Critical Heat Flux of Boiling Heat Transfer in aModerate Narrow Space

Boiling heat transfer process is analyzed in a moderate narrow space consisted of two horizontal plates.The main difference between this process and the conventional unconfined pool boilillg is the liquidsupply mechanism which is absolutely prevented by the growth of coalescence bubble along with theheated surface in the narrow space. As a result, the macrolayer becomes thinner due to the evaportion of the individual bubbles within the macrolayer during the period of bubble coalescence, with orwithout dryout that depends on both the gap size of narrow space and the size of heated surface. Fora specified size of the heated surface, the initial thickness of the liquid layer has a critical value whichapproaches a constant while the space height is larger than a critical value. The individual bubblebehaviors and tlie heat transfer can be considered as the same as that in the unconfined pool boiling, ifthe space gap is large. However, the individual bubbles do not generate in the last period of the bubblecoalescence and a lower maximum heat fiux will be resulted if the space gap is reduced. In such a case,the macrolayer is dryout.

Forced Convection Boiling Heat Transfer and Dryout Characteristics in Helical Coiled Tubes with Various Axial Angles

对水和水蒸汽汽液两相流体在螺旋轴呈各种倾角放置的螺旋管内强制对流沸腾传热与烧毁特性进行了系统地试验研究，试验中系统及结构参数范围如下：压力：Ｐ＝０．４～３．０ＭＰａ质量流速：Ｇ＝１００～２４００ｋｇ／ｍ２·ｓ进口水温：Ｔ＝３０～８０℃出口干度：ｘ＝－０．０５～１．２管内壁面热负荷：ｑ＝０～５４０ｋＥ／ｍ２试验段结构参数：总长Ｌ＝６４４８ｍｍ，管内径ｄ＝１１ｍｍ，螺旋直径Ｄ＝２５６ｍｍ，螺旋升角β＝４．２７°螺旋管轴向放置倾角：水平位置（０°）、向上倾斜４５°（＋４５°）、垂直向上（＋９０°）、向下倾斜４５°（－４５°）．共进行了１０５０个工况的试验．试验结果表明，螺旋管内汽液两相流强制对流沸腾传热可以划分为核态沸腾区、两相流强制对流区、烧毁及烧毁后传热区等３种区域．通过数据处理和分析总结，给出了３区域间转变的边界方程，和３个区域内两相流传热系数的计算公式．根据试验观察和数据结果，对烧毁现象及烧毁点或区域发生的条件及机理进行了深入的分析研究，发现一系列有关现象的规律和特点，指出了其主要影响因素，并给出了烧毁点临界质量干度的预报公式．

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 alu- minium 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 borizontally 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.

Accurate prediction of the critical heat flux for pool boiling on the heater substrate

While the influence of liquid qualities,surface morphology,and operating circumstances on critical heat flux(CHF)in pool boiling has been extensively studied,the effect of the heater substrate has not.Based on the force balance analysis,a theoretical model has been developed to accurately predict the CHF in pool boiling on a heater substrate.An analytical expression for the CHF of a heater substrate is obtained in terms of the surface thermophysical property.It is indicated that the ratio of thermal conductivity(k)to the product of density(ρ)and specific heat(cp)is an essential substrate property that influences the CHF.By modifying the well-known force-balance-based CHF model(Kandlikar model),the thermal characteristics of the substrate are taken into consideration.The bias of predicted CHF values are within 5%compared with the experimental results.

Experimental Study on Jet Impingement Boiling Heat Transfer in Brass Beads Packed Porous Layer

Jet impingement boiling has been widely used in industrial facilities as its higher heat transfer coefficient(HTC)and critical heat flux(CHF)can be achieved in comparison with the pool boiling.By covering beads packed porous layer on the heated wall surface,the enlarged heat transfer area and rise of nucleation sites for boiling occur,thus,the heat transfer performance of boiling can be enhanced.For the jet impingement boiling with brass bead packed porous layers,the heat transfer performance is crucially influenced by the characteristics of porous layer and working fluid flow,so the experiments were conducted to investigate the effects of the jet flow rate,fluid inlet subcooling,number of porous layer and brass bead diameter of porous layer.Comparison study shows that impingement boiling promotes the HTC and CHF as 1.5 times and 2.5 times respectively as pool boiling at similar conditions.Higher heat transfer performance can be obtained in the cases of a higher jet flow rate and a higher fluid inlet subcooling,and there exist the optimal layer number and bead diameter for heat transfer.Particularly,a double-layer porous layer results in an increase of 39%in heat flux at superheat of 30 K compared with a single-layer case;a single porous layer at d=8 mm brings an increase of 23%in heat flux at superheat of 30 K compared with that of bare plain surface.Besides,the actual scene of jet impingement boiling was recorded with a camera to investigate the behavior evolution of vapor bubbles which is highly correlated to the heat transfer process.

Investigation of Enhanced Boiling Heat Transfer from Porous Surfaces

Experimental investigations of boiling heat transfer from porous suffaces at atmospheric pressure were performed. The porous surfaces are plain tubes covered with metal screens, V-shaped groove tubes covered with screens, plain tubes sintered with screens, and V-shaped groove tubes sintered with screens.The experimental results show that siatering metal screens around spiral V-shaped groove tubes can greatly improve the boiling heat transfer. The boiling hysteresis was observed in the experiment. This paper discusses the mechanism of the boiling heat transfer horn those kinds of porous surfaces stated above.

PRESSUREEFFECTONTHEHEATTRANSFERINBATHOFSUPERFLUIDHELIUM

Considering the basic heat transfer rules of superfluid helium and the phase diagram of helium, pressure effects on heat transfer to HeⅡ are especially studied. If the bath pressure is less than λ pressure ( P λ ), special peak heat flux density relations are shown to correlate bath pressure, hydrostatic head and modified pressure items (Van der waals pressure and fountain pressure). If the bath pressure is greater than λ pressure( P>P λ), a generalized formula of peak flux density of HeⅡp bath is shown.

Rewetting and Flow Film Boiling Along Hot Surface

The recent investigations on the rewettmg and film boiling of liquid flowing along a hot/heated surface are briefly reviewed and discussed.Some advanced theoretical analyses are conducted and new conclusions achieved.These investigations describe the fundamental characteristics of liquid flow boiling and further the complicated rewetting phenomena,and have resulted in considerable insight intothe mechanism.

Effect of the Geometrical Parameter of OpenMicrochannel on Pool Boiling Enhancement

High heat dissipation is required for miniaturization and increasing the power of electronic systems.Pool boiling is a promising option for achieving efficient heat dissipation at low wall superheat without the need for moving parts.Many studies have focused on improving heat transfer efficiency during boiling by modifying the surface of the heating element.This paper presents an experimental investigation on improving pool boiling heat transfer using an open microchannel.The primary goal of this work is to investigate the impact of the channel geometry characteristics on boiling heat transfer.Initially,rectangular microchannels were prepared on a circular copper test piece with a diameter of 20 mm.Then,the boiling characteristics of these microchannels were compared with those of a smooth surface under saturated conditions using deionized water.In this investigation,a wire-cutting electrical discharge machine(EDM)machine was used to produce parallel microchannels with channel widths of 0.2,0.4,and 0.8 mm.The fin thicknesses were 0.2,0.4,and 0.6 mm,while the channel depth remained constant at 0.4 mm.The results manifested that the surface featuring narrower fins and broader channels achieved superior performance.The heat transfer coefficient(HTC)was enhanced by a maximum of 248%,and the critical heat flux(CHF)was enhanced by a maximum of 101%compared to a plain surface.Eventually,the obtained results were compared with previous research and elucidated a good agreement.

A comparison study on viscosity and heat transfer property of fluorinebearing and fluorine-free mold flux for casting silicon steel

A comparison study was conducted on viscosity and heat transfer property of the traditional fluorine-bearing(F-bearing)mold flux and the newly designed fluorine-free(F-free)mold flux for casting silicon steel.The results showed that the viscosity of F-free mold flux is lower than that of F-bearing mold flux,as the apparent activation energy of F-free mold flux is also lower than that of F-bearing mold flux confirmed by the kinetics results for viscous flow,which meant that the energy barrier of F-free mold flux that the migration of ion clusters needs to overcome is smaller,and thus leads to a lower viscosity of F-free mold flux.Besides,the research results of the heat transfer suggested that the comprehensive heat transfer coefficient of F-free mold flux is lower than that of F-bearing mold flux in all directions.One reason is that the true density of F-free mold flux increases from glassy state to crystalline state,while that of F-bearing mold flux is opposite.Another reason is that the air gap gets increased due to the volume shrinkage of F-free mold flux from glassy state to crystalline state and the solidification of the molten steel.These introduce a larger thermal resistance of F-free mold flux than that of F-bearing mold flux.Through the comparison,the designed F-free mold flux shows a better lubrication and capacity to control the heat transfer than the traditional F-bearing mold flux.

Experimental Study and Analysis on Heat Transfer Coefficient of Radial Heat Pipe

The experimental study was performed on five eccentric radial heat pipes with two outer-tube diameters.The test range can be given as follows,working fluid filling ratio Ω=44%～83%,heat flux q=10000W/m2～32000W/m2,and working temperature tv=50 ℃～120 ℃.The correlations between radial heat pipe heat transfer performance and filling ratio,heat flux,working temperature were studied in the experiment.Based on linear regression of experimental data,the relationship between heat pipe equivalent heat resistance R and working temperature tv,heat flux q and filling ratio Ω was obtained.

机加工铜表面可视化池沸腾传热特性实验研究

在标准大气压下,采用经机械加工得到的不同粗糙度铜基表面的紫铜棒在自主设计制造的实验台上进行可视化池沸腾实验,使用高级摄像机记录铜基表面气泡的产生、生长和脱离的沸腾过程,同时用数据采集器记录铜棒不同节点处的温度.经过对比分析得出了光滑表面与不同粗糙程度的机械加工表面的换热特性.结果发现:在低热流密度时,光滑表面最先沸腾,其次是相对较宽的螺纹槽表面开始沸腾;在高热流密度时,槽宽0.3 mm的铜表面气泡脱离频率最快,换热效果最好,槽宽0.1 mm的铜表面产生的气泡较大,脱离频率相对较慢,总体上经机械处理后的表面的气泡脱离频率均强于未经处理过的光滑表面的.对实验数据进行处理,整理出了不同表面对临界热流密度和传热系数的影响趋势.

铜表面-氧化石墨烯纳米涂层池沸腾换热特性实验

石墨烯材料因其优异的导热性能和化学惰性,在核反应堆先进换热设备领域有很大的应用前景。为探索石墨烯材料的强化换热性能,本文采用沸腾沉积法制备氧化石墨烯纳米涂层,开展了氧化石墨烯涂层-去离子水池沸腾换热特性实验。氧化石墨烯纳米流体工质浓度为0.001、0.003、0.005 mg/mL,加热时间控制为1 h和2 h,通过沸腾沉积在裸露铜表面制备6种氧化石墨烯涂层。以此为基础开展池沸腾实验,结果发现氧化石墨烯涂层对池沸腾换热系数的影响很小,但可显著增强池沸腾临界热流密度。从涂层表面润湿性、热导率及形貌结构变化等角度分析了氧化石墨烯涂层对池沸腾换热特性的影响机理。

老化效应对涂层表面池沸腾传热性能的影响

压力容器外部冷却(ERVC)是第三代压水堆核电站严重事故缓解策略的关键技术,其表面临界热流密度(CHF)决定了压力容器的完整性。为探究压力容器真实材料SA508钢表面老化对涂层表面CHF特性的影响,开展了SA508钢光表面和基于SA508钢的TC4钛合金多孔涂层表面在不同老化状态下的沸腾换热特性实验研究。结果表明:SA508钢光表面在水中会快速氧化,随着老化的加深,其CHF呈上升趋势,上升幅度在20%以内;CHF增强的原因在于SA508钢氧化生成了能改善表面润湿性的Fe3O4磁性微纳米颗粒层。随着氧化的加深,TC4涂层表面由超亲水特性变为疏水特性,基体SA508钢生成的氧化物易堵塞涂层孔隙,CHF呈恶化趋势。

基于Rohsenow关联式的池沸腾HTC预测修正模型

针对去离子水池沸腾传热系数(HTC)预测,当前理论研究主要是基于Rohsenow关联式,采用固定数值的方式对关联式中的经验参数进行优化。本文基于Rohsenow关联式,通过拟合法建立关联式中两个经验参数与过热度的联系,进而搭建了池沸腾HTC预测修正模型。与3组去离子水铜表面池沸腾实验结果对比,修正模型对HTC的预测平均误差为17.79%、5.35%、18.14%,优于Rohsenow关联式模型和Li关联式模型。研究工作可以为HTC的预测提供一定的理论帮助。

高参数小管径内煤油的传热特性研究

本文对酸碱水洗煤油在高热流密度、高流速、高压、小直径圆管条件下进行了试验及理论研究,试验参数为热流密度q=0.8-50 MW/m2、亚临界压力p=2 MPa、超临界压力p=5,15 MPa、质量流速pw=8500-51000 kg/m2·s、圆管内径φ=1.7 mm。通过试验研究,获得了酸碱水洗煤油在亚临界及超临界压力条件下的传热特性、拟沸腾发生的界限条件以及传热恶化时的临界热流密度值及相关影响因素。在对试验结果分析的基础上,建立了临界热流密度关联式,计算结果与试验结果吻合良好。

逆向法确定铝合金连铸喷水冷却的换热系数

确定适当的冷却制度,尤其是对水冷换热系数的研究,对于提高连铸坯质量意义重大,为此,建立了铝合金连铸过程中喷水冷却换热系数的二维计算模型,采用试验测试铸坯冷却过程温度场和计算机模拟相结合的方法,可以比较准确地计算出冷却水的换热系数hα。结果表明,换热系数随着喷水量的增加而增大;随表面温度的升高呈先增后减的趋势。铸坯表面温度为50~550℃、喷水量为2.0~3.8m3/h条件下,冷却水的换热系数hα=0.52(3759.61+24.39T-0.059T2)W0.94。

电场强化乙酸乙酯自然对流和沸腾换热的实验研究

对沉浸于强极性有机液体工质乙酸乙酯中的平板表面上的自然对流和沸腾换热的外加电场强化进行了实验 ,得出了自然对流和沸腾换热的表面传热系数、强化效果与电场电压、热通量的关系 .实验数据表明 ,外加电场对平板表面自然对流换热的强化效果好于对沸腾换热的强化效果 .实验条件下 ,当施加外电场时 ,平板表面的自然对流换热的传热系数最大可达到无外电场时的 6～ 7倍 ,沸腾换热的传热系数最大可达到无外电场时的 4～

同轴径向热管传热特性实验研究

为了解热管径、轴向外壁温度的分布情况及不同充液率条件下热管的换热系数,对新型碳钢-水同轴径向热管在5种不同充液率下的传热性能进行实验研究,分析不同热流密度下热管管壁的温度分布情况;并根据最小二乘法原理,推导热管传热系数与充液率及热流密度的实验关联式。研究结果表明:在实验条件下,50%充液率的热管传热性能最佳;在不同充液率条件下,热管的换热系数相对误差不超过7.92%。

变截面翅片通道内过冷沸腾可视化试验

以水为介质,对三角多孔翅片(TP)和横排锯齿翅片(TDS)两种变截面翅片通道的过冷沸腾进行了可视化试验。研究了体积流量、过冷度与热通量对过冷沸腾起始位置的影响,通过观察变截面翅片流道内单个气泡形成、生长与合并或脱离的过程,对比分析两种变截面翅片对流沸腾传热强化机理。试验结果表明:TDS翅片通道内气泡从出现到消失的平均周期约为TP翅片的一半;过冷沸腾起始位置随着体积流量的增大,逐渐向流道出口处移动;而随着热通量的增大和过冷度的减小逐渐靠近流道入口处;过冷度对过冷沸腾起始位置的影响比热通量对其的影响更大。