1、2009 ASHRAE 407ABSTRACTThe focus of this study is to outline the general require-ments to accurately simulate the air quality in the breathing zone of a person using CFD when steep gradients of velocity, temperature, and contaminants are present near the person. In particular, these steep gradients
2、may result from the pres-ence of personal ventilation devices, or from body-emitted bio-effluents. Two configurations are discussed in the paper: a person sitting in an infinite domain with no nearby ventila-tion system (buoyancy-driven flow alone), and the case of a person sitting in a room with a
3、combined displacement and personal ventilation system. The latter case compares the computational results with test data for validation purposes. Issues discussed in this paper include: (1) the importance of proper physical representation of the person near the breath-ing zone including the shoulder
4、s, neck and chin, (2) the effects of the thermal boundary conditions, and (3) the effect of grid resolution. CFD results are obtained using the steady Reyn-olds-Averaged Navier-Stokes (RANS) method with the k- and k- families of turbulence models.INTRODUCTIONThe air quality in the Personal Micro-Env
5、ironment (PE) of a person, defined as the region around a person affecting the inhaled air, depends strongly on both the ventilation system, and the strength and location of pollutant sources. When pollutant sources are far away from the person and a mixing type ventilation system is employed, the w
6、ell-mixed condition may be adequate. For example, if we consider a person sitting at a desk in the middle of a large room with a mixing ventila-tion system and the pollutant sources are from the walls, then the pollutants have sufficient time to mix with the surrounding air to achieve the well-mixed
7、 condition by the time the contaminated air reaches the PE. However, if the pollutant sources are in close proximity to the person, e.g., pollutants emitted from the desk or bio-effluents emitted from the body, then the well-mixed condition is often inappropriate even if the room has a mixing ventil
8、ation system. The situation is more complex when Personal Ventilation (PV) systems are deployed near the person to deliver clean and unpolluted air directly to the persons breathing zone (BZ), either with or without a mixing ventilation system in the room. In this situ-ation, in addition to the pres
9、ence of a steep gradient of the pollutant concentration near the person, there are also large gradients in temperature and velocity surrounding the person that can affect the air quality in the PE. For example, the inter-action between the persons thermal plume (which carries pollutants such as bio-
10、effluents) and the jet flow from the PV system (which carries clean air) can be very significant, depending on the relative strength of the momentum of these two air streams (Khalifa et al., 2008; Russo et al., 2008). In situations where there exist significant gradients of velocity, temperature, an
11、d pollutants in the PE, the common practice of single-point measurement to quantify the air quality in the PE, often used in situations where the well-mixed approximation is valid, is no longer adequate. Measurements must now be taken at many points and can be very time-consuming and expensive, and
12、the choice of the measurement locations is not a trivial task. As an example, the experimental work of Khalifa et al. (2008) on the effectiveness of PV systems showed that when a simple round jet from a PV system is aimed at the face of a manikin, the PV jet is deflected and distorted by the thermal
13、 plume around the person. The resulting air quality in the BZ is found to exhibit significant gradients, and a strong Modeling of the Human Body to Study the Personal Micro EnvironmentRyan K. Dygert Jackie S. RussoThong Q. Dang, PhD H. Ezzat Khalifa, PhDMember ASHRAERyan K. Dygert and Jackie S. Russ
14、o are PhD students and Thong Q. Dang and H. Ezzat Khalifa are professors in the Department of Mechan-ical and Aerospace Engineering, Syracuse University, Syracuse, NY.LO-09-037 2009, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE
15、 Transactions 2009, vol. 115, part 2. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission.408 ASHRAE Transactionsdependence on many local flow and thermal parameters (e.g. PV flow rate
16、 and manikins body temperature).For problems where steep gradients of velocity, temper-ature and pollutants exist in the PE and the well-mixed condi-tion is not appropriate, CFD methods can provide very powerful and efficient tools to study the air quality in the PE. CFD studies around a Computer Si
17、mulated Person (CSP) have been carried out by many researchers. These studies include examining the flow and temperature fields around the body (Topp et al. 2002; Murakami, 2004), heat and mass transfer with the environment (Murakami et al. 2000; Sorensen Kilik Hayashi et al., 2002; Gao Deevy et al.
18、, 2008) and person to person contaminant spread (Gao Hayashi et al., 2002), the RNG k-model (Gao Khalifa et al., 2006), the v2-f model (Sideroff Topp et al. 2002; Sorensen the first was the baseline CSP discussed above and the second CSP, called multi-block CSP, was constructed from multiple rectang
19、ular boxes which include a gap between the legs and thighs, a larger rectangular box representing the upper body and arms, and a rectangular box representing the neck and head. The total exposed surface area of the two “half” CSPs is identical at 0.69m2(7.43 ft.2) (the back and the bottom are not ex
20、posed to the environment) We note that the multi-block CSP geometry employed here contains two important features. The first one is the gap between the legs and thighs which was identified by Topp et al. (2002) to be important, at least in the presence of room ventilation airflow. The second one is
21、the inclusion of the shoulder region, which is of interest when PV or ventilation systems located on the seat are employed (e.g. Dang, 2007; Melikov, 2008; Nielsen, 2008). With respect to the PE, the primary geometrical differences between the baseline CSP and the multi-block CSP are the neck/chin r
22、egion (Zhu et al., 2005) and the curvatures of the head, neck and shoulder. It is noted that, to isolate the effects of grid resolution, both CSP geometries were meshed with identical boundary layer resolution to yield the same average y+for the multi-block CSP as for the baseline CSP. The grid size
23、 of the multi-block CSP consisted of 500,000 cells, and the grid characteristics are summarized in Table 1.As the two CSP geometries are significantly different, it is most appropriate to provide qualitative comparisons of velocity, temperature, and concentration fields rather than values at discret
24、e points. Figure 2 shows the pathlines released from a plane located at the CSPs abdomen location extending 0.1 m (0.33 ft.) from the body. The figure shows that, in the baseline CSP, most of the pathlines very close to the CSP go around the BZ because of the protruding chin and the curva-tures of t
25、he shoulder and the head. For the baseline CSP, path-lines which enter into the BZ emanate from farther away from the CSP surface. In the case of the multi-block CSP, however, the pathlines close to the CSP enter the BZ. To illustrate the effects of CSP geometry on exposure, body-emitted bio-effluen
26、ts are released from the CSP surface at a constant flux, and the distributions of bio-effluents in the PE at the BZ are examined. The results are depicted as an air quality index (AQI), defined as:where Ce, C, and Cjare the exhaust, local, and PV jet concen-trations, respectively. For the natural-co
27、nvection case being examined, the entire outflow from the domain leaves through the pressure boundary above the CSPs head, therefore, the exhaust concentration is taken as the mass-flux averaged concentration at this boundary. Also, since no PV system is implemented, Cjis zero. Figure 3 shows a comp
28、arison of contours of AQI between the baseline CSP and the multi-block CSP on a horizontal plane cutting through the mouth region. Here, AQI of zero represents well-mixed air, and highly negative AQI values represent highly contaminated air. The figure clearly shows that in the case of the baseline
29、CSP, the highest concentration of bio-effluents is located behind the head, and it is much lower in the BZ region. This contour plot clearly shows a large portion of the bio-effluents emitted from the lower part of the body are transported by the thermal plume around the head/neck region toward the
30、shoulder AQICeCCeCj-=Table 1. Grid Details of Buoyancy Flow Cases, Representing Half of the CSP with SymmetryCSP CaseSurface ElementsSurface Grid Volume Grid Prism B.L.First Cell Height, mm (in.)Avg. y+ Max y+Baseline 19000 tri tetrahedral Yes, 5 layer 0.5 (0.0197) 1.5 3Multi-Block 7400 tri tetrahed
31、ral Yes, 5 layer 0.5 (0.0197) 0.5 6Coarse Multi-Block 800 quad Hexahedral No 20 (0.7874) 9 21ASHRAE Transactions 411regions, as indicated in Figure 2. On the other hand, in the case of the multi-block CSP, the opposite is true; the highest concentration of bio-effluents is found to be in the BZ with
32、 very little being transported over the shoulders, which is also consistent with the pathlines shown in Figure 2. These conclu-sions are in agreement with the earlier combined experimental and CFD work of Zhu et al. (2005). Another interesting difference in the flow field between the two CSP geometr
33、ies is in the regions above the shoulders. Figure 4 shows the velocity vector field on a plane cutting through the center of the shoulder. In the case of the baseline CSP, the curvature of the shoulder along with the migration of a large portion of the thermal-plume flow into this area result in the
34、 presence of a flow with high vertical momentum in the region. The opposite is true in the case of the multi-block CSP; here, the presence of a “sudden-expansion” region results in a re-circulating zone with very low momentum. This rather large difference between the two results can be important whe
35、n PV and extraction devices are integrated in the seat. Effects of Grid ResolutionIn the above study, in order to isolate the effects of grid resolution, a rather fine grid was used in the multi-block CSP (y+ 0.5). In practice, this type of simplified CSP geometry is used to reduce the grid size. To
36、 quantify the effects of grid resolution on the results, the same multi-block CSP geometry was meshed with the number of grid points reduced by a factor of 10, resulting in a y+value in the range of 10-20 (further grid Figure 2 Streamlines released from horizontal plane above lap region.Figure 3 Con
37、tours of AQI; horizontal plane passing through mouth.412 ASHRAE Transactionsinformation can be found in Table 1). In the coarsened multi-block CSP case, the standard wall function was used due to the increased y+. Although this range of y+is not recommended for use with any wall treatment options (e
38、nhanced wall treatment recommends y+less than 4-5, standard wall function recom-mends y+greater than 30), it is a range commonly used in coarse grid calculations involving the indoor environment. The grid size of the coarsened multi-block CSP consisted of 14,000 cells, and the grid characteristics a
39、re summarized in Table 1. The thermal boundary conditions here were set the same way as the baseline case discussed previously (32C (89.6F) isothermal surface).Figure 5 shows a comparison of the vertical velocity component at the mouth level along a horizontal line on the symmetry plane. Here it is
40、noted that the coarsened multi-block CSP misses the peak velocity nearest the CSP surface by 25% and under-predicts the overall plume velocity. To quan-tify the effects of grid resolution on the results, the net momen-tum flux at the exit boundary above the head (a measure of the strength of the the
41、rmal plume) along with the net convective heat flux on the surface of the CSP were calculated and are summarized in Table 2. When the coarsened mesh is used, the CFD results yield a much lower value for the net convective heat flux (59% of the baseline-mesh value), and hence the corresponding value
42、for the net momentum flux is also lower (72% of the baseline-mesh value). Thus, when using a coarse grid with large y+and a temperature boundary condition on the CSP surface, one under-predicts the strength of the thermal plume. This can be important when there is strong flow-inter-action between th
43、e thermal plume and certain types of venti-lation systems (e.g. PV systems). Effects of Turbulence ModelUsing the baseline CSP setup, the CFD results using five turbulence models available in the commercial software FLUENT are compared. The turbulence models examined here include the k- family of tu
44、rbulence models (standard, realizable, and RNG) and the k- family of turbulence models (standard and SST). Figure 6 shows the comparison of the vertical velocity component as a function of horizontal distance from the mouth location on the symmetry plane. The results show that the predictions from t
45、he k- family of turbu-lence models are very similar, while the k- family of turbu-lence models predict as much as 25% higher vertical velocity component along the plume centerline. Table 3 yields the comparisons of net convective heat flux and momentum flux between the five turbulence models studied
46、. Here it is noted that the momentum flux of both k- models is slightly lower than that of the k- models despite the higher velocity seen along the centerline in Figure 6. This is explained by a differ-ence in the plume shape and overall velocity distribution when comparing the k- and k- models; a n
47、arrower plume with higher velocity core is predicted by the k- models as compared to a wider, more diffuse plume predicted by the k- models.Effects of Thermal Boundary Condition on the CSP SurfaceThermoregulation models for the human body have been developed to study thermal comfort (e.g. Smith, 199
48、1; Sakoi, et al. 2005, 2006). When the focus of the modeling is on the calculation of contaminant exposure or air quality in the PE, the question arises as to whether one must include an accurate Figure 4 Velocity vectors; vertical plane passing through shoulder centerline.ASHRAE Transactions 413Fig
49、ure 5 Vertical velocity; horizontal line at mouth on symmetry plane.Figure 6 Vertical velocity; horizontal line at mouth on symmetry plane.Table 2. Summary of Fluxes of Buoyancy Flow Cases, Half DomainCSP CaseNet Convective Heat Flux,W (Btu/h)Net Momentum Flux; Top Boundary, N (lbf)Baseline 17.06 (58.21) 2.87e-3 (6.46e-4)Multi-block 16.27 (55.52) 2.94e-3 (6.61e-4)Coarse Multi-block 9.57 (32.65) 2.13e-3 (4.78e-4)414 ASHRAE Transactionsthermal boundary condition on the CSP surface such as a human-body thermoregulation model, or if a constan
