Recent developments in EHD Enhanced Heat Transfer

This paper considers that the case for using electrohydrodynamic (EHD) enhancement of heat transfer has been established, especially in thermodynamic renewable energy applications where temperature levels are relatively low. It goes on to establish the basis on which nucleate boiling heat transfer is enhanced by EHD forces at surfaces designed to improve condensation, giving experimental results for a six-tube, shell/tube heat exchanger boiling R12 at “Io-fin” surfaces as well as for single-tube tests using “Thermoexcel” and “Gewa-T” surfaces.     

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.     

Two-Phase Flow and Boiling Heat Transfer in Small-Diameter Tubes

For the development of a high-performance heat exchanger using small channels or minichannels for air-conditioning systems, it is necessary to clarify the characteristics of vapor‐liquid two-phase flow and heat transfer of refrigerants in small-diameter tubes. In this keynote paper, the related research works that have already been performed by the author and coworkers are introduced. Based on the observations and experiments of R410A flowing in small-diameter circular and noncircular tubes with hydraulic diameter of about 1 mm, the characteristics of vapor‐liquid two-phase flow pattern and boiling heat transfer were clarified. In low quality or mass flux and low heat flux condition, in which the flow was mainly slug, the “liquid film conduction evaporation” heat transfer peculiar to small-diameter tubes prevailed and exhibited considerably good heat transfer compared to nucleate boiling and forced convection evaporation heat transfer. The effects of the tube cross-sectional shape and flow direction on the heat transfer primarily appeared in the region of the “liquid film conduction evaporation” heat transfer. A new heat transfer correlation considering all of three contributions has been developed for small circular tubes.     

Bubble Dynamics and Heat Transfer during Pool and Flow Boiling

A large number of studies of bubble growth rate and departure diameter have been reported in the literature. Because of uncertainty in defining the shape of an evolving interface, empirical constants are invariably used to match the model predictions with data. This is especially true when force balance is made on a vapor bubble to determine the departure diameter. In this paper, the results of an alternate approach based on a complete numerical simulation of the process are given. Single and multiple bubbles are considered for both pool and flow boiling. The simulations are based on the solution of the conservation equations of mass, momentum, and energy for both phases. Interface shape is captured through a level set function. A comparison of bubble shape during evolution, bubble diameter at departure, and bubble growth period is made with data from well-controlled experiments. Among other variables, the effect of magnitude of gravity and contact angle is explicitly investigated.     

Pressure Drop,Boiling Heat Transfer and Flow Patterns during Flow Boiling in a Single Microchannel

The boiling heat transfer and two-phase pressure drop of water in a microscale channel were experimentally investigated. The tested horizontal rectangular microchannel had a hydraulic diameter of 100 μ m and length of 40 mm. A series of microheaters provided heat energy to the working fluid, which made it possible to control and measure the local thermal conditions in the direction of the flow. Both the microchannel and microheaters were fabricated using a micro-electro-mechanical systems (MEMS) technique. Flow patterns were obtained from real-time flow visualizations made during the flow boiling experiments. Tests were performed for mass fluxes of 90, 169, and 267 kg/m²s and heat fluxes from 200 to 500 kW/m². The effects of the mass flux and vapor quality on the local flow boiling heat transfer coefficient and two-phase frictional pressure gradient were studied. The evaluated experimental data were compared with existing correlations. The experimental heat transfer coefficients were nearly independent of the mass flux and vapor quality. Most of the existing correlations did not provide reliable heat transfer coefficient predictions for different vapor quality values, nor could they predict the two-phase frictional pressure gradient except under some limited conditions.     

Correlation of the Flow Pattern and Flow Boiling Heat Transfer in Microchannels

Flow boiling in microchannels is characterized by the considerable influence of capillary forces and constraint effects on the flow pattern and heat transfer. In this article we utilize the features of gas–liquid flow patterns in rectangular microchannels under adiabatic conditions to explain the regularities of refrigerants flow boiling heat transfer. The flow-pattern maps for the upward and horizontal nitrogen–water flow in a microchannel with the size of 1500 × 720 μm were determined via dual-laser flow scanning and compared with corrected Mishima and Ishii prediction. Flow boiling heat transfer was studied for vertical and horizontal microchannel heat sink with similar channels using refrigerants R-21 and R-134a. The data on local heat transfer coefficients were obtained in the range of mass flux from 33 to 190 kg/m²-s, pressure from 1.5 to 11 bar, and heat flux from 10 to 160 kW/m². The nucleate and convective flow boiling modes were observed for both refrigerants. It was found that heat transfer deterioration occurred for annular flow when the film thickness became small to suppress nucleate boiling. The mechanism of heat transfer deterioration was discussed and a model of heat transfer deterioration was applied to predict the experimental data.     

Refrigerant Condensation Flow Regimes in Enhanced Tubes and Their Effect on Heat Transfer Coefficients and Pressure Drops

Flow regimes influence the heat and mass transfer processes during two-phase flow, implying that any statistically accurate and reliable prediction of heat transfer and pressure drop during flow condensation should be based on the analysis of the prevailing flow pattern. Many correlations for heat transfer coefficient and pressure drop during flow condensation completely ignored flow regime effects and treated flows as either annular or non-stratified flow or as stratified flow. This resulted in correlations of poor accuracy and limited validity and reliability. Current heat transfer coefficient, pressure drop, and void fraction models are based on the local flow pattern, though, resulting in deviations of around 20% from experimental data. There are, however, several inconsistencies and anomalies regarding these models, which are discussed in this paper. A generalized solution methodology for two-phase flow problems still remains an elusive goal, mainly because gas-liquid flow systems combine the complexities of turbulence with those of deformable vapor-liquid interfaces. The paper focuses on the state of the art in correlating flow condensation in micro-fin tubes and proposes flow regime-based correlations of heat transfer coefficient and pressure drop for refrigerant condensation in smooth, helical micro-fin, and herringbone micro-fin tubes.     

On the Prediction of Heat Transfer in Micro-Scale Flow Boiling

Xu et al. have recently published a set of results for boiling heat transfer measurements in a multi-channel micro-scale evaporator for flow boiling of acetone in triangular cross-section channels (hydraulic diameter of 155.4 mm). In the present collaboration, we assess our current capability to predict this independent flow boiling data set with a fluid not in the original database and also much smaller in size using the phenomenological three-zone model of Thome, Dupont, and Jacobi. The method models boiling in small diameter channels in the elongated bubble/slug flow regime. The boiling data falling in this regime are identified here using a new micro-scale flow pattern map proposed by Revellin in order to utilize only test data corresponding to the elongated bubble flow mode. The decrease of the measured wall temperature due to the heat spread by longitudinal conduction through the heat sink was investigated through a finite differences analysis. In addition, a data reduction procedure different than that one used by Xu et al. was used and, consequently, some differences in the heat transfer behavior were found. Based on the present database, a new set of empirical parameters for the three-zone model was proposed. The conjugated effect of flow pattern and bubble/slug frequency on the heat transfer coefficient was also investigated.     

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.     

缸内对流换热与气体流动的计算分析

摘要本文将内燃机燃烧室简化成轴对称的二维空间,将计算缸内流动的二维模型与边界层模型相结合,分析了缸内气体的迁移特性与对流换热.文中介绍了在内燃机工作过程中缸内气体边界层的分布与变化、边界层对对流换热的影响,给出了对流挟热系数沿燃烧室表面的分布与变化.与实测结果的比较表明,本文的模型具有较高的精度.     

Flow Patterns and Heat Transfer for Flow Boiling in Small to Micro Diameter Tubes

An overview of the recent developments in the study of flow patterns and boiling heat transfer in small to micro diameter tubes is presented. The latest results of a long-term study of flow boiling of R134a in five vertical stainless-steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1, and 0.52 mm are then discussed. During these experiments, the mass flux was varied from 100 to 700 kg/m²s and the heat flux from as low as 1.6 to 135 kW/m². Five different pressures were studied, namely, 6, 8, 10, 12, and 14 bar. The flow regimes were observed at a glass section located directly at the exit of the heated test section. The range of diameters was chosen to investigate thresholds for macro, small, or micro tube characteristics. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapor quality for low quality values. At higher quality, the heat transfer coefficients decreased with increasing quality, indicating local transient dry-out, instead of increasing as expected in macro tubes. There was no significant difference between the characteristics and magnitude of the heat transfer coefficients in the 4.26 mm and 2.88 mm tubes but the coefficients in the 2.01 and 1.1 mm tubes were higher. Confined bubble flow was first observed in the 2.01 mm tube, which suggests that this size might be considered as a critical diameter to distinguish small from macro tubes. Further differences have now been observed in the 0.52 mm tube: A transitional wavy flow appeared over a significant range of quality/heat flux and dispersed flow was not observed. The heat transfer characteristics were also different from those in the larger tubes. The data fell into two groups that exhibited different influences of heat flux below and above a heat flux threshold. These differences, in both flow patterns and heat transfer, indicate a possible second change from small to micro behavior at diameters less than 1 mm for R134a.     

Bubble Dynamics,Flow, and Heat Transfer during Flow Boiling in Parallel Microchannels

Significant efforts have recently been made to investigate flow boiling in microchannels, which is considered an effective cooling method for high-power microelectronic devices. However, a fundamental understanding of the bubble motion and flow reversal observed during flow boiling in parallel microchannels is lacking in the literature. In this study, complete numerical simulations are performed to further clarify the boiling process by using the level-set method for tracking the liquid–vapor interface which is modified to treat an immersed solid surface. The effects of contact angle, wall superheat, and the number of channels on the bubble growth, reverse flow, and heat transfer are analyzed.     

Heat Transfer Coefficient Correlation for Convective Boiling Inside Plain and Microfin Tubes Using Genetic Algorithms

Two-phase flow heat transfer has been exhaustively studied in recent years. However, in this field, several questions remain unanswered. Heat transfer coefficient prediction related to nucleate and convective boiling has been studied using different approaches—numerical, analytical, and experimental. In this study, an experimental analysis, data representation, and heat transfer coefficient prediction of two-phase heat transfer in nucleate and convective boiling are presented. An empirical correlation is obtained, based on a genetic algorithms search engine, of a dimensional analysis of the two-phase flow heat transfer problem.     

Heat Transfer Correlations for Elongated Bubbly Flow in Flow Boiling Micro/Minichannels

An improved conventional-to-micro/minichannel criterion was proposed by using the Bond number and the liquid Reynolds number. In micro/minichannels, bubbles tend to be confined and elongated in the channel and the conventional two-phase flow theory loses its applicability. As significant disagreement in experimental trends and heat transfer mechanisms was reported for flow boiling in micro/minichannels in the literature, it is not possible to explain the discrepancy and predict all data points by a single correlation without considering the different flow patterns. In this study, heat transfer correlations for elongated bubbly flow in flow boiling micro/minichannels were developed based on a collected micro/minichannel heat transfer database. The newly developed correlations not only can present a decent overall accuracy, but also estimate the parametric trends correctly. More than 97% of the data points can be predicted by the proposed correlations within a ±50% error band for elongated bubbly flow. Also, a flow-pattern-based model can be developed by combining the developed elongated bubbly flow correlations with previous annular flow correlations for predicting flow boiling heat transfer in micro/minichannels.     

换热管周围声流强化对流传热的数值模拟

为研究稳态声流成分对换热管对流换热特性的影响,建立了行波场中单换热管外声流强化传热的数值计算模型.采用分离时间尺度的数值方法模拟了声流和非均匀温度场的耦合作用,分析了换热管局部努塞尔数和平均努塞尔数在不同激励频率(10～1500 Hz)和声压级(103～127 dB)作用下的变化规律.结果 表明:平均努塞尔数随着激励频...     

Flow Boiling Heat Transfer Characteristics in Minitubes With and Without Hydrophobicity Coating

Abstract

Wettability plays an important role during flow boiling inside micro and mini channels. The present work focuses on the flow boiling heat transfer characteristics inside copper minitube (inner diameter of 3?mm) coated internally to render the inside surface nearly hydrophobic. Electroless Galvanic Deposition technique is employed for hydrophobic coating inside the copper tube. Both single phase heat transfer and two-phase flow boiling heat transfer and pressure drop characteristics were investigated in regular and internally coated hydrophobic copper minitubes. The experiments were performed with deionized water as a working fluid and the mass flux was varied from 100 to 650?kg/m²s. The two-phase heat transfer characteristics was observed to be both functions of mass flux as well as heat flux. The two phase heat transfer has been observed to be augmented due to the wettability within the tubes. The two-phase pressure drop has also been observed to increase when compared to the regular, uncoated tube; however, the proportional increment is lower than the augmentation achieved in two-phase heat transfer. The enhanced heat transfer effects observed have been explained on the basis of wetting physics.     

Prediction of Heat Transfer Coefficient in Saturated Flow Boiling Heat Transfer in Parallel Rectangular Microchannel Heat Sinks: An Experimental Study

This study focuses mainly on the prediction of saturated flow boiling heat transfer in microchannels. A wide range of experiments has been carried out with de-ionized water to obtain a comprehensive data set. Experiments of mass fluxes of 51–728.7 kg/m²s, wall heat fluxes of 36–221.7 kW/m², vapor qualities of 0.01–0.69, liquid Reynolds number of 7.72–190, aspect ratios of 0.37–5.00 (with a constant hydraulic diameter of 100 µm) and hydraulic diameters of 100–250 µm (for constant aspect ratio = 1). A new correlation including the aspect ratio effect is proposed to predict the heat transfer coefficient for saturated flow boiling in microchannels. The proposed correlation shows very good predictions with an overall mean absolute error of 16.9% and 86.4%, 96.2% and 99.5% of the predicted data falling within ±30, ±40 and ±50% error bands, respectively.     

Facile Fabrication of Nanostructured Microchannels for Flow Boiling Heat Transfer Enhancement

Flow boiling in microchannels promises high heat transfer due to the combined effect of latent heat of vaporization and forced convection in confined spaces. However, flow boiling based miniaturized thermal management devices are limited due to instability induced dryout. While several efforts have been made to delay instabilities via advanced surface modification techniques, there is a need to expand the scope of applications by developing low-cost and scalable fabrication technologies for commonly used heat exchanger materials. In this paper, we use a facile and self-limiting chemical oxidation technique for fabricating sharp needle-like superhydrophilic CuO nanostructures within six parallel 500 × 250 µm² microchannels spread uniformly over a 1 × 1 cm² area in a copper heat sink. We demonstrate heat transfer enhancement with nanostructured microchannels (NSM) without any appreciable change either in the average pressure drop or the fluctuations in comparison to baseline plain wall microchannels (PWM). Analysis of the high-speed images was performed to attribute the enhancement with NSM to the presence of a capillarity-fed thin-film evaporation regime, which otherwise was absent in PWM. We believe that these results are encouraging and suggest that the heat sink geometry can be optimized to investigate the true potential of nanostructured microchannels.     

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.     

多孔微通道流动沸腾换热特性的实验研究

本文通过实验的方法对烧结的多孔微通道和铜基微通道的沸腾换热性能和流动不稳定进行研究.实验工质选用去离子水,采用的铜粉粒径分别为30μm、50μm、90 μm,烧结底厚为200 μm和400 μm.采取控制变量的方式,研究改变入口温度、铜粉粒径大小、入口流量对多孔微通道和铜基微通道换热性能的影响.研究表明:多孔微通道最优的厚度粒径比在2～5之间,在此区间的多孔微通道可以提高沸腾传热的性能.其中厚度粒径比为2和4的多孔微通道的最大换热系数是铜基微通道的换热系数的5倍.多孔微通道相对于铜基微通道有更好的换热能力,有着较低的壁面温度.