Study on boiling heat transfer in liquid saturated particle bed and fluidized bed

An investigation on the effects of solid particles on boiling heat transfer enhancement is performed. The range of particle diameter is from millimeter to nanometer. The experimental results show that boiling heat transfer can be enhanced greatly by adding the solid particle into the liquid whether in fixed particle bed or in fluidized particle bed. The boiling enhancement is closely related to the particle size, the initial bed depth and the heat flux applied. The experiments show that boiling characteristics are greatly changed when a particle layer is put on the heated surface. The major effects of fixed particle bed on nucleate pool boiling heat transfer are the nucleation, bubble moving and thermal conductivity effect. A boiling heat transfer correlation is obtained to predict the boiling heat transfer coefficients in a liquid saturated porous bed. A volumetric convection mechanism of boiling heat transfer enhancement by fluidized particles is proposed. The calculated results from the model suggested in this paper agree reasonably with the experimental values.     

Experimental Investigations on Boiling Heat Transfer Inside Miniature Circular Tubes Immersed in FC-72

To investigate the size effect on the characteristics of boiling heat transfer, boiling behavior of FC-72 in heated vertical miniature circular tubes immersed in a liquid pool was experimentally studied. Two AISI 304 stainless steel tubes with inner diameters of 1.10 mm and 1.55 mm correspondingly, were heated by swirled Ni-Cr wire heaters and sealed in Lucite blocks by silicon adhesive. Both the top and the bottom ends of the circular test sections were open to the liquid pool. The boiling curves and heat transfer coefficients were obtained experimentally. The boiling behaviors at the outlets of the miniature tubes were also visualized with a digital video camera. Experimental results show that the tube geometry has a significant effect on the boiling characteristics. Vapor blocking at the outlet of the smaller circular tube with a diameter of 1.10 mm caused severe boiling hysteresis phenomena. The CHF decreased with reducing in tube size.     

狭缝中流动沸腾传热过冷沸腾起始点的实验研究

以间隙为1．0mm和1．5mm的环形狭缝通道中流动沸腾传热的实验数据为基础，分析了影响过冷沸腾起始点热负荷的主要因素，给出了计算环形狭缝通道中流动沸腾传热过冷沸腾起始点的经验关联式，并将计算结果与实验值进行了比较。该关联式可以用来预测实验范围内的过冷沸腾起始点的热负荷。     

Flow boiling of liquid nitrogen in micro-tubes: Part II – Heat transfer characteristics and critical heat flux

This paper is the second portion of a two-part study concerning the flow boiling of liquid nitrogen in the micro-tubes with the diameters of 0.531, 0.834, 1.042 and 1.931 mm. The contents include the heat transfer characteristics and critical heat flux (CHF). The local wall temperatures are measured, from which the local heat transfer coefficients are determined. The influences of heat flux, mass flux, pressure and tube diameter on the flow boiling heat transfer coefficients are investigated systematically. Two regions with different heat transfer mechanism can be classified: the nucleate boiling dominated region for low mass quality and the convection evaporation dominated region for high mass quality. For none of the existed correlations can predict the experimental data, a new correlation expressed by Co, Bo, We, K_p and X is proposed. The new correlation yields good fitting for 455 experimental data of 0.531, 0.834 and 1.042 mm micro-tubes with a mean absolute error (MAE) of 13.7%. For 1.931 mm tube, the flow boiling heat transfer characteristics are similar to those of macro-channels, and the heat transfer coefficient can be estimated by Chen correlation. Critical heat flux (CHF) is also measured for the four tubes. Both the CHF and the critical mass quality (CMQ) are higher than those for conventional channels. According to the relationship that CMQ decreases with the mass flux, the mechanism of CHF in micro-tubes is postulated to be the dryout or tear of the thin liquid film near the inner wall. It is found that CHF increases gradually with the decrease of tube diameter.     

Film boiling heat transfer around a vertical finite‐length cylinder with a convex hemispherical bottom

The film boiling heat transfer around a vertical silver cylinder with a convex hemispherical bottom was investigated experimentally in quiescent water at atmospheric pressure. The experiments have been carried out using a quenching method. The diameter and length of the test cylinder are 32 mm and 48 mm, respectively. The test cylinder was heated to about 600 °C in an electric furnace and then cooled in saturated or subcooled water with an immersion depth of about 100 mm. The degree of liquid subcooling was varied from 0 K to 30 K. The analytical solutions for saturated and subcooled boiling are obtained by applying a two‐phase boundary layer theory for vapor film with a smooth interface. The experimental data correlates within ±15% based on the proposed prediction method. Also, the lower limit of film boiling was examined in terms of wall heat flux and degree of superheating. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20289     

竖直矩形窄缝两相同向分相流动沸腾传热模型

首次对竖直矩形窄缝内的汽液分相流动区提出一维两相同向分相流动沸腾传热模型 ,并进行了数值计算 ,得到不同质量流速下液膜厚度变化和沸腾传热系数等结果。沸腾传热系数的模型预测值初步与已有实验关联式进行了比较 ,两者基本吻合 ,偏差在± 1 4% ;从而证实了液膜导热是竖直矩形窄缝内汽液分相流动区沸腾传热的主导机理。     

微通道内流动沸腾特性研究

对国内外微通道流动和换热的研究实验作了总结,阐述了影响微通道换热系数的因素,如热流密度、过热度和干度等.对去离子水在内径为0.65 mm、长为102 mm的圆形管道内流动沸腾换热进行了实验研究,得到了局部换热系数随干度的变化关系,进而根据换热系数的变化趋势讨论了饱和流动沸腾区微通道内主导的换热机制.结果表明:从换热系数随干度的变化关系很难判定主导的换热机制;将实验数据与已发表的预测关联式进行了比较,发现大多关联式都失效,说明基于常规理论的模型不再适用于微通道.     

Effect of tube bundles on nucleate boiling and critical heat flux

In order to elucidate boiling heat transfer characteristics for each tube and the critical heat flux (CHF) for tube bundles, an experimental investigation of pool and flow boiling of Freon-113 at 0.1 MPa was performed using two typical tube arrangements. A total of fifty heating tubes of 14 mm diameter, equipped with thermocouples and cartridge heaters, were arrayed at pitches of 18.2 and 21.0 mm to simulate both square in-line and equilateral staggered bundles. For the flow boiling tests the same bundles as were used in pool boiling were installed in a vertical rectangular channel, to which the fluid was supplied with an approach velocity varying from 0.022 to 0.22 m/s. It was found in this study that the boiling heat transfer coefficient of each tube in a bundle was higher than that for an isolated single tube in pool boiling. This enhancement increases for tubes at higher locations, but decreases as heat flux is increased. At heat fluxes exceeding certain values, the heat transfer coefficient becomes the same as that for an isolated tube. As the heat flux approaches the CHF, flow pulsations occurred in the pool boiling experiments although the heat transfer coefficient was invariant even under this situation. The approach velocity has an appreciable effect on heat transfer up to a certain level of heat flux. In this range of heat flux, the heat transfer coefficient exceeds the values observed for pool boiling. An additive method with two contributions, i.e., single phase convection and boiling, was used to predict the heat transfer coefficient for bundles. The predicted results showed reasonable agreement with the measured results. The critical heat flux in tube bundles tended to increase as more bubbles were rising through the tube clearance. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(4): 312–325, 1998     

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.     

Boiling heat transfer in a narrow space controlled by punched interference plate

An experiment on pool boiling in methanol was performed for a case in which the boiling space was controlled by an interference plate with many holes. The narrow space, 0.12 mm in thickness, between the heat transfer surface and the interference plate was hermetically sealed at the perimeter. Therefore, the vapor and liquid were only exchanged through the holes in the interference plate. The degree of superheat at the onset of boiling was 0.7 K without overshoot at 10‐mm plate thickness, 1‐mm hole diameter, and 3.85‐mm hole pitch. The critical heat flux obtained was the same value without the interference plate mentioned above. The interference plate disturbed free convection and a superheat layer was provided under small heat flux on the heat transfer surface. The critical bubble diameter for the onset of boiling was decreased as the temperature of the superheat layer was increased. Thus, the degree of superheat at the onset of boiling was decreased. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(7): 462–471, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20028     

Boiling Heat Transfer Enhancement Using Micro-Machined Porous Channels for Electronics Cooling

Boiling heat transfer enhancement for a passive electronics cooling design is presented in this paper. A novel pool boiling enhancement technique is developed and characterized. A combination of surface modification by metallic coating and micro-machined porous channels attached to the modified surface is tested and reported. An experimental rig is set up using a standard BGA package with 12 mm × 12 mm thermal die as a test surface. The limiting heat flux for a horizontally oriented silicon chip with fluorocarbon liquid FC-72 is typically around 15 W/cm². Boiling heat transfer with the designed enhancement techniques is investigated, and the factors influencing the enhancement are analyzed. The metallic coated surface at 10°C wall superheat has a heat flux six times larger than an untreated chip surface. Micro-machined porous channels with different pore sizes and pitches are tested in combination with the metallic coated surface. The boiling heat flux is seven times larger at lower wall superheat compared to the plain chip surface. Maximum critical heat flux (CHF) of 38 W/cm² is obtained with 0.3 mm pore diameter and 1 mm pore pitch. A ratio of pore diameter and pore pitch is found to correlate well with the heat transfer enhancement obtained by experiments. Structures with smaller pore diameter to pitch ratio and larger pore opening are found to have higher heat transfer enhancement in the tested combination.     

Theoretical analysis of the stability of vapor film in subcooled film boiling on a horizontal wire

Linear stability analysis of a thin vapor film in subcooled film boiling on a horizontal cylinder is reported. The effects of liquid inertia, vapor viscosity and compressibility, and heat transfer were taken into account. Theoretical predictions of the heat transfer coefficient at the neutral stability point were compared with experimental data at the minimum-heat-flux point that was obtained during rapid quenching of thin horizontal wires in water and ethanol. At high liquid subcooling, the experimental value was 60% of the theoretical prediction irrespective of the wire diameter and quenching liquid. This difference was considered to be due to the nonuniformity of the vapor film which was neglected in the theoretical analysis. © 1998 Scripta Technica. Heat Trans Jpn Res, 26(4): 219–235, 1997     

Behavioral study of alumina nanoparticles in pool boiling heat transfer on a vertical surface

Experiments were carried out to investigate the pool boiling of alumina‐water nanofluid at 0.1 g/l to 0.5 g/l of distilled water, and the nucleate pool boiling heat transfer of pure water and nanofluid at different mass concentrations were compared at and above the atmospheric pressure. At atmospheric pressure, different concentrations of nanofluids display different degrees of deterioration in boiling heat transfer. The effect of pressure and concentration of nanoparticles revealed significant enhancement in heat flux and deterioration in pool boiling. The heat transfer coefficient of 0.5 g/l alumina‐water nanofluid was compared with pure water and clearly indicates deterioration. At all pressures the heat transfer coefficients of the nanofluid were lower than those of pure water. Experimental observation revealed particles coating over the heater surface and subsequent SEM inspection of the heater surface showed nanoparticles coating on the surface forming a porous layer. To substantiate the nanoparticle deposition and its effect on heat flux, investigation was done by measuring the surface roughness of the heater surface before and after the experiment. While SEM images of the heater surface revealed nanoparticle deposition, surface roughness of the heater surface confirmed it. Based on the experimental investigations it can be concluded that an optimum thickness of nanoparticles coating favors an increase in heat flux. Higher surface temperature due to the presence of nanoparticles coating results in the deterioration of boiling heat transfer. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20365     

Enhanced boiling heat transfer in mesochannels

Heat transfer characteristics are studied for a hybrid boiling case that combine features of spray cooling and flow boiling. In such a hybrid system, a liquid is atomized and the surrounding vapor is entrained into the droplet cone to provide an initial quality for enhanced boiling. An in-house experimental setup was developed to obtain surface temperature and heat flux measurements in a series of converged mesochannels for hybrid boiling. To compare the heat transfer performance of this hybrid technique, a flow boiling module was also developed using the same series of converged mesochannels. The inlet and exit hydraulic diameter of the mesochannels was 1.55 and 1.17 mm, respectively. The heat flux was in the range of 15–45 kW/m² and the estimated mass flux varied from 45 kg/m²s at the channel inlet to 110 kg/m²s at the channel outlet. Moreover, a model was presented to predict surface temperatures and heat transfer coefficients for flow boiling and hybrid boiling in mesochannels. This model was developed based on Chen’s formulation (1966) [21] but with two essential modifications. First, the laminar entry length effect was taken into consideration for heat transfer coefficient calculation. Second, the boiling enhancement factor was calculated based on the fluid properties. The model was compared to the experimental data and several other correlations for both cases. This model shows good agreement with the experimental data (mean deviations on the order of 12–16%).     

Investigation on the characteristics of boiling heat transfer through narrow annular channels

Factors concerning the characteristics of boiling heat transfer are analyzed theoretically. Based on the experimental data of boiling heat transfer through annular channels with the gaps of 1–2 mm, three correlations which will be used to calculate the heat transfer in the similar conditions are given. The results obtained from these correlations are compared with experimental data. The main factors having influence on boiling heat transfer through narrow channels, and the desirable correlation are determined. This correlation can be used to predict the flow boiling heat transfer within the range of this experiment. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(2): 78–84, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20049     

Study of nucleate boiling heat transfer to slush hydrogen and slush nitrogen

Slush hydrogen is a mixture of liquid hydrogen and solid hydrogen particles, and is being considered as a spaceplane fuel or as a means of transport for hydrogen used as a source of clean energy. This paper describes nucleate boiling heat transfer characteristics of slush hydrogen and slush nitrogen. For the visual observation of heat transfer states, a heat transfer unit was placed in a glass Dewar designed to minimize the heat loss from an atmospheric environment. The heat transfer unit used was a circular flat plate 0.025 m in diameter made of electrolytic tough pitch copper. During testing, three different orientations of the heat transfer surface were used: horizontal facing up, vertical, and horizontal facing down. Heat transfer data for the normal boiling point (NBP) of liquid hydrogen, the triple point (TP) of liquid hydrogen, the NBP of liquid nitrogen, and the TP of liquid nitrogen were obtained up to the critical heat flux (burnout). These data for slush hydrogen and nitrogen, including the results of observation of the heat transfer surface were compared. This clarified the nucleate boiling heat transfer characteristics of slush hydrogen and slush nitrogen, which have rarely been investigated. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(1): 13–28, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10068     

Enhancement of nucleate boiling on composite surfaces

A composite heating surface composed of materials with different thermal conductivities can be expected to enhance heat transfer in nucleate boiling. This is because the end surface, with higher conductivity, will attain a higher temperature and as a result will serve to provide preferential nucleation sites. To confirm this idea, several composite surfaces were fabricated by uniaxially imbedding thin copper cylinders in the heat flow direction on a stainless steel circular plate 30 mm in diameter and 5 mm thick. The imbedded copper cylinders ranged from 1 mm to 4 mm in diameter and one to 77 in number. The heat transfer performance of these composite surfaces was investigated for pool boiling of saturated water at atmospheric pressure. It was confirmed that the copper cylinder surfaces exposed to water functioned as local hot spots to initiate preferential nucleate boiling, leading to higher boiling heat transfer coefficients than those on a homogeneous stainless steel surface. The measured void fraction above the heating surface verified intensive bubble generation on the surface of the copper cylinders. This situation continued up to a certain heat flux level and was then followed by nucleation on the mother surface of stainless steel around the copper cylinders. A numerical analysis of heat conduction within a composite wall simulated the temperature distribution within the wall and the variation in surface heat flux at the time of boiling incipience. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(3): 216–228, 1998     

强润湿性液体池沸腾传热的实验研究和机理分析

对强润湿性液体的池沸腾传热实验而言,本文提出了行之有效的实验程序,并严格按照实验程序进行了R113池沸腾传热的实验研究,具体研究了表现老化和液体过冷度对池沸腾传热曲线及起沸点的影响,实验中观察到了三个反常现象,最后,从强润湿性液体的沸腾传热机理的角度对其给出了相应的解释。     

强化传热管束狭窄空间内R_11的沸腾换热特性

对紧凑型滚压面传热管管束狭窄空间内R－11的沸腾强化换热进行了实验研究,确认了由紧凑型滚压强化管束组成的满液式蒸发换热器具有良好的换热性能。其原理是利用强化传热管管束狭窄空间提前从自然对流换热转换为旺盛核沸腾换热,实验结果确认了管束形成的狭窄空间和强化传热面两种强化技术对沸腾换热的强化效果不能简单叠加。     

表面活性剂对水工质超长重力热管传热性能的影响

超长重力热管是近年来被提出的用于干热岩地热能开采的一种新技术。该技术方案通过工质的沸腾-冷凝相变来进行热量传输从而在地面获得地下数千米深的热量,突破了常规热管的热力输运距离。表面活性剂能降低液体的表面张力,从而改变液体工质的沸腾特性,能在一定程度上提升常规热管的热力性能,但在超长重力热管中的作用仍有待研究。本文在自行搭建的超长重力热管实验系统（L = 40 m,D = 7 mm）中,以不同浓度的十二烷基硫酸钠（SDS）水溶液为工质,研究了表面活性剂的加入对超长重力热管采热性能的影响。实验发现SDS的加入降低了热管的最佳注液量。纯水工质的最佳注液率为30%（注液高度为6 m）,随SDS浓度升高,最佳注液率降低至10%（注液高度为2 m）。实验还发现,SDS的影响在不同注液量条件下有很大区别:注液量较低（2 m）时,加入SDS后热管性能改善,随着SDS溶液浓度的升高,热管的采热性能提高;注液量较高（6 m）时,加入SDS后热管性能下降,随着SDS溶液浓度的增大,采热性能下降。分析热管测温点温度波动发现,加入SDS对不同注液高度工况的影响机制并不相同。注液量较低时,SDS的加入使得热管整体壁温下降,这可能和常规热管中一样,是由于沸腾相变更加剧烈,壁面润湿性提升,从而提高了热管的采热性能。但在注液量较高时,加入SDS后,温度波动幅度减小且沿高度迅速衰减,这可能是由于工质表面张力的降低,沸腾时气泡聚并减弱,热管工作时产生的间歇沸腾减弱或消失,导致超长重力热管的采热性能下降。因为间歇沸腾在一定程度上有利于降低热管注液段与外界环境的温差,减少散热。