ORIGINAL

O. Kaynakli Æ E. Pulat Æ M. Kilic

Thermal comfort during heating and cooling periods in an automobile

Received: 9 September 2003 / Published online: 17 September 2004� Springer-Verlag 2004

Abstract Most vehicles have a heating, ventilation andair conditioning (HVAC) device to control the thermalenvironments of interior of the vehicle. But, under hotsummer season or cold winter conditions, it is difficult toachieve and maintain thermal comfort in an automobilefrom the start up to the steady-state conditions. Duringthese transition periods, an understanding of humanthermoregulatory processes facilitates the design anddevelopment of improved heating and cooling systems.This study presents a model of thermal interactionsbetween a human body and the interior environment ofan automobile. The model is based on the heat balanceequation for human body, combined with empiricalequations defining the sweat rate and mean skin tem-perature. Simulation has been performed by the use oftransient conditions. The effects of both heating andcooling processes on the thermal comfort inside theautomobile are investigated. Results are compared withthe present measurements and available experimentaldata in the literature. It is shown that the agreementbetween the experimental data and the model is verygood.

List of symbols

A surface area, m2

cp specific heat, J/(kg K)CSIG cold signalf correction factorh heat transfer coefficient, W/(m2 K)i segment numberj air or fabric layers numberk conductive heat transfer coefficient,W/(m K)

L heat load, W/m2

m body mass, kg_m mass flow rate from per unit area, kg/(s m2)M metabolic heat production rate, Wnl number of layers covering segmentp water vapor pressure, kPaQ heat transfer rate, Wr outer radius of fabric layerR thermal or evaporative resistance, (m2 K)/W

or (m2 kPa)/WS heat storage, Wt time, s (unless specified in minutes)T temperature,�CTS thermal sensationV air velocity, m/sw skin wettednessW humidity ratio, kgH2O/kg dry air_W external work rate accomplished, WWSIG warm signalx thickness, mm

Greek symbols

a ratio of skin layer mass to total body massg permeation efficiency

Subscripts

a airal air layerb bodybl bloodcd conductioncl clothingcr corecv convectiondif diffusione exposed to convective and radiant environmentev evaporation

O. Kaynakli Æ E. Pulat Æ M. Kilic (&)Faculty of Engineering and Architecture,Department of Mechanical Engineering, Uludag University,Gorukle Campus, 16059 Bursa, TurkeyE-mail: mkilic@uludag.edu.trTel.: +90-224-4429183Fax: +90-224-4428021

Heat Mass Transfer (2005) 41: 449–458DOI 10.1007/s00231-004-0558-9

ex exhaledf fabricint interface between outer clothing surface and a

solid (such as the seat or back support)max maximumn neutralres respirationrd radiations saturatedsk skinsw sweatt total

1 Introduction

The comfort of the driver and passengers in a vehicle ispartially dependent on the quality and temperature ofair in the vehicle. Three interrelated systems are used toprovide the desired air temperature and quality. Theseare the ventilating system, the heating system and the airconditioning system. The purpose of the heating, venti-lation and air conditioning (HVAC) system of a vehicleis to provide complete thermal comfort for its occu-pants. Hence, it is necessary to understand the thermalaspects of the human body in order to design an effectiveHVAC system.

In order to estimate the thermal comfort level,accurate information on the thermal environment isessential. The thermal environments can be roughlyrepresented by the temperature, velocity and humidity inthe automobile interior. In this interaction, heat andmass transfer take place together. Complete model ofhuman comfort consist of energy balances, equations forthe thermophysical properties of the fluids and materi-als, and heat and mass transfer characteristics.

The passenger compartment of an automobile isheated in the winter months by circulating hot enginecoolant through a coolant-to-air heat exchanger thatwarms the compartment’s air. The heating system isdesigned to operate in conjunction with the air venti-lating system to provide the desired air temperature.

With progressive reductions in engine size, stemmingfrom considerations of fuel economy, and correspondingreductions in the heat available for the passenger heatingsystem, there is interest in the development of moreeffective systems to ensure passengers thermal comforteven in extreme conditions by considering market situ-ation. It is difficult to achieve and maintain passengerthermal comfort under extremely hot or cold drivingconditions. Some auxiliary heating or cooling apparatusmay greatly reduce the time needed to attain thermalcomfort. But, power requirements associated with thisapparatus are substantial.

In hot season, air conditioning units are used. Whenair conditioning is mentioned, the first thought thatcomes to mind is cooling and, refreshing of air. Actually,

the automobile air conditioning system not only coolsthe air but also cleans, dehumidifies, and circulates itfor the health and comfort of the passengers. Theseprocesses are performed by working in conjunction withthe heating and ventilating systems.

Human thermal comfort has been the subject ofconsiderable previous study, and much of the availableinformation documented and codified [3]. In the litera-ture, most of the studies have considered the thermalconditions are nearly uniform and steady over the entirebody of occupant. Less attention appears to have beendirected to comfort in an automobile, where conditionsare highly nonuniform and transient over the body ofthe occupant.

Yigit [18] is to calculate the heat loses for each bodysegments and total loss for the whole body for five dif-ferent clothing ensembles. However heat losses from theparts of the body were not considered and clothingresistances were not evaluated for the thermal comfort.

McCullough et al. [13, 14] published the insulationvalue, evaporative resistances of representative cloth-ing ensembles that were measured with a thermalmanikin. These parameters were also measured forcomponent fabrics using a sweating hot plateapparatus. A computer model was developed thatestimates the resistance to dry and evaporative heattransfer. Olesen et al. [15] studied five different cloth-ing ensembles with the same total thermal insulation,but very different distributions of the insulation on thebody in experiments with 16 sedentary subjects. Theasymmetry was ranging from nude upper part to nudelower part of the body. Their experimental study willprovide a method for quantifying the nonuniformityof a clothing ensemble and examine how it influenceslocal thermal discomfort.

Tanebe et al. [16], investigated sensible and latentheat loss from several parts of the human body by theuse of a manikin. For each considered part of thebody, total heat transfer coefficient and thermal resis-tance were found. Since their study was performed inconstant temperature environment, it did not give anyresult about the thermal comfort. Kaynakli et al. [11]reported a study in which the human body is dividedinto 16 sedentary segments, a computational model ofthermal interactions between each of 16 body segmentsand the environment is developed. By the use of themodel, skin wettedness and latent (sweating, diffusion)and sensible (conduction, convection, radiation) heatlosses from each body segment and whole body arecalculated for both sitting and standing postures.Kaynakli et al. [12] presented a numerical model of theheat and mass transfer between the human body andthe environment. In their study, the required environ-mental and personal conditions for satisfaction of thepeople obtained under steady-state conditions, andtotal sensible and latent heat losses, skin temperature,wettedness, predicted mean vote (PMV) and predictedpercentage of dissatisfied (PPD) values were calculatedvia simulation.

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Chakroun and Al-Fahed [7] presented a study ofthe temperature variation and thermal comfort insidea car parked in the sun during the summer months inKuwait. They also considered the effect of using dif-ferent combinations of internal covering on the tem-perature inside the car. Burch et al. [4] reported theresults of a series tests on passenger thermal comfortduring warm-up under severe winter driving condi-tions. They found that low-power electric heating padsinstalled on the seat and back support greatly reducethe time needed to attain thermal comfort. Furtherreductions in warm-up time can be achieved byinstalling electric heaters in the air ducts, although thepower requirements associated with this method aresubstantial. In addition to their experimental study,they presented an analytical study on this subject inthe paper of Burch et al. [5].

Heating and cooling periods from the start up ofthe vehicle require some time to reach steady-stateconditions. During these periods, conditions are highlynonuniform over the body of the occupant. Thevehicle passenger experiences localized chilling due tocontact with an initially cold seat or steering wheel,nonuniform radiant heat transfer with the surround-ings, localized solar irradiation, and nonuniform airvelocities that vary depending on the location of theair registers and dashboard control settings. Thus, inaddition to the air temperature, several other factorshave a bearing on the thermal comfort of the pas-senger. Consequently, there is substantial interest inthe development of more efficient techniques forachieving and maintaining passenger thermal comfortin an automotive environment.

This study presents a model of thermal interactionsbetween a human and the interior environment of anautomobile. Since, segmental analysis permits thedetermination of local discomforts by considering theclothing insulation asymmetry effects in the relativelysmall volumes such as automobile cabin, the presentmodel is based on the heat balance equation for humanbody by dividing it into 16 segments. By combiningGagge et al.’s [10] and Olesen et al.’s [15] approaches,all body segments are considered as two-concentriccylinders and required new data such as surface areasof body segments and their masses are refined from theexisting literature. In this way, apart from the Gaggeet al.’s [10] model, it is tried to determine the localdiscomforts by calculating the thermal interactions ofeach segment and the skin temperature and wettedness.Simulation has been performed by the use of transientconditions. The effects of both heating and coolingprocesses on the thermal comfort inside the automobileare investigated. Experiments were also conducted forcooling periods. Until the thermal comfort reached inthe automobile compartment, the temperature and thehumidity changed dramatically. Driver and passengersare greatly affected by these changes. The simulationresults and experimental data were compared, in orderto validate the present model.

2 Mathematical model

The velocity of conditioned air that flow over passengeris very important from comfort point of view in smallcompartments that have large heating and coolingcapacity especially such as automobile cabin. Flowingair over driver and passengers injected by inlet vents hasnot same value on any occupant’s body. Although it is agood approximation to take average velocity for typicalindoor conditions, this results important mistakes byconsidering automobile interior. Local air velocities onthe body of sitting passenger were determined experi-mentally by Burch et al. [5] (Table 1). In this study,determination of heat losses from various regions ofpassenger is based on these velocity values.

The model used in this study is based on the sameapproach described in the study of Olesen et al. [15]. Inthis study, human body is divided by 16 regions byconsidering clothing groups and local air velocities onthe body in order to investigate the effects of thermalenvironment to occupants especially driver in detail forboth winter and summer condition. In Table 2, surfaceareas and their fractions of total body surface area aregiven.

To compute temporal temperature variations byusing stored energy in the body segments it is requiredthe masses of these segments. The masses of body seg-ments and their fractions of the total body mass areshown in Table 3.

By considering the human body as whole, mean skintemperature gives an idea from thermal comfort point ofview but the temperatures of the extremities such ashand, foot and face or naked parts of human body mayincrease or decrease unwanted values. By using thedeveloped model, time rate of changes of the parametersthat affect the thermal comfort such as sensible andlatent heat losses each of 16 regions, skin temperaturesand skin wettedness may be examined.

2.1 Thermal and physiological modeling of human body

A two-compartment transient energy balance modeldeveloped by Gagge et al. [10] represents the body astwo concentric cylinders the inner cylinder represents the

Table 1 Local air velocities on the body [5]

Region Air velocity(m/s)

Head 0.13Trunk 0.11Right shoulder 0.12Left shoulder 0.13Legs 0.11Right knee 0.18Left knee 0.21Right ankle 0.66Left ankle 0.62

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body core (skeleton, muscle, internal organs) and theother cylinder represents the skin layer. This model, byconsidering instantaneous heat storage of the core andthe skin compartment, assumes that temperatures ofthese compartments change with time. The thermalmodel is described by two coupled heat balance equa-tions, one applied to each compartment [3]:

ScrðiÞ ¼ MðiÞ � 7pt _W ðiÞ � QresðiÞ þ Qcr;skðiÞ� �

ð1Þ

SskðiÞ ¼ Qcr;skðiÞ � QcdðiÞ þ QcvðiÞ þ QrdðiÞ þ QevðiÞð Þð2Þ

where, M rate of metabolic heat production, 7pt _W rateof mechanical work accomplished, Qres total rate ofrespiratory heat loss, Qcr,sk rate of heat transport fromcore to skin, Qcn, Qcv, Qrd rate of heat loss from skin toenvironment by conduction, convection and radiation

respectively. Scr and Ssk that denotes stored energies incore and skin layer causes instantaneous temperaturechanges in these compartments. These effects are ex-pressed with following equations:

dTcrðiÞdt

¼ ScrðiÞð1� aÞ mðiÞcp;b� � ð3Þ

dTskðiÞdt

¼ SskðiÞamðiÞcp;b� � ð4Þ

where m is the body segment mass, cp,b is the specificheat of the body. Qcv and Qrd terms in Eq. 2 are the heattransfers with convection and radiation and can be cal-culated with following relation:

ðQcv þ QrdÞðiÞ ¼TskðiÞ � ToðiÞð Þ AeðiÞ

RclðiÞ þ 1=ðhcvðiÞ þ hrdÞ fclðiÞ½ � ð5Þ

where, Ae is the surface area of the body segmentsexposed to the environment (total area minus the area incontact with seat, back support, etc.), fcl is the ratio ofthe surface areas of the clothed body and the nude body.Operative temperature value (To) that includes averageradiation and ambient air temperature is given asfollows:

ToðiÞ ¼hrd�Trd þ hcvðiÞTa

hrd þ hcvðiÞð6Þ

For radiative heat transfer coefficient the value of4.7 W/(m2 K) is used since it is sufficiently accurate forinternal conditions [3] and convective heat transfercoefficient values of each segment of the body are taken

Table 2 Surface areas of the body segments [15]

Body segments Segment number

Surface area [m2]

Fraction of total body

surface area [%]

Left foot 1 0.062 3.5 Right foot 2 0.062 3.5 Left fibula 3 0.140 8.0 Right fibula 4 0.140 8.0 Left thigh 5 0.160 9.1 Right thigh 6 0.160 9.1 Pelvis 7 0.080 4.6Head 8 0.180 10.4Left hand 9 0.050 2.9 Right hand 10 0.050 2.9 Left forearm 11 0.062 3.5 Right forearm 12 0.062 3.5 Left upperarm 13 0.077 4.4 Right upperarm 14 0.077 4.4 Chest 15 0.185 10.6Back 16 0.204 11.7The whole body 1.751 100.0

Table 3 Mass of the body segments [17]

Bodysegments

Segmentnumber

Mass(kg)

Fraction of totalbody mass (%)

Foot 1–2 1.16 1.45Fibula 3–4 3.72 4.65Thigh 5–6 8.00 10.00Pelvis 7 6.78 8.48Head 8 6.48 8.10Hand 9–10 0.48 0.60Forearm 11–12 1.28 1.60Upperarm 13–14 2.24 2.80Trunk 15–16 32.98 41.22The whole body 80.00 100.00

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as described in de Dear et al. [8]. The total latent heatloss from the skin due to evaporation, Qev, is given by

QevðiÞ ¼wðiÞ psk;sðiÞ � pa

� �AðiÞ

RclðiÞ=gclLRð Þ þ 1=hcvðiÞ fclðiÞ LRð Þ ð7Þ

where, w is the wettedness ratio, psk,s is the saturatedwater vapor partial pressure at the skin temperatureand pa is the water vapor partial pressure in theambient air, gcl is permeation efficiency of the clothingand LR is the Lewis Relation which is the ratio of theevaporative heat transfer coefficient to the convectiveheat transfer coefficient. McCullough et al. [14] havebeen found an average value of gcl=0.34 for commonindoor clothing.

The total skin wettedness (w), includes wettednessdue to regulatory sweating (wsw) and to diffusionthrough to skin (wdif) is given by

wswðiÞ ¼hfg _mswðiÞQev;maxðiÞ

ð8Þ

wdifðiÞ ¼ 0:06 1� wswðiÞð Þ ð9Þ

wðiÞ ¼ wswðiÞ þ wdifðiÞ ð10Þ

Maximum evaporation potential, Qev,max occurs whenthe skin surface is completely wetted (w=1).

In an automobile, a significant portion (15–20%) ofthe body surface area is in contact with a seat, backsupport and steering wheel [5]. This portion does notlose heat by convection and radiation. The heat lossfrom the skin due to conduction is given by

QcdðiÞ ¼TskðiÞ � Tintð Þ

RclðiÞAcdðiÞ ð11Þ

In the two-node model, heat exchange between thecore and the skin occurs by direct contact andthrough the skin blood flow. A constant averagethermal conductance, Kcr,sk=5.28 W/(m2 K) is as-sumed over the body. The heat flow from core to skinis as follows:

Qcr;skðiÞ ¼ Kcr;sk þ cp;bl _mbl

� �TcrðiÞ � TskðiÞð Þ AðiÞ ð12Þ

The specific heat of the blood, cp,bl is 4,187 J/(kg K).Respiratory heat loss is approximately 10% of total heatloss [9]. The heat loss due to respiration is given by

Qres ¼ _mres cp;aðTex � TaÞ þ hfgðWex � WaÞ� �

Ab ð13Þ

where _mres is the mass flow rate of air inhaled, Tex and Ta

are the exhaled air and the ambient air temperatures,respectively. Wex and Wa are the exhaled air and theambient air humidity ratio, respectively. The heat ofvaporization (hfg) is 2.43·106 J/kg.

_mres ¼ 2:58� 10�6� �

M ð14Þ

Tex ¼ 32:6þ 0:066Ta þ 32Wa ð15Þ

Wex ¼ 0:0277þ 0:000065Ta þ 0:2Wa ð16Þ

The ratio of the skin mass to total body mass (a) ismodeled as the following function of core to skin bloodflow:

a ¼ 0:0418þ 0:745

ð3; 600 _mbl þ 0:585Þ ð17Þ

The blood flow between the core and the skin per unit ofskin area is expressed as

_mbl ¼6:3þ 200WSIGcrð Þ= 1þ 0:5CSIGskð Þ½ �

3; 600ð18Þ

The rate of sweat production per unit of skin area isestimated by

_msw ¼ 4:7� 10�5WSIGb expWSIGsk

10:7

� �ð19Þ

The average temperature of human body can be pre-dicted by the weighted average of the skin and coretemperatures:

Tb ¼ aTsk þ ð1� aÞTcr ð20Þ

The neutral body temperature is calculated from theneutral skin and core temperatures in the same man-ner.

The body is divided into 16 segments which are uni-formly clothed. The total thermal resistance and thetotal evaporative resistance for each segments are asfollows [14]:

RtðiÞ ¼ RaðiÞrði; 0Þrði; nlÞ

þXnl

j¼1Ralði; jÞ

rði; 0Þrði; j� 1Þ þ Rfði; jÞ

rði; 0Þrði; jÞ

� �ð21Þ

Rev;tðiÞ ¼ Rev;aðiÞrði; 0Þrði; nlÞ

þXnl

j¼1Re;alði; jÞ

rði; 0Þrði; j� 1Þ þ Re;fði; jÞ

rði; 0Þrði; jÞ

� �

ð22Þ

It is assumed that heat transfer through air layersbetween clothing layers occurs by conduction and radi-ation. In this case, thermal resistance of an air layer isgiven by

Ral ¼1

hrd þ k=xað23Þ

where xa is air layer thickness. The values of hrd and kwere taken as hrd=4.9 W/(m2 K) and k=0.024 W/(mK)[14]. Similar equation can be written for the evaporativeresistance. Evaporative resistance of an air layer is givenby:

453

Rev;al ¼ a 1� exp�xa

b

h ið24Þ

where a and b are constants. The values of a and b are0.0334 (m2 kPa)/W and 15 mm, respectively [14]. Theouter surface exposed to the environment is treated alittle differently. The thermal resistance of the outerlayer is then:

Ra ¼1

hcv þ hrdð25Þ

The evaporative resistance of the outer layer can bedetermined from the convective heat transfer coefficientand the Lewis Relation:

Rev;a ¼1

hcvLRð26Þ

2.2 Prediction of thermal sensation

The above equations describe thermal exchange betweenthe human body and the environment and thermoregu-latory control mechanisms. Combination of the thermalenergy on the body, thermal load, affects the humanthermal comfort in the thermal energy exchange (tran-sition) between the body and its environment. If thethermal load (L) on the body is nearly zero, then neu-trality or thermal comfort is achieved. Combinations ofactivity, clothing and the four environmental variables(air temperature, mean radiant temperature, air velocityand humidity) all affect thermal comfort. The mostwidely used thermal comfort index is the thermal sen-sation (TS) value is given by Eq. 27.

TS ¼ 0:303 exp�0:036M

Ab

� �þ 0:028

� �L ð27Þ

where Ab is the total surface area of the body. The TSscale is given in the Table 4.

2.3 Assumptions and initials conditions

Nude body surface area is taken as Ab=1.751 m2. Bodymass (m) is 80 kg and initial values of core and skintemperatures are taken as 36.8 and 33.7�C respectively[3].

Summer clothing insulation, winter clothing insula-tion, clothing area factor for summer clothing, clothing

area factor for winter clothing and metabolic activity aretaken as 0.5 clo, 1.5 clo, fcl = 1.1, fcl = 1.15 and75 W/m2, respectively [5, 6].

Table 4 Scale of TS values

0 thermal neutrality

1 slightly warm �1 slightly cool2 warm �2 cool3 hot �3 cold4 very hot �4 very cold5 painfully hot �5 painfully cold

Fig. 1 Automobile interior air temperature during heating process

Fig. 2 Temperatures inside the automobile and human bodycontact surfaces

Fig. 3 Relative humidity values during cooling process inside theautomobile

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Local air velocities on the body is given in the Table 1and mean air temperature (Ta) for heating and coolingprocesses is taken as given in Figs. 1 and 2. Relativehumidity in heating period is taken as 0.35 [5] and incooling period it is taken as given in Fig. 3. Meanradiant temperature in heating period is taken as�Trd ¼ 0:94Ta � 1:38 and in cooling period is taken as�Trd ¼ �0:007752T 2

a þ 1:625778Ta � 6:879288:Surface temperatures of solids in contact with the

body (Tint) in heating period (t is time from start-up inminutes) were [5]:

Seat

Tint ¼ 41 1� exp�t4

� 20 for t � 15

Tint ¼ 20þ 0:367ðt � 15Þ for t > 15

Clothed area in contact with seat: 0.07 m2

Back support

Tint ¼ �20þ 30t for t � 1

Tint ¼ 14:6ð1� exp �ðt � 1Þ5

� �þ 10 for 1\t\10

Tint ¼ 22:2þ 0:065ðt � 10Þ for t � 10

Clothed area in contact with back support: 0.07 m2

Steering wheel

Tint ¼ 40 1� exp�t6

� 20

Clothed area in contact with steering wheel: 0.01 m2.In cooling period, surface temperatures of solids in

contact with the body (Tint) are found as follows as aresult of performed experiments. (where t is in minutes)(Table 5).

Tint ¼ at2 þ bt þ c ð28Þ

3 Results and discussions

In order to investigate the effects of automobile interiorconditions resulted by heating and cooling process, theequations given in Mathematical model section areconducted to computer medium by using the program-ming language Delphi 6. For heating period, requiredexperimental input data such as automobile interior air

temperature and humidity, mean radiant temperature,seat, back support and steering wheel surface tempera-tures are taken from Burch et al. [5]. In their experi-mental studies, interior air has been heated from �20 to20�C as seen from Fig. 1.

Table 5 Constants in the Eq. 28

For t £ 2 For t>2

a b c a b c

Seat 3.10 �15.30 65.20 0.0051 �0.3870 46.0496Back support 2.55 �13.65 62.10 0.0049 �0.3306 43.7763Steering wheel 2.50 �16.00 67.00 0.0064 �0.4618 44.6285

Fig. 4 Comparison of body heat losses in heating process

Fig. 6 Average body skin temperature in heating process

Fig. 5 Comparison of thermal sensation during heating process

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Required experimental data for cooling process aremeasured in 1991 Toyota Corona Sedan automobileequipped with a 2,000-cc engine. Automobile is parkedin the sun and it is observed that the increase of tem-perature inside car is 64�C with the ambient temperatureof about 30�C. Later, standard cooling process is startedby running the air conditioning unit. During this processtemperature inside car, relative humidity, seat, backsupport and steering wheel surface temperatures aremeasured. Measured parameters are shown Figs. 2 and3. Since relative humidity decreases from 50 to 11%during the increase of temperature inside car to 64�C,relative humidity in cooling process is started from 11%.

Heat losses from body to the environment duringwarm-up process are given in Fig. 4 comparatively.Since the model of Burch et al. [5] and the present modelexhibit some principal differences (e.g., the body isdivided into four segments in Burch’s et al. [5] model,whereas it is divided into 16 segments in our model.),some discrepancies appear at the beginning period.Apart from this relatively small time interval, theagreement between the results can be acceptable range.Conduction heat losses to the seat, back support andsteering wheel is rather low in comparison to the totalvalue of convective and radiative heat losses because theareas of body segments in contact with solid surfaces aresmaller than other body surfaces. In the beginning ofwarm-up process, conductive, convective and radiativeheat losses are high since the temperature inside theautomobile and the interior surface temperatures arerather low. Even total of these heat losses are higherthan metabolic heat generation. For this reason, coreand skin temperatures of the body a little decreases. But,the decrease in skin temperature is higher than the de-crease in core temperature. It is observed that rapiddecrease in these heat losses due to increase in the tem-perature inside the automobile. In this process, bodytries to keep respiration and evaporation heat losses inminimum to balance heat losses.

Comparative variation of TS values in warm-upperiod is given in Fig. 5. By inspection of Fig. 5, there isa good agreement with the study of Burch et al. [5].These calculations are performed by considering thesame conditions described in the experimental andanalytical studies of Burch et al. [4, 5]. In their experi-ments, TS values were obtained by using jury data, andthe mean TS and standard deviation (r) of the jury datawere calculated versus time. The present study calcula-tions are fall within the range of TS±1r, and the valueof r is given as 0.62. In the beginning, time heat lossesfrom the body to the environment is very high due tolow temperature inside the car. For this reason, TSindices that considers thermal load on the body has beenstarted from very low values. And then, TS hasimproved with increasing inside temperature andinterior surface temperatures.

One of the parameters that indicate the effects ofenvironmental conditions on human comfort during thewarm-up period of automobile cabin is mean body skin

temperature and its variation with time is shown inFig. 6. In early minutes, average skin temperatureinstantly decreases due to very low temperatures of bothinside the car and interior surfaces. Since temperatureinside car increases with time, mean skin temperatureincreases after its values drops a minimum value of

Fig. 7 Temperature of body parts that contact with solid surfacesin heating process

Fig. 8 Heat flow between body and environment in cooling process

Fig. 9 Variation of thermal sensation during cooling process

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32�C. Although average skin temperature gives an ideaabout the human comfort condition, it must be paidattention local discomforts on human body. The tem-peratures of back, thigh and hand of the body thatcontact with solid surfaces are given in Fig. 7. Thetemperatures of back and thigh are not much moreaffected from inside temperatures, and so they don’tvary importantly. But the hand-skin temperaturedecreases to a value of 17.5�C that can be evaluated as arather low temperature. In literature, it is mentionedthat hand-skin temperature of 20�C causes a report ofuncomfortably cold; and 15�C, extremely cold [3].

Heat transfer from the body in cooling process isgiven in Fig. 8. Since inside temperature and interiorsurface temperatures are high at the beginning, sensibleheat flow (conduction, convection and radiation) occursfrom environment to the body. This situation contrib-utes to increase in core and skin temperatures. To con-tinue vital functions and in addition to ensure comfortconditions, the heat from environment to body andmetabolic heat generation of body must be emitted toenvironment. For this reason, body increases the sweatgeneration, and then a large portion of the body iscovered by sweat. In this way, evaporative heat loss in-creases as shown in Fig. 8. Whereas respiration loss isnot affected by ambient conditions and it stays about10 W.

The variation of TS for cooling period is givencomparatively with Chakroun and Al-Fahed’s [7]study in Fig. 9. In Chakroun and Al-Fahed’s [7]paper, detailed ambient conditions were not given, soour model could not be applied directly to theirmeasurement conditions. Therefore, this figure presentsonly a qualitative comparison. In their study, it isensured the temperature inside car reaches up toapproximately 65�C by parking in the sun. Then,cooling process is investigated by running the A/Cunit. But experiments are performed in relatively hotclimate and ambient temperature is about 45�C.However it is about 30�C in our experiments. And theradiation from the sun is also stronger than our cases.For this reason, temperature profile inside the car isdifferent and depending on this TS values are alsodifferent. In early minutes, due to high inside tem-peratures, heat is transferred from environment to thebody by conduction, convection and radiation. Forthis reason, since the body has a significant thermalload, TS starts from very high values. Then, thermalload decreases with decreasing the temperature insidecar and the surface temperatures and comfort condi-tion gets better.

The variation of mean body, feet, and hand-skintemperatures during the cooling process is given inFig. 10. In early times, since the temperature insidethe car is very high (�64�C), the temperatures of theall body segments rise. But, the temperature rise of thehand that directly contacts with air is higher thanother segments. Since inside the car cools with timethis rise decreases. Hands are most affected from

ambient conditions, so obvious decrease in tempera-ture occurs on hand. Similar situation is valid forhead. Since shoes are important insulation element,feet are not affected from the interior temperaturevariations. For this reason, the highest temperature atthe end of cooling process occurs at feet. Mean skintemperature of the body varies between hand and feettemperatures.

One of the important parameters that affect thecomfort sensation is skin wettedness and its variationwith time is given in Fig. 11. At the early stage ofcooling process, since temperature inside car is high,sweat generation rate is increased to increase heat lossfrom body. So skin wettedness on the body increases.The most rapid increase occurs at feet due to shoes.Since there is no clothing that prevents evaporation ofsweats on the head, and hands that do not contact withsteering wheel, the skin wettedness is the lowest on theseparts of the body. Whereas mean body skin wettednessrises to the maximum value of 0.6 between the wetted-ness of head and feet.

Fig. 10 Mean body, hand and feet skin temperatures duringcooling process

Fig. 11 Mean body, hands and feet skin wettedness in coolingprocess

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4 Conclusions

In this study, the effects of interior environmentalconditions on the human physiology and comfort inheating and cooling processes are introduced. Basicheat exchange equations and empirical relations thatexpress thermoregulatory control mechanisms are usedin heat and mass transfer between human body andenvironment. During these processes, by consideringtemperature and relative humidity inside car and thetemperatures of solid surfaces that contact with body,variations of heat transfer, body segments skin tem-peratures and wettedness and TS values are given.

In early minutes of warm-up period, heat loss frombody to environment is very high due to low inside airand surface temperatures. During this time, evaporativeheat losses are kept at minimum by thermoregulatorycontrol mechanisms of body. The average skin temper-ature of the body falls to 32�C and contact temperatureof hand with steering wheel also falls to very low valueas 17.5�C. Since it becomes important heat loss frombody to environment, TS has been started from �4.5,and then it has been improved by increasing insidetemperature.

In early minutes of cooling period, opposite to thewarm-up period, sensible heat exchange occurs fromenvironment to the body due to high automobile in-side and surface temperatures. For this reason, TSstarts from rather high value of eight and then it de-creases by increasing inside temperature. Sweat gen-eration is increased in order to balance heat exchangebetween body and environment, and so latent heatlosses increase. By considering both warm-up andcooling periods it is seen that respiratory heat lossesare not affected by ambient conditions. Skin wetted-ness on the body increases with increasing sweatgeneration. Skin wettedness is high on the body sur-face that has high clothing thermal insulation (such asfeet and pelvis), on contrary, for naked body surfaces(such as head and hands) it is low. Similarly, thesenaked surfaces are the segments that are also affectedrapidly from environmental conditions.

In addition, it must be mentioned that measurementswere taken in a parked automobile with only driver in-side. The automobile in motion or automobile withpassengers may affect the measurements.

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