表面微结构对流动沸腾中核沸腾抑制因子的影响

用两根内表面微结构不同的水平光滑管环状流区流动沸腾换热实验数据，采用叠加模型分别建立了流动沸腾换热关系式，并比较它们的抑制因子。结果表明，表面微结构对抑制因子有显的影响；当表面的平均凹腔半径较大时，抑制因子明显增大。表明表面微结构改变对流动沸腾换热能起到较好的强化作用。     

A General Correlation for Heat Transfer During Dispersed-Flow Film Boiling in Tubes

A general correlation for heat transfer during film boiling in tubes is presented. It is based on the two-step model. It has been verified with data for nine fluids flowing up in tubes. The fluids include water, cryogens, refrigerants, and chemicals. The range of data includes pressures from 1 to 215 bar, reduced pressures from 0.0046 to 0.97, mass velocities from 4 to 5,176 kg/m² s, tube diameters from 1.1 to 24.3 mm, and qualities from 0.1 to 2.4. The 546 data points are predicted with a mean deviation of 15.2%. Deviation is defined as the difference between the measured and predicted heat transfer coefficients divided by the measured heat transfer coefficient, the heat transfer coefficients being based on the saturation temperature. Three other well-known correlations are also compared to the same data and found to have much larger deviations. The correlation is also compared with a limited amount of data from horizontal tubes; the results are encouraging.     

紧凑满液型叉排管束蒸发换热器内水的沸腾强化换热特性

采用紧凑满液型蒸发换热器,利用水平传热管叉排管束狭窄空间内早期沸腾强化换热机理将中小热负 荷条件下的自然对流换热转化为旺盛核沸腾换热,换热性能大大优于传统的降膜式蒸发换热器。对水平传热管 管束在受限空间内沸腾强化换热进行实验研究,确认了紧凑满液式水平管蒸发换热器具有良好的换热性能,传 热管在管束中的位置对换热特性已经没有明显影响,随着压力增加,受限空间内沸腾强化换热强化效果显著增 加。     

Flow boiling characteristics of refrigerant mixture R22/R114 in the annulus of enhanced surface tubing

The paper presents an experimental study of flow boiling heat transfer characteristics of refrigerant mixture R22/R114 in the annuli of a horizontal enhanced surface tubing evaporator. The test section had an inner tube bore diameter of 17.3 mm, an envelope diameter of 28.6 mm and an outer smooth tube of 32.3 mm internal diameter. The ranges of heat flux and mass velocity covered in the tests were 5–25 kW/m² and 180–290 kg/m²/s, respectively, at a pressure of 570 kPa. The enhanced surface tubing data shows a significant enhancement of the heat transfer compared with an equivalent smooth tube depending on the mixture components and their concentrations. Correlations are proposed to predict such heat transfer characteristics as the average heat transfer coefficients as well as pressure drops of R22/R114 nonazeotropic refrigerant mixture 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 composition.     

Experimental Investigation on Flow Boiling Heat Transfer of CO2 at Low Temperatures

There is a growing use of CO₂ refrigeration to achieve low temperatures, particularly in the food industry; however, very limited information is available in the open literature on its boiling heat transfer characteristics below –30°C. This paper investigates experimentally the flow boiling heat transfer of CO₂ at low temperatures down to –40°C. The experimental data were collected from a novel experimental rig, specifically designed to achieve low temperatures, using a 4.5 m long horizontal stainless steel tube of 4.57 mm inner diameter. The effects of heat and mass fluxes and saturation temperature on the flow boiling heat transfer coefficient are also analyzed. Furthermore, this paper highlights the limitations of existing empirical correlations by comparing their predictions with the experimental boiling heat transfer coefficients. It is expected that the data presented in this study would be beneficial to industry and designers of compact heat exchangers for CO₂ at low temperatures.     

Heat Transfer and Crisis Phenomena at the Film Flows of Freon Mixture over Vertical Structured Surfaces

Results on experimental investigation of heat transfer in the liquid films dichlorofluoromethane R21 and dichlorotetrafluoroethane R114 Freon mixture over the vertical tubes are presented. We have studied the film flow over the outer surface of tubes with 50-mm diameter and different configurations: smooth surface, horizontal ribs, and diamond-shape knurling. Heat transfer coefficients were measured under the conditions of evaporation and nucleate boiling together with wave characteristics of the falling film, binary mixture composition, and critical heat fluxes corresponding to dry spots formation. The film Reynolds number at the inlet to the test section was varied from 15 to 250. At evaporation regime the heat transfer coefficient for a smooth surface decreases classically with an increase of Reynolds number. Dependence of heat transfer coefficient on irrigation density for the surface with diamond-shape knurling is similar to dependence for the smooth surface with insignificant heat transfer intensification. The heat transfer coefficients at nucleate boiling for the studied structured surfaces are close to those obtained for the smooth tube. Development of critical phenomena is determined by regularities of dry spots formation typical for evaporation of the wavy liquid film.     

Flow boiling of liquid nitrogen in micro-tubes: Part II – Heat transfer characteristics and critical heat flux

This paper is the second portion of a two-part study concerning the flow boiling of liquid nitrogen in the micro-tubes with the diameters of 0.531, 0.834, 1.042 and 1.931 mm. The contents include the heat transfer characteristics and critical heat flux (CHF). The local wall temperatures are measured, from which the local heat transfer coefficients are determined. The influences of heat flux, mass flux, pressure and tube diameter on the flow boiling heat transfer coefficients are investigated systematically. Two regions with different heat transfer mechanism can be classified: the nucleate boiling dominated region for low mass quality and the convection evaporation dominated region for high mass quality. For none of the existed correlations can predict the experimental data, a new correlation expressed by Co, Bo, We, K_p and X is proposed. The new correlation yields good fitting for 455 experimental data of 0.531, 0.834 and 1.042 mm micro-tubes with a mean absolute error (MAE) of 13.7%. For 1.931 mm tube, the flow boiling heat transfer characteristics are similar to those of macro-channels, and the heat transfer coefficient can be estimated by Chen correlation. Critical heat flux (CHF) is also measured for the four tubes. Both the CHF and the critical mass quality (CMQ) are higher than those for conventional channels. According to the relationship that CMQ decreases with the mass flux, the mechanism of CHF in micro-tubes is postulated to be the dryout or tear of the thin liquid film near the inner wall. It is found that CHF increases gradually with the decrease of tube diameter.     

Heat transfer and flow characteristics of flow boiling of CO2‐oil mixtures in horizontal smooth and micro‐fin tubes

Experiments were carried out on the flow pattern, heat transfer, and pressure drop of flow boiling of pure CO₂ and CO₂‐oil mixtures in horizontal smooth and micro‐fin tubes. The smooth tube is a stainless steel tube with an inner diameter of 3.76 mm. The micro‐fin tube is a copper tube with a mean inner diameter of 3.75 mm. The experiments were carried out at mass velocities from 100 to 500 kg/(m²·s), saturation temperature of 10 °C, and the circulation ratio of lubricating oil (PAG) was from 0 to 1.0 mass%. Flow pattern observations mainly showed slug and wavy flow for the smooth tube, but annular flow for the micro‐fin tube. Compared with the flow patterns in the case of pure CO₂, an increase in frequency of slug occurrence in the slug flow region, and a decrease in the quantity of liquid at the top of the tube in the annular flow region were observed in the case of CO₂‐oil mixtures. With pure CO₂, the flow boiling heat transfer was dominated by nucleate boiling in the low vapor quality region, and the heat transfer coefficients for the micro‐fin tube were higher than those of the smooth tube. With CO₂‐oil mixtures, the flow boiling heat transfer was dominated by convective evaporation, especially in the high vapor quality region. In addition, the heat transfer coefficient decreased significantly when the oil circulation ratio was larger than 0.1 mass%. For the pressure drop characteristics, in the case of pure CO₂, the homogeneous flow model agreed with the experimental results within ±30% for the smooth tube. The pressure drops of the micro‐fin tube were 0–70% higher than those predicted with the homogeneous flow model, and the pressure drops increased for the high oil circulation ratio and high vapor quality conditions. The increases in the pressure drops were considered to be due to the increase in the thickness of the oil film and the decrease in the effective flow cross‐sectional area. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20287     

Two-Phase Frictional Pressure Drop and Flow Boiling Heat Transfer for R245fa in a 2.32-mm Tube

Experimental two-phase frictional pressure drop and flow boiling heat transfer results are presented for a horizontal 2.32-mm ID stainless-steel tube using R245fa as working fluid. The frictional pressure drop data was obtained under adiabatic and diabatic conditions. Experiments were performed for mass velocities ranging from 100 to 700 kg m^?2 s^?1, heat flux from 0 to 55 kW m^?2, exit saturation temperatures of 31 and 41°C, and vapor qualities from 0.10 to 0.99. Pressures drop gradients and heat transfer coefficients ranging from 1 to 70 kPa m^?1 and from 1 to 7 kW m^?2 K^?1 were measured. It was found that the heat transfer coefficient is a strong function of the heat flux, mass velocity, and vapor quality. Five frictional pressure drop predictive methods were compared against the experimental database. The Cioncolini et al. (2009) method was found to work the best. Six flow boiling heat transfer predictive methods were also compared against the present database. Liu and Winterton (1991), Zhang et al. (2004), and Saitoh et al. (2007) were ranked as the best methods. They predicted the experimental flow boiling heat transfer data with an average error around 19%.     

Prediction of Local Bundle Boiling Heat Transfer Coefficients: Pure Refrigerant Boiling on Plain,Low Fin,and Turbo-BII HP Tube Bundles

New correlations predicting the local boiling heat transfer coefficients for vertical flow over a tube bundle have been developed. These correlations were developed for three different classes of tubes: smooth, low-finned, and Turbo-BII HP. These correlations are based on experiments with refrigerants R-134a, R-507A, and R-410A, at a saturation temperature of 4.4°C (40°F). Each one accounts for the local flow structure and/or local geometry particular to each tube. Each one also uses a void fraction model designed specifically for vertical two-phase flow in a tube bundle as an integral part of the correlation. The correlations have been compared to recent local bundle boiling data and show good agreement to within ± 20%.     

Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998     

Boiling Heat Transfer for Freon R21 in Rectangular Minichannel

In this paper, we study the boiling heat transfer of upward flow of R21 in a vertical mini-channel with a size of 1.6 × 6.3 mm. The heat transfer coefficient was measured as a function of heat flux for a wide range of vapor quality and for two levels of mass flow rate, G = 215 kg/m²s and G = 50 kg/m²s. The standard deviation of wall superheat over channel perimeter and in time was determined from the measurement of the wall temperature along the channel perimeter. Different heat transfer mechanisms were revealed depending on flow patterns. The main heat transfer mode for large mass flux is convective boiling. We also figure out the mode when the evaporation of thin liquid films makes the essential contribution to heat transfer. The modified models of Liu & Winterton and Balasubramanian & Kandlikar describe the experimental data well for regime when the convective boiling makes the main contribution to the heat transfer.     

Flow boiling heat transfer coefficient of R-134a/R-290/R-600a mixture in smooth horizontal tubes using varied heat flux method

An experimental study on in-tube flow boiling heat transfer of R-134a/R-290/R-600a refrigerant mixture has been carried out under varied heat flux test conditions. The heat transfer coefficients are experimentally measured at temperatures between ?8 and 5 °C for mass flow rates of 3–5 g s^?1. Acetone is used as a hot fluid which flows in the outer tube of diameter 28.57 mm while the refrigerant mixture flows in the inner tube of diameters 9.52 and 12.7 mm. By regulating the acetone flow conditions, the heat flux is maintained between 2 and 8 kW/m² and the pressure of the refrigerant is maintained between 3.2 and 5 bar. The comparison of experimental results with the familiar correlations shows that the correlations over predict the heat transfer coefficients for this mixture when stratified and stratified-wavy flow prevail. Multiple regression technique is used to evolve and modify existing correlations to predict the heat transfer coefficient of the refrigerant mixture. It is found that the modified version of Lavin–Young correlation (1965) predicts the heat transfer coefficient of the considered mixture within an average deviation of ±20.5 %.     

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.     

Boiling Heat Transfer Characteristics for Methane,Ethane and Their Binary Mixtures

Abstract

Methane (R50) and ethane (R170) are the dominated components of natural gas and the important components in mixture refrigerants for the mixture Joule–Thomson refrigeration cycle. In this article, experimental investigations on nucleate pool boiling and flow boiling heat transfer characteristics of R50, R170, and their binary mixtures are presented. The effects of saturation pressure, heat flux, mass flux, concentration, and vapor quality on heat transfer coefficients are analyzed and discussed. Firstly, the pool boiling heat transfer data were compared with six well-known correlations. Labuntsov correlation shows the best agreement with a mean absolute relative deviation (MARD) of 11.3%. Secondly, a new flow boiling heat transfer correlation for pure fluids was proposed based on the asymptotic addition of forced convection and pool boiling. The modified enhancement factor and suppression factor were developed to account for their relative contribution. In addition, in order to consider the mass transfer resistance of mixtures, a new mixture factor was deduced. The new flow boiling heat transfer correlations can well predict the experimental data with the MARD of 9.5% for pure fluids and 8.3% for mixtures.     

Flow boiling in horizontal flattened tubes: Part II – Flow boiling heat transfer results and model

Experiments of flow boiling heat transfer were conducted in four horizontal flattened smooth copper tubes of two different heights of 2 and 3 mm. The equivalent diameters of the flattened tubes are 8.6, 7.17, 6.25, and 5.3 mm. The working fluids were R22 and R410A. The test conditions were: mass velocities from 150 to 500 kg/m² s, heat fluxes from 6 to 40 kW/m² and saturation temperature of 5 °C. The experimental heat transfer results are presented and the effects of mass flux, heat flux, and tube diameter on heat transfer are analyzed. Furthermore, the flow pattern based flow boiling heat transfer model of Wojtan et al. [L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part I – A new diabatic two-phase flow pattern map, Int. J. Heat Mass Transfer 48 (2005) 2955–2969; L. Wojtan, T. Ursenbacker, J.R. Thome, Investigation of flow boiling in horizontal tubes: Part II – Development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes, Int. J. Heat Mass Transfer 48 (2005) 2970–2985], using the equivalent diameters, were compared to the experimental data. The model predicts 71% of the entire database of R22 and R410A ±30% overall. The model predicts well the flattened tube heat transfer coefficients for R22 while it does not predicts well those for R410A. Based on several physical considerations, a modified flow boiling heat transfer model was proposed for the flattened tubes on the basis of the Wojtan et al. model and it predicts the flattened tube heat transfer database of R22 and R410A by 85.8% within ±30%. The modified model is applied to the reduced pressures up to 0.19.     

Dispersed flow film boiling in vertical narrow annular gaps

Dispersed flow film boiling heat transfer in vertical narrow annular gaps with gap sizes of 1.0, 1.5 and 2.0 mm was experimentally investigated with de-ionized water as the working fluid at low mass velocities. Comparisons of the experimental data with established correlations show that the correlations are not accurate for small gaps. The influences of the heating mode (only one tube heating or both tubes heated), the gap size and the tube diameter were analyzed. The data was correlated in the form of the Groeneveld equation with a modified wall temperature factor as use in the Polomik correlation and a modified gap size factor as use in the Yun and Muthu correlation. A new correlation was developed for dispersed flow film boiling heat transfer based on the experimental data for 1.0–2.0 mm gaps.     

Generalized neural network correlation for flow boiling heat transfer of R22 and its alternative refrigerants inside horizontal smooth tubes

The correct prediction of refrigerant boiling heat transfer performance is important for the design of evaporators. A generalized neural network correlation for boiling heat transfer coefficient of R22 and its alternative refrigerants R134a, R407C and R410A inside horizontal smooth tubes has been developed in this paper. Four kinds of dimensionless parameter groups from existing generalized correlations are selected as the input of neural network, while the Nusselt number is used as the output. Three-layer perceptron is employed as the universal approximator to build the relationship between the input and output parameters. The neuron number of hidden layer is determined by the performance of model accuracy and the standard sensitivity analysis. The experimental data of the four refrigerants in open literatures are used for correlation. The results show that the input parameter group based on the Gungor–Winterton correlation is better than the other three groups. Compared with the experimental data, the average, mean and root-mean-square deviations of the trained neural network are 2.5%, 13.0% and 20.3%, respectively, and approximately 74% of the deviations are within ±20%, which is much better than that of the existing generalized correlations.     

A generalized correlation for evaporation heat transfer of refrigerants in micro-fin tubes

A generalized correlation for flow boiling heat transfer in horizontal micro-fin tubes was developed by implementing non-dimensional parameters accounting for heat transfer enhancement over smooth tubes and physical phenomena into the basic form of a smooth tube correlation. The enhancement factor in nucleate boiling consists of surface tension and turbulence effects generated by the liquid flow. A modified Reynolds number having some similarities with the roughness Reynolds number is introduced into the model to estimate heat transfer enhancement in convective boiling. The ratio of the liquid film thickness to the fin height is also employed in the correlation. The database of the present correlation includes 749 data points for five different refrigerants. The present correlation has a relatively simpler form to employ and yields closer fit to the experimental data with a mean deviation of 20.5% as compared to the existing correlations in the literature.     

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.