Monodisperse Spray Cooling of Small Surface Areas at High Heat Flux

Spray cooling is an effective method to remove high heat fluxes from electronic components. To understand the physical mechanisms, this work studies heat transfer rates from single and dual nozzle distilled water sprays on a small heated surface (1.3 mm × 2 mm). Thermal ink jet atomizers generate small droplets, 33 μm diameter, at known frequencies, leading to controlled spray conditions with a monodisperse stream of droplets interacting with the hot surface. Of particular interest in this work is the dissipated heat flux and its relation to the liquid film thickness, the surface superheat, and the cooling mass flow rate. Experimental results show the heat flux scales to the cooling mass flow rate. In comparison to published spreading–splashing correlations, these experiments indicate that the drops impinge on the liquid film and spread without generating splashing, leading to high-efficiency stable heat transfer. Surface temperatures range from 120 to 140°C. In addition, the liquid film thickness is investigated in relation to the heater superheat and a stable thin film is seen at superheats beyond 20°C. The efficiency of the spray system is inversely related to the film thickness and may be due to ejection of liquid from the surface due to bursting of vapor bubbles.     

Optimal Heat Input for Estimating Luikov's Parameters in a Heat and Mass Transfer Problem

In this article, the problem of computing an optimal heat input in Luikov's heat and mass transfer problem is detailed and analyzed. The main objective is the establishment of an optimal time-dependent heat flux profile with the goal of maximizing the temperature and moisture sensitivities of some parameters to this excitation in a drying process. Such maximization makes the estimation of the desired parameters possible, easier, and with limited uncertainty intervals. It also helps to reduce the linearity dependence between the parameters of interest and the number of temperature and moisture sensors used. The estimation of the optimal heat input is obtained with Uzawa's algorithm, while the estimation of parameters is performed with Levenberg-Marquardt's method of minimization of the ordinary least-square criterion. The six dimensionless parameters characterizing Luikov's equations are estimated successfully with this optimal heat flux profile, which also helps to reduce the number of both temperature and moisture sensors needed in the estimation procedure. By doing so, the objective of estimating simultaneously the six parameters which appear in the formulation of Luikov's physical problem is reached by using a limited transient temperature and/or moisture measurements taken anywhere in the drying medium.     

Investigation of Electrohydrodynamically-Enhanced Convective Heat and Mass Transfer from Water Surface

Enhancement of forced flow evaporation rate by applying electric field (corona wind) has been experimentally evaluated in this study. Corona wind produced by a fine wire electrode which was charged with positive high DC voltage impinges to water surface and leads to evaporation enhancement by disturbing the saturated air layer over the water surface. The study was focused on the effects of corona wind velocity, electrode spacing and air flow velocity on the level of evaporation enhancement. Two sets of experiments, i.e., with and without electric field, have been conducted. Data obtained from the first experiment were used as reference for evaluation of evaporation enhancement at the presence of electric field. Applied voltages ranged from corona threshold voltage to spark over voltage at 1 kV increments. The results showed that corona wind has great enhancement effect on the water evaporation rate, but its effectiveness gradually diminishes by increasing air flow velocity. Maximum enhancement ratios were 7.3 and 3.6 for air velocities of 0.125 and 1.75 m/s, respectively. Finally two empirical correlations were obtained for prediction of electrohydrodynamic evaporation enhancement and its coefficient of thermal performance.     

Heat and Mass Transfer in Capillary-Pumped Liquid Films in Square Cross-Section Minichannels

This study addresses heat and mass transfer during the vaporization of a liquid in a heated square cross-section mini-channel. A theoretical model is developed in steady state using the radius of curvature as a variable. One-dimensional simulations have been performed. An analysis of this model reveals that heat and mass transfer is governed by two main groups of non-dimensional numbers (i.e., Reynolds × Boiling and Weber × Boiling² numbers). Maps of heat transfer performance are thus proposed according to these non-dimensional numbers. A reduced model is finally derived, allowing the main parameters to be expressed (such as the extended meniscus length) analytically.     

Evaporative Cooling and Heat Transfer Augmentation Related to Reduced Condenser Temperatures

In recent articles evaporative cooling has proved to be the utmost air-side heat transfer augmentation. This kind of augmentation, when applied to steam condensers in power plants or to refrigerants in air conditioner systems, results in a lowering of the condensing temperature, even below ambient levels. This yields a remarkable increase of thermodynamic efficiencies of both processes and, therefore, a reduction of energy consumption. The combined heat and mass transfer processes in an evaporatively cooled device are very complex, due to the vast numbers of parameters which must be considered. Previous studies have assumed it impossible to perform any analysis unless many simplifying assumptions are made. The present article indicates that analysis can be performed where none of the assumptions originally used by Merkel are made. It is demonstrated by experiments that plate-fin tube and bare tube condenser performances can be predicted reliably. The insensitivity of the analysis to geometric design eliminates the need for extensive model tests and for production of correction and design factors for the design of evaporative condensers.     

无沸腾喷雾冷却中流量和喷头高度对换热性能的影响

采用薄膜电阻加热器进行了喷头进口压力，喷头类型，喷头高度对换热系数影响的实验研究。研究了冷却介质的质量流量对换热性能的影响，并测量了同一喷头在不同喷头高度下的换热系数大小。实验发现当喷雾面积近似等于实验用薄膜加热器面积时冷却能力达到最大。根据以上实验结果可以最优化喷雾冷却性能。     

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.     

Experimental Investigation of Microbial Fouling and Heat Mass Transfer Characteristics on Ni-P Modified Surface of Heat Exchanger

Fouling deposition problem on heat transfer surface is widely distributed in the field of energy and chemical industry,and microbial fouling is a common fouling type in heat exchanger.In this article,the surface modification was used for inhibiting or mitigating the microbial fouling deposition on heat exchange surface.Firstly,the experimental system for real-time monitoring the fouling deposition process was built,and then the Ni-P modified surface was prepared.Further,the slime forming bacteria(SFB)microbial fouling characteristics and corresponding influencing factors on Ni-P modified surface were investigated experimentally.The results indicated that Ni-P modified surface had an excellent fouling inhibition property.Comparing with carbon steel,Ni-P modified surface reduced the fouling heat resistance by 80%.Accordingly,the influencing factors of microbial fouling deposition including temperature,flow rate and microbial concentrations were discussed.With cooling water temperature increasing given in the experiment arrangement,the microbial fouling resistance was increased first and then decreased,while with bacteria concentration and flow rate increasing,the fouling resistance was increased and decreased separately.The work can provide experimental reference for the fouling inhibition surface development and fouling inhibition mechanism study.     

竖直螺旋槽管壁面液膜在蒸发/冷凝时的传热特性的研究

研究竖直螺旋槽管壁面液膜在传热条件下的液膜形成及流动特性，建立了单组分流体的物理和数学模型并得出解析解，且分析了壁面液膜在蒸发，冷凝及无热传输时的液膜厚度分布及速度分布，结果表明，液膜的形状主要受表面张力影响，在表面内弯处流膜较厚，而在槽道起始部液膜较薄，相对于光滑直管，竖直螺旋槽管壁面液膜具有均匀的厚度分布和更好的传热传质性质，特别在冷凝时壁面液膜更薄且分布更加均匀。     

太阳能热水器传热传质和强化传热研究

在低温太阳能光热光伏联合应用试验台的基础上,结合GB/T 17049—2005,利用Gambit、Ansys Fluent和Tecplot软件,对全玻璃真空管太阳能热水器进行传热传质和强化传热分析。结果表明:所建立的二维数值计算模型,能准确反映同一条件下,全玻璃真空管太阳能热水器的变化趋势;在数值模拟基础上,确定了单面受热时的最佳安装角度为51°,加装反光板类似双面受热的最佳安装角度为38°;在粗略估算和细化分析的基础上,确定了不同真空管结构的最佳导流板长度及安装位置;通过实验和数值模拟,确定了58mm×1 800mm为优化的全玻璃真空管结构。     

Experimental and Numerical Study on Heat and Mass Transfer of Cross-Flow Liquid Desiccant Dehumidifier/Regenerator

Abstract

A liquid desiccant air dehumidification system driven by heat pump was established. The performance of cross-flow dehumidifier/regenerator was experimentally investigated. The empirical correlations of Sherwood number for dehumidification/regeneration were obtained by fitting the experimental data. On the basis of the empirical correlations of Sherwood number and thermodynamics analysis of heat and mass transfer process for dehumidifier/regenerator, a cross-flow heat and mass transfer model was established. The effects of air and solution parameters on the dehumidification/regeneration performance were analyzed. The number of mass transfer units and the height-to-length ratio of the packing module were also studied. The results show that there exist optimal number of mass transfer units and height-to-length ratio in the dehumidifier/regenerator.     

Subcooled Boiling Heat Transfer of CO2 in A Horizontal Tube at Low Temperatures

This article presents an experimental investigation on the subcooled flow boiling heat transfer characteristics of CO₂ in a horizontal tube with inner diameter of 6.16 mm below ?30°C. The effects of mass and heat fluxes and saturation temperature on the heat transfer coefficient are discussed. Large deviations are noted between the predictions from previous empirical correlations and the current CO₂ experimental data. Hence a new empirical correlation is developed, which agrees within ±30% with the current CO₂ experimental data. 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.     

喷雾高度和流量对不同表面结构传热系数影响的实验研究

设计搭建了喷雾冷却实验台,以去离子水为冷却工质,研究了喷雾高度和流量对光滑表面和方肋表面传热系数的影响。喷雾高度从29 mm降低到10 mm,喷雾流量的变化范围为20~32 L/h。实验结果表明：喷雾高度从29 mm降低到14 mm,表面传热系数增加72%,而从14 mm降低到10 mm,表面传热系数仅增加2.7%;喷雾流量从20 L/h增加到32 L/h,表面温度降低2.4 ℃,表面传热系数增加10.6%;在相同的实验工况下,方肋表面的传热系数始终大于光滑表面。     

Heat and Mass Transfer in Open-Cycle OTEC Systems

The temperature difference between surface and deep water in the oceans represents a vast resource of thermal energy. A promising method of harnessing this resource is the open-cycle ocean thermal energy conversion (OC-OTEC) system, which utilizes steam evaporated from the surface water to power the turbine. In this paper the state of the art of heat and mass transfer related to evaporation and condensation of steam at low pressures in OC-OTEC is summarized and relevant research issues are discussed.     

自由表面摩擦和蒸发对过冷下降液膜传热的影响

从理论上对下降液膜在自由表面上存在反向剪切力和蒸发散热情况下的换热特性进行了分析,得到了膜厚、换热系数的无量纲关系式,讨论了剪切力、液膜雷诺数、壁面热流、蒸发率对流动和传热的影响。     

Heat and Mass Transfer Modeling and Simulation during Liquid Route Processing of SiC/Ti Filamentary Composites

To process SiC/Ti filamentary composites using a liquid route method, it is first necessary to overcome various major difficulties such as, high speed filament/matrix coupling, liquid titanium wetting of filament surfaces, and reduction of filament/matrix interaction. All of these requirements depend mainly on the heat and mass transfer, which occurs as the filament runs through a liquid titanium bath. Consequently, these transfers were modeled and simulated numerically during the different processing steps, particularly the cooling step. The results describe the physical phenomena which occur during the process: the carbon transfer from the carbon coated SiC filament to the liquid titanium, heat exchanges, formation of the TiC interphase at the filament surface, and, finally, the solidification of the titanium coating. Numerical simulation has shown the strong influence of running speed which governs the wettability of the filament by the liquid metal. Furthermore, the effects of an additional specific cooling device have been highlighted.     

Approximate Analytical Solution of Stagnation Point Flow and Heat Transfer over an Exponential Stretching Sheet with Convective Boundary Condition

This study is concerned with the stagnation point flow and heat transfer over an exponential stretching sheet via an approximate analytical method known as optimal homotopy asymptotic method (OHAM). The governing partial differential equations are converted into ordinary nonlinear differential equations using similarity transformations available in the literature. The heat transfer problem is modeled using two‐point convective boundary condition. These equations are then solved using the OHAM approach. The effects of controlling parameters on the dimensionless velocity, temperature, friction factor, and heat transfer rate are analyzed and discussed through graphs and tables. It is found that the OHAM results match well with numerical results obtained by Runge–Kutta Fehlberg fourth‐fifth order method for different assigned values of parameters. The rate of heat transfer increases with the stretching parameter. It is also found that the stretching parameter reduces the hydrodynamic boundary layer thickness whereas the Prandtl number reduces the thermal boundary layer thickness.     

Heat and Mass Transfer in Multicomponent Condensation and Boiling

This paper reviews the state of the art in design methods for multicomponent condensation and boiling. In multicomponent condensation, a range of methods of varying complexity are available to calculate the local heat and mass transfer rates; the complexity depends on the detail with which the mass transfer resistances are calculated. These methods are briefly reviewed and their interrelationships along with those of their expertmental validation are discussed. Mixture boiling research has thus far followed different lines and very little effort has gone into understanding the liquid multicomponent diffusion process in boiling. Research work has been mainly concentrated on understanding the dynamics of bubble growth; heat transfer calculations are done using empirical corrections to coefficients calculated for pure fluids. The extensive research in these areas is reviewed and also the few previous studies on convective two-phase heat transfer to mixtures are described. In conclusion, possible design recommendations are offered. It is suggested that, although condensation and boiling heat transfer in mixtures have hitherto been studied separately, a combined research approach may produce further advances.