高压过冷池沸腾换热机理的研究

目前还没有一种被广泛承认的理论能够解释高压过冷池沸腾换热,其机理尚不明确。为了揭示高压池内过冷核沸腾的物理传热机理,并获得气泡脱离频率与活化穴半径的函数关系,根据池内过冷核沸腾加热表面活化穴的分布,在统计方法的基础上,提出了高压池内过冷核沸腾的一个数学模型。从该模型中发现,池内过冷核沸腾热流密度是壁面过热度、液体过冷度、活化穴尺寸、流体的接触角以及流体物理特性的函数。对不同的过冷度,将模型预测的结果与实验数据进行了比较,两者吻合得极好,从而证明了数学模型的可靠性。该解析模型更深刻地揭示了过冷池沸腾换热的物理机理,且没有增加新的经验常数。     

非均匀电场强化水的池沸腾传热特性实验研究与分析

本研究在工质水中布置针状电极,研究非均匀电场对其池沸腾换热过程中传热系数以及临界热流密度的影响,同时测量了不同电压下沸腾表面接触角的变化,以及可视化的观察到了气泡的产生以及脱离相对于无电场情况下的变化.结果表明,在低热流密度区电场强化传热,且强化效果随热流密度的增加而减弱.在中热流密度区电场恶化传热,恶化效果随着热流密...     

超声波对池沸腾换热影响

对不同超声强度和辐射距离条件下过冷池沸腾换热特性进行了实验研究.超声波有效强化了沸腾起始段换热,对高热流密度沸腾传热也有一定的强化作用.超声辐射距离越近,强度越大,强化传热效果越好.对单相对流和沸腾起始区传热,超声波强化传热机理为空化作用;高热流密度沸腾时超声波对强化传热的主要机理是声流作用.得到了传热实验关联式.     

开孔金属泡沫池内沸腾换热特性研究

常压下用去离子水作为工质,在沸腾池底部单独或两两叠加地放置3种PPI的泡沫金属进行池内沸腾实验研究;分析了不同过热度下的热流密度和不同热流密度下池沸腾的传热系数;对沸腾汽泡在单层或双层泡沫金属表面产生、融合和脱离进行了可视化的研究。实验发现:不同PPI泡沫金属的叠加方式,会影响沸腾传热面积和产生汽泡的核心数,以及汽泡脱离受热壁面的动态特性。本实验为多孔介质池沸腾传热特性研究提供了实验指导和基础数据。     

纳米流体对流换热机理分析

考虑在纳米流体中纳米颗粒做布朗运动引起的对流换热, 基于纳米颗粒在纳米流体中遵循分形分布, 本文得到纳米流体对流换热的机理模型. 本解析模型没有增加新的经验常数, 从该模型发现纳米流体池沸腾热流密度是温度、纳米颗粒的平均直径、 纳米颗粒的浓度、纳米颗粒的分形维数、沸腾表面活化穴的分形维数、基本液体的物理特性的函数. 对不同的纳米颗粒浓度和不同的纳米颗粒平均直径与不同的实验数据进行了比较, 模型预测的结果与实验结果相吻合. 所得的解析模型可以更深刻地揭示纳米流体对流换热的物理机理.     

基于遗传算法的SOBER-SJ10池沸腾传热研究

基于遗传算法构建了沸腾过程中加热固壁一维瞬态导热反问题计算方法,采用具有精确解的半无限瞬态导热问题构造了一个校核算例,验证了该算法具有较高的准确度和抗干扰能力,能够准确反演计算沸腾传热瞬态热流密度。基于该算法分析了 SOBER-SJ10地面和空间实验中沸腾传热特性,结果表明微重力环境中单相传热被严重抑制,热流密度远小于地面数值;不同重力条件下核态沸腾传热曲线落在同一位置,显示出低热流密度时核态沸腾传热具有某种重力无关特征;但微重力条件下核态沸腾曲线起始于远低于地面的热流密度,同时在远比地面小的热流密度值时达到临界热流状态,并转变为过渡沸腾模式。     

微纳耦合表面池沸腾强化传热的实验研究

以去离子水为工质,拟在钛板表面利用线切割进行微槽处理,采用阳极氧化法制备出二氧化钛纳米管阵列的微纳耦合表面,研究该表面的强化沸腾传热性能.通过场发射扫描电镜表征其微观结构形貌,利用接触角测量仪检测表面的静态接触角.结果表明,与光板相比,微槽结构增大了传热表面,规整的纳米管阵列具有亲水特性,接触角明显减小,微纳耦合表面的传热系数和临界热流密度分别达到了15.5 kW·m~(-2)·℃~(-1)和420.1 kW·m~(-2),分别提高了158.3%和50%,结合实验现象及机理分析可知,微纳耦合表面的微通道结构为气泡继续生长提供了支撑,有效避免了换热壁面被合并的大气泡完全覆盖;过热度达到一定温度后,更小的活化中心被激活,过热度随着热流密度的上升出现下降的趋势。微纳耦合表面对池沸腾具有强化作用。     

纳米特性表面圆柱型高速喷流沸腾临界热流密度的实验研究

制备了分别具有亲水和疏水特性的2种纳米特性表面,对具有不同固液接触角的传热面上圆柱型高速水喷流沸腾的沸腾临界热流密度(CHF)进行了系统的稳态实验研究,重点考察了喷流速度,过冷度和传热面固液接触角对CHF的影响。通过研究整理了传热面固液接触角和CHF之间的实验关系。对作者过去提出的饱和液与过冷液圆柱喷流沸腾CHF的半理论关系式进行了扩展和改进,使公式扩展到高流速范围和广泛的固液接触角范围。改进的关系式与实验数据符合得很好.     

多孔表面的液氮池沸腾实验研究

文中对颗粒烧结多孔表面和泡沫金属多孔表面上的液氮池沸腾换热特性进行了实验研究,并与光滑铜表面的试验结果进行了比较。结果表明,多孔表面成核条件更好,使得沸腾起始点相对于光滑表面提早;随着热流密度逐渐增大,气泡增多,且在多孔层内部连成一片,加热表面气泡离开受到多孔层的限制,热阻增加,换热系数大幅降低,整个沸腾进入表面沸腾阶段;多孔结构所产生的毛细抽力不断补充冷却流体,使表面沸腾能够持续较长时间,实验中未观测到临界热流密度现象。在实验基础上,文中描述了多孔表面不同池沸腾换热阶段的主要换热机理,并分析了流体工质、多孔层厚度、渗透系数、孔隙率等参数对多孔表面池沸腾换热的影响。     

流动方向对超临界二氧化碳流动传热特性的影响

本文基于拟沸腾理论研究了超临界二氧化碳(sCO₂)在水平和垂直向上管中的流动和传热特性差异.比较了不同质量流量、热流密度和压力下水平管与垂直向上管的流动和换热特性差异.与以往超临界流体的经典单相流体假设不同,本文引入拟沸腾理论来处理sCO₂在两管中的流动和传热,将超临界流体视为多相结构,包括近壁区的类气层和管芯中的类液流体.结果发现,传热方面,在正常传热模式下垂直向上管内壁温和水平管底母线内壁温基本一致.当垂直向上管发生传热恶化时,垂直向上管的壁温峰值会随着超临界沸腾数(SBO)的增大超过对应焓值位置的水平管顶母线内壁温.垂直向上管中SBO区分了正常传热和传热恶化.而在水平管中,当弗劳得数小于100时,SBO主导顶底壁面最大壁温差.相比于垂直向上管,相同压力下的超临界流体在水平管内发生传热恶化需要更高的热流密度和质量流量的比值.流动方面,引起垂直向上管压降斜率增高的机理是孔口收缩效应.主导水平管压降变化的机理是分层效应,并用弗劳得数在水平管中顶底壁温差异与压降之间建立联系.     

毛细微槽内的相变传热的实验研究

本文对矩形毛细微槽竖直板的相变传热特性进行了实验研究。结果表明毛细微槽对相变换热具有很大的促进作用。当壁面过热度较小时,相变换热形式主要是三相接触线附近的蒸发换热机制。而当过热度较大时,微槽内发生剧烈的沸腾。微槽内相变换热的临界热负荷有两种产生机理:其一是当微槽长度较大时微槽内由于流动阻力而产生的液体输运临界;另一机理是当微槽长度较小时的池内沸腾临界现象,亦即由动态微液层模型决定的临界机理。实验还得到了微槽强化传热的最佳优化尺寸。     

A fractal study for nucleate pool boiling heat transfer of nanofluids

In this paper, a fractal model for nucleate pool boiling heat transfer of nanofluids is developed based on the fractal distribution of nanoparticles and nucleation sites on boiling surfaces. The model shows the dependences of the heat flux on nanoparticle size and the nanoparticle volume fraction of the suspension, the fractal dimension of the nanoparticle and nucleation site, temperature of nanofluids and properties of fluids. The fractal model predictions show that the natural convection stage continues r...     

微小空间内多孔表面上的沸腾实验研究

1引言微小空间内的沸腾在电子器件冷却、航天热控、微型换热器以及核反应堆的冷却等领域中有着广泛的应用，因此对其沸腾机理的研究具有重要的意义。过去微小空间内的沸腾研究主要是针对光滑表面［‘－‘1，已证明在微小空间里，沸腾换热受空间尺寸的影响要比大空间大。对于多孔表面在微小空间内的沸腾研究则相对较少。本文对矩形槽道表面和烧结型多孔表面在微小空间里的沸腾进行了实验研究。2实验装置实验装置如图1，实验段是3O0mm长的紫铜管（包括矩形槽道管和烧结型的多孔管），内插不同外径的不绣钢管，形成不同间隙的环形小空间。在内…     

Critical heat flux enhancement of pool boiling using a porous nanostructured coating

Surface coating techniques are commonly used to increase heat transfer and control critical heat flux. In this research, we used anodizing—an electrochemical coating process—to coat an aluminum oxide layer on the aluminum plain surface. This porous nanostructured coating has uniform, cylindrical, parallel nanochannels, and closed end pores. Next, we conducted saturated pool boiling tests on the anodized samples, using deionized water and the CHFs were measured. We found that porous nanostructured coatings, due to their improved surface characteristics, particularly wettability, increased CHF values and also critical heat flux increased linearly with decreasing the contact angle.     

NUCLEATE POOL BOILING UNDER DC ELECTRIC FIELD

This article deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. Electrohydrodynamic (EHD) effects on heat transfer coefficients for dielectric liquids are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n-pentane, R-113, and R-123 are used as working fluids and the boiling phenomenon takes place on a horizontal plane copper surface. The experimental results have shown: (1) a threefold increase of nucleate pool boiling heat transfer coefficients, (2) a threefold increase of the critical heat flux (CHF), and (3) the disappearance of the hysteresis phenomenon. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed. The results obtained by the proposed EHD model are in good agreement with the experimental results.     

BUBBLE BEHAVIOR AND HEAT TRANSFER MECHANISM IN FC-72 POOL BOILING

A transparent heater made of a thin synthetic diamond substrate along with a high-speed camera was used to investigate bubble behavior during pool boiling. The heater design, combined with the selected FC-72 liquid, overcame the difficulty of previous thin-film heater experiments where transparency and adequate heat flux could not be simultaneously achieved. It also resulted in an essentially uniform temperature field over the heater surface. The growth and merging of bubbles were visualized and quantitatively documented. The relative contribution from phase change to the overall heat flux was determined at several heat flux levels. At a heat flux level half of the critical heat flux (CHF), surface bubble nucleation was found to contribute to more than 70% of the heat transfer from the heater surface. At a similar heat flux level, the ratio of dry to wetted area was determined to exceed 1/3, significantly higher than that predicted by a recent hydrodynamic model for CHF (approximately 1/16). This result suggests that modifications are needed for the hydrodynamic model when applied to highly wetting fluid on nearly isothermal surfaces. The merging of bubbles to form vapor blankets over the heater surface was observed, as has been assumed in recent hydrodynamic models.     

Numerical simulation of pool boiling of a Lennard-Jones liquid

We performed a numerical simulation of pool boiling by a molecular dynamics model. In the simulation, a liquid composed of Lennard-Jones particles in a uniform gravitational field is heated by a heat source at the bottom of the system. The model successfully reproduces the change in regimes of boiling from nucleate boiling to film boiling with the increase of the heat source temperature. We present the pool boiling curve by the model, whose general behavior is consistent with those observed in experiments of pool boiling.     

Possible Mechanism for Formation of Nonwettable “Dry Spots” on a Heated Surface during Nucleate Pool Boiling: II. Feedwater Stop Regime

The problems of simulation of heterogeneous nucleate pool boiling on a horizontal surface on the ascending branch of the boiling curve from the formation of a steam lens (SL) to the boiling crisis are considered. The proposed hypothesis provides in a number of cases a logically consistent interpretation of experiments and outlines the organizational principle of transferring the wall–liquid–steam system into the regime of nonwettable “dry spot” formation. The model includes the following types of nucleate boiling: (a) cyclic boiling with the contact line reverse to the bubble bottom center and bubble departure from the surface (at low heat flux q and the contact angle θ < 90°); (b) single steam bubble conversion into a steam lens, i.e., local film boiling with the possibility of spreading of a single “dry spot” at the variation of the contact angle θ ≥ 90°, and substantial growth of the departure diameter D_d and SL lifetime τd; (c) formation of a single steam cluster of four SLs at a given pressure, the liquid underheating, and the average wall overheating.     

Prediction of heat transfer of nanofluid on critical heat flux based on fractal geometry

Analytical expressions for nucleate pool boiling heat transfer of nanofluid in the critical heat flux (CHF) region are derived taking into account the effect of nanoparticles moving in liquid based on the fractal geometry theory. The proposed fractal model for the CHF of nanofluid is explicitly related to the average diameter of the nanoparticles, the volumetric nanoparticle concentration, the thermal conductivity of nanoparticles, the fractal dimension of nanoparticles, the fractal dimension of active cavities on the heated surfaces, the temperature, and the properties of the fluid. It is found that the CHF of nanofluid decreases with the increase of the average diameter of nanoparticles. Each parameter of the proposed formulas on CHF has a clear physical meaning. The model predictions are compared with the existing experimental data, and a good agreement between the model predictions and experimental data is found. The validity of the present model is thus verified. The proposed fractal model can reveal the mechanism of heat transfer in nanofluid.