Enhanced Boiling Heat Transfer of the Compact Staggered Tube Bundles under Sub-Atmospheric Pressures

An experimental investigation was carried out on the boiling heat transfer enhancement of water on plain tubes in compact staggered tube-bundle evaporators under atmospheric and sub-atmospheric pressures. The experiment investigated the effects of the tube spacing and positioning and the test pressure on the boiling heat transfer characteristics in restricted spaces of compact tube bundles. The experimental results indicated that for compact tube bundles, the effect of the tube spacing is very significant on the boiling heat transfer. The boiling heat transfer has a maximum enhancement when the tube spacing is so selected as to take an optimum value. The enhanced heat transfer efficiency for the compact bundles would gradually decrease as the test pressure was reduced.     

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.     

封闭方腔内叉排管间熔融盐自然对流换热

以硝酸锂熔融盐为流体介质,对封闭方腔内交叉排布的四根热管间的介质自然对流换热进行了数值模拟,研究了热管不同位置和不同瑞利数对方腔内介质自然对流换热特性的影响。结果表明:管间距ε=0.5时,腔体内温度场和流场的对称性开始打破;ε=0.4和0.5时,下管及左右两管产生的热羽流冲刷上管热边界层,使上管局部Nu_φ分别在圆周角φ=180°、φ=280°和φ=80°处出现增强;ε=0.6时,上管不再受下管羽流影响。随着管间距增大,上管平均Nu_m越来越小,下管Nu_m越来越大;ε=0.4~0.6时,管间距对左右两管Nu_m影响很小,但ε=0.7时,左右两管平均换热均明显增强。     

多孔表面管沸腾传热试验研究

针对烧制成多孔表面管,进行了传热性能研究,试验表明：多孔管可以显著地强化多孔侧沸腾传热,民同规格光滑管传热性能试验对比,其沸腾给热系数比光滑管提高５－６倍。     

Pool Boiling Heat Transfer in Diluted Water/Glycerol Binary Solutions

Pool boiling heat transfer in water/glycerol binary solutions has been experimentally investigated on a horizontal rod heater. The experiments have been performed at various concentrations (zero to 35% mass glycerol) and heat fluxes up to 92 kW m^?2 at atmospheric pressure. The experimental values of boiling heat transfer coefficient have been compared to main existing correlations. It has been shown that the various predictions are significantly inconsistent. Based on the high difference between relative volatilities of water and glycerol, a simple model has been proposed to predict the boiling heat transfer coefficient. The applicability of this model is limited to low concentrations of glycerol and medium/low heat fluxes; however, the predictions are accurate. The proposed model is anticipated to be extendable to other binary systems in which the vapor pressure of one constituent is considerably higher when compared to the other component.     

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%.     

强化管外CFC11与HCFC123沸腾换热系数的对比试验研究

本文通过强化管外CFC11与HCFC123沸腾换热系数的对比试验分析与研究,分别得出了HCFC123与CFC11沸腾换热系数,并对这两种工质的实际应用情况进行了分析。     

Effect of Nucleation Cavities on Enhanced Boiling Heat Transfer in Microchannels

Boiling instabilities, high temperatures of the onset of boiling (ONB), and early transition to dryout are some of the insufficiently resolved issues of flow boiling in microchannels. This article addresses the flow boiling challenges with the incorporation of flow restrictors to reduce the boiling instabilities and hinder vapor backflows. In addition, the temperature of the ONB was lowered and the heat transfer coefficient was increased during boiling with the fabrication of potential nucleation cavities in the microchannel walls and bottom. Experiments were conducted with degassed double-distilled water in arrays of microchannels with the hydraulic diameter ranging from 25 to 80 µm, whereas the nucleation cavities characteristic sizes varied from 2 to 12 µm. The temperatures of the ONB were up to 35 K lower in the microchannel array with properly sized nucleation cavities compared to arrays of microchannels, in which the etched nucleation cavities were less suitable. The combined effect of fabricated nucleation cavities and interconnected microchannels increased the heat transfer coefficient from three to 10 times depending on the size of the etched nucleation cavities and the transferred heat flux in the microchannel arrays.     

Simulation Study on Flow and Heat Transfer of Cross-Arrangement Tube Bundle

The air precooler of SABRE engine(Synergetic Air-Breathing Rocket Engine) is a tube bundle type of heat exchanger consisting of thousands of small tubes.The cold helium gas flows in the small tube bundle,and the high-speed hot air circulates outside the tubes and exchanges heat with the helium in the tubes.In this study,the cross-arrangement tube bundle consisting of small tubes with outer diameter of 1mm was taken as the research object.The system simulation calculation and data sorting were ca...     

“球囊夹紧法”取出锁骨下动脉支架推送杆断裂残端一例

针对水平光滑管和微肋管,基于FLUENT平台对制冷剂管内沸腾传热特性进行了数值模拟,研究质量流量、热流密度及干度等因素对制冷剂R245fa沸腾换热系数的影响。模拟结果表明:沸腾换热系数随着制冷剂质量流速与热流密度的增加而提高;随着干度的增加,换热系数先增加再降低,并在x=0.7时达到极大值;相比光滑管,微肋管内制冷剂的沸腾传热系数能提高10%~25%。     

Flow Boiling Heat Transfer in A Minichannel with Enhanced Heating Surface

This paper presents the results of flow boiling in a 1.0-mm-deep minichannel with asymmetrical heating. The heating element for the working fluid (FC-72) is a single-sided enhanced alloy foil made from Haynes-230. Two types of enhanced heating surfaces, prepared by laser texturing and with microrecesses varied in terms of size, were used for investigations. The experimental research focused on the transition from single-phase forced convection to nucleate boiling, that is, the zone of boiling incipience and further development of boiling. Flow structure was observed through a glass pane. Owing to the liquid crystal layer placed on the opposite side of the enhanced foil surface, it was possible to observe the onset of flow boiling (as a “boiling front”) and to measure temperature distribution on the heating wall through another glass pane. The objective of the study is to determine void fractions for increasing heat fluxes supplied to the heating surface. The flow structure photos were processed in Corel graphics software and binarized. The analysis of phase volumes was developed in Techsystem Globe and NIS-Elements Advanced Research software. The results of experiments with both types of enhanced heating foil were compared.     

The Effect of Tube Flattening on Flow Boiling Heat Transfer Enhancement

The present study investigated the effect of smooth tube flattening on heat transfer enhancement in an evaporator. The tubes with internal diameter of 8.7 mm were flattened into an oblong shape with different inside heights. The test setup was basically a vapor compression refrigeration system equipped with all necessary measuring instruments. Refrigerant R-134a flowing inside the tube was heated by an electrical coil heater wrapped around it. The ranges of mass velocities were from 74 to 106 kg/m²-s and vapor quality varied from 25% to 95%. Analysis of the collected data indicated that the heat transfer coefficient elevates by increasing the mass velocity and vapor quality in flattened tubes just like the round tube. The flow boiling heat transfer coefficient increases when the flattened tube is used instead of the round tube. The highest heat transfer coefficient enhancement of 172% was achieved for the tube with the lowest inside height at mass velocity of 106 kg/m²-s and vapor quality of 85%. Finally, based on the present experimental results, a correlation was developed to predict the heat transfer coefficient in flattened tubes.     

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.     

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.     

凝汽器管束布置传热效果评价方法的探讨

通过对包头电厂实测数据的分析,对传统的评价凝汽器管束布置修正系数的方法进行了改进,根据试验结果分析并讨论了管束布置修正系数随负荷变化的规律,同时,通过对凝汽器总体传热系数的分析,提出了一种新的评价凝汽器管束布置传热效果的方法。     

换热器内弹性管束固有阵型对换热性能的影响

弹性管束换热器管束的振动可实现强化换热,为了使弹性管束换热器更好的强化换热,本文研究了弹性管束的固有阵型对管束换热特性的影响。采用work bench数值分析方法对弹性管束进行模态分析,得到管束的前十阶固有频率及阵型。采用脉动流激励弹性管束振动,得到弹性管束的类似固有振型,分析管束固有振型变化对弹性管束换热特性的影响。结果表明,对于管束的面外振型,管束固有振型的振幅出现的位置不同,对管束的换热系数影响很大;对于管束的面内振型,随着管束面内振型阶数增大,弹性管束的换热系数增大。     

Prediction of Heat Transfer Coefficient in Flow Boiling over Tube Bundles Using ANFIS

The applicability of the artificial intelligence technique called ANFIS (for adaptive neuro fuzzy inference system) to model the flow boiling heat transfer over a tube bundle is studied in this paper. The ANFIS model is trained and validated with the experimental data from literature. The heat flux, mass flux, and row height are taken as input and the flow boiling heat transfer coefficient as output. The developed model performance is evaluated in terms of performance parameters such as root mean square error, mean square error, correlation coefficient, variance accounted for, and computational time. The preceding parameters of the model are then determined for different combinations of type and number of membership functions. The model is found to predict experimental heat transfer coefficient within an error of ±5%. The developed model is also compared with the artificial neural network model and is found to be better in predicting the flow boiling heat transfer coefficient. The developed model is further used to observe the variation of heat transfer coefficient of the individual rows and bundle for intermediate value of parameters such as heat flux and mass flux that are not included in the analysis of experimental data. The analysis is able to provide complete information about variation of heat transfer coefficient of individual rows and the bundle with respect to heat flux and mass flux.     

EHD Augmented Convective Boiling: Flow Regimes and Enhanced Heat Transfer

This work investigates the influence of electrohydrodynamics (EHD) on the flow and heat transfer during convective boiling of HFE7000. A unique tube-and-shell heat exchanger has been constructed with heated water flowing on the shell side and a saturated mixture of refrigerant flowing within the tube side. The heat exchanger is novel in that it allows full visual access to the flow in the inner tube while being both thermally and electrically conductive. This permits observation of the two-phase flow regimes, which is not possible with metallic test sections. In this work the influence of EHD on the flow regimes and subsequent overall heat transfer is investigated for fixed inlet refrigerant mass flux of 100 kg/m²-s, inlet quality of 3%, and wall superheat of approximately 11.5°C. For these conditions the applied voltage across a concentric inner electrode and the outer wall of the tube was varied between 0 kV and 10 kV at 60 Hz AC. In particular, this work focuses on quantifying the level of overall enhancement that is achievable with EHD for this heat exchanger. This is done in the context of the additional heat extracted by the working fluid in the heat exchanger compared with the field-free case and the additional power penalties required to do so. Heat transfer enhancements of up to 1.8 -fold were realized in this heat exchanger. Even so, there were hydraulic power increases as well as electrical power required to achieve the heat transfer enhancement. It was found that the electrical power was the dominant penalty and that an overall enhancement of 40 times more heat power extracted than input required was achieved. Finally, a proportional–integral–derivative (PID) control system has been utilized in conjunction with a high-voltage amplifier in order to accurately control the heat transfer rate of the heat exchanger. To our knowledge this is the first solid-state control system of this type for a two-phase heat exchanger.     

微细圆管内CO2两相沸腾换热实验研究

本文对CO_2在水平微细管内流动沸腾特性进行实验研究。实验结果表明:热流密度增加对强化核态沸腾换热和高干度区域流型转变具有显著影响,随着热流密度的增加换热系数增加,对摩擦压降影响很小;质量流率对于换热系数的影响较小,但随着质量流率的增加摩擦压降大幅增加,质量流率的大小直接决定了换热过程所经历流态;饱和温度升高换热系数相应升高,摩擦压降减小,且对流态转变特性有重要影响。在同样工况下摩擦压降最大值先于换热系数最大值出现,理论分析采用的流态形式与实际CO_2管内流动流动沸腾换热流态基本一致。     

微/小通道内沸腾换热研究综述

微／小通道紧凑式蒸发器的应用越来越广泛,对其换热特性的深刻认识和进一步研究已成为当前亟待解决的课题,而目前涉及微／小尺度通道内沸腾换热特性和流动方面的研究尚处于起步阶段：本文介绍了近年来国内外微／小通道内沸腾换热方面的研究状况,并指出了该研究领域有待于深入开展研究的内容。