Effect of finite thermal conductivity of the separating wall on the performance of counterflow heat exchangers

Axial conduction is a major source of inefficiency in a compact counterflow heat exchanger. Any attempt to reduce axial conduction by using material of low thermal conductivity for the separating wall results in increased resistance to lateral heat flow, thereby reducing the overall thermal efficiency of the heat exchanger. The governing equations including axial conduction and lateral resistance due to the separating wall have been solved and an expression, for the overall efficiency of the heat exchanger has been derived in terms of relevant nondimensional parameters. Computed results have been presented which give the optimum thermal conductivity of the wall material.     

PIV experimental investigation of entrance configuration on flow maldistribution in plate-fin heat exchanger

Flow characteristics of flow field in the entrance of plate-fin heat exchanger have been investigated by means of particle image velocimetry (PIV). The velocity fields were measured using the two-frame cross-correlation technique. A series of velocity vector and streamline graphs of different cross-sections are achieved in the experiment. The experimental results indicate that performance of fluid maldistribution in conventional entrance configuration is very serious, while the improved entrance configuration with punched baffle can effectively improve the performance of fluid flow distribution in the entrance. Based on the analysis of the fluid flow maldistribution, a baffle with small holes is recommended to install in the entrance configuration in order to improve the performance of flow distribution. When the punched baffle is proper in length, the small holes is distributed in staggered arrangement, and the punched ratio gradually increases from central axis to the boundary along with the baffle length, the performance of flow distribution in plate-fin heat exchanger is effectively improved by the optimum design of the entrance configuration. The flow maldistribution parameter S in plate-fin heat exchanger has been reduced from 1.21 to 0.209 and the ratio of the maximum velocity to the minimum θ is reduced from 23.2 to 1.76 by installing the punched baffle. The results validate that PIV is well suitable to investigate complex flow pattern and the conclusion of this paper is of great significance in the optimum design of plate-fin heat exchanger.     

空调排风能量回收用热管换热器的优化设计

对用于回收空调排风热（冷）量的热管换热器的整体优化设计进行研究,提出对其整体优化的设计方法,建立寿命期总经济效益最大为目标的目标函数,并利用标准气象年数据,研究不同温度下的热管换热器的最优结构,通过经济性分析,得出整个供暖（冷）季的最优结构和最佳经济性能的热管换热器.     

CFD simulation on inlet configuration of plate-fin heat exchangers

A computational fluid dynamics (CFD) program FLUENT has been used to predict the fluid flow distribution in plate-fin heat exchangers. It is found that the flow maldistribution is very serious in the y direction of header for the conventional header used in industry. The results of flow maldistribution are presented for a plate-fin heat exchanger, which is simulated according to the configuration of the plate-fin heat exchanger currently used in industry. The numerical prediction shows a good agreement with experimental measurement. By the investigation, two modified headers with a two-stage-distributing structure are proposed and simulated in this paper. The numerical investigation of the effects of the inlet equivalent diameters for the two-stage structures has been conducted and also compared with experimental measurement. It is verified that the fluid flow distribution in plate-fin heat exchangers is more uniform if the ratios of outlet and inlet equivalent diameters for both headers are equal.     

A numerical model of thermal analysis for woven wire screen matrix heat exchanger

Woven wire screen matrix heat exchanger (WSMHE) is a kind of compact, light-weight and high-efficiency matrix heat exchanger (MHE) for cryogenic applications. This paper presented a numerical model for the design and thermal analysis of WSMHE. The influence of wall thermal resistance, axial conduction, parasitic heat load and properties variation was taken into account, which is neglected in the traditional effectiveness-NTU method but important for compact cryogenic heat exchangers. The proposed numerical method is verified by effectiveness-NTU method under specific conditions, then it is tested according to experiment data, and a well agreement was obtained. Based on this model, a detailed analysis was performed on WSMHEs. The analysis results show that the axial conduction might result in evident decrease of effectiveness at low flow rate region; the effectiveness of WSMHEs with large flow rate could be remarkably enhanced by increasing its length; the influence of parasitic heat load varied little throughout of the flow rate region. Furthermore, the numerical model presented in this paper can be developed to the design and thermal analysis of small partition wall type heat exchangers.     

Study of flow distribution and its improvement on the header of plate-fin heat exchanger

In order to enhance the uniformity of flow distribution, an improved header configuration of plate-fin heat exchanger is put forward in this paper. Based on the analysis of the fluid flow maldistribution for the conventional header used in industry, a baffle with small holes of three different kinds of diameters is recommended to install in the header. The flow maldistribution parameter S is obtained under different header configuration. When the baffle is properly installed with an optimum length, with stagger arranged and suitably distributed holes from axial line to baffle boundary, the ratio of the maximum flow velocity to the minimum flow velocity drops from 3.44-3.04 to 1.57-1.68 for various Reynolds numbers. The numerical results indicate that the improved header configuration can effectively improve the performance. The conclusion of this paper is of great significance in the improvement of plate-fin heat exchanger.     

Analysis of heat flow and “channelling” in a scraped-surface heat exchanger

Scraped-surface heat exchangers (SSHEs) are widely used in industries that manufacture and thermally process fluids; in particular, the food industry makes great use of such devices. Current understanding of the heat flow and fluid dynamics in SSHEs is predominantly based on empirical evidence. In this study a theoretical approach (based on asymptotic analysis) is presented for analysing both the flow and heat transfer in an idealised SSHE (a cylindrical annulus) for Newtonian fluids. The theory allows the effects of scraping-blade configuration, pumping rates, annular shear velocity and material properties all to be accounted for. The analysis relies on asymptotic simplifications that result from the large Péclet numbers and small geometrical aspect ratios that are commonly encountered in industrial SSHEs. The resulting models greatly reduce the computational effort required to simulate the steady-state behaviour of SSHEs and give results that compare favourably with full numerical simulations. The analysis also leads to what appears to be the first theoretical study on the undesirable phenomenon of “channelling”, where fluid passes through the device in an essentially unheated or uncooled state. A parametric study is also undertaken to investigate the general circumstances under which channelling may occur.     

金属蜂窝材料换热性能分析快速数值算法

由于存在一个易于流动的方向并具有较大的面密度 , 金属蜂窝材料在具有良好的比刚度和比强度的同时也具有良好的散热性能 , 研究强迫对流下的散热性能对其多功能化设计具有重要意义。蜂窝材料强迫对流换热是流2固耦合传热问题 , 由于具有大量微结构 , 直接采用计算流体动力学(CFD)方法进行模拟 , 计算量巨大 , 成为进行优化设计的主要障碍。发展具有广泛适用性的高效快速算法具有重要的理论意义和应用价值。针对金属蜂窝强迫对流换热结构 , 本文中提出了一种快速高效的数值算法 , 计算速度比有限体积法提高3～4个数量级。通过2个具体算例 , 验证了该方法的有效性和广泛适用性。      

An axisymmetric heat conduction model for a multi-material cylindrical system with application to analysis of carbon nanotube based composites

A model for axisymmetric steady-state heat conduction in a multi-material cylindrical system containing a thermal superconductor is presented. An analytical solution in terms of series involving Bessel functions is derived for the temperature distribution in the multi-material system. The model may be applied to analyze the thermal behaviors of carbon nanotube based composites. Some results are obtained for specific cases of the multi-material system.     

Determination of thermal diffusivity of solids by use of periodic heat flow

This paper deals with the determination of thermal diffusivity in a cylindrical specimen by the use of a periodic heat flow in the axial direction. Heat transfer from the periphery is taken into account, and its influence upon the evaluation of thermal diffusivity from measurement of phase lag and amplitude decrement, respectively, is discussed. Experimental conditions are pointed out for which the evaluation can be done as for a semiinfinite specimen. Theoretical considerations are compared with experimental results.     

Effect of number of turns and configurations on the heat transfer performance of helium cryogenic pulsating heat pipe

The pulsating heat pipe (PHP) is a potential alternative to highly conductive metals such as copper for long distance heat transfer. Effective thermal conductivity and heat transfer capacity of a PHP are two of the most critical factors for practical applications. In this paper, a helium based PHP, which consists of 48 parallel tubing sections, was developed. The lengths of the evaporator, adiabatic and condenser sections are 50 mm, 100 mm and 50 mm respectively. The condenser section was thermally anchored to a Gifford-McMahon cryocooler (GM cryocooler) with a cooling capacity of 1.5 W at 4.2 K. A maximum effective thermal conductivity of 12330 W/m∙K was obtained when 1.1 W heat was applied to the evaporator section at a fill ratio of 70.5%. With the same geometric parameters and operational parameters, the effect of the number of turns on the heat transfer performance was figured out by comparing the 48-turn PHP with an 8-turn PHP. The results show that the temperature difference between the evaporator and condenser sections of the 48-turn PHP is much smaller than that of the 8-turn PHP. The dry-out temperature response, effective thermal conductivity and heat transfer capacity of them are obtained and analyzed. Furthermore, two configurations of the 48-turn PHP, a parallel configuration and a series configuration, are defined. An optimum configuration is proposed and makes a reference to the design of a cryogenic PHP for applications.     

Analytical solution of the heat equation in a longitudinally pumped cubic solid-state laser

Knowledge about the temperature distribution inside solid-state laser crystals is essential for calculation of thermal phase shift, thermal lensing, thermally induced birefringence, and heat-induced crystal bending. Solutions for the temperature distribution for the case of steady-state heat loading have appeared in the literature only for simple cylindrical crystal shapes and are usually based on numerical techniques. For the first time, to our knowledge, a full analytical solution of the heat equation for an anisotropic cubic cross-section solid-state crystal is presented. The crystal is assumed to be longitudinally pumped by a Gaussian pump profile. The pump power attenuation along the crystal and the real cooling mechanisms, such as convection, are considered in detail. A comparison between our analytical solutions and its numerical counterparts shows excellent agreement when just a few terms are employed in the series solutions.     

Practical design of a heat exchanger for dilution refrigeration 1

A compact heat exchanger for a dilution refrigerator with a high thermal efficiency is presented. Discrete heat exchangers with by-pass channels were used to decrease the flow impedance. This heat exchanger was designed so that the thermal conductance of liquid along the stream was greatly reduced. The effective thickness of the sponge material in the heat exchanger and mixer is also discussed. The obtained minimum temperature of 12 mK was very close to the designed value of 10.8 mK. Moreover a rapid response was obtained. This is attributed to the small liquid volume of the heat exchanger.     

基于热阻网络模型的硅基微槽热沉多目标优化设计

微槽热沉具有传热效率高、可靠性强的优点,可用于对微尺度高热流密度电子元件进行冷却。为满足其性能需求和控制成本,在对微槽热沉进行设计时需要对其传热能力和流动阻力同时进行优化。传统研究采用的热阻网络模型较为简单,不能很好地反映热阻和流动阻力对微槽道截面形状拓扑变化的响应,且其优化对象通常为既定截面的形状尺寸。为此提出一种基于离散化方法的单层硅基微槽热沉热阻网络模型,将热沉鳍片细分为厚度较小的微元,根据微元热阻对微元宽度的响应及微元热阻对整体热阻的贡献来描述微槽道的整体热阻。以微泵输出功率为优化边界条件,压降和热阻为优化目标,通过SQP（sequential quadratic programming,序列二次规划）方法对层流状态下四边形等截面硅基微槽热沉进行尺寸优化,利用CFD（computational fluid dynamics,计算流体动力学）对优化结果进行模拟和验证。结果表明,当鳍片高度较低时,鳍片截面形状为矩形,随着鳍高增加,截面形状有向三角形发展的趋势。在设计区间内,微槽道截面为梯形、鳍片截面为三角形时传热效率与压降相对占优。用边界点法和理想点法优化模型求得微槽道高度、鳍底宽、槽底宽、槽顶宽的优化结果分别为500,50,64.5,114.5 μm和500,50,50,100 μm。该方法能根据设计需求调整评价函数,同时计算结果具有重要工程意义,为微槽热沉设计人员提供参考。     

Simulation and optimization of heat flow via anisotropic material thermal conductivity

This article is focused on heat flow control within a composite material that has designed anisotropic thermal conductivity. The optimal conductive heat transfer path in the composite is specified via customized local scale properties, where the physical parameter distribution is found using an iterative procedure that couples a gradient based optimization routine with a finite element solver. A sample optimization result is presented to illustrate the procedure, and the final solution is translated into a physical embodiment having heterogeneous material properties. Numerical experiments were performed both on this synthesized material and a baseline homogeneous material with the same filler volume fraction. Heat transfer results indicate a substantial reduction in overall temperature with effective concentration of thermal power density in the designed material.     

Numerical investigation on a 300 Hz pulse tube cryocooler driven by a three-stage traveling-wave thermoacoustic heat engine

A 300 Hz pulse tube cryocooler (PTC) driven by a three-stage traveling-wave thermoacoustic heat engine (TSTHE) has been proposed and studied in this paper. In the configuration, three identical thermoacoustic heat engine units are evenly incorporated in a closed traveling-wave loop, in which three pulse tube cryocoolers are connected to the branch of each thermoacoustic heat engine. Compared with the conventional thermoacoustic heat engine which involves a traveling-wave loop and a long resonator, it has advantages of compact size and potentially high thermal efficiency. A TSTHE–PTC system was designed, optimized and studied in detail based on the thermoacoustic theory. Firstly, numerical simulation was conducted to design the system thus the optimum structure parameters of the system were obtained. With the operating condition of 4 MPa mean pressure and high working frequency, a cooling power of 7.75 W at 77 K and an overall relative Carnot efficiency of 11.78% were achieved. In order to better understand the energy conversion characteristics of the system, distributions of key parameters such as acoustic work, phase difference, dynamic pressure, volume flow rate and exergy loss were presented and discussed. Then, the coupling mechanism of the system was investigated. In addition, influence of coupling position on the system performance was further studied.     

多角度肋片强化相变材料换热仿真分析

目的 针对太阳能热利用领域中相变材料的封装结构提出圆柱体相变蓄热棒,并设计多角度肋片以加快相变材料融化速率。方法 采用CFD仿真技术,分析不同形状肋片对蓄热棒中相变材料融化特性的影响,计算各模型相变材料的融化时间、温度响应速率和平均传热系数。结果 在800 W/m²的热流边界条件下,无肋片蓄热棒的相变材料完全融化需要2 813 s,设计的12组肋片中Tra–45模型性能最优,相变材料的融化时间比无肋片对照组的缩短了5.4%;Tra–45模型中相变材料温度分布集中,且最高温度上升了6 ℃,Tra–45模型的温度响应速率较对照组的提升了5%;Tra–45模型的平均换热系数达到9.97 W/(m².K),较对照组的提升了2.8%。结论 蓄热棒内增加梯形45°肋片后,相变材料融化速率加快,蓄热棒内温度分布均匀。同时,相变材料的温度响应速率提高,平均换热系数显著增加,可满足频繁充放热的需求。     

换热器热传递的有限元分析

 换热器的设计涉及许多因素,而热传递能力是检验换热器结构设计成功与否的重要标准.从理论上难以准确计算出换热器的传热效率,所以提出利用ANSYS有限元分析软件来研究换热器的热交换过程.剖析系统的热传递原理,对换热器进行优化建模,加载热载荷,并得到换热器的温度场分布;由分析结果可以看出换热器能够满足系统要求,使系统重新达到热平衡.换热器的换热性能得以肯定,继而说明了换热器的结构设计是可行的,并为后一步的优化设计奠定了基础.     

Convective heat transfer enhancement produced by secondary heat sources in a liquid nitrogen pool

This paper studies the influence of secondary heat sources on the convective heat transfer from a vertical cylindrical heater immersed in a LIN pool. Two types of secondary heat source have been used. In the first case, the heat leak through the vessel walls into the pool was varied without causing nucleation, so as to retain convective heat transfer. It was found that the convective flow loops induced in the pool produced a small enhancement of the heat transfer from the cylindrical heater. In the second case, a small horizontal heater loop was introduced into the pool below the cylindrical heater. The secondary heat flux used was such that boiling occurred at the loop to give rising bubbles surrounding the cylindrical heater. This produced a large enhancement of the convective heat transfer from the cylindrical heater. The enhancement ratio is presented as a function of the power supplied to the loop heater.     

Porous heat exchangers for continuous flow helium cryostats

An investigation was made into the effective application of porous heat exchangers of cylindrical shape through which fluid passes axially. On the basis of a theoretical analysis the conclusion derived was that the best thermal efficiency can be reached by the use of porous material with a large heat-exchanger surface, a high radial and low axial thermal conductivity (ie with a marked anisotropy of thermal conductivity), and a small radius of the heat exchanger operating at lower flows of cooling agent. The results of experiments carried out at helium and nitrogen temperatures are presented. These results have confirmed the high effectiveness of porous heat exchangers, even in comparison with chamber-type heat exchangers. For the temperature range from 1.5 to 300 K the heat exchangers composed of highly conductive metal nets (mesh gauge of the order of magnitude of 10^?1 mm) stacked perpendicularly to the direction of flow of the cooling fluid, appear to be the most promising ones.