BOILING HEAT TRANSFER OF WATER AND R-11 ON HORIZONTALLY SMOOTH AND ENHANCED TUBES ENCLOSED BY A CONCENTRIC OUTER TUBE WITH TWO HORIZONTAL SLOTS

An experimental investigation was carried out on the boiling heat transfer characteristics of water and R-11 on the outside of a horizontal heated tube in narrow spaces. Two kinds of heat transfer surfaces (roll-worked and smooth surfaces) were tested. The test section consisted of a narrow annular space formed by enclosing the heated tube in an isolated concentric outer tube with two horizontal slats on the top and bottom. The nucleate boiling heat transfer characteristics were investigated experimentally at atmospheric pressure. The experimental results indicated that a single roll-worked tube in bulk liquid showed better boiling heat transfer than a single smooth tube. In the narrow spaces, the boiling heat transfer coefficients for the smooth tube were considerably enhanced when the gap size was so selected as to take an optimum value. There was no clear optimum gap size for heat transfer enhancement for the roll-worked tube in the narrow spaces. Enhancement of boiling heat transfer in the narrow spaces for the roll-worked tube was not clearly observed in this experiment. Finally, the critical heat flux (CHF) for boiling heat transfer in narrow spaces can be predicted by using a proposed CHF correlation.     

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.     

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。     

大空间和毛细管内液氮池沸腾传热的实验研究

文中以直径50μm,长20mm的磷青铜丝作为加热丝和测温元件,采用控制热流密度的方式测量了0°,30°,60°和90°倾角下大空间和玻璃毛细管内液氮的沸腾曲线,分析了毛细管对核态沸腾传热的影响以及管径和倾角对临界热流密度的综合影响。结果表明,在实验管径内,毛细管对于核态沸腾传热有明显的强化作用;并存在一最佳管径,可在30°~90°倾角范围内获得最大的CHF值,并且其值高于大空间时的CHF。     

竖直微槽道内沸腾换热CHF实验研究

对于沸腾换热,一个主要的约束条件就是临界热流密度(Critical Heat Flux,简称CHF)。这个约束条件对沸腾换热量有一个最高值的限制。文中对矩形微槽道中的流动沸腾临界热流密度进行了实验研究。实验数据是在不同尺寸(0.15mm;0.4mm;1mm)微槽道中,在较大范围的面积质量流速和不同进口过冷度下,以去离子水为工质得到的。实验过程中发现,达到CHF时,靠近出口壁面温度会突然升高,此时传热效率迅速下降。实验数据分析结果表明:CHF随质量流量的增加而增加;进口过冷度对CHF没有明显影响;CHF随着出口干度的增加而降低。     

Flow patterns and flowboiling heat transfer of freon R318C in an annular minichannel

This paper deals with experimental results on flowboiling heat transfer of liquid moving in an annular channel with unilateral central heating under the conditions of a significant effect of capillary forces on the flow modes and heat transfer. Experiments were carried out on boiling freon R318C in an annular channel with a gap of 0.95 mm and transparent outer wall. The inner wall was heated by the electric current. The local heat transfer coefficients and flow modes are presented. The critical film thickness corresponding to suppression of nucleate boiling was determined.     

高浓度润滑油对泡沫金属池沸腾特性的影响

实验研究了高油浓度的制冷剂/油混合物在泡沫金属加热表面池沸腾换热特性。使用三种泡沫铜作为加热表面,其参数分别为10ppi/90%孔隙率、10 ppi/95%孔隙率和30 ppi/98%孔隙率,厚度均为10 mm。制冷剂为R113,润滑油为VG68,油浓度为0%～40%。实验结果表明,泡沫金属总是强化池沸腾换热,换热系数最多提高450%;润滑油恶化制冷剂在泡沫金属加热表面池沸腾换热,换热系数最多降低90%。开发了高油浓度的制冷剂/油混合物在泡沫金属加热表面池沸腾换热关联式,预测值与95%的实验值误差在±30%以内。     

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…     

BOILING CHARACTERISTICS OF R-11 IN COMPACT TUBE BUNDLES WITH SMOOTH AND ENHANCED TUBES

This study presents new data on nucleate boiling heat transfer obtained in compact horizontal tube bundles with small tube gaps. The experiment investigates the heat transfer enhancement effects by the restricted spaces comprising the compact tube bundles and the enhanced heat transfer tubes for nucleate boiling heat transfer of R-11 at atmospheric pressure. A roll-worked tube was used as a new type of enhanced heat transfer tube. The experimental results show that the small tube gaps can greatly enhance boiling heat transfer in a smooth tube bundle, while enhancement effects of small gaps were not quite significant for the enhanced tube bundle. There is a compound effect from the enhanced surface and the restricted space only for the enhanced tube bundle with the tube gap of 0.5 mm. The effects of the tube positions within both compact tube bundles on the boiling heat transfer were minor.     

Étude phénoménologique de l'ébullition en espace confinéà partir d'un site de nucléation isolé

The aim of the present work is to identify the mechanisms of heat transfer enhancement during natural convective boiling in confined spaces. The influences of gap-size (0.3 to 2 mm), of pressure (1 to 3 bar), of heating surface orientation (vertical or horizontal) and that of the number of nucleation sites (no site, single site or several sites) for boiling R-113 in a narrow channel are investigated. Results for unconfined boiling are also presented as a reference. It is shown that latent heat transfer is improved when the bubbles are squeezed against the heated wall. The heat transfer enhancement is greater for a vertical than for a horizontal orientation because the two-phase fluid velocity is increased when the gap-size decreases, which enhances the sensible heat transfer. Finally, the confinement reduces the effect of an increase in pressure, which limits the influence of confinement. Indeed, both parameters lead to contrary effects on the bubble diameter.     

Influence of nucleation on the flow boiling heat transfer coefficient of a refrigerant mixture under varied heat flux conditions

The influence of nucleation on the flow boiling heat transfer coefficient of R-134a/R-290/R-600a refrigerant mixture is experimentally studied in a smooth horizontal tube of 12.7 mm diameter. The heat transfer coefficients are experimentally measured for stratified flow patterns under a varied heat flux condition; a condition found in the evaporator of refrigerators and deep freezers. The experiments are conducted in a counter-current heat exchanger test section. By regulating the flow rate and inlet temperature of acetone, which is the heating fluid flowing in the outer tube, a varied heat flux is provided to the refrigerant flowing in the inner tube. The refrigerant mass flow rate is fixed between 3 and 5 g s⁻¹ and its inlet temperature between −8.59 and 5.33°C, which corresponds to a pressure of 3.2 to 5 bar. The significance of nucleate boiling prevailing in the above-mentioned evaporators is highlighted. The experimental heat transfer coefficients are also compared with well known heat transfer correlations.     

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.     

FC-72在竖直壁面上及微小三角型通道内的沸腾传热

本文对浸在FC-72液池中的竖直壁面及两个微小三角形通道进行了沸腾实验研究,考虑了管道尺寸对沸腾传热特性的影响。两个微小三角形通道的边长分别为1.5和2.5 mm,水力直径分别为0.87和1.44 mm,长度50 mm,采用铜块上开V型沟槽,再覆盖上透明的玻璃片构成。热流密度由贴在铜块背后的膜状加热器提供。实验得到了沸腾曲线和传热系数,并用DV摄影机拍摄到了沸腾状况。实验结果显示,管道尺寸对沸腾传热特性有显著的影响,CHF值随通道尺寸的减小而减小,小通道在低热流密度时传热系数较大。     

液氮中导线加热丝的沸腾传热特性研究

本文以50μm的磷青铜丝作为加热丝和测温元件,采用控制热流密度的方式测量了0°-90°倾角下加热丝在液氮中的沸腾曲线,结果表明:核态沸腾在增加热流密度时存在滞后现象;Bromley公式能准确的预测出膜态沸腾换热曲线的斜率,Zuber模型和Kutateladze公式预测水平细加热丝的CHF误差在15%以内;对于Leidenfrost热流密度的预测,常温流体的计算模型并不适用;CHF随倾角的变化较大,且大于加热平面在相同倾角下的变化幅度。     

Wide range parametric study for the pool boiling of nano-fluids with a circular plate heater

The characteristics of boiling and critical heat flux (CHF) behavior of nano-fluids with alumina and silver nano-particles suspended in de-ionized water (pure water) were studied with circular plate heaters in the present study. Enhancements of CHF in nano-fluids in the wide range of particle sizes and concentrations were compared with those in pure water. Also, the effects of the particle deposition on CHF enhancement were investigated. All experiments were performed at the atmospheric pressure condition. The results show that the measured boiling curves in nano-fluids were shifted to the right and CHF were significantly enhanced for different nano-particle sizes and concentrations. The CHF of nano-fluids was increased as the size of the nano-particles decreased. On the other hand, nano-particle concentration value showing the maximum CHF had a critical value. In each pool boiling experiment of nano-fluids, nano-particles were deposited on the heater surface. Assuming that this phenomenon caused the CHF enhancement, pool boiling experiments of pure water were carried out with these nano-particle deposited heaters. The results of these tests were similar to those of the test of the nano-fluids for the CHF enhancement. The main cause of CHF enhancement was found to be the change of the heater surface structure. In order to analyze boiling phenomena of pure water and Al₂O₃ nano-fluids, boiling process was visualized by using a high speed camera.     

FC-72在浸泡于液池中的微小圆管内的沸腾传热

本文对浸泡于FC-72液池中的两个微小圆管进行了沸腾实验研究,得到了沸腾曲线和传热系数,并用DV摄像机拍摄到了圆管出口处的沸腾状况,研究了管道尺寸对沸腾传热特性的影响。实验结果显示,管道尺寸对沸腾传热特性有显著的影响。传热系数和CHF随着管道尺寸的缩小而减小.直径为1.10mm的圆管出口处在低热负荷加热时发生了汽泡阻塞,并导致了剧烈的沸腾滞后现象。     

Phenomenology of boiling: A coupled map lattice model

A minimal model for boiling is proposed. With increasing temperature of a bottom plate, the model shows three successive phases; conduction, nucleate, and film boiling. In the nucleate regime the heat flux increases with the temperature of the bottom plate, while it decreases in the film boiling regime. In the boiling phase, the maximum Lyapunov exponent is positive, implying that the boiling phenomena are spatiotemporally chaotic.     

ENHANCED POOL BOILING HEAT TRANSFER WITH SURFACE ATTACHMENT

Abstract

A pool boiling experiment with either water or Freon-113 was conducted to investigate nucleate boiling from a heated wall with either a spherical or a cylindrical attachment. The result revealed that nucleate boiling can be enhanced by applying a horizontal cylindrical attachment to a vertical heated wall, owing to the favorable thermal environment characterized by a small-gradient liquid temperature profile within the restricted regions between the attachment and the heated wait. Nucleate boiling is enhanced in terms of a lower wall superheat required for incipient boiling and more bubbles generated than from an open heated wall. As a result of the enhanced nucleate boiling, heal transfer of the vertical heated wall above the attachment was improved due to excessive bubbles moving upward along the heated wall, causing removal of the thermal layer near the wall and evaporation of the thin liquid film between the bubbles and the wall. The boiling curve hysteresis with Freon-113 was significantly reduced as a result. The effects of diameter, length and surface roughness of the cylindrical attachment were also investigated.     

BOILING HEAT TRANSFER MECHANISMS IN A HORIZONTAL TUBE BUNDLE

The effect of thermal environment on boiling heat transfer performance in a section of a horizontal tube bundle was investigated using R-113 as the working fluid. The in-line tube bundle has five columns and 27 rows with a pitch-to-diameter ratio of 1.3. Heal transfer coefficients obtained from the instrumented tube in the tube bundle with only one tube heated while the other tubes remained unhealed and with all the lubes in the bundle heated are reported for a range of heat flux, pressure, mass flow rate, and quality. The results showed that heat transfer coefficient of a tube in a heated bundle is slightly higher than that in an unhealed bundle, with the variation of heat transfer coefficient decreasing as heat flux, mass flow rate, or pressure increased. It was also found that higher quality would tend to improve the heat transfer. However, the effect of quality disappeared as heat flux, mass flow rate, and pressure increased. Based on the experimental data, the mechanism of the heat transfer augment due to thermal environment was analyzed. It was proposed that fluid agitation and thin liquid film formation are two main factors for a heated bundle to have better transfer performance than an unhealed bundle,     

COMBINED EFFECTS OF SUBCOOLING AND SURFACE ORIENTATION ON POOL BOILING OF HFE-7100 FROM A SIMULATED ELECTRONIC CHIP

The effects of orientation and subcooling on pool boiling of the HFE-7100 dielectric liquid near atmospheric pressure (0.085 MPa) from a 10 × 10 mm smooth copper surface are investigated experimentally. Results are obtained for inclination angles θ = 0° (upward-facing), 30°, 60°, 90°, 120°, 150°, and 180° (downward-facing) and liquid subcoolings ΔT_sub = 0, 10, 20, and 30 K. Increasing θ decreases the saturation nucleate boiling heat flux at high surface superheats (ΔT_sat > 20 K), but increases it only slightly at lower surface superheats. The critical heat flux (CHF) decreases slowly with increasing θ from 0° to 90°, and then deceases faster with increasing θ to 180°. CHF increases linearly with increased subcooling, but the rate increases from 0.016 K^?1 at 0° to 0.048 K^?1 at 180°. At θ = 0° and ΔT_sub = 30 K, CHF is ～ 36 W/cm² and 24.45 W/cm² for saturation boiling, while at θ = 180° CHF = 10.85 W/cm² at ΔT_sub = 30 K and only 4.30 W/cm² at saturation. The developed correlation for CHF of HFE-7100, as a function of θ and ΔT_sub, is within ±10% of the present data. The recorded still photographs of the boiling surface in the experiments illustrate the effects of liquid subcooling and surface orientation at different nucleate boiling heat fluxes and surface superheats on vapor bubble accumulation and/or induced mixing at the surface.