竖直狭缝通道中液氮沸腾强化换热的实验研究

实验表明，狭缝间隙对液氮自然对流核态沸腾换热有明显的影响，在低热流密度下，间隙小的狭缝沸腾换热比间隙大的狭缝明显增强，当狭缝间隙小于实验压力下气泡的脱离直径时，对于同样的热流密度，传热温差减小一个数量级以上，沸腾换热系数提高十几倍到二十倍以上，当热流密度增加一定程度（＞4W/cm^2)时，间隙小的狭缝沸腾换热比间隙大的狭缝有所减弱。     

饱和液氮中临界热流密度和最小膜态沸腾热流密度的实验研究

用直径为５０ｍｍ、厚度分别为１２，２０，３０ｍｍ的铜平板在液氮中进行了不同热面方位角（热面水平向下为０°，热面垂直为９０°）的淬冷沸腾实验研究。方位角对临界热流密度、最小膜态沸腾热流密度及其壁面过热度的影响较大。在一定的方位角下，临界热流密度随平板厚度的增加而增加，但厚度达到一定大小时，其值与厚度无关。最小膜态沸腾热流密度与厚度呈离散关系。平板厚度对临界沸腾密度和最小膜态沸腾热流密度所对应的表面过热度的影响较小     

深冷文献消息(国内部分)

20 0 3110 1　竖直狭缝通道中液氮沸腾强化换热的实验研究郭廷玮等　《低温工程》　 2 0 0 2　№ 4　 1～ 4实验表明 ,狭缝间隙对液氮自然对流核态沸腾换热有明显的影响。在低热流密度下 ,间隙小的狭缝沸腾换热比间隙大的狭缝明显增强。当狭缝间隙小于实验压力下气泡的脱离直径时 ,对于同样的热流密度 ,传热温差减小一个数量级以上 ,沸腾换热系数提高十几倍到二十倍以上 ,当热流密度增加一定程度 (>4Ｗ/ｃｍ2 )时 ,间隙小的狭缝沸腾换热比间隙大的狭缝有所减弱。2 0 0 3110 2　ＰＳＡ空气分离吸附过程中的压降特性研究龚建英 ,张玉文　《低…     

CO2流动沸腾换热干涸研究进展

针对CO2在亚临界管内流动沸腾换热过程中所表现出来的干涸现象研究进展进行了综述,描述了在CO2沸腾换热过程中的干涸现象及其产生的影响因素,分析了热流密度、质量流量、饱和温度、管径等因素对干涸产生的影响及机理.提出CO2流动沸腾换热过程中临界热流密度,流态变化,干涸干度的预测以及抑制干涸提前发生的相应措施是今后研究的方向.     

NH_3-LiNO_3与NH_3-NaSCN在水平管内沸腾换热特性的试验对比分析

通过试验对比分析氨盐工质对在水平管内的沸腾换热特性,分析热流密度、质量流量及出口干度对沸腾换热系数的影响。试验结果表明,热流密度对于氨盐在水平管内的沸腾换热系数有主要作用,核态沸腾机制占主导地位;在相同工况下,NH_3-NaSCN溶液的沸腾换热系数比NH_3-LiNO_3溶液的沸腾换热系数略大,主要原因是前者黏度相对较小。Jiang J.等拟合的NH_3-LiNO_3工质对在水平管内的沸腾换热系数的关联式能够准确预测NH_3-NaSCN的沸腾换热系数。试验结果对于氨盐溶液在余热废热驱动的吸收式空调中的应用有着积极的促进作用。     

两种不同工质在微通道内沸腾换热特性的实验研究

为研究流体物性、流动和换热过程的状态参量对微通道内沸腾换热特性的影响规律,本文采用去离子水和无水乙醇在当量直径为0.293 mm的矩形微通道进行了不同质量流量和热流密度条件下的沸腾换热实验研究,通过对实验数据的计算和处理,分析总结了流体的热物性、质量流量、热流密度、干度和Bo数等参量对沸腾换热系数的影响规律。结果表明:沸腾换热系数随着热流密度、干度和Bo数的增大而降低,核态沸腾占主导地位;相同的质量流量和热流密度条件下,去离子水的沸腾换热系数明显高于无水乙醇的沸腾换热系数,并且前者的换热系数随质量流量的增大而增大,而后者变化不明显。根据考虑了通道尺寸效应及流体物性参量总结出的换热系数关联式进行了计算,计算结果对去离子水和无水乙醇的平均绝对误差分别为14.2%和16.6%,可认为该关联式适用于微通道内沸腾换热系数的预测。     

逆向载荷下矩形通道内水流动沸腾临界热流密度

临界热流密度(CHF)是流动沸腾过程中一个重要的参数,在旋转平台上以蒸馏水为工质,采用单侧加热的矩形通道,对逆向载荷下两种不同加热方位下的流动沸腾CHF特性进行了实验研究,获得了逆载下发生临界换热时的质量流速、入口压力、实验段压降和壁温的变化特性。研究讨论了逆载、入口温度、质量流速和加热方位对CHF的影响。结果表明:临界换热现象发生时,壁温迅速上升,有效热流密度迅速减少,实验段压降增大,质量流速减小;逆载和质量流速越大,CHF越大;入口温度越高,CHF越小,同时加热方位对CHF也有明显影响。     

加热方位对流动沸腾临界热流密度影响

为了研究重力场对流动沸腾临界热流密度的影响,搭建了两相沸腾换热实验系统。以蒸馏水为工质,采用单侧加热的窄缝通道,通过改变质量流速、入口过冷度和重力场与加热方位的夹角,考察不同加热方位临界热流密度特性和实验段流阻特性。分析了质量流速、入口过冷度、加热方位对流动沸腾临界热流密度的影响,并将实验数据与Ivey-Morris模型、Sudo模型和Wojtan模型的计算值进行了验证对比。结果表明:加热面呈0°放置时的临界热流密度最大,呈180°放置时最小,质量流速和入口过冷度的增大会加大临界热流密度。Sudo模型对本实验条件不适用;Ivey-Morris模型和Wojtan模型在加热面呈0°放置时与实验值符合情况良好,相对误差约在30%以内,其他加热方位时,计算值均大于实验值。     

R404A在小管径管内流动沸腾换热特性研究

R404A在小管径管内的流动沸腾换热过程是一个极其复杂的物理现象。目前对R404A换热特性的研究大多集中在大管径上,对小管径换热特性的研究较少,且对不同实验现象的机理分析也不尽相同。因此R404A在小管径管内换热特性的理论研究仍需要大量具体的实验数据来支撑。本文通过搭建小管径内螺纹铜管蒸发实验台,研究R404A在小管径管内流动沸腾换热过程中不同热流密度、不同蒸发干度、不同质流密度、不同饱和温度对表面传热系数的影响,研究表明:热流密度、干度、质流密度、饱和温度均对R404A在小管径管内换热特性的影响较大,干涸现象发生前后这些因素产生的影响也不同。此外,这些因素对管内干涸现象发生的起始干度、沸腾主要换热形式以及干涸现象是否发生具有直接影响。     

蒸发温度对强化换热管管外核态池沸腾换热性能的影响

对R134a在水平强化管(Φ25 mm)外核态池沸腾进行了实验研究。通过Wilson图解法求得管内换热准则关系式,通过改变蒸发温度(5.6℃,0℃,-2℃,-4℃,-6℃,-8℃)和热流密度(4~55 k W/m2),得到了管外沸腾换热系数随热流密度和蒸发温度变化的规律。实验表明,管外沸腾换热系数随着热流密度和蒸发温度的升高而增加。结合实验数据,提出了一个新的管外池沸腾换热关联式,该关联式与实验数据点的偏差显示,95%的数据点的相对误差在±20%以内。     

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.     

The effect of lubricant concentration, miscibility, and viscosity on R134a pool boiling

This paper presents pool boiling heat transfer data for 12 different R134a/lubricant mixtures and pure R134a on a Turbo-BII™-HP surface. The mixtures were designed to examine the effects of lubricant mass fraction, viscosity, and miscibility on the heat transfer performance of R134a. The magnitude of the effect of each parameter on the heat transfer was quantified with a regression analysis. The mechanistic cause of each effect was given based on new theoretical interpretation and/or one from the literature. The model illustrates that large improvements over pure R134a heat transfer can be obtained for R134a/lubricant mixtures with small lubricant mass fraction, high lubricant viscosity, and a large critical solution temperature (CST). The ratio of the heat flux of the R134a/lubricant mixture to that of the pure R134a for fixed wall superheat was given as a function of pure R134a heat flux for all 12 mixtures. The lubricant that had the largest CST with R134a exhibited the greatest heat transfer: 100%±20% greater than that of pure R134a. By contrast, the heat transfer of the mixture with the lubricant that had the smallest viscosity and the smallest CST with R134a was 55%±9% less than that of pure R134a. High-speed films of the pure and mixture pool boiling were taken to observe the effect of the lubricant on the nucleate boiling.     

Heat transfer in boiling neon in vertical capillaries

Values for the burnout heat flux and critical temperature difference are measured in boiling neon on the internal surface area of vertical capillaries. A hysteresis is observed as power increases and decreases, which becomes weaker after several increases of power up to the critical heat flux point. The characteristics of the dependences obtained are discussed.     

Numerical method of thermal shock resistance estimation by quenching of samples in water

A temperature dependence of a transient heat transfer for cylindrical and ball samples (of different surface roughness) of 3–60 mm diameters heated up to the temperature range from 150 to 1200° C and quenched in a water bath of large volume was established. The measurement errors of the transient heat transfer defined by different methods with regard to hysteresis and statistical nature of boiling phenomena were evaluated. The study revealed, that the transition point from bubble to film boiling and vice versa differs essentially. The transient heat transfer in the field of bubble boiling did not depend on the size and the shape of the samples, their surface roughness and thermo-physical properties. But the magnitude of hysteresis in changing between the boiling regimes were substantially governed by the geometrical and thermo-physical characteristics of the samples. The examples of thermal stresses estimation which caused quenching damage to ZrC samples, heated up to a wide range of temperature from 150 to 1200 C, are given. The obtained data on the transient heat transfer and proposed recommendation on the temperature regimes of quenching for convenient sample sizes can form a basis of a standard for the numerical evaluation of the thermal shock resistance.     

High heat flux transport by microbubble emission boiling

In highly subcooled flow boiling, coalescing bubbles on the heating surface collapse to many microbubbles in the beginning of transition boiling and the heat flux increases higher than the ordinary critical heat flux. This phenomenon is called Microbubble Emission Boiling, MEB. It is generated in subcooled flow boiling and the maximum heat flux reaches about 1 kW/cm²(10 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s for a small heating surface of 10 mm×10 mm which is placed at the bottom surface of horizontal rectangular channel. The high pressure in the channel is observed at collapse of the coalescing bubbles and it is closely related the size of coalescing bubbles. Periodic pressure waves are observed in MEB and the heat flux increases linearly in proportion to the pressure frequency. The frequency is considered the frequency of liquid-solid exchange on the heating surface. For the large sized heating surface of 50 mm length×20 mm width, the maximum heat flux obtained is 500 W/cm² (5 MW/m²) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s. This is considerably higher heat flux than the conventional cooling limit in power electronics. It is difficult to remove the high heat flux by MEB for a longer heating surface than 50 mm by single channel type. A model of advanced cooling device is introduced for power electronics by subcooled flow boiling with impinging jets. Themaxumum cooling heat flux is 500 W/cm² (5 MW/m²). Microbubble emission boiling is useful for a high heat flux transport technology in future power electronics used in a fuel-cell power plant and a space facility.     

Effect of neighboring boiling sections on the critical heat flux in forced channel flow

It is assumed that the critical heat flux is affected by nucleate boiling in neighboring channel sections upstream from the burnout point.     

New type of dry out crisis in free falling boiling liquid films

Here we present measurements of heat-transfer rate and critical heat flux as well as high-speed visualization at intensively evaporating and boiling falling liquid film flow. Research were carried-out at heated surfaces 67 mm wide and 20, 42, 64 mm long along the film flow over the range of inlet Reynolds number from 100 to 2000. Appreciably different crisis phenomena scenarios are observed depending on the heated surface length. Direct experimental measurements and visualization have shown the existence of previously unexplored surface dry out crisis development regime which is characterized by the upstream extrusion of the bubble boiling from the heated surface with the drying front. This type of crisis occurs under the certain range of the operating conditions and heated surface lengths. When the critical heat flux density occurs, large dry spots merge at the lower part of the stream. As a result of the dry spots merging, unstable temperature disturbance is formed. Subsequently it spreads over the whole surface, causing its drying, and critical heat flux is no longer determined by known calculation dependencies and is characterized by significantly smaller values. At such regimes critical heat flux is controlled not by hydrodynamic boiling crisis, but by equilibrium heat flux at which dry spots become unstable. This undesired critical heat flux reduction is potentially avoidable if measures can be taken for artificial liquid redistribution in transversal direction in order to decrease dry spots initial size (thus increase equilibrium heat flux).     

Heat exchange processes near the boundary of a film boiling nucleus

Results are presented of an experimental investigation of the change in temperature, heat flux to the liquid, and rate of displacement of the isotherms near a film boiling nucleus propagating over a plane surface. The experiment was carried out in a liquid nitrogen bath at atmospheric pressure on the saturation line. The heater was a sapphire plate 1.2 mm thick having a heat transfer surface area of 77×22 mm². The following facts were established: 1) near the boundary of the film boiling nucleus a new heat exchange mechanism takes place caused by the instability of the liquid microlayer; 2) the maximum heat flux to the liquid is considerably greater than the critical heat flux; 3) the vapor film in the film boiling region grows gradually with increasing distance from the boundary, i.e., there is a smooth transition in terms of heat exchange intensity before the equilibrium film boiling level is reached. Pis’ma Zh. Tekh. Fiz. 25, 39–46 (November 12, 1999)