Bubble Dynamics and Heat Transfer During Pool Boiling on Wettability Patterned Surfaces

Surfaces with spatial wettability patterns have been proven to enhance heat transfer coefficient and critical heat flux in pool boiling. To understand the physical mechanism behind this phenomenon and obtain the correlation among some critical parameters (bubble departure frequency, bubble size, nucleation site density, surface tension), pool boiling experiments were conducted. A Pyrex glass with a layer of indium-tin-oxide was used as the substrate. Hydrophobic patterns will serve as nucleation sites. Experiments were conducted in deionized water under atmospheric pressure at a relatively low heat flux. The processes of nucleation, growth, and departure of individual bubbles were visualized by using a high speed camera through the bottom of the heater surface. It has been found that the patterned surface performed the best in heat transfer for subcooled pool boiling when compared with hydrophilic and hydrophobic surfaces. The nucleation site density of the biphilic surface was much higher, when compared with that of the homogeneous surface. The individual bubbles always nucleate on the edge of the hydrophobic and hydrophilic area, and then move onto the hydrophobic pattern. Most of the individual bubbles detach from the wettability patterned surface in the diameter range from 300 µm to 450 µm (around 77.3%). The bubble departure periods scatter in the range from 80 ms to 1500 ms.     

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.     

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.     

On the Relation between Nucleation Site Density and Critical Heat Flux of Pool Boiling

ABSTRACT

It is traditionally accepted that the critical heat flux (CHF) decreases with increasing nucleation site density (NSD). However, such a CHF-NSD relation was no longer observed in the BETA-B experiment performed on nano-film heaters; instead the increase of NSD resulted in a gain in CHF. To address this seeming contradiction in the relation between critical heat flux and nucleation site density, the present work employed probabilistic analysis to reveal the different tendencies. A concept of effective NSD was proposed, which concerns the active nucleation sites appear within a bubble lifetime, and the resulting bubbles have the chance of direct interaction. We assumed that the boiling crisis on a heater surface is mainly induced by two mechanisms: dry spot expanding in isolated bubble regime for low-NSD surface, coalescence of dry spots under multiple bubbles in fully developed nucleate boiling regime for high-NSD surface, or a combination of the two in the transition regime for medium-NSD surface. Accordingly, we estimated the critical heat flux of each boiling regime at which the boiling crisis occurs. The result indicated that there is a threshold of nucleation site density below which the increase of NSD is contributing to CHF enhancement, while the trend is inverted beyond the threshold.     

Critical Heat Flux During Flow Boiling in Microchannels: A Parametric Study

The application of flow boiling in microchannels in copper cooling elements for very high heat flux dissipation from microprocessor chips is one of the promising technologies to replace air cooling and water cooling of these units, particularly in mainframes and servers. Recently, the authors have proposed a new theoretical model to predict the critical heat flux (CHF) in microchannels, and it is used here to perform a parametric study to investigate the effects of fluid, saturation temperature, mass flux, inlet subcooling, microchannel diameter, and heated length on CHF for this application. The parametric study shows that CHF is increased by: (i) decreasing channel length, (ii) lowering saturation temperature, (iii) increasing mass flux, (iv) increasing inlet subcooling, and (v) increasing microchannel diameter. The best coolant is water, but water is not feasible for the present application because of its very low saturation pressure at 30–40°C. Of the other four fluids simulated, their order of merit from best to worst is as follows: R-245fa, R-134a, R-236fa, and FC-72. FC-72, however, has a low saturation pressure (in fact, it would operate under vacuum at the saturation temperatures of 30–40°C envisioned here) and is not a candidate fluid for the flow boiling coolant here. Furthermore, the authors have also recently proposed a diabatic flow map for microchannels based on their database for R-134a and R-245fa in 0.5- and 0.8-mm channels. The new CHF model has been incorporated into their map here to predict the transition from annular flow to dry-out, which is a critical design limitation for microprocessor coolers. Importantly, this map then provides the feasible operating range of such coolers with flow boiling as the cooling process, in terms of mass flux and maximum vapor quality at the outlet to avoid CHF.     

Numerical Simulation of Single Bubble Dynamics During Flow Boiling Conditions on a Horizontal Surface

Complete three-dimensional numerical simulations of single bubble dynamics during flow boiling conditions are carried out using the computational fluid dynamics code FLOW3D based on the volume-of-fluid method. The analyses include a numerically robust kinetic phase-change model and transient wall heat conduction. The simulation approach is calibrated by comparison with available experimental and theoretical data. It is found that the observed hydrodynamics (i.e., bubble shape, departure, and deformation) are simulated very well. The comparison with high-resolution transient temperature measurements during a heating foil experiment indicates that the modeling of the spatiotemporal heat sink distribution during bubble growth requires major attention. The simulation tool is employed for single bubble dynamics during flow boiling on a horizontal heating wall, and the agreement is excellent with published experimental data. The numerical results indicate how bulk flow velocity and wall heat transfer influence the bubble dynamics and heat transfer characteristics.     

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.     

Numerical Simulation of Bubble Growth and Heat Transfer During Flow Boiling in a Surface-Modified Microchannel

Flow boiling in a microchannel without or with surface modifications, such as fins, grooves, and cavities, has received significant attention as an effective cooling method for high-power microelectronic devices. However, a general predictive approach for the boiling process has not yet been developed because of its complexity involving the bubble dynamics coupled with boiling heat transfer in a microscale channel. In this study, direct numerical simulations for flow boiling in a surface-modified microchannel are performed by solving the conservation equations of mass, momentum, and energy in the liquid and vapor phases. The bubble surfaces are determined by a sharp-interface level-set method, which is modified to include the effect of phase change at the liquid–vapor interface and to treat the no-slip and contact-angle conditions on immersed solid surface of microstructures. This computation demonstrates that the surface-modified microchannel enhances boiling heat transfer significantly compared to a plain microchannel. The effects of various surface modifications on the bubble growth and heat transfer are investigated to find better conditions for boiling enhancement.     

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.     

An Experimental Investigation of Nucleate Pool Boiling Heat Transfer of Nanofluids From a Hemispherical Surface

An experimental investigation of nucleate pool boiling heat transfer is carried out using SiO₂ nanofluid in atmospheric pressure and saturated conditions. The results show that the nucleate boiling heat transfer coefficient (HTC) of the nanofluids is lower than that of deionized water, especially in high heat fluxes. In addition, the experimental results indicate that the critical heat flux (CHF) improves up to 45% with the increase of the nanoparticle volume concentration. Atomic force microscopy images from the boiling surface reveal that the nanoparticles are deposited on the heating surface during the nanofluid pool boiling experiments. It is found that the boiling HTC deteriorates as a result of the reduction in active nucleation sites and the formation of extra thermal resistance due to blocked vapor in the porous structures near the heating surface. Furthermore, the improvement of the surface wettability causes an increase in CHF. Based on the experimental investigations, it can be concluded that the changes in the properties of the boiling surface are mainly responsible for the variations in nanofluids boiling performance.     

气化炉液池内单个高温气泡传热、传质的数值模拟

建立了气、液间传热传质数学模型,对单个热气泡在上升过程中与处于蒸发阶段的水之间的传热、传质规律进行了数值研究,获得了气泡半径、气泡温度、水蒸发速率以及蒸发量随时间的变化规律.研究表明:在水恒温蒸发阶段,由于传热传质的共同作用,气泡半径随着气泡的上升而变小,并逐渐趋于稳定;随着气、液间温度差的增大,气泡半径缩小得越快;在气泡与水开始接触时,水的蒸发速率及气泡内的水蒸汽增量最大;气泡温度在较短时间内急剧下降,并趋于稳定;随着气泡半径的缩小,气泡冷却速率提高;但随着接触时间的继续增加,对气体的冷却效果却无明显作用.     

气泡尺寸对气缸盖沸腾换热的影响

在应用欧拉多相流模型仿真计算气液两相流沸腾换热时,离散相的气泡尺寸常常被看作常数,而实际上往往气泡具有不同的形状和尺寸,因此研究气泡尺寸大小对仿真计算结果的影响显得至关重要.以ANSYS Workbench为仿真计算平台,在计算流体动力学模块CFX中,用气液两相流沸腾换热计算模型,对不同气泡尺寸下柴油机气缸盖与冷却水腔所组成的流固耦合传热系统进行了整场离散、整场求解,得到了冷却水腔中气液两相流流场分布特性和气缸盖温度场分布,通过与试验结果的对比分析证明了计算模型的有效性.结果表明,在气泡尺寸大小为1,mm的情况下,仿真结果更接近试验结果,并且考虑气液两相流沸腾换热能够有效地降低气缸盖火力面排气道鼻梁区的最高温度,以此降低此处的热负荷.     

Dynamic Analysis of Bubble Attachment and Sweeping on Microwire in Subcooled Nucleate Pool Boiling

With the development of industrial technology,heat transfer at the microscale has attracted more and more attention.In this work,200 μm platinum wire and 150 μm...     

闪急沸腾条件下的乙醇气泡生长数值模拟

采用能量方程和动量方程耦合求解的方法,开展了闪急沸腾条件下的乙醇气泡生长数值模拟研究.通过与过热水和三氟三氯乙烷气泡生长的实验数据对比,验证了数值模拟方法的准确性.在此基础上,通过改变环境压力和过热度,研究了不同初始状态下乙醇气泡半径、生长速度、生长加速度、不同的力、热边界层温差和厚度等参数随时间的变化规律.结果表明,乙醇气泡生长过程中表现出的生长特性是抑制生长的表面张力、黏性力、流动阻力和促进生长的气泡内外压差及热反馈效应相互竞争的结果.雅各比数Ja对不同阶段的气泡生长特性有较大影响,随雅各比数增大,乙醇气泡生长过渡阶段由热传递控制逐渐转变为惯性力控制;环境压力一定时,雅各比数越大,乙醇气泡在表面张力控制阶段的生长延迟时间越短,最大加速度越大,在热传递控制阶段的生长速度也越快.     

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.     

Measurement of a Heated Surface Temperature Using a High-Speed Infrared Camera During Critical Heat Flux Enhancement by a Honeycomb Porous Plate in a Saturated Pool Boiling of a Nanofluid

Abstract

This article presents an experimental study to investigate the critical heat flux (CHF) enhancement mechanism using honeycomb porous plate (HPP). The CHF enhanced significantly with combination of the HPP and nanofluid, up to 3.2?MW/m² at maximum compared to a plain surface, 1.0?MW/m². The mechanism by which the CHF is improved in this system was elucidated by measuring the temperature of the heated surface using an indium tin oxide (ITO) heater and a high-speed infrared camera. The pool boiling experiment of water and nanofluid is performed under saturated temperature and atmospheric pressure conditions. The CHF values obtained using ITO heater is in good agreement with a conventional CHF pool boiling experiment with HPP attachment. High-speed infrared camera is analyzed to understand the behavior of local temperature at various locations over time. It is observed at the burnout condition, the highest average temperature is occurred at the intersection of HPP wall. Moreover, the reversible dry spots were initiated in the cell part of the HPP, and small dry spots coalesced into a growth of large irreversible dryout that leads to burnout. Further CHF enhancement could be realized if the initiation of the dryout region could be suppressed.     

微细通道CO_2流动沸腾换热临界热流密度研究

CO2在微细通道内流动沸腾换热过程所具有的临界热流密度(CHF)对于其换热系数有着重要影响。根据国内外现有发表的公开文献的实验数据分析了质量流量、饱和温度、管径等对临界热流密度的影响,并对理论模型与试验数据进行误差分析。发现Bowring预测关联式对小于3 mm管径内临界热流密度预测精度较高,在30%误差范围内可以达到70%预测精度,Wojtan预测关联式具有较小的平均绝对误差。提出了今后CO2在微细通道内沸腾换热CHF的研究方向。     

Critical Heat Flux (CHF) Correlations for Subcooled Water Flow Boiling at High Pressure and High Heat Flux

The subcooled water flow boiling is beneficial for removing the high heat flux from the divertor in the fusion reactor,for which an accurate critical heat flux(CHF)correlation is necessary.Up to now,there are many CHF correlations mentioned for subcooled water flow boiling in the open literatures.However,the CHF correlations’accuracies for the prediction of subcooled water flow boiling are not satisfactory at high heat flux and high pressure for reactor divertor.The present paper compiled 1356 CHF experimental data points from 15 independent open literatures and evaluated 10 existing CHF correlations in subcooled water flow boiling.From the evaluation,the W-2 CHF correlation performs best for the experimental CHF data in all existing critical heat flux correlations.However,the predicted mean absolute error(MAE)of the W-2 correlation is not very ideal for all database and the MAE of the W-2 correlation is from 30%to 50%for some database.In order to enhance the CHF prediction accuracy in subcooled water flow boiling at high heat flux and high pressure,the present paper developed a new CHF correlation.Compared with other existing CHF correlations,the new CHF correlation greatly enhances the prediction accuracy over a broad range of pressures and heat fluxes which are desired in the cooling of high heat flux devices,such as those in the fusion reactor divertor.The validation results show that the new correlation has a MAE of 10.05%and a root mean squared error(RMSE)of 16.61%,predicting 68.1%of the entire database within±10%and 81.5%within±15%.The MAE of the new CHF correlation is 7.4%less than that of the best existing one(W-2 correlation),further confirming its superior prediction accuracy and reliability.Besides,the new CHF correlation works well not only for a uniform power profile but also for a non-uniform power profile in subcooled water flow boiling at high pressure and high heat flux.     

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.     

Experiment Analysis of Pool Boiling Heat Transfer in Bead Packed Porous Layers

The heat transfer of pool boiling in bead packed porous layers was experimentally investigated to analyze the effects of the bead material, bead diameter and the layer number of the porous bed on the transport of flux and the heat transfer coefficients. The glass and copper bead, the bead sizes of 4 mm and 6 mm as well as the bead packed porous structures ranging from one to three layers were chosen in the experiments. The pool boiling heat transfer in the bead packed porous structures and that on the plain surface were compared to analyze the enhancement of pool boiling heat transfer while the bead packed porous layers were employed. The maximum relative error between the collected experimental data of the pure water on a plain surface and the theoretical prediction of pool boiling using the Rohsenow correlation was less than 12%. Besides, the boiling bubble generation, integration and departure have a great effect on the pool boiling and were recorded with a camera in the bead stacked porous structures of the different layers and materials at different heat flux. All these results should be taken into account for the promotion and application of bead packed porous structures in pool boiling to enhance the heat transfer.