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Studies of air-flow and temperature fields inside a passenger compartment for improving thermal comfort and saving energy. Part II: Simulation results and discussion
Applied Thermal Engineering 29 (2009) 2028–2036
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Studies of air-flow and temperature fields inside a passenger compartment for improving thermal comfort and saving energy. Part II: Simulation results and discussion Huajun Zhang *, Lan Dai, Guoquan Xu, Yong Li, Wei Chen, Wenquan Tao School of Energy and Power Engineering, Xi’an Jiaotong University, 28 Xian Ning Road, Xi’an, Shaanxi 710049, PR China
a r t i c l e
i n f o
Article history: Received 30 October 2007 Accepted 19 October 2008 Available online 29 October 2008 Keywords: Automobile air conditioner Temperature field Air-flow field Numerical simulation Environment simulation test Thermal comfort (amenity)
a b s t r a c t In the first paper the simulation model, numerical methods and experimental measurement validation for a vehicle compartment with two chairs and one long bench have been provided. In this paper, the simulated results are presented. Simulation results with a variety of conditions reveal that: (1) a good choice for decreasing the cooling load in the summer time is increasing the inlet air temperature, not reducing the volume flow rate of the inlet air; (2) the thermal comfort in a compartment with given conditions depends on the number of persons in it. For the compartment studied, when there are two passengers in the compartment both of them should sit in the backside; (3) the outside temperature has appreciable effect on the cooling load. While change of the vehicle speed hardly affects the cooling load of air conditioner when good seal of the compartment is assumed; (4) to decrease the cooling load one can change the material of the window (reducing its transmissivity), and improve thermal insulation on the vehicle body; (5) a better flow circulation near the compartment bottom is favorable to improve the uniformity of temperature field around the driver’s foot zone. The inlet air direction should be kept horizontal. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In the first paper, the simulation model, numerical methods and experimental measurement validation for the vehicle compartment with two chairs and one long bench have been provided. The experimental work includes two processes, Process 1 and Process 2. For the convenience of presentation the two processes are briefly restated as follows. Both Process 1 and Process 2 are run in the internal circulation mode, of which the air cooled by the evaporator is the returning air inside the compartment. Process 1 is conducted according to the standard cooling experiment of Volkswagen Company, and the tested car is Pasat 1.8 L. In the process, the temperatures at 37 locations are measured, including eight points on four passengers’ head, four point on passenger’s knee, 10 points on driver and co-pilot’s foot, six points on passengers’ foot on the rear seat, eight points for inlets and one point for returning air outlet. The test is run under the circumstance of outside temperature 43 °C, relative humidity 15% and the sun load 1 kW/m2, being all constant during the test process. The cooling test is started after the car standing in the environment with high temperature and intense sunlight (1 kW/m2) for a long time and reaching steady state situation. Vehicle is first operated on 32 km/h for an hour and then on 0 km/h for 1 h with the natural * Corresponding author. Tel./fax: +86 29 8266 9106. E-mail address: [email protected] (H. Zhang). 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.10.006
wind speed 10 km/h. In the test nobody is in the vehicle. The total test time is 120 min and the temperature is recorded every 5 min. Process 2 adopts GMW3037 experiment standards which tests the max cooling performance of air conditioner, and the test vehicle is Buick 1.8 L. The test is run under the circumstance of which the environment temperature is 38 °C, relative humidity is 40% and sun load is 1 kW/m2. The environment conditions are remained unchanged during the test process. The vehicle is operated on 50 km/h for 20 min, then on 80 km/h for 20 min. The air-conditioning system starts to work when test begins, and the air temperature in the compartment is gradually decreased. The total test time duration is 162 min, and for the first 110 min the data are collected. No passenger is in the compartment. In the following, the simulation results without passengers will be first presented, followed by the results with four passengers. Then the effects of passenger numbers, and the boundary conditions on the flow and temperature fields and energy consumption will be provided in detail.
2. Simulation results with or without passenger 2.1. Investigation of flow field in the chamber without persons The major simulation results for Process 1 with the vehicle inlet temperature of 7 °C (when the process approaching the steady
H. Zhang et al. / Applied Thermal Engineering 29 (2009) 2028–2036
state) are listed in Table 1. The predicted average temperature inside the whole compartment is 23.9 °C and the load of the automobile air conditioner is 3.302 kW. The simulated flow fields and temperatures fields are presented in Figs. 1–5. Figs. 1 and 2 show the air flow from the two side-inlets and two middle-inlets, respectively. It can be seen that the inlet air from the two side-inlets goes along the compartment side wall, arrives at the latter half of the compartment, then it turns back to the middle and front of the compartment and leave the compartment via the outlet. From the stream routes it can be observed that these two streams can cool the latter half of the compartment and when the streams just leave the inlets, they can cool the driver zone and co-pilot zone nearby immediately. Fig. 2 shows that the stream from the middle-right inlet on the front plane can cool the co-pilot, while that from middle left inlet helps to cool the driver position. Air-flow stream from middle left inlet is divided into several parts. One part circles in the middle part of the compartment and returns to the air outlet without cooling the passengers or driver. For the middle-right inlet the major part of the air flow circulates in the front part around the co-pilot and only small part of the stream arrives at back seats. From above we can find out the total air-flow direction map from the four inlets. Figs. 3 and 4 show the temperature distribution of seat zone. In the figure the position of z = 0 is set in the middle longitudinal plane between the front two seats. The place with z > 0 is at the
Table 1 Calculation results of Process 1 without persons. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s)
Location Driver
Company
Back left
Back right
22.28 0.784 27.65 0.373 26.74 0.322
19.82 0.753 24.57 1.418 27.00 0.578
24.04 0.341 23.51 0.307 26.22 0.181
21.70 0.277 22.80 0.348 27.63 0.156
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driver side and that with z < 0 at the co-pilot side. From the figure, it can be seen that the air in this zone has been well cooled and the local temperature uniformity is quite good. Broadly speaking, there are several approximately isothermal regions: the center part has the lowest temperature, where the driver and passengers are supposed to be positioned, and around this region is a region with a bit higher temperature, being around 30 °C, is located; then the outside region which located near the top or the bottom has about 31–32 °C. Such a distribution map should be considered quite reasonable. Fig. 5 shows the velocity vectors in the longitudinal plane at z = 0.35 m which goes through the outlet of the circulating air stream. From the figure, it can be seen that in the most part of the plane the air speeds are quite low, the wind speed appears relatively high only at the air return outlet region (local velocity may reach 5–8 m/s). It is to be noted that the coordinates origin is set at the center line in Fig. 2 with the positive direction of z towards the driver side. Being near the returning outlet, the wind speed around company’s knee is over 1 m/s and the velocity on the driver’s and company’s head is a bit larger. The air velocity distributions around the seats are rather uniform. Because of the obstruct of the front row seats, an air recirculation is formed in the driver zone which helps to unify the air temperature while in the back side part of the compartment air flow becomes relatively uniform. The velocity distribution in the cross section of z = 0.35 where the driver position locates is not shown because the quite good uniformity of the velocity distribution makes the figure seems quite usual and no other specific information can be withdrawn. The above results are obtained without persons. It can be expected that the air speed will be decreased when driver and passengers are on seats. 2.2. Investigation of flow field with four persons This is also the steady state calculation. Fig. 6 displays the geometric model of four passengers. The boundary conditions at the surfaces of the passenger and driver are set as the second kind. Constant heat flux is prescribed. The total heat transfer rate of each
Fig. 1. Air flow from side-inlets.
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Fig. 2. Air flow from the middle-inlets.
Fig. 3. Temperature distributions of driver side surface (Z = 0.35 m). Fig. 5. Velocity distributions of company side surface (Z =
Fig. 4. Temperature distributions of company side surface (Z =
0.35 m).
passenger is set as 116 W and the heat dissipation capacity of the diver is set as 176 W [1]. These heat transfer rates are assumed to
0.35 m).
Fig. 6. Geometric model of four passengers.
be uniformly distributed along their surfaces. The air stream inlet temperature and the load of air conditioner can be expected to
H. Zhang et al. / Applied Thermal Engineering 29 (2009) 2028–2036
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Table 2 Calculation results of Process 1 with four persons. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Location Driver
Company
Back left
Back right
23.68 0.499 28.56 0.303 32.77 0.206 25.90
23.21 0.463 27.01 1.267 27.28 0.529 23.78
25.57 0.608 25.35 0.312 25.51 0.355 26.41
25.19 0.588 24.73 0.348 24.41 0.381 25.85
*
The predicted load of automobile air conditioner is 3.545 kW.
be higher than that without passengers. So the inlet temperature increases to 8.5 °C and the other boundaries are the same as those in Process 1 with no passenger. As indicated in the first paper, many investigations on the thermal comfort of the vehicle compartment have been conducted and conditions of thermal comfort have been proposed. Based on these investigations, especially the results in [2,3], it is the authors’ consideration that for the thermal comfort the compartment temperature should be with the range of 24–29 °C and the local velocity should not be larger than 0.5 m/s. The major simulated results are presented in Table 2. From Table 2, it is found that the temperature of driver’s foot is higher than that of the others and the wind speeds on company’s knee and passenger’s head on the back seats are higher than the required values. The other predicted parameters meet the requirements for the above-mentioned amenity quite well. The simulated heat transfer rate on the compartment surfaces except the bottom of the compartment is less than that without passengers. This can be explained as follows. Because of the exist of passengers in the compartment, the compartment environment temperature increased from 23.9 to 26.2 °C and the differences between the compartment inside and outside decrease, hence decreasing the temperature difference for the overall heat transfer process. In addition since people in the compartment occupy certain space, the air-flow field is changed, the wind speed decreases in many zones, and the related convection heat transfer decreases. However, the situation is the opposite on the bottom. When passengers’ feet are on it, the air flow is disturbed, and the convection heat transfer is enhanced. The effect of the enhanced heat transfer
Fig. 7. Temperature distributions of head (Y =
0.26 m) on horizontal plane.
Fig. 8. Temperature distributions of chest (Y =
0.6 m) on horizontal plane.
is larger than that of the effect of the decreasing temperature difference, resulting in the increase in heat transfer rate. The temperature and velocity distributions of compartment are shown in Figs. 7–14 by clouds, and in Fig. 15 velocity vector distributions in the horizontal plane are provided. The position of the zero y coordinate is set at the ceiling of the compartment, thus every horizontal plane below the ceiling has negative y coordinate. The numerically predicted average temperature inside the compartment is 26.2 °C. The standard temperature range commonly used in Chinese automobiles design is 28–29 °C [2]. The average temperature of 26.2 °C is a bit lower. Thus from energy saving point of view, the average temperature inside the compartment can be increased by 1–2 °C. From the predicted results for temperature distributions (Figs. 7–10), all locations except the driver’s foot zone meet the requirements for amenity and human physiology. Figs. 11–14 present the velocity distributions of four horizontal planes. From the figures, it can be observed that the wind speed on all the locations except the air stream inlets and the return air stream outlet is not high, and the air flow is quite gentle. As shown in Fig. 15, the air speed on the passengers’ head on the back seats is a bit higher than 0.5 m/s. In summary, the predicted results of flow field and temperature distribution with four passenger basically meet the requirements of thermal comfort except the following locations: the local air
Fig. 9. Temperature distributions of knee (Y =
0.85 m) on horizontal plane.
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Fig. 10. Temperature distributions of foot (Y =
1.1 m) on horizontal plane.
Fig. 13. Velocity distributions of knee (Y =
Fig. 14. Velocity distributions of foot (Y = Fig. 11. Velocity distributions of head (Y =
0. 85 m) on horizontal plane.
1.1 m) on horizontal plane.
0.26 m) on horizontal plane.
are a bit too high. The parameters at these locations are shown by boldface numbers in Table 2.
3. Effects of cooling load and numbers of persons For the convenience of later presentation the two simulated situations presented above will be called Case 1 (without persons) and Case 2 (with four persons), while the results presented later will be named in the subsequent order. 3.1. Investigation of decreasing cooling load
Fig. 12. Velocity distributions of chest (Y =
0.6 m) on horizontal plane.
temperature around the driver’s foot and the local air velocities around the co-pilot knee and passengers’ head on the back seats
There are two ways to decrease the cooling load: one is increasing the inlet air temperature and the other is decreasing the inlet air-flow rate. Under Case 3, the inlet temperature is increased from 8.5 to 11 °C. While under Case 4 the inlet air-flow rate is decreased to 84% of that of Case 2. The other conditions are remained the same as Case 2. Results of numerical simulation are shown in Tables 3 and 4. The average temperature inside the vehicle under Case 3 is 28.21 °C and that of Case 4 is 28.25 °C which both are about 2 °C higher compared with Case 2 (Process 1 with four passengers).
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Fig. 15. Velocity vectors distributions of head on back seats (Y =
ment in good condition, with increase temperature being the better choice.
Table 3 Calculation results of Case 3. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Location Driver
Company
Back left
Back right
25.86 0.498 30.53 0.304 34.59 0.206 28.02
25.43 0.464 29.01 1.275 29.11 0.532 25.96
27.67 0.608 27.43 0.310 27.51 0.355 28.52
27.34 0.587 26.90 0.342 26.43 0.382 28.02
Driver
Company
Back left
Back right
25.69 0.418 30.91 0.255 35.42 0.175 28.11
25.24 0.390 29.21 1.078 29.15 0.451 25.81
27.78 0.507 27.46 0.260 27.44 0.299 28.75
27.45 0.491 26.93 0.287 26.20 0.321 28.25
Table 4 Calculation results of Case 4. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
0.26 m) on horizontal plane.
Location
As indicated above such averaged compartment temperature can still satisfy the thermal comfort condition in China [2]. It is usually estimated [4] that in the summer time increasing 1 °C of compartment temperature will lead to about 10% decrease in cooling load. The uniformity of the flow field and cooling effect of Case 3 are a bit better than that of Case 4. Numerical practice revealed that for Case 4 increasing air volume flow rate results in some improvement of velocity field in the compartment. Therefore, in the summer time by either increasing the inlet air temperature or decreasing the inlet air-flow rate appropriately one can save energy consumption appreciably while still keeping the compart-
of
the
inlet
air
3.2. Effect of persons number As shown in Table 2 and Figs. 7–15, generally speaking, the simulated results for Case 2 have a quite good satisfaction with the requirements for the amenity, and the main flaws of this case are that there are higher temperatures of the driver’s foot and higher wind speed around company’s knee which results in the amenity on back seats being better than that of the front row. In addition, for the two passengers on the back seat, the temperature on the right one’s foot is 0.78 °C lower than that of his head, conflicting with the physiology of human beings which requires that the temperature of human foot should be a bit higher than its head. Thus, the amenity of the left is better. Apart from Process 1 with four persons, we also study the cases with three persons and two persons. Calculation results are shown in Tables 5–7 for the cases of three passengers with three variants of locations. It can be found that all calculation results of the three variants of locations with three persons meet the requirements for the amenity, while the comfort of Case 5 is the best. All computational conditions are the same as Case 2. This implies that when there are two passengers in the compartment both of them should sit in the backside. Table 8 presents the calculation results of the conditions with two persons (the diver and a passenger) in compartment (Case 8). We can see that the right seat in the back is the best choice
Table 5 Calculation results of Case 5 (none passenger in the front row). Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Location Driver
Back left
Back right
24.69 0.489 30.43 0.249 30.29 0.201 26.58
25.47 0.589 24.86 0.305 26.46 0.267 26.28
24.47 0.534 23.94 0.322 26.01 0.267 25.29
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Table 6 Calculation results of Case 6 (none on the back left seat). Parameters
Location
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Driver
Company (co-pilot)
Back right
23.95 0.483 27.67 0.316 29.70 0.276 25.73
23.17 0.425 26.33 1.269 27.03 0.571 23.45
25.10 0.530 23.85 0.346 24.43 0.376 25.58
Table 7 Calculation results of Case 7 (none passenger on the back right seat). Parameters
Location
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Driver
Company (co-pilot)
Back left
23.15 0.464 29.62 0.273 31.48 0.195 25.34
21.35 0.517 25.49 1.261 27.72 0.513 22.29
26.05 0.526 25.32 0.268 26.48 0.288 26.88
Table 8 Calculation results of Case 8 (two persons). Parameters
Location Passenger on company
Temperature of head/°C) Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Passenger on back left
Passenger on back right
Driver
Passenger
Driver
Passenger
Driver
Passenger
21.33
20.90
25.83
26.63
24.84
24.58
0.466 25.87 0.246
0.446 25.57 1.227
0.407 31.81 0.186
0.524 25.03 0.281
0.467 28.87 0.268
0.472 23.34
show that the average temperature in the compartment decreases by 0.58 °C, 0.99 °C, 1.61 °C, 2.02 °C, respectively. And the corresponding cooling load decreases about 6%, 10%, 16%, 20%, respectively. The outside temperature has an appreciable effect on the cooling load, hence on the energy consumption. 4.2. Changing outside wind speed From mechanics it is well known that the moving car leads to an upwind stream, with the same speed but in opposite direction. Simulation shows that with the increase in vehicle speed, the surface convection heat transfer coefficient increases. The integrative temperature on the top gets lower, which means that the air stream takes away part of the incident radiation heat. For the windscreens, the side walls, and the bottom, the enhanced convection heat transfer reduces thermal resistance, leading to more heat transfers into the compartment. The average temperature in the compartment affects the heat transfer from the engine bay. The decrease of inside temperature results in more heat transfer, while the increase on inner temperature leads to less heat transfer. For Case 2 with four passengers the speed of the vehicle is advanced from 32 to 36, 40, 50, 65, 80, 100, 120 km/h and the relative heat transfer coefficients on the outside wall of the vehicle are adjusted by Eq. (2) in the companion paper [5]. The other conditions are remained the same. Calculation results display that the heat transferred into the compartment from the top reduces by 8.2 W, 16.4 W, 32.3 W, 48.6 W, 56.4 W, 66.7 W, 77.7 W, respectively, compared with Case 2 and the heat transferred through the other surfaces increases by 4.9 W, 9.1 W, 19.1 W, 29.1 W, 35.4 W, 43.3 W, 49.2 W, respectively. Meanwhile average temperature inside the compartment at these conditions fluctuates by +0.03 °C, 0.00 °C, 0.03 °C, 0.07 °C, 0.07 °C, 0.11 °C, 0.13 °C, respectively, compared with Case 2. Thus it can be seen that the cooling load hardly changes. From above simulation results it can be concluded that with the assumption of good seal of the compartment, change of the vehicle speed hardly affects the cooling load of air conditioner. However, if the compartment seal is bad, then increase in vehicle speed will lead to the increase of the cooling load because of the increased air leakage.
0.332
4.3. Varying thermal insulation of the vehicle surface 31.55 0.206 23.63
27.02 0.524 21.85
30.98 0.200 27.21
27.70 0.225 27.05
31.02 0.224 26.18
26.44 0.219 25.43
when only one passenger is in the car. The temperature of head in the front row is low, so some measures should be taken in order to increase the temperature in this zone. All these cases are based on the steady state. During the cooling process if one wants to select a good position where the cooling rate is faster, then from Figs. 5–8 of the companion paper [5], following sequence of the cooling speed from max to minimum can be obtained: company, driver, back right, and back left. 4. Effects of boundary conditions 4.1. Changing outside environment temperature For Case 2 with four persons, simulations were re-conducted by changing the temperature outside from 43 to 40, 38, 35, 33 °C while other conditions were kept the same. Numerical results
As indicated before, the boundaries of the vehicle surfaces are set as the third boundary condition. The overall heat transfer process usually can be divided into three sections in series [6]. For the present study, these are the external heat transfer, the conduction through the compartment wall including insulation (if any) and the internal heat transfer. From calculation results of the Case 2, the thermal resistance outside the surface is trivial and the thermal resistance of the inside surface is the predominant one. The conduction thermal resistance is also not important, being only 10– 20% of the inner thermal resistance. Thus, the further increase in wind speed may enhance the outside heat transfer but cannot change the total thermal resistance appreciably, as indicated by the results in 4.2. However, increasing the thermal insulation performance will effectively increase the total thermal resistance, hence, reduce the cooling load. In this paper, numerical simulation is conducted by increasing the conduction thermal resistance by five times as that of the Case 2, being approximately the same as the internal thermal resistance, while the other boundary conditions are remained the same. The heat transferred from outside decreases form 1386 to 703 W and the average temperature inside the vehicle reduces from 26.2 to 22.9 °C. Meanwhile the cooling load reduces 33% compared with Case 2. Although great changes of the thermal insulation of the compartment surfaces are rather
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difficult, the present calculation results at least indicate that there is some room for reducing the cooling load in this regard, especially for the front wall and the top where the heat flux is quite large. 4.4. Varying penetrating radiation into the compartment For Case 2, simulations are re-performed by reducing the penetrating radiation from 1635 to 1300, 1000, 700, 400 W while other conditions are remained the same. Calculation results show that the average temperature of the compartment reduces by 1.28 °C, 2.46 °C, 3.64 °C, 4.82 °C, respectively, and the cooling load decreases about 13%, 25%, 36%, 48%, respectively. In order to reduce the radiation heat into the compartment, we can reduce the size and the glass transmissivity of the window. The present results show that it is a powerful tool to reduce the heat input through the window to save energy.
Table 9 Calculation results of Case 9. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Parameters
4.6. Air flow from the left inlet leaning towards driver feet Based on Case 2, simulation is re-conducted by changing the direction of the inlet air flow of the driver’s side from the horizontal to 45° downward (Case 10), so that the driver would feel more oncoming stream. The simulated results are shown in Table 10.
Driver
Company
Back left
Back right
24.14 0.475 28.07 0.324 29.46 0.244 25.78
23.41 0.444 26.2 1.32 27.2 0.479 23.86
24.18 0.625 24.28 0.322 25.56 0.336 25.27
24.56 0.606 23.8 0.348 25.6 0.362 25.2
Driver
Company
Back left
Back right
28.95 0.453 29.06 0.475 29.31 0.257 28.86
24.05 0.408 27.87 1.341 29.74 0.699 24.96
26.65 0.245 25.12 0.566 25.95 0.727 27.27
26.54 0.429 23.85 0.419 25.65 0.563 26.91
Table 10 Calculation results of Case 10.
4.5. Improving the air flow around the foot As shown in Table 2, the temperature on the driver’s foot is higher of Process 1 with four persons (Case 2). The reasons may be as follows. The foot zone in the front row gains heat from the side wall, the fore wall and the bottom wall. The only air outlet is arranged in the company side and there is a clapboard between the driver and company which results in a bad flow field around the diver foot. Thus, we may partially remove the clapboard to improve the air flow around driver’s foot zone. Such a situation is numerically simulated, and is named as Case 9. Based on the conditions of Case 2, Case 9 removes one part of the clapboard (see Fig. 16). And calculation results are shown in Table 9. From Table 9, it can be seen that the temperature of driver’s foot is 29.46 °C, which is 3.31 °C lower than that of Case 2, thus the amenity of the foot is improved. Compared with the results of Table 2, the amenity of back right is improved and the temperature of foot is increased which is suitable for the physiology of human beings (cooler around head and warmer around feet). It thus can be concluded that a better flow circulation near the compartment bottom is favorable to improve the uniformity of temperature field.
Location
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Location
Form the table, it is seen that the temperature of the driver’s foot decreases greatly but the temperature of driver’s head increases to 28.95 °C. It is a too high temperature for the driver. So the amenity of the compartment of Case 10 is deteriorated. Thus the change of the directions of inlet air flow at the driver’s side is not acceptable, and horizontal direction of flow inlet is a good choice. 5. Conclusions In the companion paper, environment simulation tests for the automobile with no passengers are carried out, and the necessary data for the comparison of simulation results are obtained. Both 3D steady and unsteady states are simulated, and the correctness of the models are validated. In this paper, a comprehensive investigation on the thermal comfort of a compartment is conducted by numerical simulation with the commercial software of FLUENT. The simulations are conducted for variety of cases. The major conclusions can be summarized as follows:
Fig. 16. Clapboard difference between Case 2 and Case 9.
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H. Zhang et al. / Applied Thermal Engineering 29 (2009) 2028–2036
1. The predicted results with four passenger in the compartment studied basically meet the requirements of thermal comfort except the local air temperature around the driver’s foot and the local air velocities around the co-pilot knee and passengers’ head on the back seats, all of which are a bit too high. 2. A good choice for decreasing the cooling load in the summer time is increasing the inlet air temperature, not reducing the volume flow rate of the inlet air. 3. The thermal comfort in a compartment with given conditions depends on the number of persons in it. For the compartment studied, when there are two passengers in the compartment both of them should sit in the backside. 4. The outside temperature has appreciable effect on the cooling load, hence on the energy consumption. While change of the vehicle speed hardly affects the cooling load of air conditioner when good seal of the compartment is assumed. 5. To decrease the cooling load one can change the material of the window (reducing its transmissivity), and improve thermal insulation on the vehicle body. 6. A better flow circulation near the compartment bottom is favorable to improve the uniformity of temperature field around the driver’s foot zone. The inlet air direction should be kept horizontal.
Acknowledgments This work was supported by Shanghai Delphi Automotive Air Conditioning Systems Co., Ltd. It was also partially supported by the Key Project of the National Natural Science Foundation of China (50636046) and the Key Project of Fundamental Research of R&D in China (973) (2007CB206902). References [1] Jiangping Chen, Shaopu Sun, Xiongcai Que, Chen Zhijiu, Liu Weihua, Numerical simulation of air-flow field and temperature distributions inside the passenger compartment, Automotive Engineering 21 (5) (1999) 309–313 (in Chinese). [2] Zengshe Zhang, Environment parameters valued in the compartment, Technology and Investigation of Car 18 (2) (1996) 73–76. 82, (in Chinese). [3] Xiongcai Que, Jiangping Chen, Guoqi Yao, Weihua Liu, Automotive Air Condition Technology, China Machine Press, Beijing, 2003. p. 66 (in Chinese). [4] Baozhi Wu, Auto Air Conditioner, Space Press, Beijing, 1991. pp. 10–27 (in Chinese). [5] Huajun Zhang, Lan Dai, Guoquan Xu, Yong Li, Wei Chen, Wen-Quan Tao. Studies of air-flow and temperature fields inside a passenger compartment for improving thermal comfort and saving energy, Part I: test/numerical model and validation, Applied Thermal Engineering 29 (2009) 2022–2027. [6] Yang Shi-Ming, Tao Wen-Quan, second ed., Heat Transfer, Higher Education Press, Beijing, 2006. p. 14 (in Chinese).
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Studies of air-flow and temperature fields inside a passenger compartment for improving thermal comfort and saving energy. Part II: Simulation results and discussion Huajun Zhang *, Lan Dai, Guoquan Xu, Yong Li, Wei Chen, Wenquan Tao School of Energy and Power Engineering, Xi’an Jiaotong University, 28 Xian Ning Road, Xi’an, Shaanxi 710049, PR China
a r t i c l e
i n f o
Article history: Received 30 October 2007 Accepted 19 October 2008 Available online 29 October 2008 Keywords: Automobile air conditioner Temperature field Air-flow field Numerical simulation Environment simulation test Thermal comfort (amenity)
a b s t r a c t In the first paper the simulation model, numerical methods and experimental measurement validation for a vehicle compartment with two chairs and one long bench have been provided. In this paper, the simulated results are presented. Simulation results with a variety of conditions reveal that: (1) a good choice for decreasing the cooling load in the summer time is increasing the inlet air temperature, not reducing the volume flow rate of the inlet air; (2) the thermal comfort in a compartment with given conditions depends on the number of persons in it. For the compartment studied, when there are two passengers in the compartment both of them should sit in the backside; (3) the outside temperature has appreciable effect on the cooling load. While change of the vehicle speed hardly affects the cooling load of air conditioner when good seal of the compartment is assumed; (4) to decrease the cooling load one can change the material of the window (reducing its transmissivity), and improve thermal insulation on the vehicle body; (5) a better flow circulation near the compartment bottom is favorable to improve the uniformity of temperature field around the driver’s foot zone. The inlet air direction should be kept horizontal. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction In the first paper, the simulation model, numerical methods and experimental measurement validation for the vehicle compartment with two chairs and one long bench have been provided. The experimental work includes two processes, Process 1 and Process 2. For the convenience of presentation the two processes are briefly restated as follows. Both Process 1 and Process 2 are run in the internal circulation mode, of which the air cooled by the evaporator is the returning air inside the compartment. Process 1 is conducted according to the standard cooling experiment of Volkswagen Company, and the tested car is Pasat 1.8 L. In the process, the temperatures at 37 locations are measured, including eight points on four passengers’ head, four point on passenger’s knee, 10 points on driver and co-pilot’s foot, six points on passengers’ foot on the rear seat, eight points for inlets and one point for returning air outlet. The test is run under the circumstance of outside temperature 43 °C, relative humidity 15% and the sun load 1 kW/m2, being all constant during the test process. The cooling test is started after the car standing in the environment with high temperature and intense sunlight (1 kW/m2) for a long time and reaching steady state situation. Vehicle is first operated on 32 km/h for an hour and then on 0 km/h for 1 h with the natural * Corresponding author. Tel./fax: +86 29 8266 9106. E-mail address: [email protected] (H. Zhang). 1359-4311/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2008.10.006
wind speed 10 km/h. In the test nobody is in the vehicle. The total test time is 120 min and the temperature is recorded every 5 min. Process 2 adopts GMW3037 experiment standards which tests the max cooling performance of air conditioner, and the test vehicle is Buick 1.8 L. The test is run under the circumstance of which the environment temperature is 38 °C, relative humidity is 40% and sun load is 1 kW/m2. The environment conditions are remained unchanged during the test process. The vehicle is operated on 50 km/h for 20 min, then on 80 km/h for 20 min. The air-conditioning system starts to work when test begins, and the air temperature in the compartment is gradually decreased. The total test time duration is 162 min, and for the first 110 min the data are collected. No passenger is in the compartment. In the following, the simulation results without passengers will be first presented, followed by the results with four passengers. Then the effects of passenger numbers, and the boundary conditions on the flow and temperature fields and energy consumption will be provided in detail.
2. Simulation results with or without passenger 2.1. Investigation of flow field in the chamber without persons The major simulation results for Process 1 with the vehicle inlet temperature of 7 °C (when the process approaching the steady
H. Zhang et al. / Applied Thermal Engineering 29 (2009) 2028–2036
state) are listed in Table 1. The predicted average temperature inside the whole compartment is 23.9 °C and the load of the automobile air conditioner is 3.302 kW. The simulated flow fields and temperatures fields are presented in Figs. 1–5. Figs. 1 and 2 show the air flow from the two side-inlets and two middle-inlets, respectively. It can be seen that the inlet air from the two side-inlets goes along the compartment side wall, arrives at the latter half of the compartment, then it turns back to the middle and front of the compartment and leave the compartment via the outlet. From the stream routes it can be observed that these two streams can cool the latter half of the compartment and when the streams just leave the inlets, they can cool the driver zone and co-pilot zone nearby immediately. Fig. 2 shows that the stream from the middle-right inlet on the front plane can cool the co-pilot, while that from middle left inlet helps to cool the driver position. Air-flow stream from middle left inlet is divided into several parts. One part circles in the middle part of the compartment and returns to the air outlet without cooling the passengers or driver. For the middle-right inlet the major part of the air flow circulates in the front part around the co-pilot and only small part of the stream arrives at back seats. From above we can find out the total air-flow direction map from the four inlets. Figs. 3 and 4 show the temperature distribution of seat zone. In the figure the position of z = 0 is set in the middle longitudinal plane between the front two seats. The place with z > 0 is at the
Table 1 Calculation results of Process 1 without persons. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s)
Location Driver
Company
Back left
Back right
22.28 0.784 27.65 0.373 26.74 0.322
19.82 0.753 24.57 1.418 27.00 0.578
24.04 0.341 23.51 0.307 26.22 0.181
21.70 0.277 22.80 0.348 27.63 0.156
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driver side and that with z < 0 at the co-pilot side. From the figure, it can be seen that the air in this zone has been well cooled and the local temperature uniformity is quite good. Broadly speaking, there are several approximately isothermal regions: the center part has the lowest temperature, where the driver and passengers are supposed to be positioned, and around this region is a region with a bit higher temperature, being around 30 °C, is located; then the outside region which located near the top or the bottom has about 31–32 °C. Such a distribution map should be considered quite reasonable. Fig. 5 shows the velocity vectors in the longitudinal plane at z = 0.35 m which goes through the outlet of the circulating air stream. From the figure, it can be seen that in the most part of the plane the air speeds are quite low, the wind speed appears relatively high only at the air return outlet region (local velocity may reach 5–8 m/s). It is to be noted that the coordinates origin is set at the center line in Fig. 2 with the positive direction of z towards the driver side. Being near the returning outlet, the wind speed around company’s knee is over 1 m/s and the velocity on the driver’s and company’s head is a bit larger. The air velocity distributions around the seats are rather uniform. Because of the obstruct of the front row seats, an air recirculation is formed in the driver zone which helps to unify the air temperature while in the back side part of the compartment air flow becomes relatively uniform. The velocity distribution in the cross section of z = 0.35 where the driver position locates is not shown because the quite good uniformity of the velocity distribution makes the figure seems quite usual and no other specific information can be withdrawn. The above results are obtained without persons. It can be expected that the air speed will be decreased when driver and passengers are on seats. 2.2. Investigation of flow field with four persons This is also the steady state calculation. Fig. 6 displays the geometric model of four passengers. The boundary conditions at the surfaces of the passenger and driver are set as the second kind. Constant heat flux is prescribed. The total heat transfer rate of each
Fig. 1. Air flow from side-inlets.
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Fig. 2. Air flow from the middle-inlets.
Fig. 3. Temperature distributions of driver side surface (Z = 0.35 m). Fig. 5. Velocity distributions of company side surface (Z =
Fig. 4. Temperature distributions of company side surface (Z =
0.35 m).
passenger is set as 116 W and the heat dissipation capacity of the diver is set as 176 W [1]. These heat transfer rates are assumed to
0.35 m).
Fig. 6. Geometric model of four passengers.
be uniformly distributed along their surfaces. The air stream inlet temperature and the load of air conditioner can be expected to
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Table 2 Calculation results of Process 1 with four persons. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Location Driver
Company
Back left
Back right
23.68 0.499 28.56 0.303 32.77 0.206 25.90
23.21 0.463 27.01 1.267 27.28 0.529 23.78
25.57 0.608 25.35 0.312 25.51 0.355 26.41
25.19 0.588 24.73 0.348 24.41 0.381 25.85
*
The predicted load of automobile air conditioner is 3.545 kW.
be higher than that without passengers. So the inlet temperature increases to 8.5 °C and the other boundaries are the same as those in Process 1 with no passenger. As indicated in the first paper, many investigations on the thermal comfort of the vehicle compartment have been conducted and conditions of thermal comfort have been proposed. Based on these investigations, especially the results in [2,3], it is the authors’ consideration that for the thermal comfort the compartment temperature should be with the range of 24–29 °C and the local velocity should not be larger than 0.5 m/s. The major simulated results are presented in Table 2. From Table 2, it is found that the temperature of driver’s foot is higher than that of the others and the wind speeds on company’s knee and passenger’s head on the back seats are higher than the required values. The other predicted parameters meet the requirements for the above-mentioned amenity quite well. The simulated heat transfer rate on the compartment surfaces except the bottom of the compartment is less than that without passengers. This can be explained as follows. Because of the exist of passengers in the compartment, the compartment environment temperature increased from 23.9 to 26.2 °C and the differences between the compartment inside and outside decrease, hence decreasing the temperature difference for the overall heat transfer process. In addition since people in the compartment occupy certain space, the air-flow field is changed, the wind speed decreases in many zones, and the related convection heat transfer decreases. However, the situation is the opposite on the bottom. When passengers’ feet are on it, the air flow is disturbed, and the convection heat transfer is enhanced. The effect of the enhanced heat transfer
Fig. 7. Temperature distributions of head (Y =
0.26 m) on horizontal plane.
Fig. 8. Temperature distributions of chest (Y =
0.6 m) on horizontal plane.
is larger than that of the effect of the decreasing temperature difference, resulting in the increase in heat transfer rate. The temperature and velocity distributions of compartment are shown in Figs. 7–14 by clouds, and in Fig. 15 velocity vector distributions in the horizontal plane are provided. The position of the zero y coordinate is set at the ceiling of the compartment, thus every horizontal plane below the ceiling has negative y coordinate. The numerically predicted average temperature inside the compartment is 26.2 °C. The standard temperature range commonly used in Chinese automobiles design is 28–29 °C [2]. The average temperature of 26.2 °C is a bit lower. Thus from energy saving point of view, the average temperature inside the compartment can be increased by 1–2 °C. From the predicted results for temperature distributions (Figs. 7–10), all locations except the driver’s foot zone meet the requirements for amenity and human physiology. Figs. 11–14 present the velocity distributions of four horizontal planes. From the figures, it can be observed that the wind speed on all the locations except the air stream inlets and the return air stream outlet is not high, and the air flow is quite gentle. As shown in Fig. 15, the air speed on the passengers’ head on the back seats is a bit higher than 0.5 m/s. In summary, the predicted results of flow field and temperature distribution with four passenger basically meet the requirements of thermal comfort except the following locations: the local air
Fig. 9. Temperature distributions of knee (Y =
0.85 m) on horizontal plane.
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Fig. 10. Temperature distributions of foot (Y =
1.1 m) on horizontal plane.
Fig. 13. Velocity distributions of knee (Y =
Fig. 14. Velocity distributions of foot (Y = Fig. 11. Velocity distributions of head (Y =
0. 85 m) on horizontal plane.
1.1 m) on horizontal plane.
0.26 m) on horizontal plane.
are a bit too high. The parameters at these locations are shown by boldface numbers in Table 2.
3. Effects of cooling load and numbers of persons For the convenience of later presentation the two simulated situations presented above will be called Case 1 (without persons) and Case 2 (with four persons), while the results presented later will be named in the subsequent order. 3.1. Investigation of decreasing cooling load
Fig. 12. Velocity distributions of chest (Y =
0.6 m) on horizontal plane.
temperature around the driver’s foot and the local air velocities around the co-pilot knee and passengers’ head on the back seats
There are two ways to decrease the cooling load: one is increasing the inlet air temperature and the other is decreasing the inlet air-flow rate. Under Case 3, the inlet temperature is increased from 8.5 to 11 °C. While under Case 4 the inlet air-flow rate is decreased to 84% of that of Case 2. The other conditions are remained the same as Case 2. Results of numerical simulation are shown in Tables 3 and 4. The average temperature inside the vehicle under Case 3 is 28.21 °C and that of Case 4 is 28.25 °C which both are about 2 °C higher compared with Case 2 (Process 1 with four passengers).
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Fig. 15. Velocity vectors distributions of head on back seats (Y =
ment in good condition, with increase temperature being the better choice.
Table 3 Calculation results of Case 3. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Location Driver
Company
Back left
Back right
25.86 0.498 30.53 0.304 34.59 0.206 28.02
25.43 0.464 29.01 1.275 29.11 0.532 25.96
27.67 0.608 27.43 0.310 27.51 0.355 28.52
27.34 0.587 26.90 0.342 26.43 0.382 28.02
Driver
Company
Back left
Back right
25.69 0.418 30.91 0.255 35.42 0.175 28.11
25.24 0.390 29.21 1.078 29.15 0.451 25.81
27.78 0.507 27.46 0.260 27.44 0.299 28.75
27.45 0.491 26.93 0.287 26.20 0.321 28.25
Table 4 Calculation results of Case 4. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
0.26 m) on horizontal plane.
Location
As indicated above such averaged compartment temperature can still satisfy the thermal comfort condition in China [2]. It is usually estimated [4] that in the summer time increasing 1 °C of compartment temperature will lead to about 10% decrease in cooling load. The uniformity of the flow field and cooling effect of Case 3 are a bit better than that of Case 4. Numerical practice revealed that for Case 4 increasing air volume flow rate results in some improvement of velocity field in the compartment. Therefore, in the summer time by either increasing the inlet air temperature or decreasing the inlet air-flow rate appropriately one can save energy consumption appreciably while still keeping the compart-
of
the
inlet
air
3.2. Effect of persons number As shown in Table 2 and Figs. 7–15, generally speaking, the simulated results for Case 2 have a quite good satisfaction with the requirements for the amenity, and the main flaws of this case are that there are higher temperatures of the driver’s foot and higher wind speed around company’s knee which results in the amenity on back seats being better than that of the front row. In addition, for the two passengers on the back seat, the temperature on the right one’s foot is 0.78 °C lower than that of his head, conflicting with the physiology of human beings which requires that the temperature of human foot should be a bit higher than its head. Thus, the amenity of the left is better. Apart from Process 1 with four persons, we also study the cases with three persons and two persons. Calculation results are shown in Tables 5–7 for the cases of three passengers with three variants of locations. It can be found that all calculation results of the three variants of locations with three persons meet the requirements for the amenity, while the comfort of Case 5 is the best. All computational conditions are the same as Case 2. This implies that when there are two passengers in the compartment both of them should sit in the backside. Table 8 presents the calculation results of the conditions with two persons (the diver and a passenger) in compartment (Case 8). We can see that the right seat in the back is the best choice
Table 5 Calculation results of Case 5 (none passenger in the front row). Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Location Driver
Back left
Back right
24.69 0.489 30.43 0.249 30.29 0.201 26.58
25.47 0.589 24.86 0.305 26.46 0.267 26.28
24.47 0.534 23.94 0.322 26.01 0.267 25.29
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Table 6 Calculation results of Case 6 (none on the back left seat). Parameters
Location
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Driver
Company (co-pilot)
Back right
23.95 0.483 27.67 0.316 29.70 0.276 25.73
23.17 0.425 26.33 1.269 27.03 0.571 23.45
25.10 0.530 23.85 0.346 24.43 0.376 25.58
Table 7 Calculation results of Case 7 (none passenger on the back right seat). Parameters
Location
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Driver
Company (co-pilot)
Back left
23.15 0.464 29.62 0.273 31.48 0.195 25.34
21.35 0.517 25.49 1.261 27.72 0.513 22.29
26.05 0.526 25.32 0.268 26.48 0.288 26.88
Table 8 Calculation results of Case 8 (two persons). Parameters
Location Passenger on company
Temperature of head/°C) Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Passenger on back left
Passenger on back right
Driver
Passenger
Driver
Passenger
Driver
Passenger
21.33
20.90
25.83
26.63
24.84
24.58
0.466 25.87 0.246
0.446 25.57 1.227
0.407 31.81 0.186
0.524 25.03 0.281
0.467 28.87 0.268
0.472 23.34
show that the average temperature in the compartment decreases by 0.58 °C, 0.99 °C, 1.61 °C, 2.02 °C, respectively. And the corresponding cooling load decreases about 6%, 10%, 16%, 20%, respectively. The outside temperature has an appreciable effect on the cooling load, hence on the energy consumption. 4.2. Changing outside wind speed From mechanics it is well known that the moving car leads to an upwind stream, with the same speed but in opposite direction. Simulation shows that with the increase in vehicle speed, the surface convection heat transfer coefficient increases. The integrative temperature on the top gets lower, which means that the air stream takes away part of the incident radiation heat. For the windscreens, the side walls, and the bottom, the enhanced convection heat transfer reduces thermal resistance, leading to more heat transfers into the compartment. The average temperature in the compartment affects the heat transfer from the engine bay. The decrease of inside temperature results in more heat transfer, while the increase on inner temperature leads to less heat transfer. For Case 2 with four passengers the speed of the vehicle is advanced from 32 to 36, 40, 50, 65, 80, 100, 120 km/h and the relative heat transfer coefficients on the outside wall of the vehicle are adjusted by Eq. (2) in the companion paper [5]. The other conditions are remained the same. Calculation results display that the heat transferred into the compartment from the top reduces by 8.2 W, 16.4 W, 32.3 W, 48.6 W, 56.4 W, 66.7 W, 77.7 W, respectively, compared with Case 2 and the heat transferred through the other surfaces increases by 4.9 W, 9.1 W, 19.1 W, 29.1 W, 35.4 W, 43.3 W, 49.2 W, respectively. Meanwhile average temperature inside the compartment at these conditions fluctuates by +0.03 °C, 0.00 °C, 0.03 °C, 0.07 °C, 0.07 °C, 0.11 °C, 0.13 °C, respectively, compared with Case 2. Thus it can be seen that the cooling load hardly changes. From above simulation results it can be concluded that with the assumption of good seal of the compartment, change of the vehicle speed hardly affects the cooling load of air conditioner. However, if the compartment seal is bad, then increase in vehicle speed will lead to the increase of the cooling load because of the increased air leakage.
0.332
4.3. Varying thermal insulation of the vehicle surface 31.55 0.206 23.63
27.02 0.524 21.85
30.98 0.200 27.21
27.70 0.225 27.05
31.02 0.224 26.18
26.44 0.219 25.43
when only one passenger is in the car. The temperature of head in the front row is low, so some measures should be taken in order to increase the temperature in this zone. All these cases are based on the steady state. During the cooling process if one wants to select a good position where the cooling rate is faster, then from Figs. 5–8 of the companion paper [5], following sequence of the cooling speed from max to minimum can be obtained: company, driver, back right, and back left. 4. Effects of boundary conditions 4.1. Changing outside environment temperature For Case 2 with four persons, simulations were re-conducted by changing the temperature outside from 43 to 40, 38, 35, 33 °C while other conditions were kept the same. Numerical results
As indicated before, the boundaries of the vehicle surfaces are set as the third boundary condition. The overall heat transfer process usually can be divided into three sections in series [6]. For the present study, these are the external heat transfer, the conduction through the compartment wall including insulation (if any) and the internal heat transfer. From calculation results of the Case 2, the thermal resistance outside the surface is trivial and the thermal resistance of the inside surface is the predominant one. The conduction thermal resistance is also not important, being only 10– 20% of the inner thermal resistance. Thus, the further increase in wind speed may enhance the outside heat transfer but cannot change the total thermal resistance appreciably, as indicated by the results in 4.2. However, increasing the thermal insulation performance will effectively increase the total thermal resistance, hence, reduce the cooling load. In this paper, numerical simulation is conducted by increasing the conduction thermal resistance by five times as that of the Case 2, being approximately the same as the internal thermal resistance, while the other boundary conditions are remained the same. The heat transferred from outside decreases form 1386 to 703 W and the average temperature inside the vehicle reduces from 26.2 to 22.9 °C. Meanwhile the cooling load reduces 33% compared with Case 2. Although great changes of the thermal insulation of the compartment surfaces are rather
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difficult, the present calculation results at least indicate that there is some room for reducing the cooling load in this regard, especially for the front wall and the top where the heat flux is quite large. 4.4. Varying penetrating radiation into the compartment For Case 2, simulations are re-performed by reducing the penetrating radiation from 1635 to 1300, 1000, 700, 400 W while other conditions are remained the same. Calculation results show that the average temperature of the compartment reduces by 1.28 °C, 2.46 °C, 3.64 °C, 4.82 °C, respectively, and the cooling load decreases about 13%, 25%, 36%, 48%, respectively. In order to reduce the radiation heat into the compartment, we can reduce the size and the glass transmissivity of the window. The present results show that it is a powerful tool to reduce the heat input through the window to save energy.
Table 9 Calculation results of Case 9. Parameters
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Parameters
4.6. Air flow from the left inlet leaning towards driver feet Based on Case 2, simulation is re-conducted by changing the direction of the inlet air flow of the driver’s side from the horizontal to 45° downward (Case 10), so that the driver would feel more oncoming stream. The simulated results are shown in Table 10.
Driver
Company
Back left
Back right
24.14 0.475 28.07 0.324 29.46 0.244 25.78
23.41 0.444 26.2 1.32 27.2 0.479 23.86
24.18 0.625 24.28 0.322 25.56 0.336 25.27
24.56 0.606 23.8 0.348 25.6 0.362 25.2
Driver
Company
Back left
Back right
28.95 0.453 29.06 0.475 29.31 0.257 28.86
24.05 0.408 27.87 1.341 29.74 0.699 24.96
26.65 0.245 25.12 0.566 25.95 0.727 27.27
26.54 0.429 23.85 0.419 25.65 0.563 26.91
Table 10 Calculation results of Case 10.
4.5. Improving the air flow around the foot As shown in Table 2, the temperature on the driver’s foot is higher of Process 1 with four persons (Case 2). The reasons may be as follows. The foot zone in the front row gains heat from the side wall, the fore wall and the bottom wall. The only air outlet is arranged in the company side and there is a clapboard between the driver and company which results in a bad flow field around the diver foot. Thus, we may partially remove the clapboard to improve the air flow around driver’s foot zone. Such a situation is numerically simulated, and is named as Case 9. Based on the conditions of Case 2, Case 9 removes one part of the clapboard (see Fig. 16). And calculation results are shown in Table 9. From Table 9, it can be seen that the temperature of driver’s foot is 29.46 °C, which is 3.31 °C lower than that of Case 2, thus the amenity of the foot is improved. Compared with the results of Table 2, the amenity of back right is improved and the temperature of foot is increased which is suitable for the physiology of human beings (cooler around head and warmer around feet). It thus can be concluded that a better flow circulation near the compartment bottom is favorable to improve the uniformity of temperature field.
Location
Temperature of head/°C Wind speed of head/(m/s) Temperature of knee/°C Wind speed of knee/(m/s) Temperature of foot/°C Wind speed of foot/(m/s) Average temperature of passenger surface/°C
Location
Form the table, it is seen that the temperature of the driver’s foot decreases greatly but the temperature of driver’s head increases to 28.95 °C. It is a too high temperature for the driver. So the amenity of the compartment of Case 10 is deteriorated. Thus the change of the directions of inlet air flow at the driver’s side is not acceptable, and horizontal direction of flow inlet is a good choice. 5. Conclusions In the companion paper, environment simulation tests for the automobile with no passengers are carried out, and the necessary data for the comparison of simulation results are obtained. Both 3D steady and unsteady states are simulated, and the correctness of the models are validated. In this paper, a comprehensive investigation on the thermal comfort of a compartment is conducted by numerical simulation with the commercial software of FLUENT. The simulations are conducted for variety of cases. The major conclusions can be summarized as follows:
Fig. 16. Clapboard difference between Case 2 and Case 9.
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H. Zhang et al. / Applied Thermal Engineering 29 (2009) 2028–2036
1. The predicted results with four passenger in the compartment studied basically meet the requirements of thermal comfort except the local air temperature around the driver’s foot and the local air velocities around the co-pilot knee and passengers’ head on the back seats, all of which are a bit too high. 2. A good choice for decreasing the cooling load in the summer time is increasing the inlet air temperature, not reducing the volume flow rate of the inlet air. 3. The thermal comfort in a compartment with given conditions depends on the number of persons in it. For the compartment studied, when there are two passengers in the compartment both of them should sit in the backside. 4. The outside temperature has appreciable effect on the cooling load, hence on the energy consumption. While change of the vehicle speed hardly affects the cooling load of air conditioner when good seal of the compartment is assumed. 5. To decrease the cooling load one can change the material of the window (reducing its transmissivity), and improve thermal insulation on the vehicle body. 6. A better flow circulation near the compartment bottom is favorable to improve the uniformity of temperature field around the driver’s foot zone. The inlet air direction should be kept horizontal.
Acknowledgments This work was supported by Shanghai Delphi Automotive Air Conditioning Systems Co., Ltd. It was also partially supported by the Key Project of the National Natural Science Foundation of China (50636046) and the Key Project of Fundamental Research of R&D in China (973) (2007CB206902). References [1] Jiangping Chen, Shaopu Sun, Xiongcai Que, Chen Zhijiu, Liu Weihua, Numerical simulation of air-flow field and temperature distributions inside the passenger compartment, Automotive Engineering 21 (5) (1999) 309–313 (in Chinese). [2] Zengshe Zhang, Environment parameters valued in the compartment, Technology and Investigation of Car 18 (2) (1996) 73–76. 82, (in Chinese). [3] Xiongcai Que, Jiangping Chen, Guoqi Yao, Weihua Liu, Automotive Air Condition Technology, China Machine Press, Beijing, 2003. p. 66 (in Chinese). [4] Baozhi Wu, Auto Air Conditioner, Space Press, Beijing, 1991. pp. 10–27 (in Chinese). [5] Huajun Zhang, Lan Dai, Guoquan Xu, Yong Li, Wei Chen, Wen-Quan Tao. Studies of air-flow and temperature fields inside a passenger compartment for improving thermal comfort and saving energy, Part I: test/numerical model and validation, Applied Thermal Engineering 29 (2009) 2022–2027. [6] Yang Shi-Ming, Tao Wen-Quan, second ed., Heat Transfer, Higher Education Press, Beijing, 2006. p. 14 (in Chinese).