低雷诺数流动错位翅片传热和压降特性的实验研究

通过对油水板翅式换热器进行的性能试验，得到了低雷诺数流动下板翅式换热器翅片侧传热与阻力特性的数据，在此基础上获得了错位翅片传热因子与摩擦系数的准则关系式，传热因子和摩擦系数的最大计算误差分别为0．62％和1．44％。根据这些准则关系式提出了一个衡量翅片质量的经济系数。     

翅片形状对板翅式机油散热器性能影响

本文主要阐述了板翅式换热器的翅片形状对换热效果及阻力特性的影响，并给出了二种不同形状散热翅片的试验数据及曲线。     

逆流板翅式换热器动态性能的建模与仿真

对间冷回热燃气轮机关键部件逆流板翅式换热器进行了动态性能的仿真研究。在考虑气体工质压缩性的基础上，根据换热器内部冷、热流体的压力、流量和温度的变化和冷热流道间隔板的热平衡，建立了逆流板翅式换热器的动态数学模型，在EASY5平台上搭建了逆流板翅式换热器的分布参数模型，并进行了动态的仿真试验计算。分析结果表明该逆流板翅式换热器模型较好地实现了对换热器内部流体流动和换热的模拟，可以作为部件模块，用来时间冷回热燃气轮机系统性能的仿真研究。     

基于燃气轮机的板翅式换热器特性分析及结构优化

针对微型燃气轮机板翅式换热器结构参数与燃气轮机性能之间的耦合关系,建立了板翅式换热器多目标分析和优化模型。在此基础上,分析了2种设计条件(定燃烧室吸热量和定涡轮出功)下板翅式换热器关键参数对板翅式换热器和燃气轮机性能的影响。结果表明:影响燃烧室吸热量和涡轮出功的主要因素为换热器压损而并非换热器效能,2种设计条件下各参数的变化趋势一致(除燃烧室吸热量和涡轮出功外);在对翅片结构进行优化后,涡轮出功增大了6.8%,燃烧室吸热量减少了5.1%;相对于基本参数,优化后翅片厚度、翅片间距和波纹角减小,翅片高度增加,保证了板翅式换热器具有较小的压损;采用■耗散最小和采用熵产最小为优化目标时无明显区别。     

翅片形状对板翅式机油散热器热性能影响

本文主要阐述了板翅式换热器的翅片形状对换热效果及阻力特性的影响,并给出了二种不同形状散热翅片的试验数据及曲线.     

高温紧凑板翅式换热器稳态和动态性能的研究

建立了紧凑板翅式换热器的动态数学模型.为了满足系统快速动态仿真的需要,采用容阻特性建模技术建立了紧凑板翅式换热器的仿真模型.在相同的运行条件下,对不同的高温板翅固体材料进行了稳态和动态仿真研究.结果表明:在不同的材料条件下,高温板翅式换热器的稳态性能基本相同;但不同材料的动态特性时间不同,硅碳陶瓷材料具有较小的延迟时间,常规高温合金材料延迟较大.     

多股流板翅式换热器温度交叉的数值分析

以平行多股流板翅式换热器为研究对象,给出了考虑翅片旁通作用的多股流板翅式换热器流体和翅片的能量方程。在改变多股流板翅式换热器各通道的流体参数、流动方式及换热器的结构参数等情况下,对能量方程进行数值求解,获得了各通道的流体温度分布情况及相邻通道的流体温度差,并分析了流体参数、流动方式和结构参数的变化对相邻通道流体温度交叉的影响。     

多股流换热器动态过程场协同分析

建立了多股流板翅式换热器动态数学模型,通过换热器入口温度及流量阶跃的改变,模拟过渡过程中温度场的动态响应,利用温差场均匀性因子对多股流换热器过渡过程动态特性进行了评价,通过分析内部温度场与速度场的协同关系,揭示温差场在动态过程中的变化特征.将温差场均匀性因子与过渡时间结合,建立了自组织能力系数,并对多股流换热器控制品质进行了分析.多股流换热器在流量阶跃时,温差场均匀性因子平缓迁移,而温度阶跃时变化剧烈且存有极值.多股流换热器自组织系数越大,越易达到新的热平衡.     

LNG板翅式换热器板翅结构热应力分布规律分析

为保证LNG板翅式换热器冷箱安全运行,建立了板翅结构热应力有限元耦合分析物理模型,采用热-力直接耦合方法分析了大型LNG板翅式换热器冷箱正常运行时板翅结构的热应力分布规律,分析结果表明:实际运行工况下板翅结构的第一主应力、第三主应力以及基于第三强度理论的等效应力在钎焊位置处变化梯度较大,并且等效应力最大值出现在翅片和隔板钎焊位置处,从而使钎焊位置可能发生疲劳破坏;对于整个板翅结构,结构最薄弱区在最外层隔板与翅片的钎焊位置处。上述研究成果将为大型LNG冷箱内板翅式换热器结构设计和安全可靠运行提供重要参考依据。     

百叶窗板翅式换热器换热与阻力性能数值模拟研究

为了解决百叶窗板翅式换热器的内部性能优化问题,通过对层流稳态下换热器燃气侧的典型流动换热单元进行建模及流动换热分析,得到了单元体内部速度、流线及温度的分布特性,并通过对换热系数、科尔本传热因子、进出口单位压降、范宁摩擦系数的比较,获得了不同燃气入口速度下翅片间距及翅片角度对换热器换热性能及流动阻力的影响。结果表明:在百叶窗翅片角度及其他尺寸参数不变时,当百叶窗间距为0. 7 mm时其换热性能最优,阻力随间距增大而减小;在翅片间距等参数不变而角度变化时,换热性能与阻力均随角度增大而增大,当百叶窗角度从15°增加至30°时,换热性能的增加幅度较为明显。     

WINGLET-TYPE VORTEX GENERATORS FOR PLATE-FIN HEAT EXCHANGERS USING TRIANGULAR FINS

Vortex generators in the form of delta winglet pairs have already been proposed by many researchers for enhancement of the heat transfer rate in plate-fin heat exchangers. In this work, the enhancement potential of triangular fins (which are widely used inserts between the plates of the plate-fin heat exchanger) having delta winglets mounted on their slant surfaces has been computed. The performance of this combination is evaluated for varying angles of attack of the winglet and different thermal boundary conditions. The performance of the combination of triangular fins and winglets with stamping on the slant surfaces also has been evaluated.     

Fluid Flow Distribution and Heat Transfer in Plate-Fin Heat Exchangers

Single-phase and two-phase flow distribution in plate-fin heat exchangers and the influence of nonuniform fluid flow distribution on the thermal performance of such heat exchangers were experimentally investigated. The experimental results show that flow maldistribution can be a serious problem in plate-fin heat exchangers because of nonoptimized header configurations. The uneven distribution of two-phase flow in plate-fin heat exchangers is more pronounced than that of single-phase flow. It is shown that the uneven distributions result in a significant deterioration of the heat transfer performance. The relationship between the flow maldistribution characteristics and the resulting loss in heat exchanger effectiveness has been studied in this work. Certain improved header configurations with perforated plates were proposed in order to solve the maldistribution problem. It was found that the new header configurations could effectively improve the thermal performance of plate-fin heat exchangers. By changing the header configuration, the degree of flow and temperature nonuniformity in the plate-fin heat exchanger was reduced to 16.8% and 74.8%, respectively, under the main test condition.     

Study on forced air convection cooling for electronic assemblies

The slotted fin concept was employed to improve the air cooling performance of plate-fin in heat sinks. Numerical simulations of laminar heat transfer and flow pressure drop were conducted for the integral plate fin, discrete plate fin and discrete slotted fin heat sinks. It is found that the performance of the discrete plate fin is better than that of the integral continuum plate fin and the performance of slotted fin is better than that of the discrete plate fin at the same pumping power of the fan. A new type of heat sink characterized by discrete and slotted fin surfaces with thinner fins and smaller spaces between fins is then proposed. Preliminary computation shows that this type of heat sink may be useful for the next generation of higher thermal load CPUs. The limit of cooling capacity for air-cooling techniques was also addressed.     

Heat and mass transfer in plate-fin sinusoidal passages with vapor-permeable wall materials

Laminar forced flow and heat mass transfer in sinusoidal plate-fin small passages encountered in compact heat mass exchangers are investigated. The duct is similar to a traditional plate-fin heat exchanger, but vapor-permeable materials like polymer membranes, paper, and ceramics can be used as the duct materials so both sensible heat and moisture can be exchanged simultaneously. Heat conduction and mass diffusion in the fins and heat and moisture convection in the fluid are analyzed simultaneously as a conjugate problem. Their fully developed Nusselt and Sherwood numbers under various aspect ratios and fin conductance parameters are calculated. The results found that though fins extend the heat transfer area, they are less effective compared to a traditional compact heat exchanger with metal foils. Most unfortunately, fin efficiencies for moisture transfer are even much smaller than those for heat transfer due to the low fin mass conductance parameters. For such heat mass exchangers, the use of fins can be regarded mostly as supporting materials, rather than as mass intensification techniques.     

Dynamics of plate heat exchangers subject to flow variations

A predictive model has been presented to suggest the transient response of plate heat exchangers, subjected to a step flow variation. The work also brings out the effect of the port to channel maldistribution on the performance of plate heat exchangers under the condition of flow variation. The results indicate that flow maldistribution affects the performance of the plate heat exchangers in the transient regime. A wide range of the parametric study has been presented which brings out the effects of NTU and heat capacity rate ratio on the response of the plate heat exchanger, subjected flow perturbation.To verify the presented theoretical model, appropriate experiments have been carried out. Experiments include the responses of the outlet temperatures subjected to inlet temperature transient in the circuit followed by a sudden change in flow rate in one of the fluids. Simulated performance has been compared to the performance measured in the experiments. Comparisons indicate that theoretical model developed for flow transient is capable of predicting the transient performance of the plate heat exchangers satisfactorily, under the given conditions of changed flow rates.     

Thermofluid characteristics of frosted finned-tube heat exchangers

The performance of frosted finned-tube heat exchangers of different fin types is investigated by experiments in this paper. The effects of the air flow rate, the air relative humidity, the refrigerant temperature, and the fin type on the thermofluid characteristics of the heat exchangers are discussed. The time variations of the heat transfer rate, the overall heat transfer coefficient, and the pressure drop of the heat exchangers are presented. The heat transfer rate, the overall heat transfer coefficient, and the pressure drop for heat exchangers with re-direction louver fins are higher than those with flat plate fins and one-sided louver fins are. The amount of frost formation is the highest for heat exchangers with re-direction louver fins.     

Heat Transfer and Pressure Drop Under Dry and Humid Conditions in Flat-Tube Heat Exchangers With Plain Fins

Flat-tube heat exchangers could be an interesting alternative to make indirect cooling of display cabinets more energy-efficient. This application involves low air velocities in combination with condensation of water vapor on the air side, so plain fins could be suitable. Two different heat exchangers having flat tubes and plain fins on the air side were evaluated experimentally. One of the heat exchangers had continuous plate fins, and the other had serpentine fins. The performances during dry and wet test conditions were compared and related to theoretical predictions for different assumptions. The influence of air velocity, air humidity, and inclination angle was investigated. The results show that, in most cases, the heat transfer performance is somewhat reduced under wet conditions in comparison with dry test conditions, and that wet heat transfer surfaces lead to an increased pressure drop. At the lower air velocity range that was investigated, the heat exchanger having continuous plate fins drained better than the one with serpentine fins.     

Laminar flow and heat transfer in plate-fin triangular ducts in thermally developing entry region

Laminar forced flow and heat transfer in plate-fin isosceles triangular ducts encountered in compact heat exchangers is investigated. The flow is hydrodynamically fully developed, but developing thermally under uniform temperature conditions. Heat conduction in the fin of finite conductance and convection in the fluid are analyzed simultaneously as a conjugate problem. The study covers a wide range of apex angles from 30° to 120°, and fin conductance parameters from 0 to infinitely large. Nusselt numbers in the developing and fully developed regions for various apex angles and fin conductance parameters are obtained, which can be used in estimation of heat transfer characteristics in plate-fin compact heat exchangers with fins of various conductivities and thickness.     

A general solution for one-dimensional multistream heat exchangers and their networks

A mathematical model for predicting the steady-state thermal performance of one-dimensional (cocurrent and countercurrent) multistream heat exchangers and their networks is developed and is solved analytically for constant physical properties of streams. By introducing three matching matrices, the general solution can be applied to various types of one-dimensional multistream heat exchangers such as shell-and-tube heat exchangers, plate heat exchangers and plate-fin heat exchangers as well as their networks. The general solution is applied to the calculation and design of multistream heat exchangers. Examples are given to illustrate the procedures in detail. Based on this solution the superstructure model is developed for synthesis of heat exchanger networks.     

Thermal performance of plate-fin vapor chamber heat sinks

Since vapor chambers exhibit excellent thermal performance, they are suited to use as bases of heat sinks. This work experimentally studies the thermal performance of plate-fin vapor chamber heat sinks using infrared thermography. The effects of the width, height and number of fins and of the Reynolds number on the thermal performance are considered. Experimental data are compared with corresponding data for conventional aluminum heat sinks. The results show that generated heat is transferred more uniformly to the base plate by a vapor chamber heat sink than by a similar aluminum heat sink. Therefore, the maximum temperature is effectively reduced. The overall thermal resistance of the vapor chamber heat sink declines as the Reynolds number increases, but the strength of the effect falls. The effect of the fin dimensions on the thermal performance is stronger at a lower Reynolds number. At a low Reynolds number, a suitable number of fins must be chosen to ensure favorable thermal performance of the vapor chamber heat sink. However, at a high Reynolds number, the thermal performance improves as the fin number increases.