Experimental Study on Occupant’s Thermal Responses under the Non-uniform Conditions in Vehicle Cabin during the Heating Period

ZHANG Wencan,CHEN Jiqing,and LAN Fengchong, *

1 School of Mechanical &Automotive Engineering,South China University of Technology,Guangzhou 510640,China

2 School of Mechanical &Electrical Engineering,Foshan University,Foshan 528000,China

3 Key Laboratory of Guangdong Province of Automotive Engineering,South China University of Technology,Guangzhou 510640,China

1 Introduction*

Thermal comfort in vehicular cabins is one of the major concerns by car’s owners,researchers and automotive industries.It has become an important criterion for a consumer to select a new car.As known,the thermal conditions in a vehicle cabin are notably sensitive to the climate conditions,especially in a very cold or hot day.That is,if a vehicle has been exposed to the hot or cold environment for a period of time,it has to take more than 10 min for the HVAC system to cool down or heat up to an acceptable interior temperature in the cabin.During these cooling or heating periods,conditions are transientchanging over time and highly non-uniform over the vehicle cabin and the occupants’body.It may lead to an unwanted local cooling or warming at body segments.For instance,the local discomfort likely occurs due to the vertical temperature difference between the foot and the head level.Unfortunately,most HVAC systems equipped in vehicles are controlled by a uniform temperature.There is no design optimization according to local thermal sensation of occupants.Therefore,the thermal comfort,particularly the local thermal comfort of an occupant is hard to achieve and maintain in the transiently-changing and highly non-uniform conditions.On the other hand,an HVAC system in vehicle cabin wastes a large amount of energy,which is used to cool or heat the entire interior of the vehicle cabin rather than just occupants.In a cooling period,a person can produce only about 150 W of heat.However,a typical cooling system in a vehicle provides about 6000 W of cooling.Concerning to a heating period,the heating energy for a conventional gasoline vehicle cabin is free to some extent,which uses hot engine coolant as the source of heater energy.But,for an electric powered vehicle,the situation is quite different,which has no thermal source.All the energy used for the cabin heating in an electric vehicle is thus supplied by the main battery which is expected to be used to drive.Operation of the heating system thus has a major influence on battery’s cruising range,which is one of the bottlenecks to restrict electric powered vehicle’s development.

With the urgent need to ensure the thermal comfort of occupants and reduce the economic and environmental cost of energy consumption,many investigations covering various aspects related to thermal comfort in vehicle cabin environment have been carried out using experiments or numerical analysis.LEE,et al[1],experimentally investigated the effect of ventilation modes on the thermal environments for compartment heating.In the study,the color image processing system and particle streak method were applied to visualize and measure the instantaneous temperature field and flow field variations in a 1/10 scale passenger compartment.The findings from this study can be used for designing HVAC systems to improve the thermal comfort of occupants.CURRLE[2]numerically investigated the influence of relevant flow and geometrical parameters on the microclimate and thermal comfort of the occupants in a vehicle cabin during the heat-up period.The flow field and temperature distribution were calculated using the commercial CFD program STAR-CD.The parameters as the air ducts,air nozzles and louvers were investigated and optimized to improve the thermal comfort for occupants.CHIEN,et al[3],performed a study to evaluate the thermal comfort of a car passenger in a warm cabin during the cooling period.In this study,the temperature filed and flow conditions in vehicle cabin were transient numerically analyzed and verified by experiments.The local thermal comfort of occupant was evaluated by the predicted mean vote(PMV)method proposed by FANGER[4].The result showed that the flow filed became very complex and the average temperature reduced quickly in the initial cooling period,and started to stabilize after 10 minutes.And the local PMV values indicated that only the feet were still uncomfortable after 20 min.SEVILGEN,et al[5],presented a three-dimensional transient numerical analysis to investigate the airflow and heat transfer characteristics inside a vehicle cabin during a heating period.They considered all the possible thermal loads of the cabin and the influence of the glazed surfaces,pertinent physical and thermal properties of the car body.Also,a virtual manikin model was used to determine the heat transfer between the human body and the vehicle cabin ambient.The numerical results were in good agreement with the available experimental and theoretical data in the literature,and indicated that non-uniform temperature distributions presented in the cabin and the flow and temperature fields reached steady-state conditions about 30 min.ZHANG,et al[6–7],used the commercial software FLUENT to simulate 3D temperature distributions and flow field in a compartment.The numerical model was validated by the experimental data and used to investigate the influence of different factors on the thermal comfort and the energy consumption,such as passenger numbers,cooling load,ambient air temperature,insulation of car body.The main findings from their studies can be summarized as:compare to reducing volume flow rate of inlet air,increasing inlet air temperature is a better choice to decrease the cooling load in cabin;for a fix boundary condition,the number of passengers and their sitting arrangement have a significant effect on thermal comfort in a cabin;the outside ambient air temperature plays a vital role on the cooling load the energy consumption;reducing the transmissivity of window glass can improve thermal insulation of the vehicle body and decrease the cooling load;keeping the inlet air direction horizontal can result in a better flow circulation near the compartment bottom and improve the uniformity of temperature field around the driver’s foot zone.ZHANG,et al[8],carried out a numerically study to evaluate the effects of different heat transfer processes on the temperature profile in a vehicle cabin.The results indicate that the solar radiation is the main factor that affects the thermal conditions in a vehicle cabin.The transmission rate of glaze and the object absorption rate have significant impact on a vehicle cabin temperature profile.A proper transmittance for windshield glaze can significantly reduce the temperature inside the compartment,which can improve the occupant thermal comfort and potentially reduce vehicle air conditioning energy consumption.

Most studies mentioned above mainly focus on the thermal environment conditions,for example,the air temperature,surface temperature,air velocity,relative humility in vehicle cabin.Less attention appears to direct to the thermal comfort or thermal sensation of occupants,even to the relationship between thermal conditions and thermal sensation.However,in order to design more effective HVAC systems for vehicles,it is necessary to understand how the thermal conditions affect the thermal sensation of occupants.The PMV-PPD index[4],which is calculated from several environment variables,was ever applied to evaluate mean responses of a group of people exposed to the same environment.It may be useful in the uniform conditions,but applied in non-uniform conditions such as the cases in vehicles cabins,it is not able to provide accurate predictions[9–10].Some studies[11–13]indicated that the skin temperature has a strong association with thermal comfort of people exposed to uniform or non-uniform conditions,and extensive investigations[14–15]have been devoted to the relationship between the skin temperature and thermal sensation.Among these studies,most were conducted in controlled environment chambers which can provide repeatability condition for different subjects and the possibility of elaborate measurement strategies.However,the findings from carefully controlled conditions will sometimes give incorrect results in realistic daily life conditions[16].Therefore,in order to get the skin temperature and thermal sensation responses of occupants in real life traffic,it is very important to perform studies to achieve the solutions for real car interior conditions.

When the occupants enter the vehicle cabin from outdoor winter climate conditions and started the HVAC heating process,they are confronted with instantaneous change in interior air temperature and thermal responses.The purpose of the present study is to investigate the variation in skin temperature and occupant’s thermal sensation,as well as their relationship during this heating period in realistic daily life conditions.This paper is organized as follows.The next section contains the experiment design,subjects,conditions,test items and equipments.Section 3 is devoted to results obtained from experiments.The final section summarizes the major conclusions.

2 Experiment Method

2.1 Experiment design

Experiments were designed to simulate the HVAC heating period starting from occupants entering a parked vehicle from outdoor winter environment.There were two spaces considered in this study,which is as the vehicle’s exterior space,the underground parking garage in this experiment and the vehicle interior space.The occupants first stayed in the garage until they reached steady status,in this stage they were limited to have sedentary activities only,and then entered the vehicle cabin.When occupants entered the cabin,start the engine and turn on the heating HVAC system simultaneously.

During the HVAC heating process,the transient interior air temperatures at the appointed positions were monitored.Simultaneously,each occupant’s thermal responses,such as the skin temperature and the subject thermal perception were also evaluated.

Each test would last for 50 min,which comprises 30 min preparation time(including 10 min for occupants sitting rest,reading or light conversation)and 20 min for measuring interior air temperature,skin temperature and thermal sensation of each occupant in test in the vehicle cabin.

2.2 Subjects

Six graduate students from the university,including 3 males and 3 females,volunteered as subjects were selected to participate in the present study.Table 1 shows the basic characters and distribution of the subjects.All of them understood the experimental purpose,method,and procedure of this study.At least within 18 h,they were prohibited alcohol and strenuous exercise before the experiment.In order to avoid the influence of clothing difference on the test results,the occupants were required to wear the uniform clothes comprising a long-sleeved shirt,a jacket,a pair of trousers,socks and shoes.

Table 1.Characters and distribution of subjects

2.3 Experimental conditions

The experiments were carried out in an underground parking garage at South China University of Technology in winter February.A Honda FIT Sedan with standard HVAC system parked in the garage(Fig.1)was used for testing.As shown,the HVAC vents are in the dashboard in the front;no vents in the rear.According to the HVAC vents arrangement,it might easily lead to the horizontal non-uniform temperature distribution over the cabin during the HVAC system working process.

Fig.1.Vehicle used in experiments

The interior air temperature measurement and occupant’s thermal responses test were to perform under the cabin interior condition.Prior to commencing each test,the test vehicle had to park in the garage for 30 min with all side doors open to ensure the conditions in vehicle cabin were equal to the ambient conditions.Therefore,the initial conditions for all tests can be considered to be uniform.Each test would be started when the subjects entered the vehicle cabin and sat well,and meanwhile started the engine,turned on the HVAC system.

2.4 Test items and equipments

The ambient temperature,relative humility and air speed of the underground garage,the flow temperature and speed at the vent outlet,which were on behalf of the environment and the HVAC system operation conditions in the present investigation,were first measured.

During the experiment,in order to further understand the non-uniform conditions and their effects on the responses of occupants,8 testing points in the vehicle cabin were selected for measuring different interior air temperature,and 9 body segments were selected for measuring the skin temperature.As shown in Fig.2,due to the symmetry of the vehicle cabin,only the right sides of both front and rear parts of the cabin were monitored.Similarly,the limbs’skin temperature was obtained only from the right limb segments.They were simultaneously measured in real time.The data were collected every 5 s.The testing equipments for each test item are shown in Table 2.

Fig.2.Interior air vertical level

The subjective thermal sensation responses to the transient and non-uniform conditions during the initial heating period were evaluated by using a comfort survey questionnaire proposed by ASHRAE Standard[17].As shown in Fig.3,a seven-point scale was used to rate the thermal sensation of the occupants,the value“+3”,“+2”,“+1”,“0”,“-1”,“-2”and“-3”responding to the subject thermal sensations of“hot”,“warm”,“slightly warm”,“neutral”,“slightly cool”,“cool”and“cold”,respectively.During the experiment,the subjects need to rate their thermal sensation every minute for their head,body,upper limb and lower limb in the first 5 min,and then every 2 min for the next 8 min,and then every 3 min and 4 min for the last 7 min(Fig.3).The voting intervals in the experiment took into account the fact that the conditions in vehicle changed rapidly in the first 5 min,and then became more and more gently.

Table 2.Test items and equipments

Fig.3.Thermal sensation scale and thermal sensation test

3 Results and Discuss

The tests were carried out under a typical winter climate in South China.The environment conditions of the underground garage for all tests were approximate as follows:ambient temperature,12°C;relative humility,60%;air velocity,＜0.1 m/s.According to the experiment design,the environment conditions in the underground garage also were the initial conditions in the cabin of the test vehicle.The heating HAVC system settings were:flow temperature at vent outlet,48 °C;flow velocity,3 m/s.These parameters were keeping unchanged in all tests,and occupant control was not allowed during each test.Each subject would participate in two tests:one was for response in the front seat,and the other was for the rear seat.When the occupants entered the cabin,turned on the HVAC system simultaneously,and began data acquisition.

3.1 Interior air temperature

The interior air temperature variations on different vertical levels at both front and rear sides during the heating period are depicted in Fig.4.As can be seen,the transient and non-uniform temperature field in the vehicle cabin is presented.At both front and rear sides,the temperature variations have three stages:in the first few minutes,the air temperature increases rapidly as the air conditioning process begins which brings a huge amount of heat source into the cabin;then the variation rate of temperature slows down due to that the temperature difference between the interior and the vent outlet becomes smaller;finally,the temperature reaches steady state.In general,air temperature at the head level is higher than the other levels,while the foot level has the lowest temperature during heating period.The maximum air temperature difference between these measured levels is approximately 4 ℃.Furthermore,though the temperature variation profiles are similar in both front and rear sides,durations of each stage are different.The rapid transient change stage in the front appears within the first 5 min,while it lasts about 10 min in the rear;in the front the air temperature will reach steady state after about 15 min,while it needs 20 min or more in the rear.It can be noted that the front,which is near the vent outlets,is warmed up rapidly and needs shorter time to reach steady state.From Fig.4(c),it also can be seen that the arithmetic mean temperature of 4 vertical levels in the front is higher than that in the rear,the difference becomes larger in the first 15 min,and then decreases in the last 5 min,the maximum difference during the heating period is about 5 °C.The vertical and horizontal temperature difference will cause unwanted local skin temperature and discomfort during the heating period.

Fig.4.Air temperature variation inside vehicle cabin at different vertical levels during the heating period

3.2 Skin temperature

In order to find out the difference of occupant’s thermal responses caused by vertical and horizontal temperature difference in vehicle cabin during the initial heating period,the skin temperatures obtained from the tests were divided into 2 groups.One group was measured from the sedentary occupants in the front seats;the other group was from the rear seats.Fig.5 shows the arithmetic mean skin temperature at different body segments of total 6 subjects.

Fig.5.Skin temperature variation at each body segment during the heating period

At the beginning of the heating period,the skin temperatures of both groups are almost the same.However,the initial local skin temperatures are quite different among the segments.The head,lower arm,hand,and lower limb have lower temperatures as compared to the body’s other segments.The maximum temperature difference between measured body segments is about 6 ℃.When the heating HVAC system switched on,the skin temperature increases with the increasing of interior air temperature in the cabin.The variation profile of each segment at both front and rear sides is similar.But,like the interior air temperature,the skin temperature change at the rear also lags behind that of the front.It may lead to different thermal sensations between the occupants sitting at different locations during the heating period.

As depicted in Fig.5,the skin temperature variation profile over time for each body segment is quite different.For the head and hand,which have no clothes covered,the temperatures increase rapidly in the first few minutes because these two segments were warmed up directly by the heat flow.Within the first 5 min,the skin temperatures have increased by 3℃ and 6.5℃ at head and hand respectively in the front,and by 2.9℃ and 5.4℃respectively in the rear.Because of the human thermal regulation system,the skin temperature will not increase unlimited.After the rapid changing stage,the skin temperatures increase slowly and then reach steady state,which is about 36℃ in these 2 segments.Similar to the head and hand,skin temperatures of the upper arm and lower arm also have a short rapidly increasing stage and a long steady state stage.It is noted that the skin temperatures of segments discussed above are very sensitive to the interior air temperature.

For the body segments,namely the chest and abdomen,they have higher initial temperatures and greater clothing thermal insulation than the other segments.Therefore,influence of the interior air temperature on their skin temperatures is not significant.As depicted in Fig.5(a),the skin temperatures increase slowly over time and then reach steady state in about 15 min.During the heating period,the skin temperatures only increases by 2℃ at chest and 1.3℃ at abdomen respectively,which are much lower than those of the other segments.

The skin temperatures of the thigh,calf and foot increase slowly during the heating period and take a long time to achieve steady state.Under the present environment conditions and HVAC settings,this process lasts nearly 18 min.The main reason for this phenomenon is the knowledge that the interior air temperature at the foot level is low and its increase lags behind the other levels as shown in Fig.4.From the above discussion,it can be seen that more than the interior air temperature,the skin temperature is also affected by the clothes resistance and the human thermal regulation system.Only controlling the interior temperature would lead to improper skin temperature.

3.3 Thermal sensation

The subject thermal sensation data measured by the survey questionnaire were partitioned into 2 groups on the basis of their sitting location,at the front or the rear.Differences of thermal sensation among segments and between the two groups in response to temperature variation during the heating period were evaluated by analysis of variance(ANOVA).In this study,differences were considered statistically significant at the level of p＜0.05.The statistical analysis was conducted using the software SPSS version 19(SPSS Inc.,Chicago,USA).

Fig.6 shows the change of mean thermal sensation of all subjects over time for the head,body,upper limb,and lower limb,where the asterisk(*)indicates the significant differences between segments(P＜0.05)is found.At each survey point,significant difference between the segments is observed(P＜0.05).At the beginning of the heating period,cool or cold local thermal sensation is voted by the subjects due to low air temperature in the cabin.However,a warmer thermal sensation is perceived by the body due to greater clothing thermal insulation than the other segments.During the first 5 min heating process,faster increases are observed at the head and upper limb.Both segments perceive neutral thermal sensation in about 5 min,while the other segments also feel cool or cold.Then,the thermal sensation of the upper limb is stable,and the thermal sensation index of head still increases fast.At the end of the test,the body,upper limb,and lower limb feel neutral or slightly warm.But,unwanted hot sensation is perceived by the head.It reveals that the thermal sensation is significant different among segments during the heating period under the non-uniform conditions in vehicle cabin.The current uniform temperature control strategy cannot guarantee all local thermal comfort of occupant.Moreover,the heating energy which causes unwanted local hot sensation is kind of wasted energy.

Fig.6.Mean local thermal sensation variation during the heating period

Fig.7 shows the time course of changes in thermal sensation for the head,body,upper limb,and lower limb for both of the front and the rear sides.

Fig.7.Local thermal sensation variation at each body segment for both front and rear side during the heating period

In Fig.7,the bars show the standard deviations;and the asterisk(*)indicates the significant differences between the front side and the rear side(P＜0.05)is found.As shown,at the beginning and the end of the heating period,there is no significant difference found at any segment between the front and rear side.But during the heating process,the subjects sitting at the front seat felt significantly warmer at the head,body,and upper limb than sitting at the rear.This phenomenon is caused by the horizontal air temperature difference as shown in Fig 4.However,no significant difference is observed at the lower limb,and the thermal sensation is almost the same during the whole heating period.

3.4 Relationship between skin temperature and thermal sensation

Fig.8 shows the relationship between local skin temperature and thermal sensation for the head,body,upper limb,and lower limb during the heating period in vehicle cabin.For all segments,robust correlation is found(R2＞0.85,P＜0.05),reflecting that the skin temperature is in sync with the subjective perception of thermal status.As shown in Fig.8,the regression line for each segment is similar,the relationship between the skin temperature and thermal sensation is liner.But the thresholds and slopes of the regression lines are different.Thus,the sensitivity of thermal sensation to changes in skin temperature is different among segments.From the regression lines it also can be seen that the thermal sensation may be different at different segments even with the same skin temperature.

The neutral temperature is another major issue concerning in vehicle cabin thermal comfort investigation,which may be useful and necessary for setting up an intelligent HVAC control strategy.Adjusting the skin temperature in the proper range is benefit for thermal comfort and energy saving.From the regression equations shown in Fig.8,the neutral temperature for the head,body,upper limb,and lower limb is 34.8℃,34.4℃,33℃,and 33.2℃,respectively.Table 3 shows comparison of the present neutral temperature result with other studies[13,18].It can be seen that the present results are between those in these two studies,and closer to Olesen’s results.The reasons for the difference could be caused by the subject’s living environment,test conditions and evaluation segments.Therefore,from the viewpoint of comfort and energy saving,controlling the skin temperature for the different segment separately is necessary.

4 Conclusions

The presented study focused on the non-uniform conditions and occupant’s responses inside a vehicle cabin during the heating period.In addition to investigating the interior air temperature distribution,the skin temperature variation and the subject thermal sensation were also studied by experiments.Furthermore,the relationship between the skin temperature and the thermal sensation was analyzed and discussed through statistics method.This study results can be summarized into following points.

Fig.8.Relationship between the skin temperature and the thermal sensation

Table 3.Comparison of neutral skin temperature with other studies

(1)The interior environment conditions become transient and non-uniform over the vehicle cabin when the heating process switched on.Three stages were observed during the whole heating period:rapid increase,slow increase and steady state stage.But,duration of each stage is different between the front and the rear.

(2)The skin temperature of each segment increases during the heating period.However,the variation profiles over time for different segments are quite different.The head and hand segment with no clothes covered are very sensitive to air temperature,and their skin temperature increases rapidly in the first few minutes and then slowly down to stabilization.The body and the lower segment are less sensitive,and their skin temperatures increase quite slowly and take a long time to achieve steady state.On the other hand,the skin temperature changes at the rear lag behind that of the front.

(3)The local thermal sensation of each body segment during the heating process is different.Similar to the skin temperature,the head and the hand segments are more sensitive to air temperature than the other segments.The thermal sensations of head,body,and upper limb are significantly different between the front and the rear during the heating period.However,there is no significant difference observed at the lower limb.

(4)Robust correlation is found between the thermal sensation and the skin temperature at segments.But the sensitivity of thermal sensation to changes in skin temperature is different.The neutral skin temperature is also different for each segment.

From the above discussion,the subject local thermal responses in vehicle cabin during the heating period are different between each body segment and affected by the occupant’s sitting position.The current uniform temperature control strategy cannot meet the requirements of thermal comfort and energy saving.This strategy implies that the difference of segments response to the non-uniform conditions is negligible.The skin temperature is a robust index to evaluate the subject’s thermal sensation in a vehicle cabin.Applying the skin temperature as the control parameters may improve the thermal comfort and reduce energy consumption.Future work will investigate the skin temperature control scheme and implement it into actual vehicle cabin.

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