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Abstract: The paper presents a comprehensive overview of the most recent research works on heat transfer (and pressure drop) with natural refrigerants in minichannels, aimed at proper design of heat transfer equipment. About boiling heat transfer, experimental HT results are mainly fitted by empirical correlations referring to the common mechanisms used in more conventional geometries (nucleate and convective boiling); evaporation heat transfer through thin liquid film around vapor plugs is at times considered. About shear dominated condensation heat transfer, suggested design tools again mostly refer to the extension of the semi-empirical correlations earlier established for conventional geometries. For CO2, heat transfer at supercritical conditions, such as in a gas cooler, is also treated. Finally the concept of the Penalty Factor is applied to shear condensation in minichannels to establish the heat transfer performance of the different working fluids, the superior effectiveness of minitubes over macrotubes, and the optimization of minichannel condensers. [Copyright &y& Elsevier]

A 0.96 mm circular minichannel is used to measure both heat transfer coefficients during condensation and two-phase pressure losses of the refrigerants R32 and R245fa. Test runs have been performed at around 40°C saturation temperature, corresponding to 24.8 bar saturation pressure for R32 and 2.5 bar saturation pressure for R245fa. The pressure drop tests have been performed in adiabatic flow conditions, to measure only the pressure losses due to friction. The heat transfer experimental data are compared against predicting models to provide a guideline for the design of minichannel condensers. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents Performance Evaluation Criteria (PEC) dubbed Penalty Factor (PF) and Total Temperature Penalization (TTP) for condensation inside tubes. This general methodology allows for the ranking of refrigerants for condenser applications based on their heat transfer potentials. Among halocarbons, higher pressure fluids have lower PF and TTP values than medium and lower pressure refrigerants, with ammonia outperforming all refrigerants. These PEC are also used to develop simple theoretical models for the optimization of condenser refrigerant circuitry. As examples, two condenser optimization cases are considered, with the theoretical predictions comparing favorably with simulation results from a detailed heat exchanger model. [Copyright &y& Elsevier]

The design of an innovative shell-and-tube heat pump condenser using 2 mm ID minichannels is presented. This condenser has been designed aiming at the minimization of the charge, which can be required by safety or environmental restrictions. Nevertheless, minichannels represent also a solution to the high-pressure challenge when using carbon dioxide as a refrigerant. The present prototype was realized for the use with propane in the framework of the European project SHERHPA, concerning the development of heat pumps working with natural fluids. Experimental data for heat transfer and pressure drop are reported in the present paper. The measurements have been obtained using refrigerant R22, which displays a temperature versus pressure saturation curve pretty close to the one of propane. The data have also been compared against a computational procedure for shell-and-tube heat exchangers design. The refrigerant charge has been computed by means of different void fraction correlations, showing that the expected charge is less than half the quantity required by a brazed plate condenser giving the same capacity. [ABSTRACT FROM AUTHOR]

Metal foams are cellular structure materials that present open cells, randomly oriented and mostly homogeneous in size and shape. Cellular structure materials, and particularly open-cell metal foams, have been proposed as possible substitutes for traditional finned surfaces in electronics cooling applications. This article presents the heat transfer and pressure drops measurements obtained during air flow through an aluminum foam, which has 40 pores per inch with 0.63 mm pore diameter. The specimen has been inserted in a new open-circuit type wind channel with a rectangular cross-section that has recently been built at the Department of Fisica Tecnica of the University of Padova. The experimental heat transfer coefficients and pressure drops have been collected by varying the air flow rate supplied by the screw compressor that provides a variable volumetric air flow ranging between 0-90 m3h-1 at a constant gauge pressure of 7 bar. The specific heat flux has been simulated by powering with a 25-kWm-2 copper heater attached at the bottom of the aluminum foam base plate. The experimental results are reported in terms of heat transfer coefficients, mean normalized wall temperatures, and pressure drops. [ABSTRACT FROM AUTHOR]

Abstract: The measurement of the condensation heat transfer coefficient inside micro- and minichannels is still somewhat elusive due to the difficult task of getting accurate values of the heat transfer coefficients during the condensation process, particularly when studied within single minichannels. The present paper reports local heat transfer coefficients obtained from the measurement of the local heat flux and the direct measurement of the saturation and wall temperatures during condensation of R134a and R32 within a single circular 0.96mm diameter minichannel. Except for the lowest mass velocity, the test results do not show significant discrepancy from the trends expected for macroscale tubes. [Copyright &y& Elsevier]

Abstract: Microfin tubes are widely used in air cooled and water cooled heat exchangers for heat pump and refrigeration applications during condensation or evaporation of refrigerants. In order to design heat exchangers and to optimize heat transfer surfaces, accurate procedures for computing pressure drops and heat transfer coefficients are necessary. This paper presents a new simple model for the prediction of the heat transfer coefficient to be applied to condensation in horizontal microfin tubes of halogenated and natural refrigerants, pure fluids or nearly azeotropic mixtures. The updated model accounts for refrigerant physical properties, two-phase flow patterns in microfin tubes and geometrical characteristics of the tubes. It is validated against a data bank of 3115 experimental heat transfer coefficients measured in different independent laboratories all over the world including diverse inside tube geometries and different condensing refrigerants among which R22, R134a, R123, R410A and CO2. [Copyright &y& Elsevier]

This paper proposes a new method to determine the condensation heat transfer coefficient of fluids flowing into horizontal smooth tubes with internal diameters D > 3 mm. The method has been drawn up as simply as possible and is ready to use in heat exchanger modeling and design applications. It is also suitable to work very well with old and new fluids used in the refrigeration, air conditioning, and heat pump industries. Particular attention is given to accuracy: it has been tested over a wide updated experimental database and comes from many different independent researchers with reduced experimental uncertainties. In order to obtain an easy structure, only two equations are employed, related respectively to ∆T-independent and to ∆T-dependent fluid flows. All the parameters that influence the condensation heat transfer have been included. A comparison has been conducted against HCFCs, HFCs, HCs, carbon dioxide, ammonia, and water data. Zeotropic mixtures with two and three components are also considered in the comparison by applying the Bell and Ghaly [1] correction to calculate the relative heat transfer penalization. A model has been developed with the idea of getting high accuracy through an easy structure, and the results show a very satisfactory agreement with experimental data: average deviation e R = +2%, absolute mean deviation e AB = 14%, and standard deviation σ N = 19% for the total number of 5478 data points. [ABSTRACT FROM AUTHOR]

The present paper reviews published experimental work focusing on condensation flow regimes, heat transfer, and pressure drop in minichannels. New experimental data are available with high (R410A), medium (R134a), and low (R236ea) pressure refrigerants in minichannels of different cross-section geometries and with hydraulic diameters ranging from 0.4 to 3 mm. Because of the influence of flow regimes on heat transfer and pressure drop, a literature review is presented to discuss flow regimes transitions. The available experimental frictional pressure gradients and heat transfer coefficients are compared with semi-empirical and theoretical models developed for conventional channels and models specifically created for minichannels. Starting from the results of the comparison between experimental data and models, the paper will discuss and evaluate the opportunity for a new heat transfer model for condensation in minichannels; the new model attempts to take into account the effect of the entrainment rate of droplets from the liquid film. [ABSTRACT FROM AUTHOR]

Abstract: The aim of this paper is to investigate, both experimentally and theoretically, the potential of improving the cycle efficiency through two stage compression with intermediate cooling, at operating conditions typical of air conditioning. The experimental set-up consists of two closed loop air circuits acting as heat sink and heat source for gas-cooler and evaporator, respectively. The tested refrigerating circuit includes two tube-and-fin heat exchangers as gas-cooler and evaporator, a back-pressure valve as throttling device and a double-stage semi-hermetic compound, two-piston, reciprocating compressor equipped with oil separator and intercooler. A full set of thermocouples, pressure transducers and flow-meters allows measurement of all the main parameters of the CO2 cycle, enabling to perform heat balance both air and refrigerant side. Tests were run at fixed evaporation pressure, evaporator outlet superheating and gas-cooler outlet temperature, varying the gas-cooler outlet pressure in the range 8–11MPa. The optimal gas-cooler pressure for this application as well as the effect of the intercooler efficiency on the cycle performance were investigated. A FORTRAN code for the simulation of an improved two-stage cycle was validated against the experimental results; a theoretical analysis performed with this code is proposed for optimisation and energy performance evaluation of such a cycle. [Copyright &y& Elsevier]

In this paper, the experimental heat transfer coefficients measured during condensation of RI34a and R410A inside multiport minichannels are presented. The frictional pressure gradient was also measured during adiahatic two-phase flow. The need for experimental research on condensation inside multiport minichannels comes from the wide use of those channels in automotive air-conditioners. The perspective for the adoption of similar channels in the residential air conditioning applications also calls for experimental research on new high pressure refrigerants, such as R410A. Experimental data are compared against models to show the accuracy of the models in the prediction of heat transfer coefficients and pressure drop inside minichannels. [ABSTRACT FROM AUTHOR]

Condensation in minichannels is widely used in air-cooled condensers for the automotive and air-conditioning industry, heat pipes, and compact heat exchangers. The knowledge of pressure drops in such small channels is important in order to optimize heat transfer surfaces. Most of the available experimental work refers to measurements obtained within multiport smooth extruded tubes and deal with the average values over the number of parallel channels. In this context, the present authors have set up a new test apparatus for heat transfer and fluid flow studies in single minichannels. This paper presents new experimental frictional pressure gradient data, relative to single-phase flow and adiabatic two-phase flow of R134a and R32 inside a single horizontal minitube, with a 0.96 mm inner diameter and with not-negligible surface roughness. The new all-liquid and all-vapor data are successfully compared against predictions of single-phase flow models. Also the two-phase flow data are compared against a model previously developed by the present authors for adiabatic flow or flow during condensation of halogenated refrigerants inside smooth minichannels. Surface roughness effects on the liquid-vapor flow are discussed. In this respect, the friction factor in the proposed model is modified, in order to take into consideration also effects due to wall roughness. [ABSTRACT FROM AUTHOR]

Abstract: The objective of this work is to present new condensation heat transfer coefficients measured inside a single square cross section minichannel, having a 1.18mm side length, and compare them to the ones previously measured in a circular minichannel. Tests have been performed with R134a at 40°C saturation temperature, at mass velocity ranging between 200 and 800kgm−2 s−1. As compared to the heat transfer coefficients measured in the circular cross section channel, for the same hydraulic diameter, in the square minichannel the authors find a heat transfer enhancement at the lowest value of mass velocity, which must be due to the effect of surface tension. No heat transfer coefficient increase has been found at the highest values of the mass velocity where condensation is shear stress dominated. [Copyright &y& Elsevier]

Abstract: The present paper reports local heat transfer coefficients measured during condensation of R1234yf within a single circular 0.96 mm diameter minichannel and compares them to the ones of R134a. This experimental work is carried out in a unique test apparatus which allows to determine the local heat flux extracted from the condensing fluid from the temperature profile of the coolant. For this purpose, the temperatures of the coolant and of the wall are measured along the test section. The saturation temperature is determined from the saturation pressure which is measured at inlet and outlet of the test channel. Condensation tests are carried out at mass fluxes ranging between 200 and 1000 kg m−2 s−1 and the heat transfer coefficients result to be lower as compared to the ones of R134a. Since the saturation temperature drop directly affects the heat transfer rate, the pressure drop during adiabatic two-phase flow of R1234yf is also measured and compared to R134a. [Copyright &y& Elsevier]

Abstract: This paper presents experimental heat transfer coefficients measured during air flow heating in seven different aluminum open-cell foam samples with different number of pores per inch (PPI), porosity and foam core height under a wide range of air mass velocity. Three imposed heat fluxes are considered for each foam sample: 25.0, 32.5 and 40.0kWm−2. The collected heat transfer data are analyzed to obtain the global heat transfer coefficient and the normalized mean wall temperature. A model from the open literature has been selected and compared with the experimental database. A new simple heat transfer model, based on the present data, for the global heat transfer coefficient and the foam-finned surface area efficiency estimations has then been developed and compared against the experimental measurements. [ABSTRACT FROM AUTHOR]

Abstract: This paper aims at investigating experimentally the pressure drop during air flow in six different aluminum open-cell foam samples with different number of pores per inch (PPI) and different porosity under a wide range of air mass flow rate. Three imposed heat fluxes are considered for each foam sample. The collected pressure drop data are analysed with reference to models available in the literature. A new simple pressure drop model, based on present data, has then been developed. The consistency of the proposed model is finally checked by comparison to data available in the literature from different laboratories. [Copyright &y& Elsevier]

A new flow pattern map and flow pattern based heat transfer model for condensation inside horizontal plain tubes are proposed in this two-part paper. In Part I, a new version of a two-phase flow pattern map, originally developed by Kattan et al. [J. Heat Transfer 120 (1998) 140] for flow boiling, is presented for condensation inside horizontal tubes while a new heat transfer model is presented in Part II. The new flow pattern map incorporates a newly defined logarithmic mean void fraction (LMϵ) method for calculation of vapor void fractions spanning from low pressures up to pressures near the critical point. Several other modifications are also made that are appropriate for condensation as opposed to evaporation. In the absence of void fraction data at high reduced pressures for these conditions, the new LMϵ method has been indirectly validated using the convective condensation model for annular flow and corresponding heat transfer test data at reduced pressures up to 0.8. The new map has also been successfully compared to some recent flow pattern observations for condensation and other existing flow transition criteria and maps. [Copyright &y& Elsevier]

A new general flow pattern/flow structure based heat transfer model for condensation inside horizontal, plain tubes is proposed based on simplified flow structures of the flow regimes, and also includes the effect of liquid–vapor interfacial roughness on heat transfer. The model predicts local condensation heat transfer coefficients for the following flow regimes: annular, intermittent, stratified-wavy, fully stratified and mist flow. The new model has been compared to test data for 15 fluids (R-11, R-12, R-22, R-32, R-113, R-125, R-134a, R-236ea, a R-32/R-125 near-azeotrope, R-404A, R-410A, propane, n-butane, iso-butane and propylene) obtained in nine independent research laboratories. The new model has been tested over the following range of conditions: mass velocities from 24 to 1022 kg/(m2 s), vapor qualities from 0.03 to 0.97, reduced pressures from 0.02 to 0.80 and tube internal diameters from 3.1 to 21.4 mm. Overall, the model predicts 85% of the heat transfer coefficients in the non-hydrocarbon database (1850 points) to within ±20% with nearly uniform accuracy for each flow regime and predicts 75% of the entire database to within ±20% when including the hydrocarbons (2771 points), the latter all from a single laboratory whose data had some unusual experimental trends over part of their test range. [Copyright &y& Elsevier]

This article considers the heat transfer and pressure drop performance potentials of halogenated propene isomers during in-tube condensation and in-tube flow boiling using the penalty factor and total temperature penalization concepts, respectively. In particular, five isomers are considered: R-1233xf, R-1233zd(E), R-1234yf, R-1234ze(E), and R-1243zf. In addition, to these five pure fluids, the heat transfer and pressure drop performance potentials are investigated for five R-32/R-1234yf blends and for twenty-seven blends being considered as part of the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) Low-GWP Alternative Refrigerants Evaluation Program. The article also presents thermophysical property estimations for the five pure fluids relative to R-134a or R-123, and the five R-32/R-1234yf blends relative to R-134a. The thermophysical properties considered are the ones that influence the heat transfer and pressure drop performance potentials, and include the thermodynamic properties temperature, pressure, density, latent heat of vaporization, and specific heat, and the transport properties thermal conductivity and viscosity. The article also presents a literature review of relevant articles for condensation and boiling heat transfer and pressure drop of fluorinated propene isomers. [ABSTRACT FROM AUTHOR]

SUMMARY Heat transfer coefficient and pressure drop correlations are used to analyse the boiling heat transfer performance potentials in plain evaporator tubes of several conventional refrigerants and two newer fluorinated propene isomers possessing low global warming potentials. These correlations are used to calculate two penalization quantities expressed in terms of the refrigerant saturation temperature drop due to pressure drop and the driving temperature difference. These penalization terms are combined into a single Performance Evaluation Criterion dubbed Total Temperature Penalization (TTP). Using the two penalization terms and the TTP, several refrigerants, including the newer alternatives R1234yf (CF3CF=CH2) and R1234ze(E) (CF3CH=CHF ), are evaluated for their boiling heat transfer performance potentials in plain evaporator tubes. Furthermore, the usefulness of the technique is illustrated through several examples of the optimization of evaporator tube length. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Nowadays, plain and microfin tubes are largely used in refrigeration and air conditioning industries for condensation inside macroscale tubes. The mechanisms of condensation heat transfer and pressure drop are tightly linked with the prevailing two-phase flow regime, but not much evidence about the effect of microfins on the condensation flow patterns is still available in the open literature. A deep overview about the relevant literature on two phase flow pattern visualization inside tube considering experimental techniques, suggested flow regime maps and models for heat transfer coefficient is presented. Furthermore, this paper presents the extensive experimental contribution of the Authors with smooth and microfin tubes. The collected data for plain tube are compared against different flow pattern maps. The flow visualizations obtained for microfin tube are compared with those of plain tube; moreover, two flow regime maps proposed by present Authors for smooth and enhanced tubes are validated. [Copyright &y& Elsevier]

Because of their interesting heat transfer and mechanical properties, metal foams have been proposed for several different applications, thermal and structural. This paper aims at pointing out the effective thermal fluid dynamic behavior of these new enhanced surfaces, which present high heat transfer area per unit of volume at the expense of high pressure drop. The paper presents the experimental heat transfer and pressure drop measurements relative to air flowing in forced convection through four different aluminum foams, when electrically heated. The tested aluminum foams present 5, 10, 20 and 40 PPI (pores per inch), porosity around 0.92-0.93, and 0.02 m of foam core height. The experimental heat transfer coefficients and pressure drops have been obtained by varying the air mass flow rate and the electrical power, which has been set at 25.0 kW m-2, 32.5 kW m-2, and 40.0 kW m-2. The results have been compared against those measured for 40 mm high samples, in order to study the effects of the foam core height on the heat transfer. Moreover, predictions from two recent models are compared with heat transfer coefficient and pressure drop experimental data. The predictions are in good agreement with experimental data. [ABSTRACT FROM AUTHOR]