Computational Analysis of Thermal Distribution within Passenger Car Cabin

Transcription

1 Cmputatinal Analysis f Thermal Distributin within Passenger Car Cabin Zeya Ahmad Quadri 1 & Jmn Jse 2 1 Department f Aerspace Engineering, University f Petrleum and Energy Studies, Dehradun 2 Department f Mechanical Engineering, The ICFAI University, Dehradun zeya.quadri@stu.upes.ac.in, j12jse@gmail.cm Abstract This paper n the thermal distributin within a passenger car cabin discusses and slves the prblem f Thermal Accumulatin within a car when it is parked under the sun fr sme perid f time.it discusses the effects f inlet and utlet vents n the temperature distributin in the cabin. Studies shw that the engine wrking temperature f the car has an impact n the cabin envirnment. In additin it has been identified that prper ventilatin within the car reduces this prblem. In this paper we use 3 dimensinal cmputer simulatins using CFD technique t examine these issues f Heat accumulatin within the car. This paper identifies the causes f this cmmn prblem that mst passengers face in their daily lives and aims t suggest ways f reducing the uncmfrtable perid f time within the car. Keywrds CFD, slar radiatin. I. INTRODUCTION Mdern technlgy has brught a lt f changes t the cars we lk arund in the traffic. The cars have evlved nt just in their exterirs but als in their interirs.the luxury segment cars are primarily abut the inner cmfrt. They intend t imprve the experience f the passenger within the car. The prblem faced by the cars tday is the initial high temperature within the car cabin as we enter int them.this phenmenn f Thermal Accumulatin is clearly evident when the car has been parked in the sun fr a few hurs.the car user is frced t wait fr a perid f arund 2-5 minutes t allw the cabin envirnment t be like the ambient. In a parked car with its Engine n, we can als ntice a temperature rise within the car because f the heat leakages frm the Engine Sectin. The cmfrt prvided by a car t the passengers travelling in it has becme a majr field f imprvement and demand in the Autmbile Industry. A perfect HVAC (Heating Ventilatin -Air Cnditining) is being explred by the research scientists. T understand this, knwledge abut the Thermal Distributin within the car is essential. In this prject a GAMBIT mdel f a tw seater car has been used t analyse in Ansys Fluent the varius factrs that affect the cabin temperatures. This study shall present befre the car engineers varius aspects t be lked int while designing a car with nt just an aesthetic value but which als delivers cmfrt t the passengers. A. Wrking Principle Thermal analysis f a passenger cmpartment invlves nt nly gemetric cmplexities but als strng interactins between airflw and the three mdes f heat transfer, namely Heat Cnductin, Cnvectin and thermal Radiatin [3]. Cnductin is a mde f transfer f heat which ccurs when a temperature gradient exists in a bdy r tw r mre bdies in the same phase. The heat transfer rate per unit area is prprtinal t the nrmal temperature gradient [5]: When the prprtinality cnstant is inserted: (1) 129

2 Where q= Heat transfer rate T/ x=temperature gradient in the directin f flw k=thermal cnductivity f the material A=The area under cnsideratin The minus sign is inserted s that the secnd principle f thermdynamics will be satisfied. The equatin is called the Furier s law f Heat Cnductin. Cnvectin is the mde f heat transfer between any tw different phases such as a slid and a liquid r a slid with a gas r vice versa. The verall effect f cnvectin is expressed by Newtn s law f Cling. q = ha T (2) Where q= Heat transfer rate h=cnvectin Heat transfer Cefficient T=Temperature gradient between the tw bdies A=Area under cnsideratin Energy transfer can als take place withut a medium this mde f heat transfer is called Radiatin.The mechanism in this case is electrmagnetic radiatin. Thermdynamic cnsideratins shw that an ideal thermal radiatr r a black bdy will emit energy at a rate prprtinal t the furth pwer f the abslute temperature f the bdy. (3) Where σ=stefan-bltzmann Cnstant (5.669 x 10-8 W/m 2.K 4 ) B. Parameters: Slar Radiatins The Slar Radiatins entering the car thrugh the windshield accunt fr the temperature rise within the car n a sunny day. The ttal heat permissin thrugh the glass is expressed as the sum f the slar radiatin transmitted thrugh the glass and the inward heat flw by cnvectin frm the glass inner surface [3]. Prlnged perids in the sun can deterirate the car interirs including trims, plastic mldings, uphlstery and seat cvers. If there are fancy leather interirs, they need t be taken care f because f the cnstant expsure t heat that can induce weathering in leather materials that may eventually result in cracking and fading f surfaces. Als, a car with leather seat-cvers parked in the sun is gd enugh t cause serius burns if a persn sits inside withut allwing it t cl. There are technlgies t reduce the temperature when parked.the Aut Cl is ne such device which cnsists f a slar-pwered car fan that aims t keep the interir f the vehicle cl even after many hurs baking under the sun. In this technlgy all a persn needs t d is place the device n tp f the windw and the fan will get t wrk as lng as there is sufficient sunlight. It maintains the security level f car intact since there wuldn t be any gap lcated at the tp. Tests prve that the Aut Cl suppsedly keeps a car cler by up t 30 degrees when used. Tinted glasses can als be used t keep the carcabin cl and hence avid the need fr cnstant use f air cnditiner while driving. As we all knw, less use f air cnditiner means mre fuel efficiency. Als, tinted mirrrs maintain decent cmfrt levels inside the car-cabin by preventing harmful heat frm entering inside. Tinted glasses can prevent the passenger s skin frm harmful effects f sun s heat and radiatins. Tinted windws als prevent the driver frm any glare emanating frm radside bjects making the drive safer. Engine Temperature The engine f the car perates at very high temperatures because f the ceaseless explsin f the fuel between pistn and the cylinder. This generates temperatures as high as F within the engine. The mdern day car design is such that the heat within the engine is dispersed t the atmsphere frm the penings in the bnnet f the car. In spite f such an efficient car design sme amunt f heat leaks in t the cabin thrugh the penings in the dashbard and ther parts thus affecting the thermal distributin within the car. In a car parked with its engine n, we can ntice the effect the Engine temperature has n the cabin climate, while fr a mving car mst f the heat generated is lst t the ambient because f its design thus negating the effect. This research intends t nte the changes this factr brings abut in the envirnment within the car. 130

3 Number and psitin f Inlets and utlets f air within the car Studies in this field have detected that the increase in the number f air inlet /utlet vents leads t decrease in the ht znes within the car [3]. Fresh air enters the cabin via the windshield cwl area. If we pen the hd and lk at the cwl we shall see penings that allw the air t enter. The number and als the psitin f these inlets and utlets prvide better ventilatin within the car. Better ventilatin within the cabin ensures that the envirnment remains almst the same as the ambient. In this experiment the inlets have been cnsidered t be lcated symmetrically n the dashbard 15 cm away frm the sidewalls and the utlets belw the rear windw 30 cm away frm the sidewalls. C. Applicatins f the study High levels f vehicle cmfrt are being increasingly demanded by users.this creates a new challenge fr climate cntrl engineers t develp the (HVAC) Heating Ventilatin and Aer-Cnditining fr a new vehicle, this study simplifies the prcess f the engineers [5]. When a vehicle is parked under the sun, accumulated heat affects many interirs, thus bringing abut their degradatin this study can be used effectively t prevent this [1]. This study will prvide the scientists in the autmbile sectr the exact causes f the temperature rise within the car.it shall als suggest methds fr imprving the envirnment within. This study can be used further in any clsed envirnments where temperature distributins play a vital rle; fr example Space shuttles etc. II. LITERATURE REVIEW Al-Kayiem [1] fund ut that when a car is parked under the sun there is degradatin f its interirs and an uncmfrtable perid fr the passengers. He experimentally with the help f thermcuples pinted ut the areas f thermal accumulatin within the car. The highest temperatures were nted in the spts near the windshields, the dashbard is seen t be a majr heat surce fr the cabin envirnment and the primary sink fr the external radiatins. He suggested the installatin f a sunshade which prved t reduce the temperature by abut 27%. Alexandrv [2] in his study indicated that bth the external and internal parameters play an imprtant rle in determining the HVAC system fr the car. He depicted the air flw patterns cnjured up within the car due t these parameters.in his study he suggested that the develpment f znes f lw air circulatin can be reduced significantly by imprving the inlet/utlet cnfiguratin. Jalil and Alwan [3] put tgether a numerical study f a tw dimensinal, turbulent, recirculating flw within a passenger car cabin. Their study demnstrated the capability f CFD t accurately simulate the air flw within the cabin. They predicted that the increase f air inlets leads t the decrease f ht znes. It als leads t lwer temperature gradient near the interir surfaces and a unifrm temperature distributin. Zhang,Dai and their team [4] in their study highlighted the prblems and challenges faced by the Air Cnditiner within the car using Fluent and applying a user defined functin in accrdance with the assumptins taken in the numerical analysis. In additin the air temperature distributin in a steady test situatin is als numerically predicted by bth transient and steady state mdel and the agreements between the data and the numerical analysis is satisfactry. III. SIMULATION MODEL, METHODS AND BOUNDARY CONDITIONS A. 3D Cabin Mdeling The entire space f the vehicle cnsists f the cmputatinal Dmain and the inner surfaces f the cmpartment are the bundary cnditins. The 3D mdel f the car was prepared using Gambit The ttal vlume f mdel is 4.26 cubic meters and a Face Area f x 10-3 m 2. This study cncentrates in the thermal distributin within a tw seater car s as t reduce the cmplexities in the flw distributin f a fur seater. The dimensins f the car are accrding t the ASHRAE standards. Inlet and utlet vents have been added t the structure at the specified lcatins. B. Meshing Criteria The meshing f the structure was cmpleted in Gambit. Unstructured grids have been adpted, and mst f the grids are tetrahedrns. The mesh has been given a spacing f.15 units fr prper and unifrm meshing. Tetrahedral meshing has been preferred ver any ther because it prduces mre accurate results than 131

4 quadrilateral due t the nn-unifrm gemetrical shape f the mdel under investigatin. The meshing yields a ttal f 6892 tetrahedral cells, faces and 1707 ndes. With taing cnsideratin f inlet and utlet vents meshing f the mdel has been dne by tetrahedral meshes f size.08 units. A ttal f cells, faces, and ndes were frmed. C. Bundary Cnditins fr Slar Radiatins The inner surfaces f the car have been given bundary cnditins. The Windw glass is smewhat transparent and the majr part f sun radiatin can g thrugh the glass. Slar radiatins have been assumed t fall n the frnt wind shield, the side windws and the rear windw. The mmentum prperties f the windws are cnsidered t be a statinary wall with n slip cnditin. The thermal prperties f the windws are set with an external temperature f 310K.The prperties are that f air. The windws participate in slar ray tracing, the Absrptivity and Transmissivity are taken as the default values frm the Fluent database. The ther parts are cnsidered as paque walls which transmit n part f the slar radiatins inside the cabin. It has been assumed that the ther parts are cmpletely adiabatic. They have zer heat flux and heat generatin rate. N cnvectin, radiatin r any ther mde f heat transfer takes place frm acrss its walls. They d nt take part in slar ray tracing. The fluid enclsed is cnsidered as air with prperties as prvided in the fluent database. Fr slving the prblem a pressure based slver with an abslute velcity frmatin and a steady time is used. The energy equatin has been utilized t achieve results. The P1 Radiatin mdel has been used t analyze the prblem. In the P1 radiatin mdel the directinal dependence in Radiative Intensity Transprt Equatins (RTE) is integrated ut, resulting in diffusin equatin fr incident radiatin. This mdel basts f benefits like lw CPU demand, ease f wrking in applicatins where ptical thickness is large. The mdel has been analyzed when the car is psitined in the latitude and lngitude f New Delhi at 2 Pm n the 10 th day f the 4 th mnth. Fair weather cnditins have been assumed t get a cnclusive result. The Sun directin vectr has been calculated frm the slar calculatr with a sunshine factr f 1.The direct Slar Irradiatin is cnsidered as 1423W/m 2 and the Diffuse Slar Irradiatin as 200W/m 2. The spectral fractin is 0.5. The residual mnitrs have been pltted with a strage capacity f 1000 iteratins. D. Bundary Cnditins fr inlet and utlet vents The windw panes are cnsidered t be semitransparent, thus allwing sme amunt f slar radiatin t enter inside the car. The bundary cnditin in this analysis is same as the abve sectin. In additin t that the Inlet vents have been set as velcity inlets and the utlets as pressure utlets. The flw within the cabin has been cnsidered as laminar and steady and the experiment has been cnducted in atmspheric pressure cnditins f Pascals. A pressure based slver has been favred in this analysis. The radiatin cnditins are same as that f abve. The P1 radiatin mdel has nce again been favred in this analysis. The material that makes up this 3D mdel is nce again air with prperties that are fund in nrmal cnditins. E. Bundary Cnditins fr Engine Temperature The Engine perates at very high temperatures. The Ignitin chambers reach up t K at the time f ignitin.the car has inbuilt cling systems which d nt allw the heat generated t affect the cabin envirnment. The Thermstat and Radiatr systems help t keep the temperature in cntrl.the heat absrbed by the cling water causes it t gain temperature. This water circulates within the cabin. The bundary temperatures fr this prblem have been taken as 353K.The ambient temperatures have been taken as 300K.The engine temperature seeps in thrugh the area under the dashbard.adiabatic cnditins prevail within the car. F. Inlet and Outlet Cnditins The air temperature and velcity at the inlets play an imprtant rle in determining cabin climate [2]. Steady state cnditins are prevalent within the cabin. The ambient temperature is cnsidered as 300K while the inner temperatures are relatively high due t the slar radiatins falling n the windw panes. The inlet velcity vents blw air at a velcity f 8m/s in the X directin, while.95m/s in the Y and Z directin at a temperature f 289K.The external back flw temperature at the pressure utlet i.e. the utlet vent is 310K.The X cmpnent f the utlet vent is.95 m/s. G. Inlet and utlet Lcatins In rder t establish the rle f the size, number and lcatin f HVAC system inlets and utlets, a 3D simulatin fr varius psitins f inlet and utlet have been run. There are tw inlets in the cabin, each f size (3X80 cm) lcated symmetrically n the dashbard 15cm away frm the sidewalls. Tw utlets f size 132

5 (7.5X3.5 cm) are prvided belw the rear windw at a distance f 30cm frm the sidewalls [2]. IV. RESULTS AND DISCUSSION A. Temperature variatin due t slar radiatin The meshed mdel was exprted t the simulatin sftware Fluent This slver yields the fllwing results: Frm the Fig 1 and Fig 2 it is evident that the passenger seats, rf f the car which had perfect adiabatic cnditins als ntice a 3K rise in temperature. The Fig 3 gives us the Crss Sectinal view f the cabin envirnment. Frm the figure we can deduce that the air near t the windshield, side windws, and back windws has very high temperatures and it decreases as we mve inside. The high temperature n the windshield face is bad fr the material n the dashbard. The dashbard tends t absrb this heat and it begins t act as an internal heat surce fr the cabin [1].The results indicate that the temperature increase due t radiatin is mre prnunced than that areas affected by cnvectin. Figure 1: Cnturs f static temperature (k) Fig 1 exhibits the cnturs f static temperature n the surface f the mdel when it has been expsed t slar radiatins. The red clr dentes the areas where slar radiatins fall n the mdel. The temperature in thse areas is the temperature f the external radiatins which is 310K.The radiatins fall n the windshield, side windws and the back windw. The blue areas represent the lcatins where the temperature is 303K, which means that these areas have experienced a rise in temperature in cmparisn t the ther nes. The initial temperature was assumed t be 300K s there has been 3K rise in temperature in these areas. Figure 3: Cnturs f static temperature (K), Crss sectin view The Fig 4 exhibits a plane cut alng the Z axis and it gives us a clear depictin f the thermal distributin within the car. A passenger sitting n the seat wuld experience ht areas arund him as high as 306K.The heat accumulated near the windws can be ventilated ut by prviding a prus slit abve them alng the length f the windw pane. Figure 2: Cnturs f static Temperature (k), Tp view Figure 4: Cnturs f static temperature (k), Cut alng the Z plane 133

6 B. Temperature variatin due t number and psitin f inlets and utlets The penings were prvided fr the inlet and utlet vents and the temperature f the stream f air frm the inlet vent is 289K.The simulatin was cnducted when the sun rays fell n the windw panes as in the previus sectin. An area f air circulatin is frmed in frnt f the passenger seat. The Fig 6 depicts the velcity prfile f air within the car. These areas f air circulatin are evident frm the differences in velcities f the air flwing within the car. Figure 5: Cnturs f static Temperature (K) with inlets The Figure 5 depicts the bundary cnditins that have been impsed n the mdel.the blue spts shw us the air inlet velcities at a temperature f 291K.These cld air inlets change the envirnment within the car. Figure 6: Cnturs f Static Temperature (K) viewed frm X Plane Figure 8: Cnturs f velcity magnitude (m/s) alng the x plane V. CONCLUSION A simulatin f a 3D car cabin mdel has been perfrmed with the help f simulatin sftware such as Fluent The car has been subjected t varius external and internal parameters.the car cabin envirnment was analyzed when it was kept in Delhi at 2 pm n the 10th f April. The Slar calculatr was used t determine the intensity f radiatins falling n the windw panes. T depict the inlet and utlet vents in a car, slits with specific dimensins were cut in the mdel and cld air was allwed t enter int the cabin. When the Slar radiatins fall n the windw panes the areas arund them experienced the highest temperatures and a prus slit psitined abve the windws culd act as a vent fr ht air. Frm the simulatin it was evident that the lcatin f the vents plays a vital rle in determining the cabin envirnment. The vents if psitined twards the center f the dashbard tend t create a cl envirnment in the frnt part f the passenger seats. The velcity inlets help t negate the high thermal accumulatin due t the incming Slar Radiatins. S the HVAC (Heating Ventilatin and Air Cnditining) system in the car plays a crucial part in determining the cabin envirnment. Future studies can take the help f the simulatin sftware t make the ventilatin system mre efficient. Figure 7: Cnturs f Static Temperature (k) when cut alng Z Plane 134

7 VI. REFERENCES [1] Hussain H.Al-Kayeim, M.Firdaus Bin M.Sidlik, Yuganthira R.A.L Munusammy,Study n the Thermal Accumulatin and Distributin Inside a Parked Car Cabin,American Jurnal f Applied Sciences, 7(2010) [2] Alex Alexandrv,Vladimir Kudriavtsev,Marcel Reggi,Analysis f Flw Patterns And Heat Transfer in Generic Passenger Car Mini- Envirnment,9 th Annual cnference f the CFD Sciety f Canada,2001. [3] Jalal M.Jalil,Haider Qassim Alwan,CFD Simulatin fr a Rad Vehicle Cabin,JKAU:Eng Sci,Vl.18 n.2, ,2007. [4] Huajun Zhang, Lan Dai, Guquan Xu, Yng Li, Wei Chen, Wen-Quan Ta,Studies f air-flw and temperature fields inside a passenger cmpartment fr imprving thermal cmfrt and saving energy, Applied Thermal Engineering 29(2009) [5] A.Mezrhab,M.Buzidi,Cmputatin f Thermal Cmfrt inside a passenger Car Cmpartment,Applied Thermal Engineering 26(2006) [6] J. P. Hlman, Heat Transfer, 9th ed., pages 1-13 (2008). [7] ASHRAE Handbk f Heating and Ventilatin,