Nucleate boiling of two and three-component mixtures

For three pure fluids and their two- and three-component mixtures, heat transfer coefficients were measured in nucleate pool boiling on the upward facing copper surface of 40 mm diameter. The more-, moderate- and less-volatile components in mixtures are refrigerants R-134a, R-142b and R-123, respectively. Heat transfer coefficients of mixtures were less than the interpolated heat transfer coefficients between pure components, with more reduction at higher heat flux. Two correlations originally developed for two-component mixtures by Thome and Shakir and by Fujita and Tsutsui reproduced well the measured heat transfer coefficients of three- as well as two-component mixtures. This result implies that the boiling range included in the correlations accounts for heat transfer reduction in mixture boiling.     

Predicting Methods for Flow Boiling Heat Transfer of a Non-Azeotropic Mixture Inside a Single Microchannel

At this time, a widely accepted model that can predict flow boiling heat transfer in microchannels with different fluids, geometries, and operative conditions is still missing. Depending on the working fluid, a predicting correlation can lead to accurate estimation or give rise to errors up to 50% and higher. The situation is further complicated when the working fluid is a zeotropic mixture of two components, due to the additional mass transfer resistance that must be estimated. In the recent years much attention has been paid to the possible use of fluorinated propene isomers in substitution for high-global-warming-potential refrigerants. The available hydrofluoroolefins cannot cover all the air-conditioning, heat pump, and refrigeration applications when used as pure fluids because their thermodynamic properties are not suitable for all the operating conditions, and therefore some solutions may be found using blends of refrigerants, to satisfy the demand for a wide range of working conditions. The adoption of new mixtures poses the problem of how to extend the correlations developed for pure fluids to the case of flow boiling of mixtures in microchannels. In this work, a mixture of R1234ze(E) and R32 (0.5/0.5 by mass) has been considered: The local heat transfer coefficient during flow boiling of this mixture in a single microchannel with 0.96 mm diameter has been measured at a pressure of 14 bar, which corresponds to a bubble temperature of around 26°C. This flow boiling database, encompassing more than 300 experimental points at different values of mass velocity, heat flux, and vapor quality, is compared with available correlations in the literature. The introduction of a correction to account for the additional mass transfer resistance is discussed, and such correction is found to be necessary and proper to provide a correct sizing of the evaporator.     

Nucleate Pool Boiling Heat Transfer of Pure Refrigerants and Binary Mixtures

Heat transfer coefficients in nucleate boiling on a smooth flat surface were measured for pure fluids of R-134a, propane, isobutane and their binary mixtures at different pressure from 0.1 to 0.6 MPa. Series of experiments with different heat flux and mixture concentrations were carried out. The influences of pressure and heat flux on the heat transfer coefficient for different pure fluids were studied. Isobutane and propane were used to make up binary mixtures. Compared to the pure components, binary mixtures show lower heat transfer coefficients. This reduction was more pronounced as the heat flux increasing. Several heat transfer correlations are obtained for different pure refrigerants and their binary mixtures.     

Experimental study on saturated flow boiling heat transfer of R290/R152a binary mixtures in a horizontal tube

An experimental study on the saturated flow boiling heat transfer for a binary mixture of R290/R152a at various compositions is conducted at pressures ranging from 0.2 to 0.4 MPa. The heat transfer coefficients are experimentally measured over mass fluxes ranging from 74.1 to 146.5 kg/(m²·s) and heat fluxes ranging from 13.1 to 65.5 kW/m². The influences of different parameters such as quality, saturation pressure, heat flux, and mass flux on the local heat transfer coefficient are discussed. Existing correlations are analyzed. The Gungor-Winterton correlation shows the best fit among experimental data for the two pure refrigerants. A modified correlation for the binary mixture is proposed based on the authors’ previous work on pool boiling heat transfer and the database obtained from this study. The result shows that the total mean deviation is 10.41% for R290/R152a mixtures, with 97.6% of the predictions falling within ±30%.     

Forced convective boiling of nonazeotropic refrigerant mixtures inside enhanced surface tubing

An experimental study on the characteristics of two phase flow boiling of pure refrigerants such as R12 and R22 as well as nonazeotropic refrigerant mixtures R22/R114 and R22/R152a inside horizontal enhanced surface tubing is presented. The enhanced surface tubing results showed a significant improvement of the heat transfer over that of an equivalent smooth tube, depending on the mixture components and their concentrations. Correlations are proposed to predict the heat transfer characteristics such as average heat transfer coefficients as well as pressure drops of nonazeotropic refrigerant mixture flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixtures pressure drop is a weak function of the mixture compositions.     

Pool boiling heat transfer of propane, isobutane and their mixtures on enhanced tubes with reentrant channels

Pool boiling heat transfer experiments were carried out on a conventional smooth tube and four enhanced tubes with reentrant surfaces using propane, isobutane and their mixtures as working fluids for six saturation temperatures. The heat transfer performance is very different for different surface-fluid combinations. Compared to the smooth tube, the mixture boiling heat transfer degradation is more significant for the enhanced tubes. The current data are compared with available literature data for the same fluids and also with data for R12 and R134a. Experimental results of boiling hysteresis and for twin-tube bundles are also provided. Further explanations for the different heat transfer performances is provided by means of visualization in an accompanying paper [Y. Chen, M. Groll, R. Mertz, R. Kulenovic, Visualization and mechanisms of pool boiling of propane, isobutane and their mixtures on enhanced tubes with reentrant channels, submitted to Int. J. Heat Mass Transfer (H/S 04016)].     

Heat transfer in forced convection boiling of oil–non-azeotropic binary refrigerant mixtures

In this paper, the results of the heat transfer, forced convection, boiling characteristics of non-azeotropic refrigerant mixtures and oil are presented. This includes heat transfer coefficients for pure and binary mixtures under boiling conditions outside enhanced surface tubing. Local convective heat transfer coefficients have been determined using a modified Wilson-plot technique. Heat transfer correlations were established as a function of the binary mixture mass flow rate, and oil concentration, as well as key flow parameters.     

Experimental investigation of condensation heat transfer of R600a/POE/CuO nano-refrigerant in flattened tubes

In this study, the effect of addition of copper oxide nanoparticles on condensing heat transfer coefficient of R600a refrigerant flowing in a flat tube condenser has been investigated experimentally. The test setup consists of a pump, condenser test, second condenser, evaporator, heaters, and flow meter. The validation of the study was done by comparing the obtained condensation heat transfer coefficients with different empirical correlations in the literature. Different fluids including pure R600a, R600a-oil with Polyester oil (POE) mass percentage of 1%, and three R600a-oil-nanoparticle mixtures with mass percentages of 0.5%, 1%, and 1.5% were studied experimentally. It was shown that adding nanoparticles will result in 4.1%, 8.11%, and 13.7% average increase in condensing heat transfer coefficient with respect to the R600a-oil mixture. The greatest amount of increase was reported for the weight fraction of 1.5%, where it was observed that the condensing heat transfer coefficient for the mixture passing through the flattened tube is averagely 109.3% higher than its corresponding value for the pure refrigerant flowing in the round tube with the same mass flux. It was also found that an increase in mass flux resulted in an increase the heat transfer coefficient at all vapor qualities.     

Experimental Study on Pool Boiling Heat Transfer for R22, R407c,and R410a on a Horizontal Tube Bundle With Enhanced Tubes

An experimental test rig for study of the pooling-boiling heat transfer performance of pure and mixed refrigerants was designed and established. The test section is a horizontal tube bundle evaporator with nine mechanically fabricated porous surface tubes in a triangular layout. With this test system, the heat transfer coefficients of the nucleate boiling in the evaporator were measured for R22, R407c, and R410a. Extensive experimental measures were made for those pure and mixed refrigerants at different heat fluxes from 10 kW m^?2 to 43 kW m^?2 at saturation temperature of 9°C. Comprehensive measured data are presented in this paper. From experimental results, it is found that the pool boiling heat transfer coefficient increases with increasing the heat flux. It is also found that boiling heat transfer coefficients for R410a are 1.25–1.81 times and 6.33–7.02 times higher than that for R22 and R407c, respectively. The experimental correlations for the pool boiling heat transfer coefficients of R22, R407c, and R410a on the present enhanced tubes bundle are developed. The thermal resistance analysis reveals that the thermal resistance of the water side is a controlling factor for the evaporator for R22 and R410a. However, for R407c, the thermal resistance of the refrigerant side is slightly higher than that of the water side. To further improve the overall heat transfer coefficient in the evaporator of R22 and R410a, the enhancement for both the inside and outside is equally important, and the effectively enhanced boiling surface must be developed for the evaporator of R407c.     

Influence of magnetic field on two‐phase flow convective boiling of some refrigerant mixtures

In this paper, an experimental study on the influence of magnetohydrodynamic (MHD) on heat transfer characteristics of two‐phase flow boiling of some refrigerant mixtures in air/refrigerant horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of MHD on the heat transfer characteristics such as average heat transfer coefficients, and pressure drops of R‐507, R‐404A, R‐410A, and R‐407C in two‐phase flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture's pressure drop is a weak function of the mixture's composition. It was also evident that the proposed correlations for predicting the heat transfer characteristics were applicable to the entire heat and mass flux, investigated in the present study. The deviation between the experimental and predicted value using new and improved correlations for the heat transfer coefficient and pressure drop were less than ±20%, for the majority of data. Copyright © 2005 John Wiley & Sons, Ltd.     

Influence of liquid injection on two‐phase flow convective boiling of refrigerant mixtures

In this paper, an analytical study on the influence of liquid injection on heat transfer characteristics of two‐phase flow boiling of some refrigerant mixtures in air/refrigerant horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of the liquid injection the thermophysical properties of refrigerant mixtures as well as the heat transfer characteristics such as average heat transfer coefficient of R‐507, R‐404A, R‐410A, and R‐407C in two‐phase flow boiling inside enhanced surface tubing. It was also evident that the proposed correlations and the experimental data that the liquid injection has significant impact on the heat transfer coefficient. In addition, the proposed correlations were applicable to the entire heat and mass flux, investigated in the present study under the liquid injection conditions. The deviation between the experimental and predicted under liquid injection were less than ±20, for the majority of data. Copyright © 2004 John Wiley & Sons, Ltd.     

Flow boiling heat transfer of HFO1234yf and R32 refrigerant mixtures in a smooth horizontal tube: Part I. Experimental investigation

HFO1234yf has been proposed for mobile air-conditioners due to its low global warming potential (GWP) and performance comparable to that of R134a. However, its performance is inferior to that of R410A. This makes it difficult to be applied to residential air-conditioners. In order to apply the low-GWP refrigerant to residential air-conditioners, refrigerant mixtures of HFO1234yf and R32 are proposed, and their flow boiling heat transfer performances were investigated at two mass fractions (80/20 and 50/50 by mass%) in a smooth horizontal tube with an inner diameter of 2 mm. The experiments were conducted under heat fluxes ranging from 6 to 24 kW/m² and mass fluxes ranging from 100 to 400 kg/m² s at the evaporation temperature of 15 °C. The measured heat transfer coefficients were compared with those of pure HFO1234yf and R32. The results showed that the heat transfer coefficients of the mixture with an R32 mass fraction of 20% were 10–30% less than those of pure HFO1234yf for various mass and heat fluxes. When the mass fraction of R32 increased to 50%, the heat transfer coefficients of the mixture were 10–20% greater than those of pure HFO1234yf under conditions of large mass and heat fluxes. Moreover, the heat transfer coefficients of the mixtures were about 20–50% less than that of pure R32. The performances of the mixtures were examined at different boiling numbers. For refrigerant mixture HFO1234yf and R32 (80/20 by mass%), the nucleate boiling heat transfer was noticeably suppressed at low vapor quality for small boiling numbers, whereas the forced convective heat transfer was significantly suppressed at high vapor quality for large boiling numbers. This indicates that the heat transfer is greatly influenced by the mass diffusion resistance and temperature glide of the mixture.     

Experimental analysis of saturated pool boiling of acetone and isopropanol with added surfactant sodium lauryl sulfate over aluminum surface

The present article reports a detailed experimental study to know the effect of adding surfactants to different base fluids. For this purpose, an aqueous solution of different concentrations of surfactant with base fluids was formed and then heated at varying heat flux in a pool boiling apparatus. Acetone and isopropanol have been used as base/boiling fluids with anionic surfactant sodium lauryl sulfate (SLS) of 200, 400, 600, 800, and 1000 PPM concentration. The data obtained by heating surfactant-based fluids has been compared to the data of heating pure base fluids to establish the role of surfactants and the importance of surfactant concentration, that is, critical micelle concentration (CMC) in pool boiling augmentation. CMC value was found in the case of pool boiling of acetone with an SLS concentration of 400 PPM. However, the CMC value is found to be a lower value of SLS concentration of 200 PPM in the pool boiling process of isopropanol. Therefore, it is interesting to investigate the possible causes of such heat transfer phenomena with different values of surfactant concentration. Variations in heat flux change the degree of superheat of the aluminum heater substrate. The heat transfer performance is studied, and finally, a semi-empirical correlation is proposed to know the heat transfer coefficient and its dependency over the concentration of surfactant.     

Transition and flow boiling heat transfer inside a horizontal tube

Experiments on transition and flow boiling heat transfer with refrigerant R114 inside a horizontal tube were performed at bubble flow, critical heat flux and in the transition region between bubble flow and film boiling at mass fluxes between 1200 and 4000 kg/m² s and in the pressure range between 5 and 15 bar. In comparison with pool boiling bubble flow heat transfer depends essentially on the mass flow rates and on the vapor quality. The critical heat flux depends less on the temperature difference than in pool boiling heat transfer and exhibits a maximal and a minimal value as a function of the pressure. The critical heat flux increases with mass flow rate as already shown by Collier. In the region of transition boiling the heat flux over the difference between wall and saturation temperature approaches a horizontal curve. Therefore in this region an evaporator may always be operated under stable conditions and burn out does not occur.     

Effects of Surfactant on Flow Boiling Heat Transfer of Ethylene Glycol/Water Mixtures in a Minitube

This study investigated effects of surfactant sodium dodecylbenzene sulfonate (SDBS) on flow boiling heat transfer of ethylene glycol/water mixtures in a vertical minitube. Experiments were performed using solutions containing 300 ppm by weight of surfactant and the results were compared with those of pure mixture. Local heat transfer coefficient was measured and found to be dependent on the mass quality. Although the surfactant intensifies the vaporization process, it doesn't necessarily enhance the heat transfer coefficient. Heat transfer coefficients were compared at two different mass fluxes, and the results might be explained based on the local flow pattern and the heat transfer mechanism. After a critical quality, higher quality tends to deteriorate heat transfer due to intermittent dryout, and therefore adding a surfactant to generate more vapor may be of negative effect on the flow boiling heat transfer in a minitube, which is contrary to the experience of enhancing nucleate pool boiling heat transfer with trace surfactants.     

Convective boiling of pure and mixed refrigerants: An experimental study of the major parameters affecting heat transfer

An experimental study is carried out to investigate the characteristics of the evaporation heat transfer for different fluids. Namely, pure refrigerants fluids (R22 and R134a), azeotropic and quasi-azeotropic mixtures (R404A, R410A, R507) and zeotropic mixtures (R407C and R417A).The test section is a smooth, horizontal, stainless steel tube (6 mm ID, 6 m length) uniformly heated by the Joule effect. The flow boiling characteristics of the refrigerant fluids are evaluated in 250 different operating conditions. Thus, a data-base of more than 2000 data points is produced.The experimental tests are carried out varying: (i) the refrigerant mass fluxes within the range 200–1100 kg/m² s; (ii) the heat fluxes within the range 3.50–47.0 kW/m²; (iii) the evaporating pressures within the range 3.00–12.0 bar.In this study, the effect on measured heat transfer coefficient of vapour quality, mass flux, saturation temperature, imposed heat flux, thermo-physical properties are examined in detail.     

Influence of gas/liquid injection on two‐phase flow convective boiling of refrigerant mixtures

An analytical study on the influence of gas/liquid injection on heat transfer characteristics of two‐phase flow boiling of some refrigerant mixtures in horizontal enhanced surface tubing is presented. Correlations were proposed to predict the impact of the gas/liquid injection on the heat transfer characteristics such as average heat transfer coefficient of R‐507, R‐404A, R‐410A and R‐407C in two‐phase flow boiling inside enhanced surface tubing. The data also revealed that gas/liquid injection is beneficial at certain gas/liquid injection ratios to the heat transfer coefficient depending upon the Reynolds number and the boiling point. It was also evident that the proposed correlations and the experimental data that the gas/liquid injection has significant impact on the heat transfer coefficient. In addition, the proposed correlations were applicable to the entire heat and mass flux, investigated in the present study under gas/liquid injection conditions. The deviation between the experimental and predicted under gas/liquid injection were less than ±20, for the majority of data. Copyright © 2004 John Wiley & Sons, Ltd.     

Prediction of convective boiling characteristics of alternatives to R-502 inside air/refrigerant enhanced surface tubing

In this paper, an experimental study on the heat transfer characteristics of two-phase flow boiling of alternative azeotropic refrigerant mixtures to R-502 on air/refrigerant horizontal enhanced surface tubing is presented. Correlations were proposed to predict the heat transfer characteristics such as average heat transfer coefficients, as well as pressure drops of alternatives to R-502; such as R-507, R-404A, R-407B, and R-408A in two-phase flow boiling inside enhanced surface tubing. In addition, it was found that the refrigerant mixture’s pressure drop is a weak function of the mixture’s composition. It was found that the correlations were applicable to the entire heat and mass flux, investigated in the present study for the proposed blends under question. The deviation between the experimental and predicted values for the heat transfer coefficient and pressure drop were less than ±20% and 35%, respectively, for the majority of data.     

Boiling heat transfer correlations for refrigerant mixtures flowing inside micro-fin tubes

Based on experimental results of ternary non-azeotropic refrigerant mixture R417A flowing and boiling in one smooth and two internally grooved horizontal tubes with different geometrical parameters, a boiling heat transfer correlations was developed for refrigerant mixtures flowing inside micro-fin tubes by applying the enhancement factor in the present modified-Kattan model which was modified by the experimental data of R417A in a smooth tube. The comparison between the calculation and the experimental results indicates that the prediction by the present correlations is in good agreement with the experiment of refrigerant mixtures inside different micro-fin tubes with a standard deviation of ± 30% for vapor qualities below 80%.     

A heat transfer correlation of flow boiling in micro-finned helically coiled tube

Two main mechanisms, nucleate boiling and convective boiling, are widely accepted for in-tube flow boiling. Since the active nuclei on the heated wall are dominant for nucleate boiling and flow pattern governs the convective boiling, the heat transfer coefficient is strongly influenced by the wall heat flux, mass flux and vapor quality, respectively. In practical industrial applications, for example, the evaporators in refrigeration, forced convective evaporation is the dominant process and high heat transfer efficiency can be obtained under smaller temperature difference between wall and liquid. Therefore, it is of importance to develop a correlation of convective boiling heat transfer with a good accuracy. In this paper, a new kind of micro-finned helically coiled tube was developed and the flow boiling heat transfer characteristics were experimentally studied with R134a. Based on the analysis of the mechanisms of flow boiling, heat transfer correlations of the specific micro-finned helically coiled tubes are obtained.