging from 26 to 28 C in summer [9]. In a more radical move, the Ministry of the Environment (MoE) of Japan has been encouraging people to set the temperature of air conditioners at offices to 28 C during the summer months [10]. A survey from The International Facility Management Association shows that many facility professionals are adjusting the thermostat to higher settings in the summer to cut energy consumption in United States [11]. Recently, stratum ventilation, a new ventilation method, was proposed by Lin et al. [12,13] for small to medium rooms. This ventilation method works by creating a layer of fresher air in occupants’ breathing zone. It delivers supply air directly to the occupants with little attention to the indoor air quality (IAQ) and thermal comfort of the upper zone (approximately > 1.5 m from the floor if the occupants are mostly sedentary). The lower zone (approximately < 0.8 m from the floor) is not the target zone of IAQ performance. This is realized by positioning supply terminal(s) at the side-walls or columns slightly above the height of occupants, standing or sitting depending on the application, whereas for displacement ventilation, air for breathing is transported by the boundary layer around the body of an occupant and the air quality is a weighted average of the air quality in the room from the breathing level to the floor level. Ventilation efficiency can be expected to increase if air is supplied directly into the breathing zone. Therefore, compared with mixing ventilation and
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Table 1 Source and diffusion coefficient for each 4.
f
Gf
1 uj k
0
m þ mt ðm þ mt Þ=dk ðm þ mt Þ=33 m=d1 þ mt =dt mt =Sct mt =Sct
3 T C
s
Sf 0 vp vx rbgj ðT T0 Þ j G r3 þ GB ðC31 G C32 r3 þ C33 GB Þ3=k þ R ST Sc
r
where 2 vuj i vui m is laminar viscosity; mt ¼ rCm k3 is turbulent viscosity; G ¼ mt vu vxj ð vxj þ vxi Þ is the mt vT turbulent production; GB ¼ gi bPrt vxi is the turbulent production due to buoyancy; Cm h3 ð1h=h0 Þ 32 k 1þbh3 vuj i Si;j ¼ 12ðvu vxj þ vxi Þ;
R ¼
is the source term from renormalization; h ¼ Sk3 , S ¼ ð2Si;j Si;j Þ1=2 , Cm ¼ 0:0845, C31 ¼ 1:42, C32 ¼ 1:68, C33 ¼ 1:68 are the model
constants’ dk ¼ 0:7194, d3 ¼ 0:7194, d1 ¼ 0:7, dt ¼ 0:9, Sct ¼ 0:7 are Prandtl or Schmidt number.
displacement ventilation, stratum ventilation may offer higher ventilation efficiency. The range of face velocity and the locations of air supply gear should be carefully optimized to break the boundary layer around the body of an occupant and to minimize the risks of draft. The required height of the breathing layer/zone depends on the nature of occupancy. The supply air terminals should be positioned at the occupants’ head level. At the same time, a quasistagnant zone is also formed between the breathing zone and the floor (0 < height < 0.8 m). The temperature within the quasi-stagnant zone should be reasonably controlled which would not be lower than that of displacement ventilation. The problem of ‘‘cold ankles’’ could be solved. Energy is also saved by avoiding overcooling of the lower zone of a room. Indoor air should be mixed well and air temperature gradient in the occupied zone should be lower than that of displacement ventilation. Because air is supplied directly to breathing zone, the air supply temperature should be higher than that of mixing ventilation. This implies that the evaporating temperature of the refrigerating plant may also be elevated accordingly, which would result in higher coefficient of performance (COP). From the view point of heat removal ability, stratum ventilation, which can be used in elevated indoor air temperature condition, does not have to remove so much heat as that of mixing ventilation and displacement ventilation. The need to remove heat is mainly to provide thermal comfort to occupants in a non-industrial room. Although the supply air temperature for stratum ventilation is higher than that of conventional ventilations, the distance between the occupants and the air terminal is shorter, which results in:
Fig. 1. Configurations of test chamber.
For both stratum ventilation and task station ventilation, occupants stay in the flow of supply air jet(s). This significantly improves the inhaled air quality. However, current task station ventilation does not provide reversed temperature gradient in the occupied
1) reversed temperature gradient in the occupied zone - the temperature is lower at the head-and-chest level (the breathing zone)and higher at the ankle level, which effectively cools the body parts that need cooling the most; 2) higher air speed for the same airflow rate, which offsets the effect of higher temperature in thermal comfort; and 3) reduced room cooling load – Because of Points 1) and 2), the mean room temperature for equal thermal comfort is significantly higher than that of conventional ventilations. The heat transmission therefore decreases.
Table 2 Heat loads for studied cases. Items
Heat load (W)
Human simulators Computer box Lamps
75 180 72 2
Fig. 2. Layout of contaminant sources and measurement positions (mm).
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
z(m)
z(m)
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
v(m/s)
Line 2: x = 1.65, y = 1.45 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
z(m)
z(m)
Line 1: x = 0.6, y = 1.45 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
v(m/s)
Line 4: x = 2.15, y = 1.45 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
z(m)
z(m)
Line 3: x = 1.85, y = 1.45 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
v(m/s)
Line 6: x = 3.2, y = 1.45 (m)
z(m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
v(m/s)
Line 7: x = 2.15, y = 1.15 (m)
z(m)
z(m)
Line 5: x = 2.95, y = 1.45 (m)
Line 8: x = 1.85, y = 1.15 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
Line 9: x = 1.325, y = 0.78(m) Fig. 3. Measured and simulated velocity profiles for various positions (m/s).
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2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
Measured Simulated
z(m)
z(m)
2038
21
22
23
24
25
o
26
27
28
29
30
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
Measured Simulated
21
22
23
24
z(m)
z(m)
Measured Simulated
21
22
23
24
25
26
27
28
29
30
o
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
21
22
z(m)
z(m) 23
24
25
o
23
24
26
27
28
29
30
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
21
22
24
23
24
25
26
27
28
29
30
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
26
27
28
29
30
25
o
26
27
28
29
30
Measured Simulated
21
22
23
24
o
25
26
27
28
29
o
t ( C)
t ( C)
Line 7: x = 2.15, y = 1.15 (m)
z(m)
z(m)
z(m) 23
25
o
Line 6: x = 3.2, y = 1.45 (m)
Measured Simulated
22
30
t ( C)
Line 5: x = 2.95, y = 1.45 (m)
21
29
Measured Simulated
t ( C)
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
28
Line 4: x = 2.15, y = 1.45 (m)
Measured Simulated
22
27
t ( C)
Line 3: x = 1.85, y = 1.45 (m)
21
26
Measured Simulated
t ( C) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
o
Line 2: x = 1.65, y = 1.45 (m)
Line 1: x = 0.6, y = 1.45 (m) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
25
t ( C)
t ( C)
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
Line 8: x = 1.85, y = 1.15 (m)
M easured Simulated
21
22
23
24
25
26
27
28
29
30
o
t ( C)
Line9: x = 1.325, y = 0.78(m) Fig. 4. Measured and simulated temperature profiles for various positions ( C).
30
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
Measured Simulated
z(m)
z(m)
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
500
600
700
800
900
1000
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
Measured Simulated
500
600
700
c(ppm)
z(m)
z(m) 600
700
800
900
1000
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
500
600
z(m)
z(m) 700
800
900
1000
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
500
600
800
900
1000
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
800
900
1000
Measured Simulated
500
600
c(ppm) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
700
800
900
c(ppm)
Line 7: x = 2.15, y = 1.15 (m)
z(m)
z(m)
z(m) 700
700
Line 6: x = 3.2, y = 1.45 (m)
Measured Simulated
600
1000
c(ppm)
Line 5: x = 2.95, y = 1.45 (m)
500
900
Measured Simulated
c(ppm)
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
800
Line 4: x = 2.15, y = 1.45 (m)
Measured Simulated
600
700
c(ppm)
Line 3: x = 1.85, y = 1.45 (m)
500
1000
Measured Simulated
c(ppm) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
900
Line 2: x = 1.65, y = 1.45 (m )
Measured Simulated
500
800
c(ppm)
Line 1: x = 0.6, y = 1.45 (m) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
2039
Line 8: x = 1.85, y = 1.15 (m) Measured Simulated
500
600
700
800
900
1000
c(ppm)
Line 9: x = 1.325, y = 0.78 (m) Fig. 5. Measured and simulated CO2 concentration profiles for various positions (ppm).
1000
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Table 3 Supply air parameters for studied cases. Ventilation method
ACH (1/h)
Inlet velocity (m/s)
Inlet temperature ( C)
SV DV
5.5 5.5
1.19 0.0975
19 19
Table 4 Contaminant sources for the studied cases. Sources
Material
Intensity
1 2 3 4 Floor Four walls
Formaldehyde Formaldehyde Formaldehyde Formaldehyde Formaldehyde Toluene
50 mg/m2/h 50 mg/m2/h 50 mg/m2/h 50 mg/m2/h 0.05 mg/m2/h 0.2 mg/m2/h
zone for energy saving. Task station ventilation is not designed for serving mobile occupants. Also the ducting and air terminals are simpler for stratum ventilation. When used in a medium-sized room (e.g. a 9 9 m2 room), multiple supply terminals or long slots may be used to realize stratum ventilation. Depending on the thermal length of the supply air jets, nature of occupancy and furniture layout, air can be supplied from one or two directions. For the latter, air can come from the opposite directions or the perpendicular directions. It is estimated that people spend about 80%–90% of their lifetime indoors [14], the IAQ is closely related to occupants’ work efficiency and health [15,16]. Therefore, it is necessary to investigate the indoor environment created by stratum ventilation, such as thermal comfort, mean local air age, gaseous contaminant’s diffusion, particle dispersion, energy consumption and etc. Research of stratum ventilation is still at the starting line. Lin et al.’s study showed that RNG k–3 turbulent model can be used to predict the flow field created by stratum ventilation [13]. Their recent research pointed out that the stratum ventilation has a potential to be used for higher indoor temperature [17]. Tian et al. investigated the indoor air quality and thermal comfort of an office room with stratum ventilation by numerical method. It showed that, if properly designed, the stratum ventilation is able to provide good indoor air quality in breathing zone and to achieve good thermal comfort quantified by PMV and PPD [18]. Tian et al. investigated the contaminant’s diffusion and thermal comfort under stratum ventilation by experimental method [19]. The results demonstrated that the flow pattern formed by stratum ventilation was able to provide good IAQ in the breathing zone. Tian et al. discussed the particle diffusion in an office room under stratum ventilation by adopting the numerical method validated by their experimental data and compared the results with a corresponding displacement
Fig. 7. Velocity field at y ¼ 1.45 m (m/s).
ventilation case of which supply air parameters, heat sources and furniture arrangements are the same. The particle concentrations in the room, and particularly in the breathing zone, under stratum ventilation are lower than those under displacement ventilation [20]. Wang et al. (2009) compared the air ages in the breathing zone for the stratum ventilation and displacement ventilation with the tracer gas concentration decay method. The experimental results show that the stratum ventilation system provides lower air age in the breathing zone and better thermal comfort for four mechanically ventilated cases [21]. This study focuses on the diffusion of gaseous contaminants, mainly volatile organic compounds. Different positions and types of contaminant sources are considered. The results of stratum ventilation and of displacement ventilation are compared.
Fig. 6. Visualized picture of flow pattern formed by stratum ventilation.
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
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Fig. 10. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, Source 1 (mg/m3).
buildings. Generally, indoor air flow is a three-dimensional turbulent flow which is driven by momentum and buoyancy. Eight different types of turbulence models were investigated by Chen to determine the most appropriate model for indoor air flow computations [23]. He concluded that the re-normalization group (RNG) k–3 model was the most accurate model among the eddyviscosity models tested. In this study, the RNG k–3 model was adopted for investigation into the performance of room air distribution, the general governing equations are as follows:
divðrV fÞ ¼ div Gf gradf þ Sf
Fig. 8. Temperature field at y ¼ 1.45 m ( C).
2. Methodology 2.1. Governing equations and numerical methods for flow field
For steady flow where f represents each of the three velocity components u, v, w, k is the kinetic energy of turbulence, 3 is the dissipation rate of the kinetic energy of turbulence, h is air enthalpy, s is the mean local air age, Gf is the effective diffusion coefficient and Sf is the source term of the general equation. The source term and the diffusion coefficient corresponding to each variables f in this study are given in Table 1. Details about RNG k–3 turbulent model are found in Choudhury (1993) [24]. The equations are discretized into algebraic equations by the second-order upwind scheme which couples pressure and velocity flow fields. The Boussinesq model is employed to consider the buoyancy effect. The air inlet is defined as an opening with uniform airflow velocity. The outlet boundary conditions are set as outflow
z(m)
The airflow can be determined by computationally solving a set of conservation equations describing the flow and energy in a system. A pioneering work on indoor flow simulation by CFD was conducted by Nielsen more than three decades ago [22]. Due to the limitations of the experimental approach and the increase in performance and affordability of high-speed computers, numerical solution of these conservation equations provides a practical option for computing the airflow and contaminants distribution in
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
Source1 0
Fig. 9. Formaldehyde concentration at z ¼ 1.1 m under stratum ventilation, Source 1 (mg/m3).
(1)
1
2 3 -3 c formaldehyde / µg.m
4
Fig. 11. Formaldehyde concentration, Source 1 (x ¼ 1.47 m; y ¼ 1.45 m).
5
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
z(m)
2042
Fig. 12. Formaldehyde concentration distribution at z ¼ 1.1 m under stratum ventilation, Source 2 (mg/m3).
of room air. The walls provide heat sources with constant heat flux. The discrete ordinates (DO) radiation model is used to simulate radiation of walls [25,26]. The standard wall function is used to describe the turbulent flow properties in the near wall region. More details could be found from Launder and Spalding [27]. 2.2. Validation of mathematical model In order to validate the RNG k–3 turbulent model for simulation of stratum ventilation, an experiment is carried out to measure the air temperature, velocity and CO2 concentration. The experiment is carried out in an environmental chamber. It consists of three rooms: a plant room accommodates the air handling unit that is capable of controlling the air temperature, relative humidity and airflow rate; a test chamber in which the measurements were made, and a climate chamber which is used to prevent the influence of outdoor environment on the test chamber. The test chamber is enclosed by the climate chamber. An air conditioner is installed for the climate chamber to maintain the temperature fluctuation within reasonable range (0.5 C) during the experimental runs. The test chamber, 3900 mm 2900 mm 2600 mm, is ventilated via a 210 mm 170 mm rectangular air grille located in the middle of right wall of the test chamber and a 600 mm 600 mm perforated ceiling exhaust with 15.3% as the effective area ratio. The occupant in the test chamber was simulated by a box, 250 mm 400 mm 1200 mm and is heated by three 25 W light bulbs. CO2 was introduced via a small hole of 10-
Fig. 13. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, Source 2 (mg/m3).
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
Source2 0
1
2 3 -3 cformaldehyde / µg.m
4
5
Fig. 14. Formaldehyde concentration, Source 2 x ¼ 1.47 m; y ¼ 1.45 m.
mm bore at the height of 1.1 m of the box to simulate exhalation from the occupant, with an initial velocity of 0.039 m/s in the horizontal direction. A box, 400 mm 400 mm 400 mm, located on the table with three 60 W light bulbs, was used to simulate a personal computer [28]. Five rectangles, one as surface (680 mm 1380 mm 60 mm) with the top at the height of 760 mm, two as drawer clusters (560 mm 380 mm 585 mm), two as feet (430 mm 70 mm 115 mm), and a thin vertical surface (520 mm 450 mm) were used to construct the desk. The test chamber is illuminated by 2 sets of fluorescent lights, 170 mm 1240 mm 70 mm, consuming a total of 2 72 W electrical power. Table 2 summarizes the information of heat loads for the two cases. A book shelf, 400 mm 800 mm 1850 mm, is in the test room (Figs. 1 and 2). The ventilation rate was calibrated to be 5.5 air changes per hour (ACH). This was realized by adjusting the damper of the HVAC system serving the test room. The volume flow rate is measured by orifice flowmeter. In order to carry out meaningful comparison, the supply air temperature for both ventilation methods is controlled at 19 C, an optimal temperature for displacement ventilation. The supply temperature is monitored by two thermocouples. A visualization system using smoke generator is used to show the flow pattern formed by stratum ventilation. The SWEMA hot sphere anemometer system is used to measure air velocity and temperature. For velocity, the measuring range is
Fig. 15. Formaldehyde concentration distribution at z ¼ 1.1 m under stratum ventilation, Source 3 (mg/m3).
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
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Fig. 18. Formaldehyde concentration distribution at z ¼ 1.1 m under stratum ventilation, Source 4 (mg/m3).
0.05 m/s to 3 m/s, the measuring error is 0.03 m/s for 0.05 m/s to 1.00 m/s and 3% of readings for 1.00 m/s to 3.00 m/s, the dynamic responding time is 0.2 s. The measuring error for air temperature is 0.3 C. Copper-constantan thermocouples were used to measure air temperature and wall surface temperature of the test chamber. The measuring error for the system is 0.3 C. The multi-point nondispersive infrared CO2 system was used to measure the concentration of CO2. The measuring range is 0 ppm to 2000 ppm. The measuring error is 20 ppm þ 3% of readings. The dynamic responding time is less than 60 s. Measurements are taken for steady state condition. When the temperature, velocity and CO2 concentration of the test room are almost unchanged, a steady state condition is believed to be achieved, this process would almost cost more than 12 hours after the HVAC system starts to work [28,29]. Simultaneously, the air conditioner of the climate chamber also worked to keep the temperature to be 22.2 C, with the fluctuation of 0.5 C. It takes one to two hours for the test chamber to reach steady state condition again after re-location of a probe-holding pole [29]. Fig. 1 shows the configurations of the test room with stratum ventilation and with displacement ventilation. Due to the limited number of anemometer and CO2 concentration sensor, the temperature, velocity and concentration along nine plumb lines in the testing space are measured line by line through shifting the probe-holding
pole with hot-sphere anemometers, thermocouples and CO2 sensor. The locations of the plumb lines are shown in Fig. 2.
z(m)
Fig. 16. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, Source 3 (mg/m3).
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
2.3. CFD simulations Because the existence of the lamps, book shelf, dummy, computer and table plate, the geometrical configuration of flow region is quite complicated. Therefore, unstructured grid was adopted for the simulation of flow field. The grid numbers for stratum ventilation and for displacement ventilation are 587344 and 576404 respectively, which were checked to be grid independent. The converged residuals for continuity equation, uj, k and 3 are 104. The converged residual for T is 107. Comparisons between the measured and simulated values of temperature, velocity and CO2 concentration for locations along Plumb Lines 1 to 9 are shown in Figs. 3–5. The patterns of the simulated temperature and velocity are in agreement with those of the measured, which demonstrates the acceptability of the CFD model used for the simulation. The discrepancies between measured and simulated CO2 concentrations are significant at some locations, which were probably due to the different background CO2 concentrations applied in the experiments and in the simulations. The background CO2 concentration in Xi’an was fluctuating during the measurement of nine locations that lasted for nearly 30 h. For simulation of CO2 concentration distribution in the room, the averaged background CO2 concentration for the 30 h is adopted as the value in the supply air.
Source3 0
1
2 3 -3 c formaldehyde / µg.m
4
Fig. 17. Formaldehyde concentration, Source 3 x ¼ 1.47 m; y ¼ 1.45 m.
5 Fig. 19. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, Source 4 (mg/m3).
z(m)
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L. Tian et al. / Building and Environment 45 (2010) 2035–2046
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
Source4 0
1
2 3 -3 cformaldehyde / µg.m
4
5
Fig. 20. Formaldehyde concentration, Source 4 x ¼ 1.47 m; y ¼ 1.45 m.
2.4. Cases studied Numerical simulations of gaseous contaminant diffusion under stratum ventilation and displacement ventilation are carried out for the room as shown in Fig. 1. The ventilation rates for both ventilation methods are 5.5 ACH. The internal heat sources are the same for the two cases, 399 W in total. The supply air temperature is at 19 C for both stratum ventilation and displacement ventilation. For displacement ventilation, the supply diffuser is 440 mm 1000 mm, corresponding to a face velocity of 0.0975 m/s. Tables 2 and 3 summarize the information of heat loads and supply air parameters for the two cases. Table 4 shows information of contaminant sources. Fig. 2 shows the position of contaminant sources S1 to S4 added for the numerical simulations. The area sources such as the floor and the four walls of the room are also studied.
3. Results and discussions 3.1. Flow field and temperature field
is necessary to obtain information on the flow field and temperature distribution. Fig. 6 gives the flow pattern of stratum ventilation by using smoke generator. Figs. 7 and 8 show respectively the velocity and temperature fields of the two ventilation methods at the plane of y ¼ 1.45 m. For stratum ventilation, the supply cold air from the grille flows through breathing zone (the region between 0.8 m high and 1.2 high) in front of the occupant and then gradually flows downward due to buoyancy which enables the air quality of breathing zone to be better than the other regions. Because of the existence of occupant and desk, several eddies are formed. In the region close to occupant, air flows upwardly due to the natural convection. The temperature of the region where air jet flows through directly is lower than that in the other regions. It is obvious that the temperature in the room below the height of 1.2 m is lower than that in the upper zone of the room and the temperature gradient below the height of 0.8 m is quite small which implies that the air in this region is either well mixed or stagnant. All the phenomena observed match the characteristics of stratum ventilation discussed above. Displacement ventilation creates thermal plume around occupant and temperature distribution is stratified which are in agreement with the characteristics of this ventilation method. The reason why air velocity on the desk is relatively high is that the computer, a heat source, is quite close to the plane y ¼ 1.45 m.
z(m)
The flow patterns, and to less extent the temperature patterns, created by different ventilation methods have great impact on IAQ, such as mean local air age and contaminant diffusion. Therefore, it
Fig. 22. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, floor source (mg/m3).
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
Floor 0
Fig. 21. Formaldehyde concentration distribution at z ¼ 1.1 m under stratum ventilation, floor source (mg/m3).
1
2 3 -3 cformaldehyde / µg.m
4
Fig. 23. Formaldehyde concentration, floor source x ¼ 1.47 m; y ¼ 1.45 m.
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L. Tian et al. / Building and Environment 45 (2010) 2035–2046
Fig. 24. Toluene concentration distribution at z ¼ 1.1 m under stratum ventilation, four walls (mg/m3).
3.2. Contaminant’s diffusion Figs. 9 and 10 show the formaldehyde concentration distribution for stratum ventilation and for displacement ventilation at the plane of z ¼ 1.1 m when only Contaminant Source 1 is in place. The formaldehyde concentration along the jet is much lower than that in other regions, which implies that if fresh air can be supplied to breathing zone directly, the breathing zone will have better air quality in this region. Because of the eddies, some air flows under the desk and takes contaminant to the right side of the room, which brings the risk of polluting the supply air. For displacement ventilation, under the influence of flow field, contaminant mainly flows to the left of the room. Because of the entrainment by heat sources (the computer and occupant), formaldehyde is induce towards the desk and the occupant. Fig. 11 shows the formaldehyde concentration along the plumb line just in front of occupant. At the height of 1.1 m, the formaldehyde concentration for stratum ventilation and displacement ventilation are almost the same. Figs. 12 and 13 show formaldehyde concentration for stratum ventilation and for displacement ventilation at the plane of z ¼ 1.1 m when only Contaminant Source 2 is in place. Similarly, the formaldehyde concentration along the jet is much lower than those in other regions for stratum ventilation. The eddies formed near the occupant bring contaminant away from occupant. However, to some extent, the supply air is polluted because of that. For displacement ventilation, because the contaminant source is quite close to occupant, the plume generated by the occupant brings
2045
contaminant to upper zone via the occupant. Therefore, formaldehyde concentration at the height of breathing zone is quite high which pollutes the air inhaled by the occupant. Shown in Fig. 14, the formaldehyde concentration for stratum ventilation is lower than that for displacement ventilation when the contaminant source is close to occupant which is also a heat source. Figs. 15 and 16 show that formaldehyde concentration for stratum ventilation and for displacement ventilation at the plane of z ¼ 1.1 m when only Contaminant Source 3 is in place. The position of Source 3 is close to the back of the occupant. For stratum ventilation, flow behind occupant is almost stagnant. Therefore, the contaminant diffusion depends on the plume. Then the contaminant is brought to upper zone. On the other hand, supply air flows to occupant and therefore the formaldehyde concentration in front of occupant is quite low. For displacement ventilation, the contaminant is brought to the breathing zone by the plume around the occupant (Fig. 17). When only Contaminant Source 4 is in place, the source is far away from the occupant. For stratum ventilation, the diffusion of formaldehyde depends on flow field. The diffusion region is bigger than the previous scenario (Fig. 18). For displacement ventilation, formaldehyde diffusion depends on plume generated by the occupant and the momentum of the supply air. Therefore, the formaldehyde concentration is higher around the occupant (Fig. 19). Fig. 20 shows that formaldehyde concentration for stratum ventilation is lower than that for displacement ventilation. Figs. 21–26 show the contaminant diffusion of area sources which are the floor and the four walls. Formaldehyde is the main contaminant from the wooden floor, whereas toluene is the main contaminant from the painted walls. Therefore they are selected as the index gaseous contaminants for this study. For the floor case, the formaldehyde concentration in front of occupant is lower for stratum ventilation, which is resulted from the difference in flow fields formed under the two ventilation methods. For stratum ventilation, supply air flows to the breathing zone directly. The air inhaled by occupant is younger. Therefore, the contaminant concentration is lower. For displacement ventilation, the cold supply air is entrained by occupant (heat source) from floor, which is also the contaminant source. Therefore, the air has been polluted by formaldehyde before it is inhaled by occupant (Figs. 21–23). When the contaminant, such as toluene, emits from the paint coating surrounding walls, the situation becomes different (Figs. 24–26). For stratum ventilation, supply air enters the room from the grille. Due to the entrainment effect of the jet, the jet velocity decays gradually and the temperature increases. One of the criteria for determining the optimal supply temperature for stratum ventilation is to realize proper thermal length [30]. To certain extent, the jet mixes with the indoor
Fig. 25. Toluene concentration distribution at z ¼ 1.1 m under displacement ventilation, four walls (mg/m3).
z(m)
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L. Tian et al. / Building and Environment 45 (2010) 2035–2046
Acknowledgements
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
The work described in this paper is supported by NSFC (50521604) and by a Strategic Research Grant of the City University of Hong Kong, Hong Kong Special Administrative Region, China (Project No. 7002481). References
Four vertical walls 0
10
20 -3 c toluene / µg.m
30
40
Fig. 26. Concentration of toluene, four walls x ¼ 1.47 m; y ¼ 1.45 m.
air. For displacement ventilation, the flow(s) are mainly generated by entrainment of heat sources (such as the occupant and computer). The room air mixing effect is not comparable to that under stratum ventilation. Therefore, the toluene concentration is higher in the occupied zone for stratum ventilation than that for displacement. It is expected that if supply temperature is increased to meet the requirement for the optimal thermal length of stratum ventilation, the influence of buoyancy on jet would decrease and more fresh air would be supplied to the occupant.
4. Conclusions For small-to-medium-sized rooms, the newly proposed stratum ventilation could be adopted as a solution to elevated indoor air temperatures recommended by the governments in East Asia. The system is expected to provide good air quality with promising potential both in energy saving and in thermal comfort. In this study, the gaseous contaminant diffusion of stratum ventilation is experimentally and numerically investigated. The results are compared with those of the results for displacement ventilation in similar conditions. The following conclusions are drawn: 1. For stratum ventilation, gaseous contaminant concentration along the jet is lower than that of the other parts of the room. If the supply air stream is stable to fill the breathing zone, air inhaled by the occupant is generally better. 2. When the location of the contaminant source is close to the occupant, the inhaled air quality for stratum ventilation is better than that for displacement ventilation. When the contaminant source locates at the upper stream of the supply air flow (e.g. Source 1 case), the formaldehyde concentrations at the height of breathing zone (1.1 m) for stratum ventilation and for displacement ventilation are comparable. 3. For floor gaseous contaminants, stratum ventilation performs better than displacement ventilation in terms of the inhaled air quality by the occupant. However, under the given supply temperature of 19 C, when the area source locates on the surrounding walls of the office, displacement ventilation is superior to stratum ventilation in terms of inhaled air quality.
[1] Pachauri RK. IPCC Director-General, The IPCC Fourth Assessment Working Group Reports: Key findings. United Nations Headquarters, New York City, 24th September 2007. [2] Arens E, Turner S, Zhang H, Paliaga G. Moving air for comfort. ASHRAE Journal 2009;51(5):18–28. [3] ASHRAE. 2008. Addenda d, e, f, and g to ANSI/ASHRAE Standard 55-2004. Thermal environmental conditions for human occupancy, Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. [4] Olesen BW. Wirsbo-VELTA GmbH, Norderstedt/Germany. Introduction to the new revised draft of EN ISO 7730. [5] Fountain ME, Arens EA. Air movement and thermal comfort. ASHRAE Journal 1993;35(8):26–30. [6] Electrical and Mechanical Services Department (EMSD), Government of Hong Kong S.A.R. Energy Conservation Charter 2006 Suitable Room Temperature. http://www.energyland.emsd.gov.hk/blue_sky/eng/index.htm. [7] National Development and Reform Commission (NDRC), the State Council of China, Citizen’s pact for deed of thrift. http://www.ndrc.gov.cn/gzdt/ t20051223_54462.htm. [8] NOW news, Taiwan. Campaign in office of president for saving energy and reducing carbon emissions. http://www.nownews.com/2008/05/24/108442279584.htm. [9] Ministry of Knowledge and Economy, the Republic of Korea. The standard of energy management, December 29, 2008. http://www.mke.go.kr/info/law/ gosiView.jsp? [Item No. 2008-219]. [10] Japanese Ministry of the Environment (MoE). National campaign to fight global warming. http://www.env.go.jp/en/press/2005/0428b.html. [11] International Facility Management Association. Temperature wars - savings vs. comfort, http://www.ifma.org; July 2009. [12] Lin Z, Chow TT, Tsang CF. Stratum ventilation? A conceptual introduction. In: Proceedings of the 10th international conference on indoor air quality and climate, Beijing, China; 4–9 September 2005. p. 3260–4. [13] Lin Z, Chow TT, Tsang CF. Validation of CFD model for research into stratum ventilation. International Journal of Ventilation 2006;5(3):345–63. [14] Zhao RY, Sun SF, Ding RY. Conditioning strategies of indoor thermal environment in warm climates. Energy and Buildings 2004;36(12):1281–6. [15] Fanger PO. What is IAQ? Indoor Air 2006;16(5):328–34. [16] Wyon DP. The effects of indoor air quality on performance and productivity. Indoor Air 2004;14(Suppl. 7):92–101. [17] Lin Z, Chow TT, Tsang CF, Fang KF, Chan LS. Stratum ventilation – a potential solution to elevated indoor temperature. Building and Environment 2009; 44(11):2256–69. [18] Tian L, Lin Z, Liu J, Wang QW. Numerical study of indoor air quality and thermal comfort under stratum ventilation. Progress in Computational Fluid Dynamics 2008;8(7/8):541–8. [19] Tian L, Lin Z, Liu J, Wang QW. Measurements of indoor air quality and thermal comfort of an office ventilated by stratum ventilation. In: Proceedings of indoor air, Copenhagen, Denmark; August 17–22, 2008. [20] Tian L, Lin Z, Wang QW, Liu J. Numerical investigation of indoor aerosol particle dispersion under stratum ventilation and under displacement ventilation. Indoor and Built Environment 2009;18(4):360–75. [21] Wang FH, Li YB, Liu XD, Wang XK. Experimental investigation on local air age and air distribution of stratum ventilation. Journal of Hunan University Natural Sciences 2009;36(s):35–9. [22] Nielsen PV. Flow in air-conditioned rooms. Ph.D. Thesis: Technical University of Denmark, 1974, Copenhagen. [23] Chen Q. Comparison of different k–3 models for indoor airflow computations. Numerical Heat Transfer 1995;28(Part B):353–69. [24] Choudhury D. Introduction to therenormalization group method and turbulence modeling, Fluent Inc. Technical Memorandum TM-107, 1993. [25] Abanto J, Barrero D, Reggio M, Ozell B. Airflow modeling in a computer room. Building and Environment 2004;39(12):1393–402. [26] FLUENT. Fluent 6.2 user’s guide. Lebanon, NH: Fluent Inc; 2005. [27] Launder BE, Spalding DB. The numerical computation of turbulent flows. Computer Methods in Applied Mechanics and Energy 1974;3:268–389. [28] Yuan X, Chen Q, Glicksman LR. Measurements and computations of room air flow with displacement ventilation. ASHRAE Trans 1999;105(1):340–52. [29] He G. Modeling indoor pollutant exposures under different ventilation schemes. Ph.D. Thesis, 2003, University of Miami, USA, 215 pages. [30] Elvsen PA, Sandberg M. Buoyant jet in a ventilated room: velocity field, temperature field and airflow patterns analysed with three different wholefield methods. Building and Environment 2009;44(11):137–45.
Contents lists available at ScienceDirect
Building and Environment journal homepage: www.elsevier.com/locate/buildenv
Comparison of gaseous contaminant diffusion under stratum ventilation and under displacement ventilation Lin Tian a, Zhang Lin b, Qiuwang Wang a, * a b
State Key Lab of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China Building Energy & Environmental Technology Research Unit, Division of Building Science and Technology, City University of Hong Kong, Hong Kong SAR, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2009 Received in revised form 3 January 2010 Accepted 5 January 2010
The gaseous contaminant diffusion under stratum ventilation is investigated by numerical method which is validated by experiments carried out. The concentration of gaseous contaminants along the supply air jet is found to be lower than the other parts of the room. Compared with displacement ventilation, the formaldehyde concentration in breathing zone is lower when a contaminant source locates close to the occupant. The concentration is at the same level when the contaminant source locates up-steam to the occupant. The concentration in the occupied zone (<1.9 m from the floor) is also lower when the contaminant source locates on the floor. At supply air temperature optimized for displacement ventilation, the toluene concentration in breathing zone for stratum ventilation is higher than that for displacement ventilation when the area source locates on the four surrounding walls of the room. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Stratum ventilation Gaseous contaminant concentration Air quality Comparison
1. Introduction The earth is already showing many signs of worldwide climate change. The Intergovernmental Panel on Climate Change (IPCC) has used its strongest language to link human activities to the current planetary warming [1]. There is international consensus to reduce CO2 emission (Kyoto Protocol, etc.). Minimizing the energy consumption by air conditioning systems would help to reduce CO2 emission. Arens et al. 2009 reported that when the room air temperature above 22.5 C, there is a small risk of draft and a strong preference for more air movement [2]. ANSI/ASHRAE Standard 552004 has been updated with new provisions that allow elevated air movement to broadly offset the need to cool the air in warm conditions [3]. In revising EN ISO 7730, Olesen adopted Fountain and Arens’ (1993) theory that higher air speed was required to offset increased indoor temperature [4,5]. Proactive actions in this regard have been taken by several governments in East Asia. The Electrical and Mechanical Services Department (EMSD) of the Hong Kong S.A.R. government issues guidelines to ensure that normally room temperature is adjusted to 25.5 C in summer [6]. The National Development and Reform Commission (NDRC) of the Chinese State Council issued a similar guideline to set the indoor temperature to 26 C in the cooling season [7]. The room temperature in the ‘‘Office of President’’ in Taipei has been set to 27 C after
* Corresponding author. E-mail address: [email protected] (Q. Wang). 0360-1323/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.buildenv.2010.01.002
Mr. Ma Ying-jeou’s inauguration in May 2008 [8]. Similarly, in the Standard of Energy Management issued in last December, the Ministry of Knowledge and Economy of the Republic of Korea recommend that room temperature of a building should be ranging from 26 to 28 C in summer [9]. In a more radical move, the Ministry of the Environment (MoE) of Japan has been encouraging people to set the temperature of air conditioners at offices to 28 C during the summer months [10]. A survey from The International Facility Management Association shows that many facility professionals are adjusting the thermostat to higher settings in the summer to cut energy consumption in United States [11]. Recently, stratum ventilation, a new ventilation method, was proposed by Lin et al. [12,13] for small to medium rooms. This ventilation method works by creating a layer of fresher air in occupants’ breathing zone. It delivers supply air directly to the occupants with little attention to the indoor air quality (IAQ) and thermal comfort of the upper zone (approximately > 1.5 m from the floor if the occupants are mostly sedentary). The lower zone (approximately < 0.8 m from the floor) is not the target zone of IAQ performance. This is realized by positioning supply terminal(s) at the side-walls or columns slightly above the height of occupants, standing or sitting depending on the application, whereas for displacement ventilation, air for breathing is transported by the boundary layer around the body of an occupant and the air quality is a weighted average of the air quality in the room from the breathing level to the floor level. Ventilation efficiency can be expected to increase if air is supplied directly into the breathing zone. Therefore, compared with mixing ventilation and
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Table 1 Source and diffusion coefficient for each 4.
f
Gf
1 uj k
0
m þ mt ðm þ mt Þ=dk ðm þ mt Þ=33 m=d1 þ mt =dt mt =Sct mt =Sct
3 T C
s
Sf 0 vp vx rbgj ðT T0 Þ j G r3 þ GB ðC31 G C32 r3 þ C33 GB Þ3=k þ R ST Sc
r
where 2 vuj i vui m is laminar viscosity; mt ¼ rCm k3 is turbulent viscosity; G ¼ mt vu vxj ð vxj þ vxi Þ is the mt vT turbulent production; GB ¼ gi bPrt vxi is the turbulent production due to buoyancy; Cm h3 ð1h=h0 Þ 32 k 1þbh3 vuj i Si;j ¼ 12ðvu vxj þ vxi Þ;
R ¼
is the source term from renormalization; h ¼ Sk3 , S ¼ ð2Si;j Si;j Þ1=2 , Cm ¼ 0:0845, C31 ¼ 1:42, C32 ¼ 1:68, C33 ¼ 1:68 are the model
constants’ dk ¼ 0:7194, d3 ¼ 0:7194, d1 ¼ 0:7, dt ¼ 0:9, Sct ¼ 0:7 are Prandtl or Schmidt number.
displacement ventilation, stratum ventilation may offer higher ventilation efficiency. The range of face velocity and the locations of air supply gear should be carefully optimized to break the boundary layer around the body of an occupant and to minimize the risks of draft. The required height of the breathing layer/zone depends on the nature of occupancy. The supply air terminals should be positioned at the occupants’ head level. At the same time, a quasistagnant zone is also formed between the breathing zone and the floor (0 < height < 0.8 m). The temperature within the quasi-stagnant zone should be reasonably controlled which would not be lower than that of displacement ventilation. The problem of ‘‘cold ankles’’ could be solved. Energy is also saved by avoiding overcooling of the lower zone of a room. Indoor air should be mixed well and air temperature gradient in the occupied zone should be lower than that of displacement ventilation. Because air is supplied directly to breathing zone, the air supply temperature should be higher than that of mixing ventilation. This implies that the evaporating temperature of the refrigerating plant may also be elevated accordingly, which would result in higher coefficient of performance (COP). From the view point of heat removal ability, stratum ventilation, which can be used in elevated indoor air temperature condition, does not have to remove so much heat as that of mixing ventilation and displacement ventilation. The need to remove heat is mainly to provide thermal comfort to occupants in a non-industrial room. Although the supply air temperature for stratum ventilation is higher than that of conventional ventilations, the distance between the occupants and the air terminal is shorter, which results in:
Fig. 1. Configurations of test chamber.
For both stratum ventilation and task station ventilation, occupants stay in the flow of supply air jet(s). This significantly improves the inhaled air quality. However, current task station ventilation does not provide reversed temperature gradient in the occupied
1) reversed temperature gradient in the occupied zone - the temperature is lower at the head-and-chest level (the breathing zone)and higher at the ankle level, which effectively cools the body parts that need cooling the most; 2) higher air speed for the same airflow rate, which offsets the effect of higher temperature in thermal comfort; and 3) reduced room cooling load – Because of Points 1) and 2), the mean room temperature for equal thermal comfort is significantly higher than that of conventional ventilations. The heat transmission therefore decreases.
Table 2 Heat loads for studied cases. Items
Heat load (W)
Human simulators Computer box Lamps
75 180 72 2
Fig. 2. Layout of contaminant sources and measurement positions (mm).
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
z(m)
z(m)
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
v(m/s)
Line 2: x = 1.65, y = 1.45 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
z(m)
z(m)
Line 1: x = 0.6, y = 1.45 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
v(m/s)
Line 4: x = 2.15, y = 1.45 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
z(m)
z(m)
Line 3: x = 1.85, y = 1.45 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
v(m/s)
Line 6: x = 3.2, y = 1.45 (m)
z(m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
v(m/s)
Line 7: x = 2.15, y = 1.15 (m)
z(m)
z(m)
Line 5: x = 2.95, y = 1.45 (m)
Line 8: x = 1.85, y = 1.15 (m)
2.6 2.4 Measured 2.2 Simulated 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
v(m/s)
Line 9: x = 1.325, y = 0.78(m) Fig. 3. Measured and simulated velocity profiles for various positions (m/s).
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2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
Measured Simulated
z(m)
z(m)
2038
21
22
23
24
25
o
26
27
28
29
30
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
Measured Simulated
21
22
23
24
z(m)
z(m)
Measured Simulated
21
22
23
24
25
26
27
28
29
30
o
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
21
22
z(m)
z(m) 23
24
25
o
23
24
26
27
28
29
30
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
21
22
24
23
24
25
26
27
28
29
30
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
26
27
28
29
30
25
o
26
27
28
29
30
Measured Simulated
21
22
23
24
o
25
26
27
28
29
o
t ( C)
t ( C)
Line 7: x = 2.15, y = 1.15 (m)
z(m)
z(m)
z(m) 23
25
o
Line 6: x = 3.2, y = 1.45 (m)
Measured Simulated
22
30
t ( C)
Line 5: x = 2.95, y = 1.45 (m)
21
29
Measured Simulated
t ( C)
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
28
Line 4: x = 2.15, y = 1.45 (m)
Measured Simulated
22
27
t ( C)
Line 3: x = 1.85, y = 1.45 (m)
21
26
Measured Simulated
t ( C) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
o
Line 2: x = 1.65, y = 1.45 (m)
Line 1: x = 0.6, y = 1.45 (m) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
25
t ( C)
t ( C)
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 20
Line 8: x = 1.85, y = 1.15 (m)
M easured Simulated
21
22
23
24
25
26
27
28
29
30
o
t ( C)
Line9: x = 1.325, y = 0.78(m) Fig. 4. Measured and simulated temperature profiles for various positions ( C).
30
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
Measured Simulated
z(m)
z(m)
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
500
600
700
800
900
1000
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
Measured Simulated
500
600
700
c(ppm)
z(m)
z(m) 600
700
800
900
1000
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
500
600
z(m)
z(m) 700
800
900
1000
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
500
600
800
900
1000
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
800
900
1000
Measured Simulated
500
600
c(ppm) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
700
800
900
c(ppm)
Line 7: x = 2.15, y = 1.15 (m)
z(m)
z(m)
z(m) 700
700
Line 6: x = 3.2, y = 1.45 (m)
Measured Simulated
600
1000
c(ppm)
Line 5: x = 2.95, y = 1.45 (m)
500
900
Measured Simulated
c(ppm)
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
800
Line 4: x = 2.15, y = 1.45 (m)
Measured Simulated
600
700
c(ppm)
Line 3: x = 1.85, y = 1.45 (m)
500
1000
Measured Simulated
c(ppm) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
900
Line 2: x = 1.65, y = 1.45 (m )
Measured Simulated
500
800
c(ppm)
Line 1: x = 0.6, y = 1.45 (m) 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 400
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Line 8: x = 1.85, y = 1.15 (m) Measured Simulated
500
600
700
800
900
1000
c(ppm)
Line 9: x = 1.325, y = 0.78 (m) Fig. 5. Measured and simulated CO2 concentration profiles for various positions (ppm).
1000
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L. Tian et al. / Building and Environment 45 (2010) 2035–2046
Table 3 Supply air parameters for studied cases. Ventilation method
ACH (1/h)
Inlet velocity (m/s)
Inlet temperature ( C)
SV DV
5.5 5.5
1.19 0.0975
19 19
Table 4 Contaminant sources for the studied cases. Sources
Material
Intensity
1 2 3 4 Floor Four walls
Formaldehyde Formaldehyde Formaldehyde Formaldehyde Formaldehyde Toluene
50 mg/m2/h 50 mg/m2/h 50 mg/m2/h 50 mg/m2/h 0.05 mg/m2/h 0.2 mg/m2/h
zone for energy saving. Task station ventilation is not designed for serving mobile occupants. Also the ducting and air terminals are simpler for stratum ventilation. When used in a medium-sized room (e.g. a 9 9 m2 room), multiple supply terminals or long slots may be used to realize stratum ventilation. Depending on the thermal length of the supply air jets, nature of occupancy and furniture layout, air can be supplied from one or two directions. For the latter, air can come from the opposite directions or the perpendicular directions. It is estimated that people spend about 80%–90% of their lifetime indoors [14], the IAQ is closely related to occupants’ work efficiency and health [15,16]. Therefore, it is necessary to investigate the indoor environment created by stratum ventilation, such as thermal comfort, mean local air age, gaseous contaminant’s diffusion, particle dispersion, energy consumption and etc. Research of stratum ventilation is still at the starting line. Lin et al.’s study showed that RNG k–3 turbulent model can be used to predict the flow field created by stratum ventilation [13]. Their recent research pointed out that the stratum ventilation has a potential to be used for higher indoor temperature [17]. Tian et al. investigated the indoor air quality and thermal comfort of an office room with stratum ventilation by numerical method. It showed that, if properly designed, the stratum ventilation is able to provide good indoor air quality in breathing zone and to achieve good thermal comfort quantified by PMV and PPD [18]. Tian et al. investigated the contaminant’s diffusion and thermal comfort under stratum ventilation by experimental method [19]. The results demonstrated that the flow pattern formed by stratum ventilation was able to provide good IAQ in the breathing zone. Tian et al. discussed the particle diffusion in an office room under stratum ventilation by adopting the numerical method validated by their experimental data and compared the results with a corresponding displacement
Fig. 7. Velocity field at y ¼ 1.45 m (m/s).
ventilation case of which supply air parameters, heat sources and furniture arrangements are the same. The particle concentrations in the room, and particularly in the breathing zone, under stratum ventilation are lower than those under displacement ventilation [20]. Wang et al. (2009) compared the air ages in the breathing zone for the stratum ventilation and displacement ventilation with the tracer gas concentration decay method. The experimental results show that the stratum ventilation system provides lower air age in the breathing zone and better thermal comfort for four mechanically ventilated cases [21]. This study focuses on the diffusion of gaseous contaminants, mainly volatile organic compounds. Different positions and types of contaminant sources are considered. The results of stratum ventilation and of displacement ventilation are compared.
Fig. 6. Visualized picture of flow pattern formed by stratum ventilation.
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
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Fig. 10. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, Source 1 (mg/m3).
buildings. Generally, indoor air flow is a three-dimensional turbulent flow which is driven by momentum and buoyancy. Eight different types of turbulence models were investigated by Chen to determine the most appropriate model for indoor air flow computations [23]. He concluded that the re-normalization group (RNG) k–3 model was the most accurate model among the eddyviscosity models tested. In this study, the RNG k–3 model was adopted for investigation into the performance of room air distribution, the general governing equations are as follows:
divðrV fÞ ¼ div Gf gradf þ Sf
Fig. 8. Temperature field at y ¼ 1.45 m ( C).
2. Methodology 2.1. Governing equations and numerical methods for flow field
For steady flow where f represents each of the three velocity components u, v, w, k is the kinetic energy of turbulence, 3 is the dissipation rate of the kinetic energy of turbulence, h is air enthalpy, s is the mean local air age, Gf is the effective diffusion coefficient and Sf is the source term of the general equation. The source term and the diffusion coefficient corresponding to each variables f in this study are given in Table 1. Details about RNG k–3 turbulent model are found in Choudhury (1993) [24]. The equations are discretized into algebraic equations by the second-order upwind scheme which couples pressure and velocity flow fields. The Boussinesq model is employed to consider the buoyancy effect. The air inlet is defined as an opening with uniform airflow velocity. The outlet boundary conditions are set as outflow
z(m)
The airflow can be determined by computationally solving a set of conservation equations describing the flow and energy in a system. A pioneering work on indoor flow simulation by CFD was conducted by Nielsen more than three decades ago [22]. Due to the limitations of the experimental approach and the increase in performance and affordability of high-speed computers, numerical solution of these conservation equations provides a practical option for computing the airflow and contaminants distribution in
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
Source1 0
Fig. 9. Formaldehyde concentration at z ¼ 1.1 m under stratum ventilation, Source 1 (mg/m3).
(1)
1
2 3 -3 c formaldehyde / µg.m
4
Fig. 11. Formaldehyde concentration, Source 1 (x ¼ 1.47 m; y ¼ 1.45 m).
5
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
z(m)
2042
Fig. 12. Formaldehyde concentration distribution at z ¼ 1.1 m under stratum ventilation, Source 2 (mg/m3).
of room air. The walls provide heat sources with constant heat flux. The discrete ordinates (DO) radiation model is used to simulate radiation of walls [25,26]. The standard wall function is used to describe the turbulent flow properties in the near wall region. More details could be found from Launder and Spalding [27]. 2.2. Validation of mathematical model In order to validate the RNG k–3 turbulent model for simulation of stratum ventilation, an experiment is carried out to measure the air temperature, velocity and CO2 concentration. The experiment is carried out in an environmental chamber. It consists of three rooms: a plant room accommodates the air handling unit that is capable of controlling the air temperature, relative humidity and airflow rate; a test chamber in which the measurements were made, and a climate chamber which is used to prevent the influence of outdoor environment on the test chamber. The test chamber is enclosed by the climate chamber. An air conditioner is installed for the climate chamber to maintain the temperature fluctuation within reasonable range (0.5 C) during the experimental runs. The test chamber, 3900 mm 2900 mm 2600 mm, is ventilated via a 210 mm 170 mm rectangular air grille located in the middle of right wall of the test chamber and a 600 mm 600 mm perforated ceiling exhaust with 15.3% as the effective area ratio. The occupant in the test chamber was simulated by a box, 250 mm 400 mm 1200 mm and is heated by three 25 W light bulbs. CO2 was introduced via a small hole of 10-
Fig. 13. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, Source 2 (mg/m3).
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
Source2 0
1
2 3 -3 cformaldehyde / µg.m
4
5
Fig. 14. Formaldehyde concentration, Source 2 x ¼ 1.47 m; y ¼ 1.45 m.
mm bore at the height of 1.1 m of the box to simulate exhalation from the occupant, with an initial velocity of 0.039 m/s in the horizontal direction. A box, 400 mm 400 mm 400 mm, located on the table with three 60 W light bulbs, was used to simulate a personal computer [28]. Five rectangles, one as surface (680 mm 1380 mm 60 mm) with the top at the height of 760 mm, two as drawer clusters (560 mm 380 mm 585 mm), two as feet (430 mm 70 mm 115 mm), and a thin vertical surface (520 mm 450 mm) were used to construct the desk. The test chamber is illuminated by 2 sets of fluorescent lights, 170 mm 1240 mm 70 mm, consuming a total of 2 72 W electrical power. Table 2 summarizes the information of heat loads for the two cases. A book shelf, 400 mm 800 mm 1850 mm, is in the test room (Figs. 1 and 2). The ventilation rate was calibrated to be 5.5 air changes per hour (ACH). This was realized by adjusting the damper of the HVAC system serving the test room. The volume flow rate is measured by orifice flowmeter. In order to carry out meaningful comparison, the supply air temperature for both ventilation methods is controlled at 19 C, an optimal temperature for displacement ventilation. The supply temperature is monitored by two thermocouples. A visualization system using smoke generator is used to show the flow pattern formed by stratum ventilation. The SWEMA hot sphere anemometer system is used to measure air velocity and temperature. For velocity, the measuring range is
Fig. 15. Formaldehyde concentration distribution at z ¼ 1.1 m under stratum ventilation, Source 3 (mg/m3).
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
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Fig. 18. Formaldehyde concentration distribution at z ¼ 1.1 m under stratum ventilation, Source 4 (mg/m3).
0.05 m/s to 3 m/s, the measuring error is 0.03 m/s for 0.05 m/s to 1.00 m/s and 3% of readings for 1.00 m/s to 3.00 m/s, the dynamic responding time is 0.2 s. The measuring error for air temperature is 0.3 C. Copper-constantan thermocouples were used to measure air temperature and wall surface temperature of the test chamber. The measuring error for the system is 0.3 C. The multi-point nondispersive infrared CO2 system was used to measure the concentration of CO2. The measuring range is 0 ppm to 2000 ppm. The measuring error is 20 ppm þ 3% of readings. The dynamic responding time is less than 60 s. Measurements are taken for steady state condition. When the temperature, velocity and CO2 concentration of the test room are almost unchanged, a steady state condition is believed to be achieved, this process would almost cost more than 12 hours after the HVAC system starts to work [28,29]. Simultaneously, the air conditioner of the climate chamber also worked to keep the temperature to be 22.2 C, with the fluctuation of 0.5 C. It takes one to two hours for the test chamber to reach steady state condition again after re-location of a probe-holding pole [29]. Fig. 1 shows the configurations of the test room with stratum ventilation and with displacement ventilation. Due to the limited number of anemometer and CO2 concentration sensor, the temperature, velocity and concentration along nine plumb lines in the testing space are measured line by line through shifting the probe-holding
pole with hot-sphere anemometers, thermocouples and CO2 sensor. The locations of the plumb lines are shown in Fig. 2.
z(m)
Fig. 16. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, Source 3 (mg/m3).
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
2.3. CFD simulations Because the existence of the lamps, book shelf, dummy, computer and table plate, the geometrical configuration of flow region is quite complicated. Therefore, unstructured grid was adopted for the simulation of flow field. The grid numbers for stratum ventilation and for displacement ventilation are 587344 and 576404 respectively, which were checked to be grid independent. The converged residuals for continuity equation, uj, k and 3 are 104. The converged residual for T is 107. Comparisons between the measured and simulated values of temperature, velocity and CO2 concentration for locations along Plumb Lines 1 to 9 are shown in Figs. 3–5. The patterns of the simulated temperature and velocity are in agreement with those of the measured, which demonstrates the acceptability of the CFD model used for the simulation. The discrepancies between measured and simulated CO2 concentrations are significant at some locations, which were probably due to the different background CO2 concentrations applied in the experiments and in the simulations. The background CO2 concentration in Xi’an was fluctuating during the measurement of nine locations that lasted for nearly 30 h. For simulation of CO2 concentration distribution in the room, the averaged background CO2 concentration for the 30 h is adopted as the value in the supply air.
Source3 0
1
2 3 -3 c formaldehyde / µg.m
4
Fig. 17. Formaldehyde concentration, Source 3 x ¼ 1.47 m; y ¼ 1.45 m.
5 Fig. 19. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, Source 4 (mg/m3).
z(m)
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L. Tian et al. / Building and Environment 45 (2010) 2035–2046
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
Source4 0
1
2 3 -3 cformaldehyde / µg.m
4
5
Fig. 20. Formaldehyde concentration, Source 4 x ¼ 1.47 m; y ¼ 1.45 m.
2.4. Cases studied Numerical simulations of gaseous contaminant diffusion under stratum ventilation and displacement ventilation are carried out for the room as shown in Fig. 1. The ventilation rates for both ventilation methods are 5.5 ACH. The internal heat sources are the same for the two cases, 399 W in total. The supply air temperature is at 19 C for both stratum ventilation and displacement ventilation. For displacement ventilation, the supply diffuser is 440 mm 1000 mm, corresponding to a face velocity of 0.0975 m/s. Tables 2 and 3 summarize the information of heat loads and supply air parameters for the two cases. Table 4 shows information of contaminant sources. Fig. 2 shows the position of contaminant sources S1 to S4 added for the numerical simulations. The area sources such as the floor and the four walls of the room are also studied.
3. Results and discussions 3.1. Flow field and temperature field
is necessary to obtain information on the flow field and temperature distribution. Fig. 6 gives the flow pattern of stratum ventilation by using smoke generator. Figs. 7 and 8 show respectively the velocity and temperature fields of the two ventilation methods at the plane of y ¼ 1.45 m. For stratum ventilation, the supply cold air from the grille flows through breathing zone (the region between 0.8 m high and 1.2 high) in front of the occupant and then gradually flows downward due to buoyancy which enables the air quality of breathing zone to be better than the other regions. Because of the existence of occupant and desk, several eddies are formed. In the region close to occupant, air flows upwardly due to the natural convection. The temperature of the region where air jet flows through directly is lower than that in the other regions. It is obvious that the temperature in the room below the height of 1.2 m is lower than that in the upper zone of the room and the temperature gradient below the height of 0.8 m is quite small which implies that the air in this region is either well mixed or stagnant. All the phenomena observed match the characteristics of stratum ventilation discussed above. Displacement ventilation creates thermal plume around occupant and temperature distribution is stratified which are in agreement with the characteristics of this ventilation method. The reason why air velocity on the desk is relatively high is that the computer, a heat source, is quite close to the plane y ¼ 1.45 m.
z(m)
The flow patterns, and to less extent the temperature patterns, created by different ventilation methods have great impact on IAQ, such as mean local air age and contaminant diffusion. Therefore, it
Fig. 22. Formaldehyde concentration distribution at z ¼ 1.1 m under displacement ventilation, floor source (mg/m3).
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
Floor 0
Fig. 21. Formaldehyde concentration distribution at z ¼ 1.1 m under stratum ventilation, floor source (mg/m3).
1
2 3 -3 cformaldehyde / µg.m
4
Fig. 23. Formaldehyde concentration, floor source x ¼ 1.47 m; y ¼ 1.45 m.
5
L. Tian et al. / Building and Environment 45 (2010) 2035–2046
Fig. 24. Toluene concentration distribution at z ¼ 1.1 m under stratum ventilation, four walls (mg/m3).
3.2. Contaminant’s diffusion Figs. 9 and 10 show the formaldehyde concentration distribution for stratum ventilation and for displacement ventilation at the plane of z ¼ 1.1 m when only Contaminant Source 1 is in place. The formaldehyde concentration along the jet is much lower than that in other regions, which implies that if fresh air can be supplied to breathing zone directly, the breathing zone will have better air quality in this region. Because of the eddies, some air flows under the desk and takes contaminant to the right side of the room, which brings the risk of polluting the supply air. For displacement ventilation, under the influence of flow field, contaminant mainly flows to the left of the room. Because of the entrainment by heat sources (the computer and occupant), formaldehyde is induce towards the desk and the occupant. Fig. 11 shows the formaldehyde concentration along the plumb line just in front of occupant. At the height of 1.1 m, the formaldehyde concentration for stratum ventilation and displacement ventilation are almost the same. Figs. 12 and 13 show formaldehyde concentration for stratum ventilation and for displacement ventilation at the plane of z ¼ 1.1 m when only Contaminant Source 2 is in place. Similarly, the formaldehyde concentration along the jet is much lower than those in other regions for stratum ventilation. The eddies formed near the occupant bring contaminant away from occupant. However, to some extent, the supply air is polluted because of that. For displacement ventilation, because the contaminant source is quite close to occupant, the plume generated by the occupant brings
2045
contaminant to upper zone via the occupant. Therefore, formaldehyde concentration at the height of breathing zone is quite high which pollutes the air inhaled by the occupant. Shown in Fig. 14, the formaldehyde concentration for stratum ventilation is lower than that for displacement ventilation when the contaminant source is close to occupant which is also a heat source. Figs. 15 and 16 show that formaldehyde concentration for stratum ventilation and for displacement ventilation at the plane of z ¼ 1.1 m when only Contaminant Source 3 is in place. The position of Source 3 is close to the back of the occupant. For stratum ventilation, flow behind occupant is almost stagnant. Therefore, the contaminant diffusion depends on the plume. Then the contaminant is brought to upper zone. On the other hand, supply air flows to occupant and therefore the formaldehyde concentration in front of occupant is quite low. For displacement ventilation, the contaminant is brought to the breathing zone by the plume around the occupant (Fig. 17). When only Contaminant Source 4 is in place, the source is far away from the occupant. For stratum ventilation, the diffusion of formaldehyde depends on flow field. The diffusion region is bigger than the previous scenario (Fig. 18). For displacement ventilation, formaldehyde diffusion depends on plume generated by the occupant and the momentum of the supply air. Therefore, the formaldehyde concentration is higher around the occupant (Fig. 19). Fig. 20 shows that formaldehyde concentration for stratum ventilation is lower than that for displacement ventilation. Figs. 21–26 show the contaminant diffusion of area sources which are the floor and the four walls. Formaldehyde is the main contaminant from the wooden floor, whereas toluene is the main contaminant from the painted walls. Therefore they are selected as the index gaseous contaminants for this study. For the floor case, the formaldehyde concentration in front of occupant is lower for stratum ventilation, which is resulted from the difference in flow fields formed under the two ventilation methods. For stratum ventilation, supply air flows to the breathing zone directly. The air inhaled by occupant is younger. Therefore, the contaminant concentration is lower. For displacement ventilation, the cold supply air is entrained by occupant (heat source) from floor, which is also the contaminant source. Therefore, the air has been polluted by formaldehyde before it is inhaled by occupant (Figs. 21–23). When the contaminant, such as toluene, emits from the paint coating surrounding walls, the situation becomes different (Figs. 24–26). For stratum ventilation, supply air enters the room from the grille. Due to the entrainment effect of the jet, the jet velocity decays gradually and the temperature increases. One of the criteria for determining the optimal supply temperature for stratum ventilation is to realize proper thermal length [30]. To certain extent, the jet mixes with the indoor
Fig. 25. Toluene concentration distribution at z ¼ 1.1 m under displacement ventilation, four walls (mg/m3).
z(m)
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L. Tian et al. / Building and Environment 45 (2010) 2035–2046
Acknowledgements
2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
SV DV
The work described in this paper is supported by NSFC (50521604) and by a Strategic Research Grant of the City University of Hong Kong, Hong Kong Special Administrative Region, China (Project No. 7002481). References
Four vertical walls 0
10
20 -3 c toluene / µg.m
30
40
Fig. 26. Concentration of toluene, four walls x ¼ 1.47 m; y ¼ 1.45 m.
air. For displacement ventilation, the flow(s) are mainly generated by entrainment of heat sources (such as the occupant and computer). The room air mixing effect is not comparable to that under stratum ventilation. Therefore, the toluene concentration is higher in the occupied zone for stratum ventilation than that for displacement. It is expected that if supply temperature is increased to meet the requirement for the optimal thermal length of stratum ventilation, the influence of buoyancy on jet would decrease and more fresh air would be supplied to the occupant.
4. Conclusions For small-to-medium-sized rooms, the newly proposed stratum ventilation could be adopted as a solution to elevated indoor air temperatures recommended by the governments in East Asia. The system is expected to provide good air quality with promising potential both in energy saving and in thermal comfort. In this study, the gaseous contaminant diffusion of stratum ventilation is experimentally and numerically investigated. The results are compared with those of the results for displacement ventilation in similar conditions. The following conclusions are drawn: 1. For stratum ventilation, gaseous contaminant concentration along the jet is lower than that of the other parts of the room. If the supply air stream is stable to fill the breathing zone, air inhaled by the occupant is generally better. 2. When the location of the contaminant source is close to the occupant, the inhaled air quality for stratum ventilation is better than that for displacement ventilation. When the contaminant source locates at the upper stream of the supply air flow (e.g. Source 1 case), the formaldehyde concentrations at the height of breathing zone (1.1 m) for stratum ventilation and for displacement ventilation are comparable. 3. For floor gaseous contaminants, stratum ventilation performs better than displacement ventilation in terms of the inhaled air quality by the occupant. However, under the given supply temperature of 19 C, when the area source locates on the surrounding walls of the office, displacement ventilation is superior to stratum ventilation in terms of inhaled air quality.
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