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Experimental analysis of the thermal entrainment factor of air curtains in vertical open display cabinets for different ambient air conditions Pedro Dinis Gaspar a L C Carrilho Gon alvesa R A Pitarmab aUniversity of Beira Interior Electromechanical Engineering Department Rua Fonte do Lameiro e Edif cio 1 das Engenharias 6201 001 Covilh Portugal bPolytechnic Institute of Guarda High School of Technology and Management Mechanical Engineering Department Avenida Dr Francisco S Carneiro n 50 6300 559 Guarda Portugal a r t i c l e i n f o Article history Received 25 November 2009 Accepted 17 November 2010 Available online 24 November 2010 Keywords Thermal entrainment factor Refrigerated display cabinet Experimental data a b s t r a c t The vertical open refrigerated display cabinets suffer alterations of their thermal performance and energy effi ciency due to variations of ambient air conditions The air curtain provides an aerothermodynamics insulation effect that can be evaluated by the thermal entrainment factor calculation as an engineering approximation or by the calculus of all sensible and latent thermal loads This study presents the vari ation of heat transfer rate and thermal entrainment factor obtained through experimental tests carried out for different ambient air conditions varying air temperature relative humidity velocity and its direction relatively to the display cabinet frontal opening The thermal entrainment factor are analysed and compared with the total sensible and latent heats results for the experimental tests From an engineering point of view it is concluded that thermal entrainment factor cannot be used indiscrim inately although its use is suitable to design better cabinet under the same climate class condition 2010 Elsevier Ltd All rights reserved 1 Introduction A large part of the refrigeration equipments installed in super markets are vertical open refrigerated display cabinets with a frontal opening to the environment thus allows the consumer to see and handle the product that intends to acquire without inconvenience The absence of this physical restriction for the consumer is accomplished by an air curtain that establishes an aerothermodynamics barrier between the conservation space where the food products are located and the surroundings However this merchandising solution contributes to the largest part of electrical energy consumption among refrigeration equip ments installed in a typical supermarket 1 The energy spent during the commercialisation of refrigerated food products is about 50 of the supermarket s total energy consumption 2 The world s demand for commercial refrigeration equipment is projected to rise 4 6 per year until 2012 value referred to 2008 being the sales market headed by reach in and walk in coolers freezers and display cabinets 3 The evaluation and analysis of the thermal performance of display cabinets in order to develop strategies and methods that promote its energy effi ciency is necessary 4 Consider the sketch of an open refrigerated display cabinet ORDC as shown in Fig 1 The air was drawn by the fans located in front of the evaporator The air passing through the evaporator is cooled below the conservation temperature of the perishable products exposed in the shelves of the equipment This air mass fl ow rate is conducted to the rear duct where part of it is dis charged inside the conservation space at low velocity across the back panel perforation PBP The other part of this air mass fl ow rate will supply the air curtain which develops vertically between discharge DAG and return RAG air grilles The aircurtain reduces the external infi ltration of air from outside at higher dry temper ature and specifi c humidity The effectiveness of this aero thermodynamics barrier varies due to thermal and mass diffusive effects that affect the thermal entrainment These effects depend on fl ow instabilities and boundary effects among others leading to a minor conservation quality of food products and greater energy consumption and costs since the ambient air infi ltration load is around 72 of the cooling load of ORDC 5 Several methods experimental and numerical are adopted by many researchers to evaluate the thermal performance of ORDC and particularly of air curtain SomeresearcherscarriedoutComputationalFluid Dynamics CFD parametric studies based on two and three dimensional models Cortella et al 6 and Navaz et al 7 evaluated Corresponding author Tel 351 275 329 759 fax 351 275 329 972 E mail addresses dinis ubi pt P D Gaspar carrilho ubi pt L C Carrilho Gon alves rpitarma ipg pt R A Pitarma Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage 1359 4311 e see front matter 2010 Elsevier Ltd All rights reserved doi 10 1016 j applthermaleng 2010 11 020 Applied Thermal Engineering 31 2011 961e969 the infl uence of DAG velocity in thermal performance quantifying the air infi ltration through the frontal opening Axell and Fahl n 8 developed a CFD parametric study to evaluate the infl uence on the thermal performance of air curtain height width ratio and inlet velocity Navaz et al 9 calculated the amount of entrained air as a function of Reynolds number based on jet width and velocity and inlet turbulence intensity to evaluate the optimum operating conditions Foster et al 10 developed 3D CFD models to analyse the effect of changing the size and position of the evaporator coil the width and angle of DAG and inserting baffl e plates into the upper duct D Agaro et al 11 carried out 2D and 3D CFD para metric studies to evaluate the infl uence of longitudinal ambient air movement display cabinet length and air curtain temperature on the extremity effects and how it refl ects in the ORDC performance Chen 12 developed a CFD parametric study of length width ratio and discharge angle of air curtains height depth ratio of the cavity and dimension and position of the inside shelveson thermal barrier performance of air curtains Ge and Tassou 13 developed corre lations for the heat transfer across air curtain with reasonable agreement with experimental data at steadystate conditions based on results obtained from a fi nite difference model Other research works were just experimental such as the study developed by Chen and Yuan 14 to evaluate the ambient air temperature and relative humidity indoor airfl ow DAG velocity PBPairfl ow and night coversapplication onthe performance of an ORDC Gray et al 15 also conducted an experimental study to evaluate the effect of the perforation pattern of PBP on the distri bution of airfl ow Among the experimental techniques used by the researchersarethermocouplethermometry hot wire fi lm anemometry laser Doppler anemometry digital particle image velocimetry hygrometry tracer gases and infrared thermography Although these experimental techniques are reliable and provide a high degree of confi dence in the CFD modelling approaches its use involves a high cost and results are dependent on the ORDC geometry DAG parameters and ambient air conditions Other method that can be used to evaluate the thermal perfor mance of an ORDC considering the thermal barrier provided by the air curtain is the thermal entrainment factor TEF calculation as proposed by 9 11 14 without PBPairfl oworas proposed byYu et al 16 to consider the PBP airfl ow on calculation This paper presents the analysis of experimental test results to evaluate the total sensible and latent heat transfers of an ORDC and the air curtain TEF for different environmental conditions The main objective is to evaluate the infl uence of environmental conditions airtemperature relativehumidityandvelocity magnitude and direction on the thermal performance of the ORDC Also it tries to clarify that TEF as it is defi ned cannot be used straightforward to assess the thermal entrainment for all air curtain applications 2 Experimental testing 2 1 Experimental apparatus The ORDC experimentally tested is a self contained system in which the condensing unit and controls are built into the cabinet structure beneath the cabinet taking up the entire lower part Its dimensions are 1900 796 1911 mm L W H It has four shelves and a well tray being its frontal opening height Hc 1209 mm see Fig 1 The evaporator is housed under the well tray and integrated with refrigerant feed and return lines It is an assembly composed by 18 tubes 16 mm of external diameter and 135 fi ns m Its dimensions are 1600 360 130 mm without tube curves stretching over the entire length of the cabinet having an Nomenclature a1to a5Constants adopted in the correlation model bAir curtain width m Cp Specifi c heat J kg 1K 1 gGravitational acceleration m s 2 hEnthalpy kJ kg 1 hfgvaporation enthalpy kJ kg 1 HHeight m HcAir curtain height m IElectrical current intensity A LLength m m Mass fl ow rate kg s 1 QHeat transfer rate kW RCentral distance between DAG and RAG m ReReynolds number RiRichardson number TTemperature K vAverage velocity m s 1 WWidth m x y zSpatial coordinates along length width height m X0Thermal entrainment factor for air curtain without PBP airfl ow XPBPThermal entrainment factor for perforated back panel airfl ow Superscripts and subscripts ambAmbient consConservation zone DAGDischarge air grille evapEvaporator inInput Upstream latLatent mMixture outOutput Downstream PBPPerforated back panel prodFood products RAGReturn air grille senSensible surfSurface Greek symbols b Perforated back panel airfl ow ratio e rDensity kg m 3 p1 p2Dimensionless geometrical parameters e 4Relative humidity qDirection of the ambient air velocity mDynamic viscosity kg m 1s 1 uAbsolute humidity kgvkga 1 Abbreviation 2DTwo dimensional 3DThee dimensional CFDComputational Fluid Dynamics DAGDischarge air grille ETExperimental Test ORDCOpen refrigerated display cabinet PBPPerforated back panel RAGReturn air grille TEFThermal Entrainment Factor P D Gaspar et al Applied Thermal Engineering 31 2011 961e969962 end clearance of about 100 mm aside It is cooled by R404A refrigerant directly expanding in multiple passes Temperature regulation and defrosting are digitally conducted by an electronic thermostat being the cooling system fan assisted four forward fans that blow air through the coil The refrigeration cycle is controlled by the cold air temperature variation This temperature is monitored by a limit positive temperature coeffi cient PTC thermistor probe located at half height of the rear duct The cut out temperature of the refrigeration system is set to 273 K The thermostatic differential is set to 276 K while the minimum and maximum values of setpoint temperature are set to 273 K and 277 K respectively The evaporator is set to defrost at specifi c intervals 180 min The controller provides direct compressor control through high power relays bystopping the coolant fl owand starting the operation of an electrical resistance Defrost period ends by time out about 30 min maximum or by reaching limit temperature setpoint of 285 K This temperature measured by a defrost limit PTC thermistor probe located on the evaporator s surface Additionally there are fl ow control devices namely a solenoid valve and an externally equalized thermostatic expan sion valve to modulate the cooling capacity delivered at the coil The air temperature peaks occur each defrost cycle and infl u ence the food temperature The temperature increases during the refrigeration cycle evaporator operation due to the cooling capacity reduction with frost accumulation on the evaporator Frost on the coil s surface reduces velocity and increases temperature since the frost acts an insulator for heat transfer During defrost cycle the refrigeration system does not work and the electrical resistance operates which refl ects into a large heat load that increases temperature Furthermore the thermal entrainment is much higher during this period since much more heat comes from outside warm environment The frost accumulation was accounted by the condensate volume removed by the defrost system each cycle Depending on the test conditions the cooling load on the coil varies This is due to RAG temperature and humidity values but also to the frost formation pattern on the coil surface The latter completely alters the patterns of fl ow and air temperature leaving the cooling coil The experimental tests ET were performed in a climate chamber Aralab e Fitoclima 650000 EDTU A Intab PC Logger 3100 data acquisition system with the test probes described in Table 1 and placed inside the cabinet as shown in Fig 1 was used to measure the physical properties such as temperature relative humidity and velocity A probe positioning system is used to evaluate the 3D effects of thermal entrainment on air curtain and properties variations along length and height of the conservation space 17 The probe posi tioning system was settled in each shelf of the equipment and it Fig 1 Confi guration of ORDC and measuring probes locations Table 1 Description of test probes and its location see Fig 1 Loc TypeAccuracyPropertyRef Location 0e4K type thermocouple 0 5 KTemperatureTconsConservation space Hygrometer odd measuring locations 3 Relative humidity4cons 5K type thermocouple 0 5 KTemperatureTDAGDAG Hot wire anemometer 0 1 m s 1 10 VelocityvDAG Hygrometer 3 Relative humidity4DAG 6K type thermocouple 0 5 KTemperatureTRAGRAG Hot wire anemometer 0 1 m s 1 10 VelocityvRAG Hygrometer 3 Relative humidity4RAG 7K type thermocouple 0 5 KTemperatureTevap outEvaporator outleta 8K type thermocouple contact 0 5 KTemp Surface Tevap inEvaporator inleta 9K type thermocouple contact 0 5 KTemp Surface TsurfInterior surfaces 10Ammeter Clamp on 0 02 A 2 Electric currentIPower source a The surface temperature of the air sidewall pipe is measured at the evaporator inlet while at the outlet is measured the air temperature P D Gaspar et al Applied Thermal Engineering 31 2011 961e969963 measured air temperature relative humidity and velocity for three positions across the air curtain width and eight vertical cross sections along the equipment s length The positioning system moved the test probes in 240 mm increments for the 1800 mm length of shelves taking 1 min to move between positions to reduce fl ow perturbation Also the properties values were acquire 1 min after reaching each position to ensure fl ow stabilization The experimental results obtained with the point measuring technique along the air curtain and conservation space lengths show a similar behaviour to the experimental results obtained by 14 15 18 It was found that the average variation in air temperaturewas 0 4 K and in air relative humidity was 4 5 which are not particularly signifi cant Based in these results the probes were distributed in the mid plane of the equipment s length 2 2 Testing procedure The experimental testing follows the procedure defi ned by EN ISO Standard 23953 19 specifi cally in what concerns the M package temperature class M1 272 15K Tprod 278 15 K The temperatures were measured according to EN ISO 23953 4 and 23953 5 The standard specifi es testroomclimateclasses imposing air temperature and relative humidity as well as ambient air movement parallel qamb 0 to the frontal opening plane of the ORDC with a magnitude of vamb 0 2 m s 1 The experimental tests ET were performed for some of these test room climate classes Were also performed experimental tests with other environmental conditions air temperature Tamb air relative humidity 4amb air velocity magnitude vamb and direction qamb relatively to the frontal opening of the ORDC The environmental conditions of the experimental tests are described in Table 2 Before the tests were started the cabinet was switched on and run during 24 h at the specifi ed climate class room air tempera ture relativehumidityandvelocity withthenight covers extended Then the cabinet was fi lled uptothe load limits with test packages and M packages After this running in period it was verifi ed that stable operating conditions were reached as the air temperature curve agreed within 0 5 K presenting no trend away from the mean temperature This condition was verifi ed for all tests except for ET7 to ET9 that consider direction and magnitude of the room air velocity different from standard The night cover was removed and then the 24 h test period was initiated Only the experimental data collected during the last 12 h of 24 h tests initiated after the M packages steady state temperatures were accomplished was considered Table 2 contains the average mean values of the parameters measured during this period 3 Heat transfer rate and thermal entrainment factor The heat transfer rate within an ORDC is given by the sum of the following individual sensible and latent thermal loads organized in a decreasing level of relevance infi ltration radiation internal sources as anti sweat heaters defrost system lighting and fan motors transmission or conduction and products subdivided into a pull down load due to their initial higher temperature before delivering the products into the cabinet and their temper ature rise after a defrost cycle and a products respiration load for fresh fruits and vegetables as exposed by 2 However Chen and Yuan 14 present simplifi ed equations to calculate the total heat transfer rate Eq 1 QT rvA DAG hRAG hDAG 1 This cooling load can be divided into its sensible Eq 2 and latent Eq 3 components The sensible portion refers to temperature driven heat into the ORDC whereas the latent portion refers to heat content of moisture entrained through air curtain and of product respiration QTsen rvA DAGCp m TRAG TDAG 2 QTlat rvA DAGhfg uRAG uDAG 3 Besides the heat transfer rate calculation other method that can be used to evaluate the thermal performance of an ORDC consid ering the thermal barrier provided by the air curtain is the thermal entrainment factor TEF calculation as proposed by 9 11 14 defi ned in Eq 4 The thermal entrainment factor will be zero if the air curtain has n