Pool boiling heat transfer of refrigerant mixtures R32/R125

Nucleate pool boiling heat transfer of new, environmentally harmless refrigerant mixtures R32/R125 has been systematically investigated in a wide range of pressure and heat flux under saturation conditions using a horizontal platinum wire (d = 0.1 mm). The platinum wire served as both heating element and resistance thermometer. The experimental results are compared with calculated values using a modified Jungnickel correlation. The comparison between all calculated data and experimental data shows good agreement. Most of the data fall within ±20% of the correlation. The mean average deviation is ±8.25%.     

混合制冷工质R1234yf/R32 PVTx性质的实验研究

利用基于Burnett法搭建的PVTx性质测试系统测量了混合制冷工质R1234yf/R32在温度为253~313 K范围内、R1234yf的质量分数为20%时的气相PVTx性质.详细介绍了该测量系统基于Burnett法的测量原理和PVTx性质测试系统的实验步骤,并对各种测量误差及不确定度进行了分析.根据实验数据拟合出混合制冷工质的气态维里方程,并比较了第二、第三维里系数与温度的关系.结果表明:实验数据与拟合得到的方程具有较好的一致性,为基础热物性研究和进一步研究替代制冷剂提供了详实的数据.     

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%.     

基于R1234yf和R134a制冷剂的太阳能-空气源复合热泵性能对比

随着化石能源的过度消耗与环境问题日益加剧,发展清洁可再生能源势在必行。针对太阳能-空气源复合热泵系统性能进行分析,结果表明:模式二时,太阳能热泵系统与空气源热泵系统的COP分别为6.55和4.81;平板式太阳能集热器的热效率高达67.86%;在相同制热功率条件下,与模式一相比,模式二时空气源热泵耗功平均降低87.81%,太阳能热泵耗功平均降低16.17%;采用R1234yf的系统耗功比R134a系统高2%~3.17%,但R1234yf的温室效应值极低。     

Experimental study of forced convective boiling for non-azeotropic refrigerant mixtures R-22/R-124 in horizontal smooth tube

In this article, the experiment is carried out with four stages of test section and new experimental data of non-azeotropic refrigerant mixtures R-22/R-124 are presented. Existing correlations are analysed and new ones are proposed for pure refrigerants. The nucleate boiling correlation of the refrigerant mixtures is modified so as to obtain the evaporation heat transfer coefficient of the refrigerant mixtures in a smooth tube. The result shows that the majority of the predictions fall within ±30% of the experiments.     

Experimental research of boiling heat transfer of smooth and screwed tube

In this paper, based on the analog theory of heat transfer research, we performed an analog experiment on boiling heat transfer in smooth tube and screwed tubes. These are widely used in the high pressure generator of lithium bromide absorption refrigeration. From the experimental research, we obtained a series of results on the boiling heat transfer of a single smooth tube and three screwed tubes. The working condition is near the zone of bubble boiling and the overheat wall temperature ranges from 2–7 °C, with a fluid medium of pure water and salt water solution. These results agreed well with the known results, and are significant for the practical design and application of a high pressure generator of lithium bromide absorption for refrigeration. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(2): 74–84, 2007; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/htj.20145     

Shell-and-tube evaporator model performance with different two-phase flow heat transfer correlations. Experimental analysis using R134a and R1234yf

This work presents a model of a shell-and-tube evaporator using R1234yf and R134a as working fluids. The model uses the effectiveness-NTU method to predict the evaporation pressure and the refrigerant and secondary fluid temperatures at the evaporator outlet, using as inputs the geometry of the evaporator, the refrigerant mass flow rate and evaporator inlet enthalpy, and the secondary fluid volumetric flow rate and evaporator inlet temperature. The model performance is evaluated using different two-phase flow heat transfer correlations through model outputs, comparing predicted and experimental data. The output parameter with maximum deviations between the predicted and experimental data is the evaporating pressure, being the deviations in outlet temperatures less than 3%. The evaporator model using Kandlikar's correlation obtains the highest precision and the lowest absolute mean error, with 4.87% in the evaporating pressure, 0.45% in the refrigerant outlet temperature and 0.03% in the secondary fluid outlet temperature.     

Carbon dioxide local heat transfer coefficients during flow boiling in a horizontal circular smooth tube

Carbon dioxide is gaining renewed interest as an environmentally safe refrigerant. In order to improve the energy efficiency of R744 systems, an accurate knowledge of heat transfer coefficients is fundamental.In this paper experimental heat transfer coefficients during flow boiling of R744 in a smooth, horizontal, circular, 6.00 mm inner diameter tube are presented. We obtained 217 experimental points in 18 operating conditions commonly encountered in dry-expansion evaporators investigating the effect of the mass flux within the range from 200 to 349 kg/m² s, the saturation temperature within the range from ?7.8 to 5.8 °C, the heat flux within the range from 10.0 to 20.6 kW/m² and the vapor quality within the range from 0.02 to 0.98.An interpretation of the experimental trends based on the local circumferential distribution of heat transfer coefficients, the flow regimes and the thermophysical properties is proposed.Besides the measured data are compared with those predicted by the Cheng et al. [L. Cheng, G. Ribatski, J.R. Thome, New prediction methods for CO₂ evaporation inside tubes: Part II – An updated general flow boiling heat transfer model based on flow patterns, International Journal of Heat and Mass Transfer 51 (2008) 125–135] and Yoon et al. [R. Yun, Y. Kim, M.S. Kim, Y. Choi, Boiling heat transfer and dryout phenomenon of CO₂ in a horizontal smooth tube, International Journal of Heat and Mass Transfer 46 (2003) 2353–2361] correlations to determine the best predictive method for the tested operating conditions.     

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.     

Comparison of R407C and R417A heat transfer coefficients and pressure drops during flow boiling in a horizontal smooth tube

The sizing of evaporators for refrigeration and air conditioning applications is essential for good cycle performance. For this reason, accurate knowledge of the heat transfer coefficients and pressures drops of refrigerants is mandatory. In order to identify the best substitute for R22, in this paper, the experimental local heat transfer coefficients and pressure drops during flow boiling of R407C and R417A were compared.

The test section was a smooth, horizontal, stainless steel tube (6.0 mm I.D., 8.0 mm O.D., 6.0 m length) uniformly heated by the Joule effect. The experimental tests were conducted by varying the evaporating pressures within the range from 3.6 to 10.3 bar, heat fluxes within the range from 7.6 to 28.5 kW/m² and refrigerant mass fluxes within the range from 195 to 706 kg/m² s.     

Flow pattern and heat transfer characteristics of R-134a refrigerant during flow boiling in a horizontal circular mini-channel

Flow boiling heat transfer of R-134a refrigerant in a circular mini-channel, 600 mm long with a diameter of 1.75 mm, is investigated experimentally in this study. The test section is a stainless steel tube placed horizontally. Flow pattern and heat transfer coefficient data are obtained for a mass flux range of 200–1000 kg/m² s, a heat flux range of 1–83 kW/m² and saturation pressures of 8, 10, and 13 bar. Five different flow patterns including slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow are observed and the heat transfer coefficient data for different flow patterns are presented. The heat transfer coefficient increases with increasing heat flux but is mostly independent of mass flux and vapour quality. In addition, it is indicated from the experiments that the higher the saturation pressure, the lower is the heat transfer coefficient. Comparisons of the present data with the existing correlations are also presented.     

High heat flux flow boiling in silicon multi-microchannels – Part I: Heat transfer characteristics of refrigerant R236fa

This article is the first in a three part study on flow boiling of refrigerants R236fa and R245fa in a silicon multi-microchannel heat sink. The heat sink was composed of 67 parallel channels, which are 223 μm wide, 680 μm high and 20 mm long with 80 μm thick fins separating the channels. The base heat flux was varied from 3.6 to 221 W/cm², the mass velocity from 281 to 1501 kg/m² s and the exit vapour quality from 2% to 75%. The working pressure and saturation temperature were set nominally at 273 kPa and 25 °C, respectively. The present database includes 1217 local heat transfer coefficient measurements, for which three different heat transfer trends were identified, but in most cases the heat transfer coefficient increased with heat flux and was almost independent of vapour quality and mass velocity. Importantly, it was found for apparently the first time that the heat transfer coefficient as a function of vapour quality reaches a maximum at very high heat fluxes and then decreases with further increase of heat flux.     

Flow boiling heat transfer in two-phase micro-channel heat sinks--I. Experimental investigation and assessment of correlation methods

This paper is the first of a two-part study concerning measurement and prediction of saturated flow boiling heat transfer in a water-cooled micro-channel heat sink. In this paper, new experimental results are discussed which provide new physical insight into the unique nature of flow boiling in narrow rectangular micro-channels. The micro-channel heat sink contained 21 parallel channels having a m cross-section. Tests were performed with deionized water over a mass velocity range of 135-402 kg/m² s, inlet temperatures of 30 and 60 °C, and an outlet pressure of 1.17 bar. Results indicate an abrupt transition to annular flow near the point of zero thermodynamic equilibrium quality, and reveal the dominant heat transfer mechanism is forced convective boiling corresponding to annular flow. Contrary to macro-channel trends, the heat transfer coefficient is shown to decrease with increasing thermodynamic equilibrium quality. This unique trend is attributed to appreciable droplet entrainment at the onset of annular flow regime development, and the increase in mass flow rate of the annular film by droplet deposition downstream. Eleven previous empirical correlations are assessed and deemed unable to predict the correct trend of heat transfer coefficient with quality because of the unique nature of flow boiling in micro-channels, and the operating conditions of water-cooled micro-channel heat sinks falling outside the recommended application range for most correlations. Part II of this study will introduce a new annular flow model as an alternative approach to heat transfer coefficient prediction for micro-channels.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures ：Part I—Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

A mechanism is proposed for nucleate pool boiling heat transfer along with a general model for both pure liquids and binary mixtures. A combined physical model of bubble growth is also proposed along with a corresponding bubble growth model for pure liquids on smooth tubes. Using the general model and the bubble growth model for pure liquids, an analytical model for nucleate pool boiling heat transfer of pure liquids on smooth tubes is developed.     

Flow boiling in horizontal smooth tubes: New heat transfer results for R-134a at three saturation temperatures

The present study presents new flow boiling heat transfer results of R-134a flowing inside a 13.84 mm internal diameter, smooth horizontal copper tube. The heat transfer measurements were made over a wide range of test conditions: saturation temperatures of 5, 15 and 20 °C, (corresponding to reduced pressures of 0.08, 0.12 and 0.14), vapor qualities ranged from 0.01 to 0.99, mass velocities of 300 and 500 kg/m² s, and heat fluxes of 7.5 and 17.5 kW/m². The experimental results clearly show that a local minimum heat transfer coefficient systematically occurs within slug flow pattern or near the slug-to-intermittent flow pattern transition. The vapor quality x_min at which the local minimum occurs seems to be primarily sensitive to mass velocity and heat flux. Thus, it is influenced by the competition between nucleate and convective boiling mechanisms that control macroscale flow boiling. The experimental results were compared to four types of predictive methods: (a) strictly convective, (b) superposition, (c) strictly empirical and (d) flow pattern based. Generally, all the methods tend to underpredict the experimental data and the higher errors occur in two particular regions: low and high vapor qualities. These vapor qualities correspond to slug and annular patterns, respectively. For slug flow, methods that require the identification of nucleate boiling related regions tend to predict the heat transfer coefficient accurately. This emphasizes that for slug flows, heat transfer is not a simple juxtaposition of nucleate and convective boiling contributions, but that the integration of these two heat transfer mechanisms is also a function of flow parameters. The comparisons between experimental and predicted data show that the best overall results are obtained with superposition and flow pattern based methods.     

Saturated flow boiling heat transfer of refrigerant R-410A in a horizontal annular finned duct

An experiment is conducted here to investigate the saturated flow boiling heat transfer characteristics of ozone friendly refrigerant R-410A in a horizontal annular finned duct. Meanwhile the associated bubble characteristics in the duct are also inspected from the flow visualization. The experimental data are presented in terms of saturated flow boiling curves, boiling heat transfer coefficients and flow photos. In addition, empirical correlation equations for the saturated flow boiling heat transfer coefficient and mean bubble departure diameter are proposed. The saturated flow boiling curves show that boiling hysteresis is insignificant in the flow and the wall superheat needed for the onset of nucleate boiling is slightly affected by the refrigerant mass flux. Besides, the boiling curves are mainly affected by the imposed heat flux and refrigerant mass flux. Moreover, the measured saturated flow boiling heat transfer coefficient increases with the imposed heat flux and refrigerant mass flux. Furthermore, at a higher refrigerant mass flux the departing bubbles are smaller.     

Experimental investigation of condensation of hydrocarbon refrigerants (R600a) in a horizontal smooth tube

In this study, the experimental results of the condensing heat transfer coefficients of R600a, a hydrocarbon refrigerant, in a horizontal smooth copper tube with an inner diameter of 4 mm and outer diameter of 6 mm are presented at different vapor quality and different mass fluxes during condensation under annular flow conditions, by adjusting the desired vapor qualities at the test area. A specially-designed sight glass has been fitted to the inlet and outlet of the test tube to identify the flow type by naked eye after the inlet vapor quality of the refrigerant to be fed to the test area during the test is adjusted in the system. Thanks to a new method developed in the measuring system, the condensing heat transfer coefficients could be calculated by measuring the difference value (T_s − T_w) directly from the data collection unit. The experimental findings have shown that the condensing heat transfer coefficients drops down with reduction in vapor quality and the coefficient rises with the increase in the mass flux at constant vapor quality. A correlation has been developed from the data obtained. The condensing heat transfer coefficients obtained from the experimental study were seen to be consistent by ± 20% with the correlations developed by Shah, Travis and Cavallini–Zecchin.     

Experimental investigation of convective heat transfer coefficient during downward laminar flow condensation of R134a in a vertical smooth tube

This paper presents an experimental investigation of laminar film condensation of R134a in a vertical smooth tube having an inner diameter of 7–8.1 mm and a length of 500 mm. Condensation experiments were performed at mass fluxes of 29 and 263 kg m^?2 s^?1. The pressures were between 0.77 and 0.1 MPa. The heat transfer coefficient, film thickness and condensation rate during downward condensing film were determined. The results show that an interfacial shear effect is significant for the laminar condensation heat transfer of R134a under the given conditions. A new correlation for the condensation heat transfer coefficient is proposed for practical applications.     

Experimental investigation of heat transfer during melting around a horizontal tube with and without axial fins

Melting phenomena around horizontal tubes with and without axial fins were investigated experimentally. While melting around a horizontal tube results in the well known deformation of the solid-liquid interface, a different heat transfer behavior is to be found by mounting nearly isothermal axial copper fins or nearly adiabatic axial PVC fins. Moreover melting patterns and heat transfer rates are influenced by the geometric fin arrangement. Mean heat transfer rates are correlated, depending on the different investigated convection schemes. Special attention was drawn to the fluid flow behavior at the interface, where Görtler vortices could be detected.     

Experimental research of pool boiling heat transfer in horizontal narrow spaces

Much progress has been made in high‐performance electronic chips, the miniaturization of electronic circuits and other compact systems recently, which brings about a great demand for developing efficient heat removal techniques to accommodate these high heat fluxes. With this objective in mind, experiments were carried out on five kinds of test elements with distilled water and ethanol as working liquids. The test elements used in these experiments consisted of five parallel discs with diameters varying from 5 mm to 40 mm. The experiments were performed with the discs oriented horizontally and uniform heat fluxes applied at the bottom surfaces. The influence of narrow spacing, space size, working liquid property, and heat flux on boiling heat transfer performance in narrow spaces has been investigated. Experimental results showed that the boiling heat transfer coefficient of a narrow space was 3 to 6 times higher than that of pool boiling when the narrow space size and heat flux combine adequately, but the critical heat flux was lower than that of pool boiling. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(5): 307–315, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20017