Experimental investigation of thermal comfort and air ... - Springer Link

Heat Mass Transfer (2012) 48:1375–1384 DOI 10.1007/s00231-012-0988-8

ORIGINAL

Experimental investigation of thermal comfort and air quality in an automobile cabin during the cooling period M. Kilic • S. M. Akyol

Received: 4 February 2011 / Accepted: 7 February 2012 / Published online: 19 February 2012 Springer-Verlag 2012

Abstract The air quality and thermal comfort strongly influenced by the heat and mass transfer take place together in an automobile cabin. In this study, it is aimed to investigate and assess the effects of air intake settings (recirculation and fresh air) on the thermal comfort, air quality satisfaction and energy usage during the cooling period of an automobile cabin. For this purpose, measurements (temperature, air velocity, CO2) were performed at various locations inside the cabin. Furthermore, whole body and local responses of the human subjects were noted while skin temperatures were measured. A mathematical model was arranged in order to estimate CO2 concentration and energy usage inside the vehicle cabin and verified with experimental data. It is shown that CO2 level inside of the cabin can be greater than the threshold value recommended for the driving safety if two and more occupants exist in the car. It is also shown that an advanced climate control system may satisfy the requirements for the air quality and thermal comfort as well as to reduce the energy usage for the cooling of a vehicle cabin.

1 Introduction Many people spend considerable amounts of time in an automobile for different purposes, such as work and travel.

M. Kilic (&) S. M. Akyol Department of Mechanical Engineering, Faculty of Engineering and Architecture, Uludag University, Gorukle Campus, Bursa 16059, Turkey e-mail: [email protected] S. M. Akyol e-mail: [email protected]

Because of the competition between automobile industries, satisfy consumer’s requirement for thermal comfort and air quality is important. In extreme hot or cold conditions, the importance of the subject necessitates using high-capacity air conditioning systems. Identically, this feature causes high fuel consumption which can be reduced by improving air-conditioner automation system. Conventional heating, ventilating, and air conditioning (HVAC) system of the automobiles run for the purpose of maintaining interior air temperature about desired values by the occupants. The system has also some free run features that allow occupants to control manually like fan speed, vent direction and air intake position. It is evident that operate HVAC system under the recirculation mode reduce power consumption for the same cooling capacity due to decreasing interior air temperature. In contrast, if interior air does not renew, CO2 concentration will raise as a result of occupants exist in the vehicle continuously exhale CO2. Therefore, vehicle air conditioning systems must be equipped with air intake position regulator. Thermal comfort has been defined as conditions of mind that expresses satisfaction with the thermal environment [1]. Although interior air temperature and air velocity around driver are effective parameters, humidity and surface temperatures of the compartment also affect thermal comfort. Human thermal comfort in cooling period of an automobile has been the subject of considerable previous studies. Most of them attempt to measure the temperature and air velocity distributions in the vehicle compartment [2–5]. They found that highly transient and non-uniform conditions occur during warm-up and cool-down periods. The effects of environmental conditions on thermal comfort were investigated by Burch et al. [6, 7] and Guan et al. [8, 9] and they predicted passengers thermal comfort perceptions either through jury testing or through mathematical modeling.

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Kilic and Akyol [10] measured air temperature and velocities around the human body segments at 11 and 17 different points respectively during the heating period of an automobile to determine local thermal comfort dissimilarities caused from non-uniform temperature distributions and air drafts. In the study, local and whole body thermal sensations were predicted theoretically with using formulations developed by Gagge et al. [11]. Besides that, two ventilation methods (console vents, jointly used windshield and foot vents) were compared in terms of the effects of thermal comfort. Collaboration with numerical analysis is particularly important to determine the three-dimensional fluid flow, temperature distribution, and heat transfer characteristics inside the cabin. Numerical evaluations, verified with experimental data obtained from Kilic and Akyol [10], were performed by Kilic and Sevilgen [12] and Sevilgen and Kilic [13]. Especially in summer seasons, cabin internal surface temperatures (ceiling, windows, etc.) and interior body components surface temperatures (steering wheel, dashboard, instrument panel, seats, etc.) rise considerably due to the solar radiation falling on them [14]. Because of the temperature differences between these surfaces and human body skin, irradiative heat transfer related with view factors also causes thermal discomfort. In general, view factors are difficult to determine. Nevertheless, some figures and correlations are given for a sitting person in ASHRAE Fundamentals [1]. The cause of the accidents is mostly driver inattention. Similar to the increasing air temperature, elevated CO2 in the region cause decrement in mental performance and loss of concentration. According to Maycock [15], car drivers in inadequate ventilated vehicles were found to have high probability of falling asleep and have a relatively high accident frequency. There are standards and procedures such as ASHRAE 62.1 [16] for indoor air-quality assessment. According to ASHRAE 62.1 [16] steady-state CO2 concentration in a space should be kept under the threshold value of 1,200 ppm (parts per million). Cheng et al. [17] investigated the air quality (CO, CO2 and NO2 emissions) in a commercial truck cabin during a 2 months monitoring period. Temperature and relative humidity measurements were also carried out due to calculate comfort level. Knibbs et al. [18] measured the amount of outdoor air entering the cabin for six different vehicles at three vehicle speeds and under four ventilation settings. It is observed that majority of such studies given above have concentrated on either thermal comfort or indoor air quality. Relatively, much less information is currently available in the open literature on the interaction between the thermal comfort and air quality. The objective of this paper is to present a comprehensive analysis for the interaction between the thermal

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Heat Mass Transfer (2012) 48:1375–1384

comfort, air quality and cooling energy usage evaluated together in a vehicle cabin. A survey and in field measurements were undertaken during the cooling period of a test vehicle for two air intake modes (recirculation and fresh air). In order to determine the frequency and the time length required for the refresh cabin air, mass balance equation of in-room CO2 level proposed by ASHRAE 62.1 [16] was arranged for the test vehicle and was confirmed by the experimental data. The effects of air intake mode on the thermal comfort air quality and cooling energy load were explained in detail. Initially, experimental setup and measurement devices are introduced in Sect. 2. Then, implementation stages of the tests and the results of the obtained data are explained in Sect. 3. Finally in Sect. 4, contribution of the article and suggestions are expressed.

2 Experimental tests and measurement devices In the present study, we tried to specify the variation of non-uniform and transient thermal comfort and air quality parameters exist inside of the automobile cabin during the cooling period, under recirculation and fresh air intake modes of HVAC system. For this purpose, field tests were performed in similar outside ambient conditions throughout the month of August in Bursa, Turkey with different male undergraduates as a human subject. The average values and standard deviations of outdoor temperature and solar radiation were calculated as; 28.97C (1.87C) and 856.63 W/m2 (13.88 W/m2) for recirculation group tests and 29.14C (3.23C) and 871.41 W/m2 (9.85 W/m2) for fresh air group tests respectively. Statistically, we have not sufficient evidence to claim that two test groups were performed in dissimilar environmental conditions. The tests were carried out on 2008 model Fiat-Linea as two steps. In the first step, the automobile was parked southerly in the blazing sun from 09.00 am to 01.00 pm. Measurement devices were also run in this step to collect temperature and CO2 data inside the cabin. In the second step, after two human subjects entered into the test vehicle, air conditioner was run for an hour at maximum power capacity and fan level while the engine was idling at 1,000 rpm. During the experiments, driver whole body and local thermal sensation votes and also some of the cabin surface temperatures were recorded for every 5 min. Throughout the tests, only console vents were kept open in straight out position and windows were closed. Subjects, who may only wear T-shirt and trouser, were provided to arrive into the test place half an hour before the experiment for the aim of sticking the thermocouples to the skin in the pre-conditioned room. Detailed notations of 12 measurement point are shown in Fig. 1. This period also contributes to achieve similar initial thermal conditions for


1377 Table 1 Mean characteristics of the human subjects Age (year)

Height (cm)

Weight (kg)

22.50

174.50

77.00

1.29

3.42

5.48

21.67

174.00

71.33

1.15

10.54

11.50

Recirculation Mean SD Fresh Mean SD

Fig. 1 Representation of skin surface temperature measurement points

all the test subjects. Mean characteristics of the human subjects are listed in Table 1. For assessment of whole and local body thermal sensations seven scaled survey was applied to the subjects. Thermal sensations and its numerical values are categorized as; neutral (0), warm (1), hot (2) and very hot (3) for warm side and cool (-1), cold (-2) and very cold (-3) for cold side. In order to monitor the distribution of air temperature, the data were collected in 10 s intervals with 16 temperature sensors located inside the cabin as shown in Fig. 2. Sensors were placed at four vertical points (head, trunk, knee, foot). Since the large number of sensors used, data were automatically saved by Digi-Sense 12 Channel Thermometer. CO2 concentration, air velocity and relative humidity data were taken from driver breath level by Testo 454 multi-function measurement device for every 10 s. Cabin internal surface temperatures were measured manually and recorded simultaneously with thermal sensation votes of human subject for every 5 min. We observed in the preliminary work that thermocouples which exposure to direct solar radiation may measure 2C higher values. So as to avoid uncertainties in experimental results we had covered all the thermocouples with aluminum foil. Other uncertainties sourced from the experimental instruments analyzed with using Moffat [19] method and maximum uncertainties were found to be within ±6%. Specification of instruments used in the experimental study is listed in Table 2.

Fig. 2 Top and side view of the air temperature, CO2, air velocity and relative humidity measurement locations

Therefore, by entering the vehicle, occupant’s skin temperature increases and occupants start to sweat with the aim of the disposal of excess heat. So, HVAC system must be run as to provide fast and a uniform cooling period until comfortable conditions are achieved. Effects of air intake positions (recirculation and fresh) on temperature and velocity distributions, variation of CO2 concentration, relationship between the human subject’s measured skin surface temperatures and survey results are discussed in this section. 3.1 Temperature distribution

3 Results and discussions At beginning of the cooling period, interior air and surface temperatures inside the test vehicle parked in the blazing sun are very high. In contrast, relative humidity is very low.

It is evident that air intake position is determinant factor on vent outlet air temperature (Tv). As it is shown in Fig. 3, vent outlet temperature has continued to cool in a continuous manner after a rapid decline under the recirculation

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Table 2 Specification of instruments used in the experimental study Measurement interval Air and surface temp.

Accuracy

Cole Palmer Digi-Sense 12 channel thermometer

Temperature probe

-200C to ?300C

\150C ± 0.25%

Air velocity

Testo 454 multi func. measurement device 0–10 m/s

±0.04 m/s

Velocity probe CO2 concentration

Testo 454 multi func. measurement device

CO2 probe

0–9,999 ppm

Relative humidity

Testo 454 multi func. measurement device

Humidity (RH) probe

0–100%

±5 ppm

±0.1%

Fig. 3 Comparison of average vent outlet air temperatures

mode. Under the fresh air mode, initially it showed a more rapid decline and then warmed slowly. This can be explained with temperature difference between the ambient and interior cabin. As long as the ambient temperature is lower than the cabin environment, fresh air mode is more suitable. In our experiments, this period took 750 s. In the following period, the difference between the two air circulation modes expanded and finally 8.1 colder air temperature was measured at the exit of the vent under the recirculation air intake mode. The results presented in Fig. 3 are the average of the data collected from the exit of the vents located on instrumental panel. In the beginning of the tests, human subjects were under the influence of highly transient and non-uniform air temperatures. At the end of the first step of the experiments, while interior air temperature (Ti) increased to 67C at head level, it reached only to 45C at foot level. Evidently, with opening the door, these values decreased to 57 and 42C. But this temperature difference disappeared in the first few minutes of the cooling period. By the reason of occurred small temperature differences throughout the

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vertical locations of the cabin during the cooling period, arithmetic mean values of the measured first eight points (1–8) and the last eight points (9–16) were taken as front side and back side average air temperatures respectively (Fig. 2). Comparison of interior air average temperatures (Ti) is given in Fig. 4. It is also clear that there is a close difference between the front and back side average air temperatures. Air temperature in the rear compartment of the automobile is slightly colder. Assessing the figures shows interior air temperature is 5 colder for recirculation mode at the end of the cooling period. Further examination from the figures indicates that to start with fresh air mode can shorten the cooling period. According to figures, fresh air mode is more favorable than recirculation mode for the first 450 s at the front side and in the rear of the automobile it is more favorable for the first 640 s. Table 3 indicates the obtained air temperature data at the end of the cooling period at 4 vertical points. It can be also seen from Table 3 that air temperature difference between the two air intake modes increases in the vertical direction. Moreover, temperature related comfortable conditions are achieved firstly at head level due to the high air circulation inside the cabin. Automobile interior volume is to be assumed as an open thermodynamic system in thermal contact with the atmospheric heat reservoir. According to the law of conservation of mass, if there is not any pressure gradient in the volume, ingoing (vent outlet air) and outgoing (infiltration air) mass flow rates (mv) have to be equal. So, cooling load rates for fresh air (Qc.f) and recirculation (Qc.r) modes can be calculated by using Eq. 1 Qc ¼ mv cp ðTv Tin Þ

½W:

ð1Þ

Infiltration temperature (Ti) was accepted as rear compartment air temperature. Ingoing air mass flow rates (mv) were measured as 0.0897 kg/s for fresh air mode and 0.0869 kg/s for recirculation mode. To illustrate the difference between the provided cooling energies (DEc) for the two air intake modes throughout a 1-h cooling period, we can write Eq. 2 DEc ¼

3600 Z

jðQc:f Qc:r Þjot ½J:

ð2Þ

0

In Fig. 5, calculated cooling load rates (Qc) and energy savings (DEc) were shown. When the outside temperature is lower than the inside temperature the usage of the fresh air intake mode results to energy saving. It is shown in the Fig. 5 that 185 kJ more cooling energy can be provided with using fresh air intake mode for the first 1,200 s. When the inlet temperature drops to below the outside temperature, switch on the recirculation mode leads less


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Fig. 5 Comparison of cooling load rates and energy saving

Fig. 4 Comparison of average air temperatures a at front side b at back side

Table 3 Air temperatures (C) measured at the end of the cooling period through vertical direction Foot

Knee

Chest

Head

Front side Recirculation

26.3

27.1

25.6

24.7

Fresh

30.5

31.7

30.7

31.9

4.2

4.5

5.1

7.2

Difference Back side Recirculation

26.7

26.3

23.3

23.3

Fresh

30.1

30.1

28.6

29.5

3.4

3.7

5.3

6.2

Difference

cooling energy. It can be seen from the Fig. 5 that it may possible to save energy up to 500 kJ during the 1 h cooling period. 3.2 Surface temperatures Besides the convectional thermal interactions, significant amount of irradiative heat transfer from the hot surfaces surrounding the human body also causes challenges to reach thermal comfort. Especially, surfaces exposed to direct solar radiation (front window, dashboard, steering wheel) and ceiling which heated by the natural convection warms more than the other internal surfaces of the cabin.

Maximum temperature values that observed upon these surfaces varied between 60 and 65C. Because of the difficulties in determining view factors we compared the arithmetic averages of the measured surface temperatures for the two air intake modes. The results of the comparison are presented in Fig. 6. Surface temperatures (Ts) showed a similar but more slowly cooled trend than air temperatures. Fresh air mode is favorable than recirculation mode for the first 10 min. As shown in Fig. 6, throughout the cooling period except first 10 min, desirable thermal conditions occurred in favor of recirculation air intake mode. 4.3C difference in temperature occurred at the end of the cooling period and radiant heat gains become the main reason to cause thermal discomfort. Table 4 presents the comparison of final surface temperatures for two air intake modes. While the difference is high for dashboard, it is low for other internal cabin surfaces such as left door, right door and front window. Maximum temperature drop was on the ceiling and the minimum temperature drop occurred in the front window. 3.3 CO2 concentration and air velocity Air quality measurements were compared in Fig. 7. CO2 concentration inside the cabin (Ccbn) reached the threshold value (1,200 ppm) recommended by ASHRAE 62.1 [3] in the first 5 min and increased up to 3,200 ppm in 1 h period. In the following period air inside the cabin must be renewed periodically to protect occupants from harmful effects of high CO2 concentration when reached to threshold value. In addition, survey results reported that human subjects began to feel indisposed from the interior air quality after the first 25 min. Here, it must be particularly emphasized that the results are applicable to a parked car. The automobile in motion or the numbers of occupants may affect the results. Assuming that the carbon dioxide level in exhale air is constant about 3.8% (38.000 ppm), mass balance equation of CO2 level inside the cabin is as follows:

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Average air velocity measured at driver breath level was 0.35 m/s for recirculation mode and it was only 0.25 m/s for fresh air mode. A better air circulation in the cabinet was provided with using recirculation mode. According to basic convectional heat transfer theory, increase in air velocity increases heat losses and gains from the human body. Consequently, residual heat of the body can be disposed to the outdoor environment more easily under the recirculation air intake mode.

Fig. 6 Comparison of average internal surface temperatures

Vcbn ðoCcbn = otÞ ¼ ðACH Vcbn ÞðCatm Ccbn Þ þ 0:038 Vexh : ð3Þ According to ASHRAE Fundamentals [2], oxygen consumption for a typical person (Vexh/0.83) varies between 0.5 to 1 l/min for moderate work activities. Infiltration rate (ACH 9 Vcbn) can be taken equal to the volumetric flow rate of the ingoing air from the ventilation holes. Many vehicles have a physical limit on the minimum aperture of the fresh air intake damper, to avoid excessive CO2 levels inside of the cabin. Therefore, ingoing air from the ventilation holes increase significantly for the vehicles in motion in proportion to vehicle velocity also in recirculation cycle. Linear regression models for calculate the air change rate (ACH) in motion vehicles according to its velocity and ventilation rates are provided in the Knibbs study. Outdoor CO2 levels Catm are usually between 350 and 450 ppm except heavily industrialized areas. Calculated values of interior cabin CO2 concentration by using Eq. 3 are given in Fig. 7. There is a good agreement between the model predictions and experimental results. The time required for the renewal of the interior ambient air (from 1,200 ppm to 450 ppm) was also calculated as about 50 s. As a result of supplying temperature regulated air via small openings, turbulent and 3 dimensional flow fields consist of inside the cabin. Therefore, airflow should also be considered as an important factor for thermal comfort. Table 4 Internal cabin and interior body components surface temperatures (C) at the end of the cooling period

3.4 Comparison of measured skin temperature and questioning of thermal comfort The results of inquiry were obtained in a wide range because of individual differences. To achieve reliable results, the arithmetic averages of the votes were taken. The results of the inquiry for the whole body are given in Fig. 8. As it seen in Fig. 8, thermally neutral conditions were provided for each two ventilation modes with small deviations. Achieving comfortable environmental conditions have taken up 30 min for recirculation mode and 35 min for fresh air mode. In highly transient and non uniform environmental conditions the results of the inquiry for the whole body depend on the local perceptions. Exposure to solar radiation and ventilation air, to be in contact with hot surfaces are the major factors which causing local dissatisfaction. In this section, in addition to determining the effects of air intake positions on local thermal sensations, the reasons of the local thermal comfort dissimilarities were also discussed. In order to establish relationship between the local perceptions and physiological responses, skin surface temperature measurements (Tsk) and thermal sensation (TS) graphics are presented comparatively in the Figs. 9, 10, 11, and 12. The interpretations of figures are listed below. •

Under the recirculation mode, thermal neutrality (TS = 0) was provided for the entire body except the back.

Internal cabin surfaces Front window

Left window

Right window

Left door

Right door

Ceiling

Recirculation

48.9

31.7

36.4

32.2

33.7

28.7

Fresh

51.3

35.7

39.5

33.6

34.8

33.5

2.3

4.0

3.2

1.4

1.2

4.9

Difference

Interior body components Steering wheel

Instrument panel

Dashboard

Glove compartment

Average

Recirculation

37.8

31.5

43.0

32.7

34.2

Fresh

42.2

37.0

52.2

37.3

38.4

4.4

5.5

9.3

4.6

4.3

Difference

123


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Fig. 7 Comparison of CO2 measurements

Fig. 9 Comparison of driver head a skin surface temperatures and b thermal sensations

Fig. 8 Comparison of whole body thermal perceptions

• •

•

•

Under the fresh air mode, thermally neutrality was only reached at the head and right arm of the subjects. For the first 20 min of the experiments, back skin temperature in contact with seat warmed rapidly and thermal perceptions could not reach thermal neutrality for both ventilation modes. Chest and abdomen skin surface temperatures were not affected significantly by environmental conditions. At trunk (mean of the chest and abdomen measurements) thermal neutrality was achieved approximately in 30 min under recirculation mode and it took 40 min for fresh air mode. Towards the end of the cooling period, due to sun exposure, trunk skin surface temperature has increased under the fresh air mode. Under the recirculation mode, the disruptive effects of the sun on thermal comfort were minimized through high air circulation inside the cabin. Thermal neutrality was firstly achieved at the right arm of the subjects in 750 s under the recirculation mode. This period took 2,100 s under the fresh air mode. But in the following period, perception results exceeded the neutral conditions (TS = 0) towards cold side. Rightarm skin temperature decreased faster than the left arm. This result is expected since the right arm is uncovered and exposed directly to the cool vent outlet air. Satisfaction from the environmental conditions surrounding the head was provided 30 min after the start

Fig. 10 Comparison of driver trunk and back a skin surface temperatures and b thermal sensations

of the tests under recirculation mode. It took 55 min under the fresh air mode. Face skin temperatures were about 35 for both ventilation modes when thermal comfort was achieved.

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Fig. 11 Comparison of driver left and right arm a, b skin surface temperatures and c, d thermal sensations

Fig. 12 Comparison of driver left and right thigh a, b skin surface temperatures c, d thermal sensations

•

Thigh skin temperature (especially right thigh) warmed sharply due to exposure to sunlight through the first 15 min of the test. When solar radiation loses its impact, skin

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temperature also decreased and this caused subjects to feel cold (Fig. 12d). The disruptive effects of the sun on thermal comfort were minimized under recirculation mode.


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3.5 Comparison of heat transfer rates from the skin Heat transfer from the skin occurs by sensible (conduction, convection and radiation) and evaporative (sweating) heat flows. Sensible heat transfer (Qs) from the skin flows through the thermal resistance of clothing insulation (Rcl) by conduction and then flows from the outer clothing surface to the environment both by convection and radiation. These sensible heat flows from the skin to the environment can be combined into single thermal resistance to describe the total sensible heat transfer in terms of an operative temperature (To) and combined heat transfer coefficient (hc ? hr) [1]. Qs ¼ ðTsk To Þ=ðRcl þ 1 = ðhc þ hr ÞÞ ½W

ð4Þ

where, To is the average of the mean radiant (Tr & Ts) and interior air temperature (Ti) temperatures weighted by their respective heat transfer coefficients To ¼ ðTi hc þ Tr hr Þ=ðhc þ hr Þ ½ C:

ð5Þ

Values of convective (hc) and radiative (hr) heat transfer coefficients for a sitting person in a moving air can be found in ASHRAE Fundamentals [1]. Evaporative heat loss from the skin (Qe) is a combination of the evaporation of sweet secreted due to the thermoregulatory control mechanism (Qrsw) and the natural diffusion of water through the skin (Qdif). The simple transient two node thermoregulatory model which considers a human as two concentric thermal compartments that represent the skin and core of the body [1], was applied in this study to predict the core (Tcr) and mean body (Tb) temperatures and the physiological responses like blood flow and the rate of regulatory sweating (Qrsw). The rate of regulatory sweating (Qrsw) can be predicted by the skin and mean body temperature deviations from their set points Qrsw ¼ 170ðTb Tb:n Þ expðTsk Tsk:n Þ=10:7

½W:

ð6Þ

Diffusion evaporative heat transfer occurs on to the portion of skin not covered with sweat. With no regulatory sweating skin wettedness due to the diffusion is approximately 6% Qdif ¼ 0:06Qe:max ð1 Qrsw =Qe:max Þ

½W:

ð7Þ

Maximum evaporation (Qe.max) occurs if the skin is completely covered with sweat and depends on the difference between the saturation water vapor pressure at the skin temperature and water vapor pressure in the ambient air. Qe:max ¼ ðPsk:sat Pi Þ=Rcl =ðicl LRÞ þ 1=ðhc LRÞ

½W

Fig. 13 Comparisons of sensible and evaporative heat transfers rates from the skin to the environment for two air circulation modes

air circulation modes were compared. Since the operative temperatures are high at the beginning, sensible heat flows occur from the environment to body. This situation contributes to increase in core and skin temperatures and the body break into perspiration to prevent the body temperatures from overheating. In the fresh air mode, owing to the insufficient sensible heat losses, core and skin temperatures and therefore sweat generation continued to increase.

4 Conclusions In this study, the effects of ventilation modes (recirculation—fresh air) on thermal comfort and air quality parameters have been evaluated experimentally. Temperature, humidity, air velocity and CO2 concentration were measured from multiple various points inside the test vehicle during the cooling period. Thermal comfort perceptions of human subjects were questioned and the results were compared with measured skin temperatures. The simple transient two node thermoregulatory model was used to predict sensible and evaporative heat transfer from the skin to the environment. Mass balance equation of in-room CO2 level and the cooling load rates were arranged for the test vehicle and were confirmed by the experimental data. The following conclusions were drawn from the study: •

ð8Þ

where icl is the moisture permeability of clothing and LR is the Lewis Ratio. At typical indoor conditions their values are equals to 0.36 and 16.5 respectively [1]. In Fig. 13, predicted sensible and evaporative heat transfers rates (Qsk) from the skin to the environment for two

•

As long as the ambient temperature is lower than the cabin environment, operate the HVAC system under the fresh air mode provides a more rapid improvement in environmental conditions inside the automobile especially around head and arms. This means less energy consumption for the same cooling load. But, thermally comfort conditions can be achieved earlier under the recirculation mode for the entire cooling process especially at head level. Although there are small temperature differences throughout the vertical locations of the cabin during

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•

•

•

•

•

the cooling period, local thermal comfort dissimilarities were observed in the tests especially at thighs, arms and back. Exposure to solar radiation and ventilation air, to be in contact with hot surfaces are the major factors which causing local dissimilarities. CO2 level inside of the cabin can be greater than the threshold value (1,200 ppm) recommended for the driving safety if two and more occupants exist in the car. So, cabin air must be renewed periodically to protect occupants from harmful effects of high CO2 concentration as it reaches threshold value. An equation proposed to calculate the cabin air renewal time for any vehicle. A better air circulation in the cabinet was provided with using recirculation mode. Increase in air velocity facilitates disposal of excess heat from the body. In the fresh air mode, sweat generation increased in order to balance heat exchange between the body and environment. Since a strong interaction exists between the thermal comfort, air quality and energy usage, an advanced control system may be used to get an acceptable thermal comfort and air quality with a substantial amount of energy savings for the vehicle HVAC systems. Finally, it must be mentioned that measurements were taken in a stationary vehicle, instead of in motion on the road. But, this is also a fact that, in major cities, a traffic jam frequently occurs in which vehicles move at very low speeds or they are stationary with idling conditions like the conditions presented in this study.

Acknowledgments The authors would like to acknowledge for granting test equipments to Technological Research Council of Turkey (TUBITAK) under the project number 105M262, and to FIAT TOFAS for supporting the test car.

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