平流式冷凝器传热流动性能理论研究

平流式换热器与常规换热器相比有其无可比拟的优点,常用作汽车空调系统中的冷凝器,其制冷剂侧的传热和流动特性与常规换热器有很大不同.本文比较了不同型式冷凝器的结构特点和工作原理,并详细分析了制冷剂在平流式冷凝器多孔扁管内的传热流动机理,希望能为该类换热器今后的研究、设计和校核提供一些依据.     

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.     

空分设备基础知识讲座 第四讲 换热器(五)

本讲为板翅式换热器,讲述了其基本结构,列举了液空过冷落、氧氮液化器和切换式换热器的形状与参数,侧重讲了切换式换热器的结构、原理与通道布置。图12表2。     

大型空调器节能装置

基于传统空调器性能测试台温度控制的原理,提出一种大型空调器节能实验装置,并简单介绍了该装置的节能原理。该试验台与传统空调器性能测试台不同的是在空气处理系统中加入了热回收换热器,有效利用室内外机室热回收换热器的热量来满足温度控制要求,从而减少了电加热器的能耗,节省了能源。     

氟塑料换热器技术在燃煤电厂的应用现状及前景分析

文章从燃煤火力发电厂的烟气余热回收及低低温烟气处理出发,指出氟塑料换热器相比于金属换热器具有耐腐蚀、传热系数大、重量轻等特点,阐述了氟塑料换热器技术的特点及其在燃煤机组中的应用现状,包含:烟气余热深度回收系统、氟塑料MGGH系统、烟气中水回收系统、氟塑料管式GGH系统,对其各自的特点展开介绍,并对氟塑料换热器技术的应用前景进行了展望分析。     

空调系统中全热换热器的实验研究

通过在空调系统中增设全热换热器,利用排风与新风进行热交换来实现热回收,能够显著减少空调系统的能耗。针对全热换热器,确定热湿交换性能实验方案,并进行了相应的实验测试。实验研究结果表明:全热换热器联合空调系统运行导致温度效率和焓效率都低于其独立运行时的温度效率和焓效率,因此空调系统与全热换热器的联合运行必须进行合理匹配。此研究为全热换热器在空调系统中的推广应用提供参考。     

浴室废水换热器的优化

浴室废水换热器是浴室热泵装置的重要部件,针对壳管式、板式等常规的浴室废水换热器存在易堵塞等问题,给出一种沉浸竖板式浴室废水换热器,介绍其工作原理和特性方程,并针对关键要素对传热量和压降的影响规律进行计算分析,为浴室废水换热器的优化提供参考。     

基于膜蒸馏原理换热器的设计与机理分析

将直接接触膜蒸馏的理念引入到换热器设计中,提出了一种适用于溴化锂吸收式制冷系统且既有传热又有传质作用的溶液热交换器。它结合了板式和框式换热器的特点,在内部设置环形挡板形成环形流道,起到扰流作用和加强换热。本文阐述了这种换热器的结构特点、工作原理和传热传质的理论分析。还考察了两流体逆流布置下,对膜换热器进行模型简化模拟分析,发现其换热效果比传统换热器好。     

半导体制冷用于房间空调器的节能性研究

对半导体制冷的原理与研究现状进行简要介绍,为提高房间空调器的能效,利用半导体制冷装置对空调系统的换热器进行冷却,提高换热器的过冷度。通过分析试验数据,得到TE制冷片的性能参数与空调系统能效的关系,为应用TE制冷片提高空调器能效的产业化研究奠定基础。     

换热器热工测试系统冷侧介质冷却方案的选择

结合换热器热工测试平台工况复杂多变特点,介绍几种常用的实现系统温度控制的冷却工艺。从工程实际出发,综合考虑冷却塔、冷水机组、蓄冰槽设备的工作原理、适用环境、设备价格等关键因素,讨论各方案的优劣,为换热器热工测试系统冷侧介质冷却方案的选择给出合理性建议。     

Enhancing flow boiling heat transfer in microchannels for thermal management with monolithically-integrated silicon nanowires

Thermal management has become a critical issue for high heat flux electronics and energy systems. Integrated two-phase microchannel liquid-cooling technology has been envisioned as a promising solution, but with great challenges in flow instability. In this work, silicon nanowires were synthesized in situ in parallel silicon microchannel arrays for the first time to suppress the flow instability and to augment flow boiling heat transfer. Significant enhancement in flow boiling heat transfer performance was demonstrated for the nanowire-coated microchannel heat sink, such as an early onset of nucleate boiling, a delayed onset of flow oscillation, suppressed oscillating amplitudes of temperature and pressure drop, and an increased heat transfer coefficient.     

低温回热制冷机内交变流动与换热的格子-Boltzmann方法模拟

采用格子-Boltzmann方法(LBM)分别通过对二维通道及多孔介质内交变流动与换热进行了数值模拟,以研究低温回热制冷机中脉冲管内及回热器内交变流动与换热规律.结果表明:二维通道交变流动结果与解析解吻合得很好.当交变流动数较大时,出现速度环形效应,温度表现为周期波动变化.多孔介质内各点速度总体均呈现交替波动变化,但振...     

脉动热管实验与理论研究进展

脉动热管(PHP/OHP)是一种新型的高效传热元件,在航天领域、电子器件冷却以及节能技术方面极具应用潜力。这里首先介绍了脉动热管的特点和工作原理,然后分别从实验研究、理论研究和实际应用等方面介绍了目前该领域的研究现状。实验研究方面着重介绍了流动可视化应用以及纳米流体和功能流体通过强化换热提高脉动热管性能等相关研究热点。同时指出,目前脉动热管的理论分析受限于两相流理论的发展,主要研究重点在于非线性分析;数值模拟方面,特别是同纳米流体以及功能流体应用相结合将会成为下一个研究热点。     

Heat transfer characteristics of oscillating flow regenerator filled with circular tubes or parallel plates

Under assumption of small perturbation, linear thermoacoustic theory was applied to analyze heat transfer characteristics of compressible oscillating flow in two kinds of simple regenerators filled with circular tubes or parallel plates. Based on the cross-sectional oscillating velocity and temperature distributions, the exact expressions of Nusselt number were derived in complex notation. The Nusselt number is the function of Prandtl number, kinetic Reynolds number Re_ω and the third dimensionless variable, D. Here, the D is defined as the ratio of heat transfer capability aroused by mean temperature gradient and gas compressibility. Both the gas compressibility and mean temperature gradient effects were discussed and two corresponding Nusselt numbers were given. In particular, simpler expressions for these two Nusselt numbers were deduced for extreme values of Re_ω and D. Finally, combined effect of gas compressibility and non-zero mean temperature gradient on heat transfer characteristics were analyzed via D. The analysis shows that the mean temperature gradient gives predominant contribution to the heat transfer performance of oscillating flow regenerator.     

Transferts thermiques en écoulements oscillants laminaires incompressibles

Phenomena concerning the temperature variations and the heat transfer are studied in the specific case of oscillating flow with null mean velocity circulating between two infinite walls. A 1D model is established and the interesting scale parameters are deduced from theoretical equations. The particular case of an oscillating laminar flow for incompressible fluid is detailed in order to illustrate and to discuss the effects of thermal interactions between the fluid and walls. Influence of wall length comparatively to the fluid displacement is studied. Conclusions for designing thermal heat exchangers of Stirling engines or PTR are proposed.     

Influence of developing flow on the heat transfer in laminar oscillating pipe flow

A theoretical analysis was performed to evaluate the influence of developing flow on the heat transfer associated with laminar oscillating pipe flow. Simplified analytical approaches are briefly discussed before an investigation based on the numerical solution of the conservation equations for mass, momentum and energy is presented, assuming constant wall temperature and an incompressible viscous fluid. Focusing on operating conditions as found in heat exchangers of regenerative thermal machines, like Stirling engines or Vuilleumier heat pumps, numerous calculations of the flow field and the heat transfer have been executed covering wide ranges of the characteristic dimensionless groups. The results are presented in tems of correlations of the mean Nusselt number averaged on the pipe length and a distribution function describing the local heat transfer. Furthermore, it is shown that the derived correlations are also applicable to compressible fluid flow, provided that the relative pressure amplitude is within the limits typical of regenerative thermal machines.     

CFD analysis of a diaphragm free-piston Stirling cryocooler

This paper presents a Computational Fluid Dynamics (CFD) analysis of a novel free-piston Stirling cryocooler that uses a pair of metal diaphragms to seal and suspend the displacer. The diaphragms allow the displacer to move without rubbing or moving seals. When coupled to a metal diaphragm pressure wave generator, the system produces a complete Stirling cryocooler with no rubbing parts in the working gas space. Initial modelling of this concept using the Sage modelling tool indicated the potential for a useful cryocooler. A proof-of-concept prototype was constructed and achieved cryogenic temperatures. A second prototype was designed and constructed using the experience gained from the first. The prototype produced 29 W of cooling at 77 K and reached a no-load temperature of 56 K. The diaphragm’s large diameter and short stroke produces a significant radial component to the oscillating flow fields inside the cryocooler which were not modelled in the one-dimensional analysis tool Sage that was used to design the prototypes. Compared with standard pistons, the diaphragm geometry increases the gas-to-wall heat transfer due to the higher velocities and smaller hydraulic diameters. A Computational Fluid Dynamics (CFD) model of the cryocooler was constructed to understand the underlying fluid-dynamics and heat transfer mechanisms with the aim of further improving performance. The CFD modelling of the heat transfer in the radial flow fields created by the diaphragms shows the possibility of utilizing the flat geometry for heat transfer, reducing the need for, and the size of, expensive heat exchangers. This paper presents details of a CFD analysis used to model the flow and gas-to-wall heat transfer inside the second prototype cryocooler, including experimental validation of the CFD to produce a robust analysis.     

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.     

Unsteady compressible 3-dimensional boundary-layer flow near an asymmetric stagnation point with mass transfer

The heat and mass transfer for unsteady laminar compressible boundary-layer flow, which is asymmetric with respect to a 3-dimensional stagnation point (i.e. for a jet incident at an angle on the body), have been studied. It is assumed that the free-stream velocity, wall temperature, and surface mass transfer vary arbitrarily with time and also that the gas has variable properties. The solution in the neighbourhood of the stagnation point has been obtained by series expansion in the longitudinal distance. The resulting partial differential equations have been solved numerically using an implicit finite-difference scheme. The results show that, in contrast with the symmetric flow, the maximum heat transfer does not occur at the stagnation point. The skin-friction and heat-transfer components due to asymmetric flow are only weakly affected by the mass transfer as compared to those components associated with symmetric flow. The variation of the wall temperature with time has a strong effect on the heat transfer component associated with the symmetric part of the flow. The skin friction and heat transfer are strongly affected by the variation of the density-viscosity product across the boundary layer. The skin friction responds more to the fluctuations of the free stream oscillating velocities than the heat transfer. The results have been compared with the available results and they are found to be in excellent agreement.     

A pumped supercritical helium flow loop for heat transfer studies

The construction and operation of a flow loop is described in which a 0.5 ls^? centrifugal pump circulates supercritical helium through a 1 m long, 18 mm id heated test section instrumented with 18 carbon resistance thermometers. Based on the heat transfer measurements obtained (published in detail elsewhere) some observations are made on deviations from the standard Dittus Boelter heat transfer correlation caused by helium's variable properties, and on possible buoyancy induced reductions in heat transfer particularly for radially inward flow in rotating machines.