Enhancement of Flow Boiling Heat Transfer with Surfactant

The surfactant additive octadecylamine (ODA) was used to enhance the flow boiling heat transfer of water in vertical copper tube, and the effects of the aqueous solution properties, mass fraction of ODA, mass flux and heat flux etc. on flow boiling heat transfer were investigated. In order to analyze the mechanism of enhancement on boiling heat transfer with ODA, the copper surface was detected by XPS, and the diagram of binding energy was obtained. The results show that ODA can be adsorbed on the surface of the copper wall, and affects the properties of the heating surfaces and enhances the flow boiling heat transfer of water. Only in low heat flux and in a suitable range of concentration, can ODA aqueous solution enhance flow boiling heat transfer, and the suitable mass fraction of ODA is in the range of 1×10 -5 5×10 -5 . In addition, compared with water, ODA aqueous solution does not increase the flow drag under the same experimental conditions.

Analysis of Bubble Behavior in a Horizontal Rectangular Channel under Subcooled Flow Boiling Conditions

Experiments on subcooled flow boiling have been conducted using water in a rectangular flow channel.Similar to the coolant channel in internal combustion engines(IC engines),the flow channel in this experiment was asymmetrically heated.Bubble images were captured using a high speed camera from the side view of the channel.The experimental conditions in terms of bulk temperature,bulk velocity,pressure and heat flux ranged from 65°C–75°C,0.25 m/s–0.75 m/s,1–1.7 bar and 490 kW/m2–700 kW/m2,respectively.On the basis of these tests,a statistical analysis of the bubble size has been conducted considering a population of 1400 samples.It has been found that the mean Sauter bubble diameter increases with the decrease of subcooling,bulk velocity,pressure and increased heat flux.A modified correlation has been finally proposed to predict the mean Sauter bubble diameter under subcooled flow boiling conditions upstream of the onset of significant void,which shows good accuracy with the experimental results.

Drag reduction of flow boiling with polymer additives

The drag reducing effect of polymer additive aqueous solution was investigated in flow boiling, and the polymer additives were two kinds of polyacrylamide (PAM) with relative molecular mass about 2.56×10 6 and 8.55×10 6. The frictional pressure drop was calculated according to the measured total pressure drop. The results show that the flow drag of flow boiling is reduced by adding a small amount of PAM to water when heat flux is in the range of 15.1 kW·m -2 to 47.0 kW·m -2 , when the mass fraction of PAM is higher than 2.0×10 -5 , the drag reducing effect is obvious. Drag reducing effect of PAM, whose relative molecular mass is 8.55×10 6, is slightly better than that of 2.56×10 6 at the same mass fraction, and the greater the flow rate of the additive solution, the better the effect of the drag reduction.

A Review on Flow Boiling of the Fluid with Lower Boiling Point in Micro-Channels

With the advancement of micro machining technology,the high-heat-flux removal from miniature electronic devices and components has become an attractive topic.Flow boiling in micro-channels is an optimal form of heat transfer and has been widely employed in high-heat-flux cooling applications.This comprehensively-reviewed article focused on the available recent literatures of experimental investigation regarding the flow boiling heat transfer and unstable behaviors of the fluid with lower boiling point in micro-channels.The thermal-fluid characteristics and potential heat transfer mechanisms of low-boiling-point fluids flow boiling in different narrow passages were summarized and discussed.The literatures regarding the pressure drop and occurrence of the unstable phenomena existing in two-phase flow boiling process were also discussed.The emphasis was given to the heat transfer enhancement methods as well as instability elimination,and various methods such as modification of surface and channel flow geometries were considered.Some future researches in the field of micro-scale flow boiling were suggested.

Flow Boiling Heat Transfer Characteristics and Delayed Dry-Out Ability of Non-Azeotropic Mixtures R245fa/R134a in Microchannels

Microchannel flow boiling heat transfer has the advantages of strong heat dissipation capacity,good temperature uniformity,and compact structure.It is an excellent way to thermally manage electronic devices,but when the heat flux exceeds CHF(Critical Heat Flux),the heat transfer performance deteriorates as the working fluid dries out.Non-azeotropic mixtures have the potential to effectively delay or avoid dry-out during the boiling process due to their temperature slide characteristics which causes the mass transfer resistance.To understand the influence of non-azeotropic mixtures on microchannel flow boiling,using the phase-change microchannel heat sink as the research object,the experiments on the flow boiling heat transfer performance of R245fa/R134a mixtures under different working conditions were carried out,and the characteristics of flow boiling heat transfer were obtained under the different working conditions,and comparison was developed with those of pure substance R245fa.The results demonstrated that a small amount of low-boiling-point components in the high-boiling-point working fluid inhibited boiling heat transfer to some extent,and lowered the average heat transfer coefficient under the non-dryout condition slightly lower than that of the pure substance;however,it also effectively delayed the onset of local dry-out and prevented significant deterioration in thermal transfer performance under the lower mass flow rate and higher heat flux,which could enhance the heat sink's stability.

Flow boiling heat transfer in copper foam fin microchannels with different fin widths using R134a

Heat sinks of copper foam fin microchannels are developed to deal with cooling challenges.The heat sinks consist of fins made of copper foam and channels.The channels are 0.5 mm in width and 1 mm in height,and the fins are 0.5 and 2.0 mm in width.Flow boiling experiments are conducted using R134a at subcooled and saturated inlet conditions.The heat flux is between 22 and 172 W/cm^(2),and the mass flux ranges from 264 to 1213 kg/(m^(2)s).The influence of the quality,the heat flux,and the mass flow rate on the heat transfer coefficient is obtained.It is found that wider fin raises the heat transfer coefficient.A correlation is developed based on heat transfer mechanisms,and it predicts the experimental result with a 12%mean absolute error.Compared with a solid fin microchannels heat sink,the heat transfer coefficient of the copper foam fin microchannels is higher(up to 60%)when the heat flux is lower than 100 W/cm^(2).The copper foam fin microchannels may enhance the heat transfer coefficient and reduce the pressure drop at the same time.

Experimental Investigation on Flow Pattern and Bubble Behavior during Subcooled Flow Boiling of R1233zd(E)in Parallel Channels

In this study,the flow pattern and bubble behavior of R1233zd(E)during subcooled flow boiling in parallel channels are experimentally investigated with visualization and thermal measurement.The test section is composed of 21 rectangular mini channels with the hydraulic diameter of 1.5 mm and the length of 140 mm.Bubbly flow,slug flow,churn flow and wavy-annular flow occur in sequence with the increase of vapor quality,while transient flow pattern transition process involving multiple flow patterns are also captured.The distribution of flow pattern is non-synchronized and axial-asymmetric,with earlier flow pattern transitions observed in peripheral channels away from the center axis.The initial nucleate site in each channel also show a random and axial-asymmetric distribution,while faster bubble growth can be noted in some channel under the comprehensive effects of liquid evaporation and bubble coalescence.The variation of heat transfer coefficient is correspondence to the flow pattern transition,showing different trends along the flow direction.The increase of mass flux can lead to delayed flow pattern transition and variation of heat transfer coefficient.In addition,higher heat transfer coefficient can be noted in channels away from the center axis.

Experimental Study on Flow Boiling Heat Transfer Characteristics of Microencapsulated Phase Change Material Suspension

Due to its core phase change characteristics,microencapsulated phase change material(MPCM)can make many base fluids have better heat transfer characteristics.In this paper,the flow boiling heat transfer characteristics of fluorinated liquid-based microencapsulated phase change material suspension(MPCMS)through vertical transparent quartz channel were studied.The effects of MPCM core phase change temperature and suspension flow velocity on boiling heat transfer coefficient and critical heat flux were discussed,respectively.The results show that the appropriate concentration of MPCMS can enhance both the boiling heat transfer coefficient and the critical heat flux.The strengthening effect becomes weak with the increase of suspension flow velocity.The maximum strengthening rates of critical heat flux appear at 0.05 m/s,which are 25%(MPCMS(70℃)),16%(MPCMS(58℃))and 10%(MPCMS(28℃)).The phase change temperature of the MPCM core has important effects on the boiling heat transfer coefficient and the critical heat flux.The results showed that the MPCM with core phase change temperature higher than the boiling temperature of base fluid has the best enhancement effect.Different bubble behavior in vertical tube with different heat flux can be observed by high-speed photography system.The particle core phase change in MPCMS inhibits the aggregation of bubbles and forms many small bubbles to enhance heat transfer.The work lays a foundation for further exploring the industrial application of MPCMS.

Flow Boiling of Ammonia in a Diamond-Made MicroChannel Heat Sink for High Heat Flux Hotspots

To solve the heat dissipation problem of electronic devices with high heat flux hotspots,a diamond microchannel heat sink consisting of 37 parallel triangular microchannels with channel lengths of 45 mm and hydraulic diameters of 280|im was designed.The flow boiling heat transfer characteristics of ammonia in the microchannels were investigated under high heat fluxes of 473.9-1000.4 W/cm^2.Saturated flow boiling experiments with saturation temperatures of 25℃,30℃,and 35℃ and mass fluxes of 98-1200 kg/m^2s were conducted,as well as subcooled flow boiling with inlet subcooling of 5℃ as a comparison.The temperature and pressure drop measurements were analyzed.The main conclusions below can be drawn.(1)At a given heat flux,the heat source temperature first decreased and then increased with the mass flux,and there existed an optimum mass flux to optimize the cooling performance of the heat sink.(2)The heat transfer performance under the saturated inlet condition was obviously better than that under the subcooled inlet condition.(3)A larger saturation temperature leaded to weakening of both the heat transfer capacity and the stability of the microchannel heat sink.Notably,with the high heat diffusion ability of the diamond substrate and the great heat transfer capacity of ammonia flow boiling in microchannels,the heat sink can achieve a heat removal capacity of up to 1000.4 W/cm^2.

Critical Heat Flux(CHF)Correlations for Subcooled Water Flow Boiling at High Pressure and High Heat Flux

The subcooled water flow boiling is beneficial for removing the high heat flux from the divertor in the fusion reactor,for which an accurate critical heat flux(CHF)correlation is necessary.Up to now,there are many CHF correlations mentioned for subcooled water flow boiling in the open literatures.However,the CHF correlations’accuracies for the prediction of subcooled water flow boiling are not satisfactory at high heat flux and high pressure for reactor divertor.The present paper compiled 1356 CHF experimental data points from 15 independent open literatures and evaluated 10 existing CHF correlations in subcooled water flow boiling.From the evaluation,the W-2 CHF correlation performs best for the experimental CHF data in all existing critical heat flux correlations.However,the predicted mean absolute error(MAE)of the W-2 correlation is not very ideal for all database and the MAE of the W-2 correlation is from 30%to 50%for some database.In order to enhance the CHF prediction accuracy in subcooled water flow boiling at high heat flux and high pressure,the present paper developed a new CHF correlation.Compared with other existing CHF correlations,the new CHF correlation greatly enhances the prediction accuracy over a broad range of pressures and heat fluxes which are desired in the cooling of high heat flux devices,such as those in the fusion reactor divertor.The validation results show that the new correlation has a MAE of 10.05%and a root mean squared error(RMSE)of 16.61%,predicting 68.1%of the entire database within±10%and 81.5%within±15%.The MAE of the new CHF correlation is 7.4%less than that of the best existing one(W-2 correlation),further confirming its superior prediction accuracy and reliability.Besides,the new CHF correlation works well not only for a uniform power profile but also for a non-uniform power profile in subcooled water flow boiling at high pressure and high heat flux.

Assessment of the Application of Subcooled Fluid Boiling to Diesel Engines for Heat Transfer Enhancement

The increasing demand of cooling in internal combustion engines(ICE)due to engine downsizing may require a shift in the heat removal method from the traditional single phase liquid convection to the application of new technologies based on subcooled fluid boiling.Accordingly,in the present study,experiments based on subcooled flow boiling of 50/50 by volume mixture of ethylene glycol and water coolant(EG/W)in a rectangular channel heated by a cast iron block are presented.Different degrees of subcooling,velocity and pressure conditions are examined.Comparison of three empirical reference models shows that noticeable deviations occur especially when low bulk subcooling and velocity conditions are considered.On the basis of the experimental data,a modified power-type wall heat flux model is developed and its ability to represent adequately reality is tested through numerical simulations against a reference rig case and a practical diesel engine.Computational results show that this modified model can effectively be used for practical engine cooling system design.

Dynamic Modeling of Phase Crossings in Two-Phase Flow

Two-phase flow and heat transfer,such as boiling and condensing flows,are complicated physical phenomena that generally prohibit an exact solution and even pose severe challenges for numerical approaches.If numerical solution time is also an issue the challenge increases even further.We present here a numerical implementation and novel study of a fully distributed dynamic one-dimensional model of twophase flow in a tube,including pressure drop,heat transfer,and variations in tube cross-section.The model is based on a homogeneous formulation of the governing equations,discretized by a high resolution finite difference scheme due to Kurganov and Tadmore.The homogeneous formulation requires a set of thermodynamic relations to cover the entire range from liquid to gas state.This leads a number of numerical challenges since these relations introduce discontinuities in the derivative of the variables and are usually very slow to evaluate.To overcome these challenges,we use an interpolation scheme with local refinement.The simulations show that the method handles crossing of the saturation lines for both liquid to two-phase and two-phase to gas regions.Furthermore,a novel result obtained in this work,the method is stable towards dynamic transitions of the inlet/outlet boundaries across the saturation lines.Results for these cases are presented along with a numerical demonstration of conservation of mass under dynamically varying boundary conditions.Finally we present results for the stability of the code in a case of a tube with a narrow section.

微多孔结构内流动沸腾及束缚水作用机理:孔隙尺度实验及换热强化

为解决高温部件的散热问题,流动沸腾技术被广泛应用于大功率设备热管理领域,采用多孔涂层和多孔材料可以进一步提高换热效果.与通道中的沸腾过程不同,由于孔隙-喉道结构复杂、流体流动方向随机性大,多孔结构内气固液三相作用规律尚不明确,换热面存在液体蒸干、壁温飞升、材料烧毁等风险.为深入研究微多孔结构内部流动沸腾传热性能,本文搭建了微观模型可视化实验系统,以揭示多孔介质内束缚水的运移机理,并量化相变过程中束缚水的运输形式及其对流动沸腾的影响规律.利用激光诱导荧光和粒子追踪技术获得了液膜在多孔骨架表面的运动速度.通过理论分析,作者发现薄液膜流动受到相邻两液桥间毛细压力差驱动,且毛细效应引起的液桥和薄液膜流动对冷却液向受热表面的输送起着重要作用.进而作者提出了一种孔喉尺寸在10~90μm的不规则多孔结构.该结构实现了传热系数5%~10%的提高,同时压降降低了5%~23%,有助于维持冷却系统高效稳定运行,为多孔材料强化流动沸腾表面结构的设计提供了重要参考.

Research on pumped two-phase single-sided cold plate of IGBT for rail transit applications

The use of pumped two-phase cooling to improve the thermal management of insulated gate bipolar transistor(IGBT)in rail transportation is a novel cooling technology.An experimental investigation on pumped two-phase cold plate of IGBT used in HXD1C locomotives was conducted at a mass flow rate of 0.1 kg/s–0.29 kg/s and a heat flux of 6.2 W/cm^(2),with R245fa as the working fluid.The experimental results showed that the base temperature nonuniformity can be controlled within 2.2℃ at flow rates of 0.14 kg/s and 0.19 kg/s,which is of great benefit to the reliability of IGBT.Based on well known correlations for saturated flow boiling in tubes,an analytical model was developed and compared with the experimental data.The model could predict the base temperature data within an error band of±3℃,as well as capture the trend of base temperature as a function of vapour quality and mass flow rate.The performance of the pumped two-phase cold plate of IGBT could be further improved with the aid of the developed model.