Nucleate Pool Boiling Heat Transfer on a Micro-Pin-Finned Surface in Short-Term Microgravity

Nucleate boiling heat transfer of air-dissolved FC-72 on a micro-pin-finned surface was experimentally investigated in microgravity by utilizing the drop tower facility in Beijing. The dimensions of the silicon chips were 10 mm × 10 mm × 0.5 mm and on these, two kinds of micro-pin-fins with the dimensions of 30 × 30 × 60 μm³ and 50 × 50 × 120 μm³ (width × thickness × height, named PF30-60 and PF50-120) were fabricated by the dry etching technique. Nucleate pool boiling on a smooth surface was also studied under both Earth gravity and microgravity for comparison. In general, the micro-pin-fins showed better heat transfer performance when compared with a smooth surface, both under Earth gravity and microgravity. In microgravity, this is mainly due to the fact that bubbles generated on micro-pin-finned surface can depart from the heater surface continuously. For micro-pin-fins, the reduced-gravity critical heat flux was about two-thirds of that in the Earth gravity experiment, but almost three times as large as that for the smooth surface, which is larger than that in the terrestrial experiment. Under different gravity levels, PF50-120 shows a little better heat transfer than that of PF30-60, mainly due to larger heat transfer area. Besides, the fin gap of PF30-60 may generate a larger flow resistance for microconvection around the fin side walls, resulting in a lower heat transfer performance.     

石墨烯纳米制冷剂核态池沸腾换热的实验研究

为分析单层石墨烯纳米片对核态池沸腾换热的影响机理,对基液为R141b、分散相为单层石墨烯纳米片的纳米制冷剂的核态池沸腾换热特征进行了测定,采用Hot Disk热物性分析仪和铂金板法分别测定了石墨烯纳米制冷剂的热导率和表面张力,采用接触角测量仪和扫描电子显微镜（SEM）观测了沸腾后加热表面的润湿性和形貌特征。实验中,单层石墨烯纳米片的质量百分含量（ω）为0.02%~0.50%,实验压力为一个标准大气压,热流密度为20~200 kW/m2。实验结果表明：单层石墨烯纳米片的加入,使制冷剂R141b的核态池沸腾换热得到强化;当ω=0.2%时,换热系数提高比例出现峰值,为57.7%。伴随ω的增加,石墨烯纳米制冷剂的热导率增大、表面张力减小,沸腾表面润湿性增强且微腔数先增后减,综合作用的结果导致存在一个最佳的单层石墨烯纳米片浓度（即ω=0.2%）使换热系数最高。     

Correlation for Heat Transfer in Nucleate Boiling on Horizontal Cylindrical Surface

This experimental investigation deals with nucleate boiling studies on horizontal cylindrical heating elements made out of copper in the medium of Forane around atmospheric conditions. The data could be successfully correlated with the system of criteria employed by the authors in their earlier study of nucleate boiling process on cylindrical heating elements. Inclusion of the data from the present experimental study on Forane and that of other investigators yielded a comprehensive correlation with an average deviation of 20% and standard deviation of 25% over a wide range of system pressures.     

Effects of Acidity and Method of Preparation on Nucleate Pool Boiling of Nanofluids

Past research has shown contradicting trends in the rate of heat transfer during pool boiling of nanofluids, which could be attributed either to their stability or to their method of preparation or to both. An experimental study has been conducted to investigate the effects of electrostatic stabilization and preparation method of nanofluids on their pool boiling rate of heat transfer. Nanofluids made from water and alumina nanoparticles at 0.1 vol% concentration were used. The effect of electrostatic stabilization was investigated by changing the pH value from 6.5, neutral, to 5, acidic. The effect of preparation method has been investigated by using nanofluids prepared from dry particles and from ready-made suspensions. Compared with water, all nanofluids investigated resulted in deterioration in the rate of heat transfer during pool boiling. Neutral nanofluids made from ready-made suspensions and from dry particles resulted into almost the same deterioration in the rate of heat transfer of 49% and 45%, respectively, with respect to that of pure water. The most significant effect of electrostatic stabilization was found in the case of acidic nanofluids made from dry particles, which resulted in deterioration in the rate of heat transfer of 31%. However, acidic nanofluids made from ready-made suspensions resulted in a deterioration of 46%, which is almost the same as that of suspension-made and dry particles-made nanofluids. These results indicate that electrostatic stabilization using acid addition is most effective with nanofluids made from dry particles.     

Experimental Investigation of the Effect of Surface Inclination Angle on Saturated Pool Film Boiling Heat Transfer in Transient Regime

In order to examine the effect of surface inclination angle on saturated pool film boiling heat transfer in transient regime, an experimental study was carried out. The experiments were performed through a cylindrical rod, made up of brass 20 mm in diameter and 75 mm in length, placed at six inclination angles about the vertical (from 0 to 50°) under atmospheric pressure. The test specimen heated at high temperatures was immersed in a distilled water pool at saturated condition. Temperature of the specimen during the cooling process was recorded using a K-type thermocouple embedded at the center of the specimen. In the experiments, the pool film boiling was observed for each inclination angle. In the film boiling region, the heat transfer coefficients were calculated by means of a lumped parameter method. The experimental results showed that the heat transfer coefficient increased as the inclination angle increased. In addition, to predict the Nusselt number, an empirical formula including the inclination angle as well as the Grashof, Prandtl, and Jakob numbers was developed, and good agreement between the predicted and experimental data for the vapor Nusselt numbers was observed.     

二氧化硅与石墨烯混合纳米流体沸腾换热实验研究

设计并搭建带冷凝回流的闭合回路沸腾换热实验装置,研究了质量分数为0.000 5％、0.001％、0.002％和0.003％的水基二氧化硅(SiO2)纳米流体、水基石墨烯(GNs)纳米流体、水基和乙二醇水基SiO2与GNs混合(SiO2/GNs-DW,SiO2/GNs-EG-DW)纳米流体沸腾换热性能.结果表明:纳米流体...     

Experimental Investigations of Pool Boiling Heat Transfer Characteristics on a Vertical Surface Using CuO Nanoparticles in Distilled Water

Experiments were conducted on pool boiling heat transfer using dilute dispersions of CuO nanoparticles in distilled water at and above atmospheric pressure. Pool boiling characteristics of CuO nanofluid were studied at different pressures and concentrations. Characterization of the heating surface was done both qualitatively and quantitatively by taking the scanning electron microscopy (SEM) images and by subsequent measurement of surface roughness of the heater. SEM images of the heater surface showed nanoparticle deposition on the heater surface, suggesting surface modification. Thorough visualization showed microcavities on the heater surface, which provide an excellent location for nucleation sites enhancing heat transfer. However, these microcavities, once filled up with the suspended nanoparticles, reduced active nucleation sites, deteriorating the boiling heat transfer coefficient. Based on the experimental investigations it was concluded that there is an optimum thickness of nanoparticles coating at which heat flux is maximum and beyond this coating boiling heat transfer coefficient decreases. At higher pressures, boiling heat transfer coefficient and specific excess temperature remained nearly the same. This showed that pressure has negligible or no role to play in boiling heat transfer using nanofluids.     

Nucleate Pool Boiling Heat Transfer of Hydro-Fluorocarbon Refrigerant R134a on TiO2 Nanoparticle Coated Copper Heating Surfaces

ABSTRACT

Pool boiling heat transfer performance of hydro-fluorocarbon refrigerant R-134a on titanium dioxide (TiO₂) nanoparticle coated surface is experimentally studied in the article. The test surfaces, viz, 100 nm, 200 nm and 300 nm thick TiO₂ nanoparticle coated surfaces over 100 nm thin film surface are used in this experimentation. The surfaces are synthesized and fabricated by simple and cost-effective electron beam evaporation method. The test surfaces were characterized by scanning electron microscope and atomic force microscope to uncover the formation of crystalline structure on coated surfaces. These surfaces are utilized in pool boiling test rig using refrigerant R134a at 10°C saturation temperatures. The result indicated that a maximum of 87.5% augmentation in the boiling heat transfer has been achieved by higher thickness of TiO₂ coated surface than the bare copper surface. In addition, the incipience wall superheat is reduced for higher thickness coated surface. The augmentation of heat transfer coefficient might be the reason for increase in micro/nano-porosity, active nucleation site density and surface area of the heating surface. It is observed that with the increase of sub-cooling temperature of liquid, the bubble departure diameter was reduced while the heat transfer coefficient has been increased.     

不凝结气体对多壁碳纳米管换热表面的池沸腾性能的影响

为揭示不凝结气体对多壁碳纳米管（Multi-walled Carbon Nanotube, MWCNT）纳米结构表面核态池沸腾过程的影响，使用气体沉积法（Chemical Vapor Deposition, CVD）在硅表面制作MWCNT纳米结构表面，并使用光滑硅表面进行对比实验研究。实验操作中，将驱气前后的工作液体应用于两种表面的池沸腾实验，换热表面过热度控制在0℃-35℃，工作液体过冷度分为40℃和50℃。实验结果表明，液体中含气量的变化对MWCNT纳米结构表面影响较小，而对光滑硅表面的影响较大；对比硅表面，MWCNT纳米结构表面能够有效提升沸腾传热效果，对于驱气后的工作液体提升效果更为明显。     

Pool Boiling Heat Transfer to Phosphoric Acid Solutions

Phosphoric acid is a weak electrolyte with complex physical properties. This complexity combined with its industrial importance has necessitated intensive studies into its heat transfer behavior. In this investigation, pool boiling heat transfer coefficients of phosphoric acid solutions have been measured over a wide range of acid concentrations. The effects of various operating parameters such as heat flux, temperature, and acid concentration have been investigated. Also, the bubble departure diameter and the number of active nucleation sites for phosphoric acid solutions are compared with those for pure water under identical conditions. A model was developed for pool boiling heat transfer of phosphoric acid solutions, which can easily be adapted for other weak electrolyte solutions. In this model, the correct boiling temperature at the vapor/liquid interface is determined rather than applying an arbitrary correction to the boiling heat transfer coefficient. The proposed model is confirmed by comparison between calculated and experimental data.     

Nucleate Pool Boiling of Pure Liquids and Binary Mixtures：PartI－Analytical Model for Boiling Heat Transfer of Pure Liquids on Smooth Tubes

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Improving Heat Transfer with Pool Boiling by Covering of Heating Surface with Metallic Spheres

Abstract

Boiling heat transfer (BHT) was investigated experimentally. Smooth copper walls were covered with single sphere layer and corresponding temperature difference and heat flux were measured. The results were compared with published data for several types of heating surfaces. Comparative analysis shows that surfaces covered with spheres have characteristics as good as the other systems, if not better.     

强润湿性液体池沸腾传热的实验研究和机理分析

对强润湿性液体的池沸腾传热实验而言,本文提出了行之有效的实验程序,并严格按照实验程序进行了R113池沸腾传热的实验研究,具体研究了表现老化和液体过冷度对池沸腾传热曲线及起沸点的影响,实验中观察到了三个反常现象,最后,从强润湿性液体的沸腾传热机理的角度对其给出了相应的解释。     

Investigation on Active Thermal Control Method with Pool Boiling Heat Transfer at Low Pressure

In order to maintain a desirable temperature level of electronic equipment at low pressure, the thermal control performance with pool boiling heat transfer of water was examined based on experimental measurement. The total setup was designed and performed to accomplish the experiment with the pressure range from 4.5 kPa to 20 kPa and the heat flux between 6 kW/m~2 and 20 kW/m~2. The chosen material of the heat surface was aluminium alloy and the test cavity had the capability of varying the direction for the heat surface from vertical to horizontal directions. Through this study, the steady and transient temperature of the heat surface at different pressures and directions were obtained. Although the temperature non-uniformity of the heat surface from the centre to the edge could reach 10℃ for the aluminium alloy due to the varying pressures, the whole temperature results successfully satisfied with the thermal control requirements for electronic equipment, and the temperature control effect of the vertically oriented direction was better than that of the horizontally oriented direction. Moreover, the behaviour of bubbles generating and detaching from the heat surface was recorded by a high-resolution camera, so as to understand the pool boiling heat transfer mechanism at low-load heat flux. These pictures showed that the bubbles departure diameter becomes larger, and departure frequency was slower at low pressure, in contrast to 1.0 atm.     

Fundamental Issues of Critical Heat Flux Phenomena During Flow Boiling in Microscale-Channels and Nucleate Pool Boiling in Confined Spaces

This article presents a comprehensive literature review on the fundamental issues of critical heat flux (CHF) during flow boiling and nucleate pool boiling in microscale channels and confined spaces. First, distinction between macro- and micro-scale channels is discussed. Then the CHF mechanisms are discussed. Next, experimental and theoretical studies of subcooled flow boiling CHF in microscale channels together with the prediction methods are reviewed and analyzed. Following this, experimental and theoretical studies on saturated flow boiling CHF together with the prediction methods are summarized and discussed. Furthermore, experimental and theoretical studies on nucleate pool boiling CHF in confined spaces together with the relevant prediction methods are reviewed as well. So far, limited studies on CHF microscale channels and confined spaces are available in the literature. There are numerous discrepancies in the existing studies on CHF results, mechanisms, and prediction methods. Furthermore, there are no generalized prediction methods for CHF in microscale channels and confined spaces. According to this review, future research needs for the experiments, mechanisms, and prediction methods of CHF phenomena in microscale channels and confined spaces have been addressed.     

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.     

Experimental and Numerical Study on Heat Transfer with Impinging Circular Jet on a Convex Hemispherical Surface

In this paper, the experimental and numerical study has been carried out to investigate the water jet impingement on a convex hemispherical surface. The pressure and skin friction coefficient distributions on the impingement surface were analyzed numerically. A great deal of attention was paid to analyze the effects of the jet impingement exit velocity and the nozzle-to-surface distance on the heat transfer characteristics. A comparison of the heat transfer coefficient was performed between the jet impingement on the convex surface and the flat plate. The results show that the Nusselt number for the convex surface is higher than that for the flat surface. The local Nusselt number is decreased monotonically from its maximum value at the stagnation point for lower Reynolds numbers. A secondary maxima occurs for higher Reynolds numbers. The experiment and simulation were performed with the following parameters: the jet impingement Reynolds number of Re = 1947–19478, and the nozzle-to-surface distance of L/D = 2.5–25.     

Enhancement of Boiling Heat Transfer Using Surface Vibration

An experimental investigation of the effect of mechanical vibrations of a copper flat circular surface on the pool boiling heat transfer coefficient of water at atmospheric pressure are presented in this paper. A vibration exciter was used to vibrate this copper test surface vertically. Effect of frequency and amplitude of vibration on the boiling heat transfer coefficient was studied. An increase in the heat transfer coefficient was observed at low frequency and amplitudes, at higher amplitude and frequency heat transfer deteriorates. Heat transfer coefficient increases up to 26% with vibration intensity, represented by vibrational Reynolds number.     

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.     

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.