[机械模具数控自动化专业毕业设计外文文献及翻译]【期刊】一个具有新的传热特性的螺旋翅片管换热器-外文文献

ORIGINALHeat transfer characteristics of a new helically coiled crimpedspiral finned tube heat exchangerKwanchanok Srisawad Somchai WongwisesReceived: 26 March 2007 / Accepted: 27 August 2008 / Published online: 27 September 2008C211 Springer-Verlag 2008Abstract In the present study, the heat transfer charac-teristics in dry surface conditions of a new type of heatexchanger, namely a helically coiled finned tube heatexchanger, is experimentally investigated. The test section,which is a helically coiled fined tube heat exchanger,consists of a shell and a helical coil unit. The helical coilunit consists of four concentric helically coiled tubes ofdifferent diameters. Each tube is constructed by bendingstraight copper tube into a helical coil. Aluminium crimpedspiral fins with thickness of 0.5 mm and outer diameter of28.25 mm are placed around the tube. The edge of fin at theinner diameter is corrugated. Ambient air is used as aworking fluid in the shell side while hot water is used forthe tube-side. The test runs are done at air mass flow ratesranging between 0.04 and 0.13 kg/s. The water mass flowrates are between 0.2 and 0.4 kg/s. The water temperaturesare between 40 and 50C176C. The effects of the inlet condi-tions of both working fluids flowing through the heatexchanger on the heat transfer coefficients are discussed.The air-side heat transfer coefficient presented in term ofthe Colburn J factor is proportional to inlet-water temper-ature and water mass flow rate. The heat exchangereffectiveness tends to increase with increasing water massflow rate and also slightly increases with increasing inletwater temperature.List of symbolsA area (m2)Cpspecific heat kJ/(kg K)d tube diameter (m)D diameter of the curvature (m)Dcdiameter of the coil (m)f friction factorF correction factorG mass flux kg/(m2s)h heat transfer coefficient W/(m2K)i enthalpy (kJ/kg)Iomodified Bessel function solution of the first kind,order 0I1modified Bessel function solution of the first kind,order 1j Colburn j factorKomodified Bessel function solution of the second kind,order 0K1modified Bessel function solution of the second kind,order 1k thermal conductivity W/(m K)L tube length (m)m mass flow rate (kg/s)Nu Nusselt numberP pitch of the helical coil (m)Pr Prandtl numberQ heat transfer rate (W)r tube radius (m)Re Reynolds numberT temperature (C176C)U overall heat transfer coefficient W/(m2K)V average velocity (m/s)d thickness (m)g fin effectivenessgooverall surface effectivenessK. Srisawad C1 S. Wongwises (&)Fluid Mechanics, Thermal Engineering and Multiphase FlowResearch Laboratory (FUTURE), Department of MechanicalEngineering, King Mongkuts University of TechnologyThonburi, Bangmod, Bangkok 10140, Thailande-mail: somchai.wonkmutt.ac.th123Heat Mass Transfer (2009) 45:381391DOI 10.1007/s00231-008-0441-1l dynamic viscosity (Pa s)q density (kg/m3)e effectivenessSubscriptsa airave averageb basec helical coilffini insidein inletLM log mean temperature differencemax maximummin minimumo outsideout outletwall wall surfacet tubetot totalw water1 IntroductionDue to the high heat transfer coefficient and smaller spacerequirement compared with straight tubes, curved tubes arethe most widely used tubes in several heat transfer appli-cations. A helically coiled tube is one of the well-knowntypes of curved-tubes which has been used in a widevariety of industries. The analysis of helically coiled tubehas been studied both analytically and experimentally.Dravid et al. 1 numerically investigated the effect ofsecondary flow on laminar flow heat transfer in helicallycoiled tubes both in the fully developed region and in thethermal entrance region. The predicted results were vali-dated against those obtained from experiments in the rangein which they overlapped. Patankar et al. 2 discussed theeffect of the Dean number on friction factor and heattransfer in the developing and fully developed regions ofhelically coiled pipes. Good agreements were obtainedfrom comparisons between the developing and fullydeveloped velocity profiles, the wall temperature for thecase of axially uniform heat flux with an isothermalperiphery obtained from calculation and those obtainedfrom experiments. In the model mentioned above, theeffects of the torsion and the Prandtl number were nottaken into account.Yang and Ebadian 3 solved the ke model to analyzethe fully developed turbulent convective heat transfer in acircular cross-section helicoidal pipe with finite pitch. Theresults showed that as the pitch of the coil increased,the temperature distribution in the cross-section wasasymmetrical. In the case of laminar flow, an increase inthe Prandtl number would diminish the effect of torsion onthe heat transfer. In addition, it was found that the pitcheffect would be augmented as the flow rate increased.Later, Lin and Ebadian 4 applied the standard ke modelto investigate three-dimensional turbulent developingconvective heat transfer in helical pipes with finite pitches.The effects of pitch, curvature ratio and Reynolds numberon the developments of effective thermal conductivity andtemperature fields, and local and average Nusselt numberswere discussed. The results obtained from the model werein good agreement with the existing experimental data. Xinet al. 5 experimentally investigated the single-phase andtwo-phase flow pressure drop in annular helicoidal pipes.Guo et al. 6 conducted the experiment to study theoscillation of steam-water two-phase flow in a uniformlyheated helically coiled tube. The study showed that thegravity has a small effect on the oscillating boundaries.They also proposed new methods to eliminate the pressuredrop oscillation. Ju et al. 7 investigated the performanceof small bending radius helical-coil pipe. The formulas forthe Reynolds number of single-phase flow structure tran-sition, and single-phase and two-phase flow friction factorwere obtained. Ali 8 proposed the pressure drop corre-lations for fluid flows through regular helical coil tubes.Generalized pressure drop correlations were developed interms of the Euler number, Reynolds number and thegeometrical group. Zhao et al. 9 studied the pressure dropand boiling heat transfer characteristics of steam-watertwo-phase flow in a small horizontal helically coiled tubingonce-through steam generator. The study showed that bothnucleation mechanism and the connection mechanism wereimportant for forced convective boiling heat transfer in asmall helically coiled tube over the full range of qualities.Kumar and Nigam 10 introduced a new device based onthe flow inversion by changing the centrifugal forcedirection in helically coiled tubes. The flow and tempera-ture fields in the device was characterized usingcomputational fluid dynamics software. The resultsobtained from the present study could be used to simulatethe developing flows in curved tubes.Rennie and Raghavan 11 performed an experimentalstudy of a double-pipe helical heat exchanger. Both parallelflow and counter flow configuration were investigated.Nusselt number in the inner tube was compared to the dataobtained in literature, and Nusselt number in annulus wascompared to the numerical results.Cioncolini and Santini 12 carried out an experiment tostudy the transition from laminar to turbulent flow inhelically coiled pipes. Twelve coils, with different ratios ofcoil diameter to tube diameter, were investigated. Theinteraction between turbulence emergence and coil curva-ture was analyzed from the obtained friction factor profiles.382 Heat Mass Transfer (2009) 45:381391123Cui et al. 13 proposed a heat transfer correlation of R134aduring flow boiling in a new kind of micro-finned helicallycoiled tube.Wongwises and Polsongkram 14 investigated the two-phase heat transfer coefficient and pressure drop of HFC-134a during evaporation inside a smooth helically coiledconcentric tube-in-tube heat exchanger. They also used thesame experimental set up to study condensation heattransfer and pressure drop of HFC134a in a helically coiledconcentric tube in tube heat exchanger 15. The resultsfrom the present experiments were compared with thoseobtained from the straight tube reported in the literature.New correlations for the evaporation and condensation heattransfer coefficient and pressure drop were proposed forpractical application.Although a number of papers are currently available onthe helically coiled tube, it can be noted that the theoreticaland experimental investigations found in literature descri-bed above focused on the study of the heat transfer andflow characteristics in a single helicoidal tube or in aconcentric double tube helical coil, no attention was paidon the heat transfer and flow characteristics of the heatexchanger fabricated from a shell and helically coiledtubes. In the present study, the main concern is to experi-mentally study the heat transfer characteristics andperformance of a new type of heat exchanger namelyhelically coiled, finned-tube heat exchanger under dry-surface conditions. The relationship between various rele-vant parameters are investigated. The experimental results,which have never been seen before, are presented.2 Experimental apparatus and methodFigure 1 shows a schematic diagram of the experimentalapparatus. The main components of the system consist of atest section, hot water loop, air loop and data acquisitionsystem. Water and air are used as working fluids. The testsection is a helically coiled finned tube heat exchanger. Inaddition to the loop component, a full set of instruments formeasuring and control of temperature and flow rate of allfluids is installed at all important points in the circuit.An open-type wind tunnel is used to conduct the air flowthrough the heat exchanger. The tunnel is fabricated fromzinc, with an inner diameter of 300 mm and a length of12 m. The duct wall is insulated with a 6.4 mm thickAeroflex standard sheet. The entering and exiting airtemperature of the heat exchanger are measured by type Tcopper constantan thermocouples extending inside the airchannel in which the air flows. The 1 mm diameter ther-mocouple probes are located at different four positions atthe same cross section, 60 cm upstream of the heatexchanger inlet and also four positions at 50 cm down-stream of the exit of the heat exchanger.The closed-loop of hot water consists of a 0.3 m3storagetank, an electric heater controlled by adjusting the voltage, astirrer, and a cooling coil immerged inside a storage tank.R22 is used as the refrigerant for chilling the water.Air is discharged by a centrifugal blower into thechannel and is passed through a straightener, guide vane,test section, and then discharged to the atmosphere. Thepurpose of straightener is to avoid the distortion of the airTSTSTSTwTwR22 LoopHot Air LoopChilled Water LoopBypass ValveExpansion ValveEvaporatorRotameterWater TankWater Heater = = Humidity TransmitterThermocoupleOrificeTemp.ControllerTCompressorCondenserWater PumpBlowerStraightener Guide VaneData LoggerAndComputerInverterGuideRHRHHeaterVaneT.S.T.S . = Test SectionTw,in TRHTw,outTwTTTTTTFig. 1 Schematic diagramof experimental apparatusHeat Mass Transfer (2009) 45:381391 383123velocity profile. The speed of the centrifugal blower iscontrolled by the inverter. Air velocity is determined fromthe flow rate obtained from an orifice meter. After thetemperature of the water is adjusted to achieve the desiredlevel, the hot water is pumped out of the storage tank and ispassed through a filter, a flow meter, a test section, and thenreturned to the storage tank. The bypass is used for passingthe excess water back to the water tank for the low waterflow rate experiment. The flow rate of the water is mea-sured by a flow meter with a range of 010 GPM.As shown in Fig. 2, the heat exchanger consists of asteel shell and a helically coiled finned tube unit. Thehelical-coil unit consists of four coils of helically coiledfinned copper tubes. Each tube is constructed by bending a9.4 mm outside diameter straight copper tube into a heli-cal-coil of seven layers. The mean diameters of eachhelical-coil are 115, 205, 285 and 365 mm, respectively.Aluminium crimped fins with thickness of 0.5 mm andouter diameter of 28.25 mm are placed helically around thetube. The edge of fin at the inner diameter is corrugated.The photograph of the helically coiled finned tube unit usedin the present study is shown in Fig. 3. Schematic diagramof fin is also shown in Fig. 4. The water inlet and outlet endof each coil are connected to horizontal manifolds withouter diameter of 28.5 mm. The copper constantan ther-mocouples are installed at the first, fourth and seventhlayers from the uppermost layer of each coil, each with twothermocouples to measure the water temperature and walltemperature. The positions installed the thermocouples areshown in Fig. 2. The water temperature is measured with1 mm diameter probe extending inside the tube in whichthe water flows. Thermocouples are soldered in a smallhole drilled 0.5 mm deep into tube wall surface and fixedwith special glue applied to the outside surface of theCoil 4Layer 1Layer 4Layer 7Coil 3Coil 2 Coil 1Water ouletWater inletLocation ofthermocouplesof Tw and Ts atLayer 1,4 and 7Air outlet Air inletFig. 2 Schematic diagramof the shell and the helicallycoil unitFig. 3 Photograph of the helically coiled finned tube unit384 Heat Mass Transfer (2009) 45:381391123copper tubing. With this method, thermocouples are notbiased by the fluid temperatures. The dimensions of theheat exchanger are listed in Table 1.In the experiment, an overall energy balance was per-formed to estimate the extent of any heat losses or gainsfrom the surrounding. Only the data that satisfy the energybalance conditions; |Qw- Qa|/Qaveis less than 0.05, areused in the analysis. The total heat transfer rate, Qave,isaveraged from the air-side heat transfer rate, Qa, and thewater-side heat transfer rate, Qw. Experiments were con-ducted with various flow rates of air and hot water enteringthe test section. The hot water flow rate was increased insmall increments while the air flow rate, inlet hot watertemperatures were kept constant. The hot water tempera-ture was adjusted to achieve the desired level by usingelectric heaters controlled by temperature controllers.Before any data were recorded, the system was allowed toapproach the steady state. The range of experimentalconditions in this study and uncertainty of the measurementare given in Tables 2 and 3, respectively.3 Data reductionIn order to determine the heat transfer characteristic of theheat exchangers from the data recorded at steady stateconditions during each test run, the Correction factor-Logarithmic-mean temperature difference method isapplied to determine the UA product.The air-side heat transfer rate can be given as8.6 mm 9.4 mm 28.25 mm Section A-A0.5 mm 2 mm AAFig. 4 Schematic diagram of crimped spiral finTable 3 Uncertainty of measurementInstruments Accuracy (%) UncertaintyOrifice meter (air velocity, m/s) 2.0 0.23Rotameter (water mass flow rate, kg/s) 0.2 0.003Thermocouple T-type, 0.1 0.03Data Logger (K) 0.04Humidity transmitter (%RH) 0.5 0.22Table 1 Dimensions of the helically coiled finned tube heatexchangerParameters DimensionsOuter diameter of tube (mm) 9.4Inner diameter of tube (mm) 8.6Diameter of spiral coil 1 (mm) 115.0Diameter of spiral coil 2 (mm) 205.0Diameter of spiral coil 3 (mm) 285.0Diameter of spiral coil 4 (mm) 365.0Helical coil pitch (mm) 16.38Diameter of shell (mm) 430Number of coil turns 7Number of helical coils 4Distance between each coil (mm) 42Length of shell (mm) 355Diameter of hole at air inlet (mm) 298Number of fins per metre 500Fin height (mm) 18.64Fin outside diameter (mm) 28.25Fin pitch (mm) 2Fin thickness (mm) 0.5Table 2 Experimental conditionsVariables RangeInlet-air temperature AmbientInlet-water temperature (K) 313323 (4050C176C)Air mass flow rate (kg/s) 0.040.13Water mass flow rate (kg/s) 0.200.40Heat Mass Transfer (2009) 45:381391 385123Qa maia;outC0 ia;inC0C11where mais the air mass flow rate, ia,inis the enthalpy of theinlet air and ia,outis the enthalpy of the outlet air.The water-side heat transfer rate can be given asQw mwCp;wTw;inC0 Tw;outC0C12where mwis the mass flow rate of water, Cp,wis the specificheat of water, Tw,inand Tw,outare the inlet and outlet-watertemperature, respectively.The total rate of heat transfer used in the calculationis averaged from the air-side and the water-side asfollowsQaveQa Qw23The air-side heat transfer coefficient, ho, of the heatexchanger is determined from the overall heat transferresistance relationship1UA1gohoAolnro=ri2pktLt1hiAi4where the overall heat transfer coefficient can be thendetermined fromUA QaveFDTLM5where the logarithmic-mean temperature difference, DTLM,is determined fromDTLMTw;inC0 Ta;outC0C1C0 Tw;outC0 Ta;inC0C1ln Tw;inC0 Ta;outC0C1C14Tw;outC0 Ta;inC0C16and F is the correction factor.The tube-side heat transfer coefficient, hi, is evaluatedfrom the Nusselt number obtained from the Gnielinskisemi-empirical correlation 16.Nu hidkwfi=8RePr1 12:7fi=8pPr2=3C0 1PrPrwallC18C190:147The Prandtl number, Pr, is evaluated at the mean fluidtemperature and Prwallis evaluated at the wall temperature.The factorPrPrwallC16C170:14was introduced into the originalequation by Schmidt 17 to take into account thetemperature dependence of the physical properties. Thefriction factor for turbulent flow in helically coiled tubes, fi,is given by Mishra and Gupta 18asfi0:3164Re0:25 0:03dDC18C190:5#lwalllC18C190:278where the diameter of the curvature, D, is related to the coildiameter Dcand the pitch, P, of the helical coil byD Dc1 PpDcC18C192#9The dynamic viscosity, l, is of water estimated at themean water temperature and the dynamic viscosity, lwallisof water evaluated at the wall temperature.The Reynold