超薄空间复杂边界条件下气体流动压降实验

随着超薄热管等元件进一步超薄化,蒸汽腔厚度减小导致蒸汽流动压降急剧增大,传热热阻增加,传输极限降低。搭建了超薄受限空间气体流动压降实验装置,开展空气流动实验,获得了不同通道高度(0.1~0.5 mm)、不同表面丝网孔径(0.036~0.104 mm)和不同流速(1~10 m/s)下的压降变化。结果表明:通道高度和流速对压降产生显著影响,而表面丝网孔径并不会;3个影响因素按显著程度依次为通道高度、流速、表面丝网孔径;随表面丝网孔径的减小,压降逐渐增大;随通道高度的减小,压降先缓慢增大,在减至0.3 mm后压降开始剧烈上升;随流速的增加,压降增大且近似呈正比例变化关系。最后基于实验数据修正了微通道层流情况下沿程阻力系数相关性预测关联式,以便更准确地预测气体压降。

基于柔性热管的笔记本电脑散热过程数值模拟研究

可折叠笔记本电脑,采用柔性热管将CPU热量传导至显示屏背板,通过自然对流将热量带走,实现笔记本电脑CPU的温度控制。针对这一结构,采用Fluent软件建立笔记本电脑热管散热结构对应的物理模型,在自然对流条件下,改变柔性热管的弯折角度,研究不同弯折角度对散热性能的影响,分析不同条件下的温度变化曲线。设计显示屏背板作为模型主要散热结构。结果表明,在柔性热管恒定导热系数和随角度变化导热系数条件下,模型散热能力均随热管弯折角度减小而增大,同时,考虑了增加辐射后对模型散热能力的影响大小,考虑辐射后,总体散热趋势无较大变化。

超临界CO_(2)印刷电路板式换热器实验研究及费用评估

目前,超临界CO_(2)印刷电路板换热器研究主要以数值模拟为主。实验研究较少,部分实验运行温度和压力远低于超临界CO_(2)布雷顿循环发电系统实际工况。此外,基于实验结果的超临界CO_(2)换热器费用评估未见报道。为此,采用印刷电路板式换热器作为超临界CO_(2)布雷顿循环发电系统回热器和冷却器,在全温全压实验条件下测试质量流量对换热器缩比样机的热导、效能、换热量和摩擦因子的影响规律,并依据实验数据对1MW超临界CO_(2)发电系统用回热器和冷却器费用进行分析。结果表明,不同质量流量条件下回热器和冷却器的热导、效能、换热量和摩擦因子变化幅度较大,从而对换热器的费用产生影响。研究结果为MW级超临界CO_(2)布雷顿循环发电系统回热器和冷却器的研发提供技术支撑。

PCHE内轴向导热对局部换热性能的影响研究

通过改变印刷电路板式换热器(PCHE)通道壁厚、直径以及冷热侧CO2进口温度差,数值分析了层流条件下,轴向导热对PCHE内超临界压力CO2流动换热的影响。结果表明,考虑轴向导热时,局部热通量沿程变化更加平缓,冷侧对流传热系数在小于拟临界温度区域变小,而热侧对流传热系数在小于拟临界温区域增大;轴向导热仅对热效率峰值附近区域有所影响,其使得峰值略有增大且向低温侧移动;传热熵产主要发生在高温区域,轴向导热可有效降低局部传热熵产。增大壁厚和直径可增大轴向导热的影响,而增大进口温差,轴向导热的影响变化不大。

A Comprehensive Review of Thermal Performance Improvement of High-Temperature Heat Pipes

Energy efficiency issues are being focused on the growing concern of global warming and environmental pollution.The high-temperature heat pipe(HTHP) is an effective and environmental-friendly heat transfer device employed in many industries,including solar power generation,high-temperature flue gas waste heat recovery,industrial furnaces,nuclear industries,and aviation.As a critical factor in HTHPs,thermal performance is mainly introduced in the entire paper.To date,most reviews have been published concerning one or several application scenarios.However,to the best of authors' knowledge,it is hard to find a review discussing how to improve the thermal performance of HTHPs comprehensively.First,the impact on the performance of three main components of HTHPs over the past 30 years is introduced:the working fluid,the HTHP structure,and the wick structure.Herein,it is a considerable review of the optimal operating conditions for each direction,and we expect this paper contribute to improving the thermal performance of HTHPs.Then,current numerical simulations and theoretical research on the heat transfer limit of HTHPs are recommended.The significant hypotheses used in numerical simulations and the present theoretical studies are compiled here.Finally,some potential future directions and tentative suggestions for HTHP research are put forward.

Optimization of Thermal Conductivity of Alumina-Filled Composites by Numerical Simulations

Understanding the correlation between the physical features of composite components and thermal conductive pathway is beneficial to optimizing the overall heat-transfer performance.Herein,we conduct numerical simulation to investigate the thermal conductivity and heat flux distributions of alumina(Al_(2)O_(3))-filled composites.The finite element model was verified by both experimental data and theoretical models.The crucial factors include the influence of the interface thermal resistance,the intrinsic thermal conductivity of the matrix and Al_(2)O_(3)filler,and the size effect of Al_(2)O_(3)fillers were investigated.For single Al_(2)O_(3)-filled composites,the results indicate that increasing the intrinsic thermal conductivity of the matrix is conductive to bridge the Al_(2)O_(3)pathway along heat-transfer direction,but there are very limited contributions by enhancing the intrinsic thermal conductivity of Al_(2)O_(3)filler,tuning the size of Al_(2)O_(3)filler,and reducing the interface thermal resistance.After introducing the multiscale fillers,it is found that the high thermal conductivity can be achieved by regulating their size matching effect.At the optimal binary ratio of 70:30(40µm:15µm)and ternary ratio of 55:35:10(40µm:15µm:10µm),the heat-conduction network presents the dominant skeleton of large-sized filler and the bridging branch of small-sized fillers features,which facilitates the formation of a complete and continuous thermal conductive network.This study gives a practical guidance for the thermal conductive design of Al_(2)O_(3)-filled composites.

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.

High Temperature Catalytic Hydrogenation of Acetone over Raney Ni for Chemical Heat Pump

Exothermic hydrogenation reaction of acetone is an important part of an IAH-CHP, and the performance of IAH-CHP is affected directly by this reaction. This paper studies the influence of space velocity, temperature, hydrogen flow rate and pressure on conversion and selectivity experimentally. The byproducts are analyzed and classified into three types: hydrogenation product, cracking products and condensation products. Both the conversion and selectivity of this reaction have the same trend with the change of space velocity, temperature and hydrogen flow rate, and has the opposite trend with the change of pressure. As the space velocity increases, the conversion curve is a gradual decline parabola but the selectivity curve is close to a straight line. Hydrogen flow rate has a more obvious influence on conversion than temperature, whereas on selectivity the situation is opposite. High pressure increases the conversion of acetone to all products, but the increment of byproducts is more than that of isopropanol, so the selectivity decreases as pressure increases.

CFD Study on Local Fluid-to-Wall Heat Transfer in Packed Beds and Field Synergy Analysis

To reach the target of smaller pressure drop and better heat transfer performance, packed beds with small tube-to-particle diameter ratio(D/dp<10) have now been considered in many areas. Fluid-to-wall heat transfer coefficient is an important factor determining the performance of this type of beds. In this work, local fluid-to-wall heat transfer characteristic in packed beds was studied by Computational Fluid Dynamics(CFD) at different Reynolds number for D/dp=1.5, 3.0 and 5.6. The results show that the fluid-to-wall heat transfer coefficient is oscillating along the bed with small tube-to-particle diameter ratio. Moreover, this phenomenon was explained by field synergy principle in detail. Two arrangement structures of particles in packed beds were recommended based on the synergy characteristic between flow and temperature fields. This study provides a new local understanding of fluid-to-wall heat transfer in packed beds with small tube-to-particle diameter ratio.

Experimental and Numerical Study of an 80-kW Zigzag Printed Circuit Heat Exchanger for Supercritical CO_(2)Brayton Cycle

In this study,an experimental system was built to investigate the global performance of an 80-k W zigzag printed circuit heat exchanger(PCHE).It could meet the requirement of the pre-cooler for the supercritical carbon dioxide(S-CO_(2))Brayton power cycle and the modified effectiveness considering the pinch point is between 61.5%and 79.3%.When the outlet S-CO_(2)temperature is near the pseudo-critical point,the thermo-physical properties have more effects on heat transfer performance compared to flow characteristics.For the local performance,the mass flow rates of both sides have crucial influences on the location where the peak of S-CO_(2)Nusselt number occurs while only the S-CO_(2)flow rate affects the variation of the peak value.In addition,the influence of the radius of curvature on the secondary-flow should not be ignored.In the end,new empirical correlations were proposed considering the drastic variations of the Prandtl number.

纳米流体在超亲水微槽道热管中的润湿特性研究

微槽道热管的性能很大程度上受限于毛细极限,改变槽道表面性质或采用新型流体工质来提升润湿长度是解决该问题的有效方法。本文基于自适应理论,发展了用于预测纳米流体在矩形微槽道中润湿长度的理论模型。结果表明:由于接触角的影响,相比于普通表面微槽道,在超亲水表面微槽道中SiO_(2)纳米流体的润湿长度显著增加;在超亲水表面微槽道中,润湿长度主要受到表面张力的影响,因此自适应润湿长度L_(a)和角流区润湿长度L_(c)随着纳米颗粒粒径的减小和体积分数的增加而减少;在普通表面微槽道中,接触角对L_(a)的影响大于表面张力,因此L_(a)随着纳米颗粒粒径的减小和体积分数的增加而上升。

A New Method for Analyzing Heat Exchangers- Matching of Temperature Field

In heat exchangers, the magnitude of Nu of each duct is influenced by the temperature field, since the ratio of heat capacity rate will influence the matching status of the temperature field between contacting ducts, the total heat transfer coefficient is related with the ratio of heat capacity rate. Considering this relationship, a new method for analyzing heat exchanger is proposed - matching of temperature field. First, for a single duct with the temperature field varying exponentially along the flow direction, its Nu is calculated. Then under the hypothesis that the thermal resistance of the wall is negligible, the matching condition was set like this: both the temperature and heat flux are equal for the hot and cold fluids at the wall, so the matching relationship of parameter that describes the temperature field of the hot and cold fluids, was obtained. Finally the relationship between the total Nu and the ratio of heat capacity rate along with the ratio of inherent thermal resistance is obtained. Compared with traditional analyzing methods, the temperature matching method can be used to get the total heat transfer coefficient directly, and also be used for optimization of heat exchanger design. For a parallel flow, the optimal ratio of heat capacity rate is reciprocal to the ratio of inherent thermal resistance, and for a counter flow, the optimal ratio of heat capacity rate is zero or infinity.

Heat Transfer and Entropy Generation Analysis of an Intermediate Heat Exchanger in ADS

The intermediate heat exchanger for enhancement heat transfer is the important equipment in the usage of nuclear energy. In the present work, heat transfer and entropy generation of an intermediate heat exchanger(IHX) in the accelerator driven subcritical system(ADS) are investigated experimentally. The variation of entropy generation number with performance parameters of the IHX is analyzed, and effects of inlet conditions of the IHX on entropy generation number and heat transfer are discussed. Compared with the results at two working conditions of the constant mass flow rates of liquid lead-bismuth eutectic(LBE) and helium gas, the total pumping power all tends to reduce with the decreasing entropy generation number, but the variations of the effectiveness, number of transfer units and thermal capacity rate ratio are inconsistent, and need to analyze respectively. With the increasing inlet mass flow rate or LBE inlet temperature, the entropy generation number increases and the heat transfer is enhanced, while the opposite trend occurs with the increasing helium gas inlet temperature. The further study is necessary for obtaining the optimized operation parameters of the IHX to minimize entropy generation and enhance heat transfer.

Influence of Thermal and Solutal Marangoni Effects on Free Surface Deformation in an Open Rectangular Cavity

Steady thermo-solutocapillary convection in a rectangular cavity with deformable free surface under microgravity condition is numerically studied, where level set method is employed to capture the free surface deformation. Both the temperature and solute concentration gradients are applied horizontally. The computational results show that, as the thermal to solutal Marangoni number ratio varies between-10 and-1(namely,-10 ≤ R_σ <-1), the flow field exists one anti-clockwise rotating convective cell driven by thermocapillary convection, and the free surface bulges out near the left end wall and bulges in near the right end wall. As-1 < R_σ < 0, the flow field exists one clockwise rotating convective cell driven by solutocapillary convection, and the free surface bulges out near the right end wall and bulges in near the left end wall. As R_σ =-1, the flow field consists of one clockwise and one anti-clockwise rotating convective cells, and the free surface bulges in at the central point and bulges out near the left and right end walls.

Film cooling with using the decomposition of an NH_3 cooling stream as a chemical heat sink

The inlet temperatures of gas turbines are generally increasing over time,so conventional cooling methods are likely to approach their useful limits.It is therefore essential to investigate novel cooling methods.Based on the theory of heat transfer enhancement,a novel film cooling method for gas turbine blades using a chemical heat sink is proposed.In this method,the endothermic reaction of an NH 3 cooling stream heated by the main gas stream takes place,reducing the convective heat transfer between the mainstream and the blades.Therefore,film cooling effectiveness is improved.To test the feasibility of the proposed method,numerical simulations were conducted,using the classical flat plate with a 30 degree angled cylindrical hole(diameter,D).Film cooling effectiveness at different blowing ratios(M = 0.5,1.0,and 1.5) were computed and compared with the results of conventional cooling methods.The simulation results show that the proposed method can enhance film cooling effectiveness not only in the stream-wise direction,but also in the span-wise direction.The span-averaged film effectiveness is improved in the presence of a chemical heat sink,with the value of X/D(the ratio of downstream distance from the center of the film hole to the diameter of the film hole) increasing downstream of the film hole.The novel film cooling approach showed the best performance at M = 0.5.

Interaction of Two Cavitation Bubbles in a Tube and its Effects on Heat Transfer

When two cavitation bubbles exist in a confined space, the interaction between the bubbles significantly affects the characteristics of bubble dynamic behaviors. In this paper, a three-dimensional(3D) model is established to study the growth and collapse of two cavitation bubbles in a heated tube and its effects on heat transfer. The liquid and gas phases throughout the calculation domain are solved by a set of Navier-Stokes equations. It is assumed that the gas inside the bubble is compressible vapor, and the surrounding liquid is incompressible water. The mass transfer between two phases is ignored. The calculated bubble profiles were compared to the available experimental data, and a good agreement has been achieved. Then, the relationship among the bubble motion, flow field and pressure distributions was analyzed. On this basis, the effects of bubble interaction on the heat transfer between the wall surface and sounding liquid were discussed. It is found that heat transfer in the centre wall region is enhanced owing to the vortex flow and micro-jet induced by the bubble contraction and collapse. In contrast, the highest surface temperature appears in the surrounding region, which is mainly attributed to the thermal resistance induced by the bubble. The present study is helpful to understand the heat transfer phenomenon with cavitation in the liquid.