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Investigation of fl ow and heat transfer for a split air conditioner indoor unit Dilek Kumlutas Ziya Haktan Karadeniz Funda Kuru Department of Mechanical Engineering Dokuz Eyl l University 35397 Buca Izmir Turkey h i g h l i g h t s A three dimensional representative thin section CFD model was introduced A psychrometric test room was used for the verifi cation of the capacity results Capacity difference between the test room data and CFD model was 0 26 Velocity distribution from the CFD analyses was compared with SPIV measurements Thin section model is useful for determining heat transfer and fl ow characteristics a r t i c l ei n f o Article history Received 25 May 2011 Accepted 24 August 2012 Available online 12 September 2012 Keywords Split air conditioner Fin and tube heat exchanger Cross fl ow fan Computational fl uid dynamics Stereo Particle Image Velocimetry a b s t r a c t Split air conditioners SACs include complicated components such as the fi n and tube heat exchanger FTHE and cross fl ow fan CFF and their complex interaction needs to be investigated in detail In this study a three dimensional representative thin section model was introduced for modelling SAC indoor units and heat transfer and fl uid fl ow analysis was made to determine the characteristics of the device In addition the numerical method was examined by comparing the results with the heat transfer capacity experiments and Stereo Particle Image Velocimetry SPIV measurements at the outlet section of the device were conducted to compare the velocity distribution The results showed that the difference between the numerical and experimental studies were within acceptable limits therefore the thin section model for the numerical study is a good assumption for determining the heat transfer and fl ow characteristics of the SACs 2012 Elsevier Ltd All rights reserved 1 Introduction Recently energy effi ciency has become one of the most impor tant design criteria for engineering applications Governments and international organisations fi rmly control the energy demand of devices through regulations and international standards Because household appliances involve a large market and constitute an important component of energy consumption their production and sales are also strictly controlled by the standards this means that producers pay more attention to the design and production processes Another important point is that consumers require more effi cient and well designed products for improving their quality of life As with other household appliances the demand for high performance split air conditioners SACs has increased rapidly Adequateairsupplyandheattransferperformancetoensurethermal comfort together with high energy effi ciency are important design parameters Indoor and outdoor SAC units need to be coherent however indoorunitsrequiremoreattentionbecauseofthenecessity forhighperformanceonasmallscale Thus thedesignsofthefi nand tubeheatexchanger FTHE andthecasingofthecross fl owfan CFF inside the SAC indoor unit are very important FTHEs are used for air conditioning systems because of their compact design and reliable heat transfer performance Many researchers have improved the performance of FTHEs and showed that the air side heat transfer is more important than the fl uid side because of the higher thermal resistance as mentioned by Wang 1 Some of the experimental and numerical studies on FTHE are by Wang et al 2 Yan et al 3 Mendez et al 4 Kim and Kim 5 and Pu et al 6 in which the effect of fi n material properties fi n pitch distance between pipes and their arrangement blasting agents the shape and orientation of fi ns air and refrigerant properties on the air side heat transfer and fl ow conditions were considered A small 3D representative part of the FTHE was modelled for the numerical studies while fl ow visualisation and wind tunnel performance testswereperformedfortheexperimentalstudiesbythe researchers Corresponding author Tel 90 232 3019253 fax 90 232 3019204 E mail address haktan karadeniz deu edu tr Z H Karadeniz Contents lists available at SciVerse ScienceDirect Applied Thermal Engineering journal homepage 1359 4311 e see front matter 2012 Elsevier Ltd All rights reserved http dx doi org 10 1016 j applthermaleng 2012 08 051 Applied Thermal Engineering 51 2013 262e272 A number of studies focused on the FTHEs inside air conditioner systems Yun and Lee 7 experimentally studied the effect of the shape of interrupted surfaces on the performance of the FTHEs in a wind tunnel through scale up and prototype experiments and an optimal fi n shape for home air conditioners was determined from their existing and newly designed models Another performance test of heat exchangers were performed in a wind tunnel by Tuztas and Egrican 8 using a similar technique to have a database of the heat exchangers with different geometries Taler 9 presented a numerical method fordetermining heat transfer coeffi cients both on the liquid and air sides for different types of FTHEs with extended surfaces The developed analytical mathematical model for determining the characteristics were offered to be used in new FTHE designs of heating ventilating and air conditioning systems and refrigeration equipment Three dimensional simulations were accomplished to compare both air side and air water side models in Borrajo Palaez et al 10 Xie et al 11 simulated the air side fl ow of FTHEs with a large number of tube rows and large tube diameters to obtain correlations for the heat transfer and fl ow characteristics To improve SAC performance both FTHE and CFF play an important role CFFs have a wide range of usage in air conditioners and ventilators and their casings affect the air fl owand thereby the heat transfer performance of the heat exchanger Researchers have always paid attention to the complex fl ow fi eld formed by the CFF and most experimental and numerical studies have been con ducted to determine the fl ow characteristics for simplifi ed CFF housing systems 12e16 Dang and Bushnell 17 showed that other than the geometry of the housing and the orientation of the CFF important points include theposition and the magnitude of the eccentric vortex formed by the rotation of the fan Experimental investigations and numerical simulations of the fl ow fi eld pattern within the impeller the eccentricity and the strength of the vortex were conducted by Toffolo et al 18 19 Gabi and Klemm 20 experimentally and numerically studied the aerodynamics of the fl ow and vortex regions inside the CFF and showed that CFD is a useful tool for design purposes There are also some recent studies which modelled the whole assembly of a SAC indoor unit Shih et al 21 used a 2D geometry for numerical simulations of a CFF in a conventional SAC indoor unit The CFD method is also successful for modelling complex geometries and similarity laws for CFFs were developed according to the simulations results A similar numerical study for CFF for a SAC indoor unit in refrigerant operating conditions to determine internal fl ow characteristics was conducted by Xue et al 22 The two gas phase dry and humid air fl ow conditions effect on CFF performance was investigated by comparing the experimental results Moukalled et al 23 constructed a 3D numerical model for the simultaneous prediction of velocity temperature and humidity distributions of air fl owing inside a rooftop air conditioning unit Through the comprehensive modellingof heatexchangers and fans which increased the grid size and computational cost the researchers obtained a reliable model and more precise results They predicted sensible and latent cooling capacities for several design conditions As mentioned in the literature the CFD method is very successful for revealing and understanding complex fl ow charac teristics The SAC indoor units components have been mostly modelled as spare parts however recent studies showed that even a whole assembly can be modelled numerically using a large amount of computational cost The 3D modelling of a device is essential for determining the air side fl ow and heat transfer together but a small representative part is adequate for example in the case of the numerical modelling of FTHEs Therefore in this study a representative thin section model was introduced for modelling SAC indoor units and heat transfer and fl uid fl ow anal ysis was made to determine the temperature and velocity distri butions In addition the ability of the numerical method was examined by comparing the results with the heat transfer capacity experimentsandStereoParticleImageVelocimetry SPIV measurements at the outlet section of the device for comparing the velocity distribution 2 Numerical study The internal construction of a conventional SAC indoor unit consists of four parts CFF rear wall vortex wall and the FTHE Both the geometry of the heat exchanger and also the form of the CFF and its cavity are important parameters for the performance of air conditioners More heat transfer and less pressure drop are the targets for a better design for the FTHE whereas supplying enough fl ow rate together with less local pressure fl uctuations less noise and vibrations at the outlet part is the main goal for the CFFs These parameters are mostly valid to defi ne a criterion for the perfor mance of SAC indoor units The parameters that affect the perfor mance of the SAC indoor units are beyond the scope of this study The fl ow inside the SAC indoor unit can be modelled two dimensionally by assuming that the heat exchanger is a porous media This assumption has the advantage of decreasing the number of nodes in the fi nite volume mesh but special attention should be paid on the pressure drop characteristics of the heat exchanger Shih et al 21 implemented experimental results of the pressure drop characteristics of FTHE on the CFD analysis to over come this problem However this could not be the case for every model because of the complex geometry of the FTHE inside SAC indoor unit In addition the inner geometry of the device and the fl ow characteristics of the CFF strongly affect the fl ow passing through the different sections of the FTHE which could not be acquired by the experiments performed on the FTHE s outside the device Moreover porous media assumption is not enough for determining the heat transfer characteristics of the device because heat transfer occurs only inside the heat exchanger Without modelling the gap between the fi ns of the FTHE variations of the fl ow between the fi ns law velocity fl ow region at the wake of the cylindrical pipes etc cannot be considered in the numerical model Therefore the homogeneous porous media assumption is not suitable for revealing the local effects of fl owon heat transfer There are almost no studies on modelling heat transfer inside a SAC indoor unit but there are many three dimensional numerical studies on modelling the FTHE alone and the CFD method is an effective tool to explore air side heat transfer and its fl ow charac teristic Therefore a three dimensional CFD method was used to solve the fl ow and heat transfer inside the SAC indoor unit The whole model of the device was taken from the producer and some modifi cations were made on the model for simplifi cation Fig 1 As performed in the common case of modelling FTHEs three dimensionally which is represented by a thin section model the Nomenclature Cp specifi c heat capacity J kg K kthermal conductivity W m K m mass fl ow rate kg s Qssensible heat capacity W Tf fi lm temperature K DTtemperature difference K rdensity kg m3 mdynamic viscosity Ns m2 D Kumlutas et al Applied Thermal Engineering 51 2013 262e272263 repetitive section of the SAC assembly was determined Fig 2a In this geometry the effects of the drain tray dust fi lter outer shell of the device and some small assembly parts were not modelled because of the need for simplifi cation This prediction was made according to the negligible effects of these elements and on the basis of having a high mesh quality However the rear wall and vortex wall which are the most important parts for the CFF performance remained the same Fig 2b The model of the SAC indoor unit was embedded into a semi circular area which represents the air at the outside of the device Fig 2a Although the nodes in the numerical model increases by adding the outside air into the model the effects of the inlet and outlet sections on the fl ow can be modelled more realistically with this method The diameter of the outside air regionwas determined to be twenty times the diameter of the fan according to information from Shih et al 21 The thickness of the 3D model was determined by the fi n thickness and the fi n pitch dimensions The resulting modelinvolvedsixgeometricalregions twohalf thickness aluminium fi ns air between the fi ns air inside the section the air between the cross fl ow fan s impellers which is the rotating region of the model the air inside the fan and the air at the outside of the device Fig 2b Another important step for numerically modelling the air conditioner is generating a proper numerical grid Different types and sizes of grids were generated and prismatic and extruded tetrahedral meshes were applied to the model The investigation of the effect of extruded layers numbers difference through the thickness and the number of surface elements were also taken into consideration The thicknesses of the extruded layers between the fi ns of the FTHE were smaller near the fi ns and increased from the fi n to the air To more accurately determine the fl ow and heat transfer smaller element sizes were chosen for the air around the fan impellers inlet outlet and in particular the air around the fi ns The details of the numerical grid which involved 2482876 pris matic elements and 1614599 nodes are given in Fig 3 To reduce the computational load of the solver process only the fi ns were modelled as solid parts because of their importance on heat transfer performance The other solid domains such as the shell of the section and the fan s impellers were modelled as adiabatic walls assuming that these regions heat transfer effects were not important for the numerical model although their effects on fl ow conditions were considerable Boundary conditions were determined according to test room data defi ned in the TS EN 14511 2007 and ISO 5151 24 standards which are the main industrial standards for determining the capacity of the split air conditioners Analyses were made for standard cooling conditions The inlet and outlet boundary conditions of the numerical model were given as the 300 K outside temperature and 0 Pa relative pressure The proper interface models between the fl uid and solid domains or the rotating and stationary domains were also specifi ed in the programme The side faces were assumed to be symmetric and the fan region was defi ned as a rotating frame to drive the CFF fl ow To simulate the characteristics of turbulent fl ow the standard Fig 2 3D numerical model of SAC indoor unit a regions of 3D model b ambient air and important fan parts Fig 1 Air conditioner indoor unit assembly D Kumlutas et al Applied Thermal Engineering 51 2013 262e272264 ke3turbulence model was used because of its wide capability This model has been implemented in most general purpose CFD codes and is considered to be the industry standard model It is stable numerically robust and has a well established regime of predictive capability The air s thermophysical properties Table 1 such as density r specifi c heat Cp dynamic viscosity m and thermal conduc tivity k were given at the fi lm temperature Tf 290 K The cooling effect of R22 Freon from the evaporator s pipes was applied to the fi ns and air s pipe side surfaces as a 280 K constant temperature The iterations of the three dimensional steady state analyses were continued until the residuals reached 10 4and the domain imbalances dropped below 0 0001 3 Results and discussion 3 1 Fluid fl ow The results of the numerical simulation were visualised for the boundary conditions and CFF velocity 1200 rpm that were indi cated in the TS EN 14511 2007 and ISO 5151 standards The fl ow conditions are given as streamlines pressure distributions and velocity vector plots and the temperature distribution is given for visualising heat transfer characteristics Thestreamlinesofthewholemodelanddetailsofthefl owinside the device are given in Fig 4 Air enters the device through the suctiongridwhererecirculationinareasshownbyAandBoccurson the front and back sides because of the closed parts of the inlet section Flowing through the fi ns of the heat exchanger which straightens the fl ow air enters the cascades of the aerofoils of the CFF Another recirculation area is shown by C in Fig 4 for which the existence strongly depends on the shape and position of the rear wall and the tongue situated on the upper part of the rear wall In addition neighbouringtherecirculatingareaC avortexshownbyD Fig 3 Mesh details of the numerical model a the numerical grid b mesh detail of the SAC indoor unit c extruded layers through the thickness and d detailed mesh view near the fan blades Table 1 Thermo physical properties of air Tf K r kg m3 Cp J kg K m Ns m2 k W m K 2901 2081006 81 796 10 50 0255 D Kumlutas et al Applied Thermal Engineering 51 2013 262e272265 occurs which is again driven by the form of the rear wall and the tongue Passing through the interior of the impeller the fl ow is directed backwards and squeezed between the rear wall and the eccentric vortex indicated by E in Fig 4 Vortexes D and E are char acteristic for the CFF fl ow although vortex D may not exist depending on the rear wall properties The eccentric vortex is the main source of the CFF fl ow because of the low pressure region indicated by E in Fig 5 which provides the air suction from the upstreamandbehaveslikeasealagainstthereversingfl owoftheair from the downstream The shape and position of the