Enhanced boiling heat transfer from microporous surfaces: effects of a coating composition and method

Composition studies (coating component and mixing ratio variations) of micro-porous coatings are performed to produce higher boiling enhancement from the flush-mounted, square heater. Two different coating methods are researched and nearly identical boiling performances are obtained with the optimized coating composition. The physical strength of the micro-porous coating is examined through a durability test and an adhesion test. Finally, the boiling performance of a micro-porous-enhanced surface is compared with that of the commercial High Flux surface.     

Enhanced flow boiling heat transfer of FC-72 on micro-pin-finned surfaces

For the purpose of cooling electronic components with high heat flux efficiently, some experiments were conducted to study the flow boiling heat transfer performance of FC-72 on silicon chips. Micro-pin-fins were fabricated on the chip surface using a dry etching technique to enhance boiling heat transfer. Three different fluid velocities (0.5, 1 and 2 m/s) and three different liquid subcoolings (15, 25 and 35 K) were performed, respectively. A smooth chip (chip S) and four micro-pin-finned chips with the same fin thickness of 30 μm and different fin heights of 60 μm (chip PF30–60) and 120 μm (chip PF30–120), respectively, were tested. All the micro-pin-finned surfaces show a considerable heat transfer enhancement compared to the smooth one, and the critical heat flux increases in the order of chip S, PF30–60 and PF30–120. For a lower ratio of fin height to fin pitch and/or higher fluid velocity, the fluid velocity has a positive effect on the nucleate boiling curves for the micro-pin-finned surfaces. At the velocities lower than 1 m/s, the micro-pin-finned surfaces show a sharp increase in heat flux with increasing wall superheat, and the wall temperature at the critical heat flux (CHF) is less than the upper limit, 85 °C, for the reliable operation of LSI chips. The CHF values for all surfaces increase with fluid velocity and subcooling. The maximum CHF can reach nearly 150 W/cm² for chip PF30–120 at the fluid velocity of 2 m/s and the liquid subcooling of 35 K.     

Enhanced boiling heat transfer in mesochannels

Heat transfer characteristics are studied for a hybrid boiling case that combine features of spray cooling and flow boiling. In such a hybrid system, a liquid is atomized and the surrounding vapor is entrained into the droplet cone to provide an initial quality for enhanced boiling. An in-house experimental setup was developed to obtain surface temperature and heat flux measurements in a series of converged mesochannels for hybrid boiling. To compare the heat transfer performance of this hybrid technique, a flow boiling module was also developed using the same series of converged mesochannels. The inlet and exit hydraulic diameter of the mesochannels was 1.55 and 1.17 mm, respectively. The heat flux was in the range of 15–45 kW/m² and the estimated mass flux varied from 45 kg/m²s at the channel inlet to 110 kg/m²s at the channel outlet. Moreover, a model was presented to predict surface temperatures and heat transfer coefficients for flow boiling and hybrid boiling in mesochannels. This model was developed based on Chen’s formulation (1966) [21] but with two essential modifications. First, the laminar entry length effect was taken into consideration for heat transfer coefficient calculation. Second, the boiling enhancement factor was calculated based on the fluid properties. The model was compared to the experimental data and several other correlations for both cases. This model shows good agreement with the experimental data (mean deviations on the order of 12–16%).     

Investigation of Enhanced Boiling Heat Transfer from Porous Surfaces

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Semi-analytical model for boiling from enhanced structures

Understanding of the fundamental mechanism of boiling from enhanced structures is currently incomplete. The main focus of the present study was to develop a semi-analytical model to predict the bubble departure diameter, frequency, and nucleation site density for a boiling enhancement structure. Existing models in literature were used as a framework to develop this. Salient feature of the model is the improvements on sub-models for bubble departure diameter, evaporation within the channels and convective heat transfer from the external surfaces of the enhanced structure. The model was used to calculate the total heat dissipated from the structures. Comparisons to experiments revealed that the bubble departure is predicted within ±10%, frequency within ±30% for all data points except a couple, nucleation site density within ±40% and the heat flux within ±50%. Sample calculations have been included to show its use in optimizing the geometrical parameters for maximizing heat transfer.     

Experimental investigation of flow boiling heat transfer of jet impingement on smooth and micro structured surfaces

Flow boiling heat transfer experiments using R134a were carried out for jet impingement on smooth and enhanced surfaces. The enhanced surfaces were circular micro pin fins, hydrofoil micro pin fins, and square micro pin fins. The effects of saturation pressure, heat flux, Reynolds number, pin fin geometry, pin fin array configuration, and surface aging on flow boiling heat transfer characteristics were investigated. Flow boiling experiments were carried out for two different saturation pressures, 820 kPa and 1090 kPa. Four jet exit velocities ranging from 1.1–4.05 m/s were investigated. Flow boiling jet impingement on smooth surfaces was characterized by large temperature overshoots, exhibiting boiling hysteresis. Flow boiling jet impingement on micro pin fins displayed large heat transfer coefficients. Heat transfer coefficients as high as 150,000 W/m² K were observed at a relatively low velocity of 2.2 m/s with the large (D = 125 μm) circular micro pin fins. Jet velocity, surface aging, and saturation pressure were found to have significant effects on the two-phase heat transfer characteristics. Subcooled nucleate boiling was found to be the dominant heat transfer mechanism.     

Nucleate boiling heat transfer from a structured surface – Effect of liquid intake

A model of the suction evaporation mode in nucleate boiling from tunnel and pore structures is presented. The model is based on the analysis by Nakayama et al. [W. Nakayama, T. Daikoku, H. Kuwahara, T. Nakajima, Dynamic model of enhanced boiling heat transfer on porous surfaces – Part II. Analytical model, ASME J. Heat Transfer 102 (3) (1980) 451–456] and L.H. Chein and R.L. Webb [A nucleate boiling model for structured enhanced surfaces, Int. J. Heat Mass Transfer 41 (14) (1998) 2183–2195]. Additionally, a detailed phenomenological model of liquid refill has been developed. It has been shown that the process of liquid refill and the time needed for it is strongly dependent on pool height. Effect of liquid pool height on bubble frequency has also been discussed. Finally, a generalized methodology is given for the prediction of boiling data from a structured surface.     

The rate of heat transfer for boiling on porous surfaces

It is shown that the constant, C_sf, introduced by Rohsenow⁶ for correlating pool boiling heat transfer data, may be used in order to characterize plates which have been made porous.     

A wall heat transfer model for subcooled boiling flow

High compactness, low weight and little space requirement are gaining attention as prominent design criteria in the development of modern cooling systems in many applications. The resulting demand for highest possible heat transfer rates has lead to the very promising concept of providing for a controlled transition from pure single-phase convection to subcooled boiling flow in thermally highly loaded regions. For its application in modern engineering design this approach requires a realistic modeling of the complex phenomena associated with the two-phase flow heat transfer. The present work proposes for the computation of the specific wall heat transfer rate a modified superposition model, where the total heat flux is assumed to be additively composed of a forced convective and a nucleate boiling component. Since the present model requires only local input quantities, it is well suited to CFD of geometrically very complex coolant flows, where the definition of global length or velocity scales would be impractical. The wall heat fluxes predicted by the present model were compared against experimental data which were obtained by in-house measurements with water being the working fluid. The overall agreement is very good, particularly, in the partially nucleate boiling regime, where the effect of the bulk flow rate on the heat transfer is significant. Deviations are primarily observed at higher wall superheats, where a strong two-way coupling between the motion of the liquid and the motion of the bubbles as well as considerable bubble–bubble interactions typically occur.     

换热表面直接与间接测温的对比性实验研究

进行了沸腾换热表面的间接测温与直接测温的对比性实验研究。以水和乙醇作工质,对大气压力下的池沸腾换热平表面,用间接测温法和直接测温法同时测量壁面过热度,对两种测温方法的结果进行了比较研究。同时,用直接测温法进行三种不同管径的光管沸腾试验,并把实验结果与Rohsenow公式进行了比较。大量实验结果表明,在一定操作条件下,直接测温法测量沸腾换热表面过热度误差较小,是一种简便而又行之有效的方法。     

多孔表面新型复杂结构优化沸腾传热的实验研究

报道了R11在烧结多孔表面开槽时沸腾传热的实验研究，实验发现，与普通槽道和双空隙层多孔表面相比，沸腾换热增强，沸腾表现为液体灌注、槽道起泡、底部蒸干三个区，对特定的多孔层，开槽可获得更好的换热效果。带槽道的多孔表面实验件与均匀多孔表面相比，在相同壁面过热度（θ）条件下，热流密度（q）提高2－10倍，临界热流密度提高2－4倍。     

Performance of different structured surfaces in nucleate pool boiling

Several structured surfaces have been developed in-house for the augmentation of boiling heat transfer using distilled water as test fluid under atmospheric pressure. These surfaces have either a number of parallel tunnels or orthogonally intersecting tunnels. Effect of design parameters like tunnel inclination and different cavity structure at the tunnel base on the boiling heat transfer has been investigated. Three different structures namely circular groove, rectangular groove and rounded base have been used at the end of the tunnels. Heat flux is varied in the range of 0–250 kW/m². Experimental results showed tunnels inclined at an angle 60° with the horizontal provide better augmentation compared to straight vertical tunnels. Amongst different base geometry the circular pocket produced most conducive condition for the boiling heat transfer. The use of tunnels also increases the degree of augmentation. The highest augmentation was obtained from the surface having intersecting inclined tunnels with a circular base.     

Studies on pool boiling heat transfer: Macrolayer thickness in transition boiling

Macrolayer thicknesses in transition boiling were determined from the energy balance relation q_tr = ρ_lH_fgδ_l·f , based on measurements of q_tr (the time-averaged heat flux in transition boiling) and f (the detachment frequency of vapor masses) for water and ethanol boiling on vertical and horizontal 15-mm-diameter surfaces under atmospheric pressure. The macrolayer thickness for the vertical surface, designed to prevent liquid contact with the periphery of the surface during the vapor mass hovering, agreed well with the correlation proposed previously by the present authors, when the heat flux at macrolayer formation is obtained from a nucleate boiling curve extrapolated to the superheat of transition boiling. The macrolayer on the horizontal surface was apparently thickened due to the inflow of bulk liquid beneath the growing vapor masses. © 1999 Scripta Technica, Heat Trans Jpn Res, 27(8): 568–583, 1998     

沸腾传热的分形学分析

分形论是当代兴起的非线性科学的一个重要组成部分。简要地阐述了分形论形的特征，解释了几何学中的维和物理学中的量纲的不同涵义。把分形论分数维的概念扩展应用到沸腾传热中的非线性问题的分析中，结合沸腾传热的操作特征和基本原理，以沸腾传热中的推动力△Tsat作为一个动力学维，进行了q-△Tsat^Df的标绘；对Df的物理意义作了分析和探讨。     

Flow boiling heat transfer in two-phase micro-channel heat sinks--II. Annular two-phase flow model

This paper is Part II of a two-part study devoted to measurement and prediction of the saturated flow boiling heat transfer coefficient in water-cooled micro-channel heat sinks. Part I discussed the experimental findings from the study, and identified unique aspects of flow boiling in micro-channels such as abrupt transition to the annular flow regime near the point of zero thermodynamic equilibrium quality, and the decrease in heat transfer coefficient with increasing quality. The operating conditions of water-cooled micro-channels fell outside the recommended range for most prior empirical correlations. In this paper, an annular flow model is developed to predict the saturated flow boiling heat transfer coefficient. Features unique to two-phase micro-channel flow, such as laminar liquid and vapor flow, smooth interface, and strong droplet entrainment and deposition effects, are identified and incorporated into the model. The model correctly captures the unique overall trend of decreasing heat transfer coefficient with increasing vapor quality in the low vapor quality region of micro-channels. Good agreement is achieved between the model predictions and heat transfer coefficient data over broad ranges of flow rate and heat flux.     

An analytic study of boiling heat transfer from a fin

Analytic expressions for the one-dimensional temperature distribution in a pin fin or a straight fin of rectangular profile are derived if various types of boiling occur simultaneously at adjacent locations on such a fin's surface. The heat transfer coefficients for the transition and nucleate boiling are taken as being the power functions of the wall superheat and that for film boiling as being constant. The number of cases analysed is 66. Some of the results obtained are compared with those of experiments carried out elsewhere. A quite reasonable degree of agreement is found between the theory and the experiment carried out in practice.     

Enhanced boiling heat transfer in restricted spaces of a compact tube bundle with enhanced tubes

In flooded-type tube bundle evaporators with smooth tubes and general tube gaps, both wall superheat and heat flux are generally quite low and boiling cannot occur on the heated tubes. But when the tube gap is quite small or the enhanced heat transfer tubes are employed, the incipient boiling can occur at low heat flux levels and results in a significant heat transfer enhancement effect. This study investigates experimentally enhancement effects by the restricted space comprising the compact tube bundle and the enhanced tubes for boiling heat transfer of pure water and salt-water mixtures under atmospheric pressure. The experimental results show that the small tube gaps can greatly enhance boiling heat transfer for the compact enhanced tube bundle.     

Pool boiling heat transfer to electrolyte solutions

Pool boiling heat transfer coefficients were measured for solutions of salts with positive solubility in water. The effect of the dissolved salts on nucleation site density, bubble departure diameter and bubble frequency was also investigated. The results show that at low heat fluxes heat transfer coefficients can be considerably lower than corresponding values for distilled water. At high heat fluxes the negative effect of the dissolved electrolyte gradually decreased and finally some improvement in heat transfer coefficient was observed. A correlation was developed for nucleate boiling of aqueous solutions from salts with positive solubility. Assuming that the mass transfer resistance is limited to the liquid side, the proposed model allows the prediction of heat transfer coefficients from boiling point data of the respective solutions. Comparison with a significant number of experimental data for different systems indicates that the model should be sufficiently accurate for most practical applications.