A simple solution for transient heat transfer during heating of a slab product

A theoretical and experimental study of transient heat transfer in the heating of an individual slab product, subjected to an air flow at a temperature of 50°C and a velocity of 1 m/s, is presented. Experimental temperature measurements at the centre of the slab product were made, and the experimental heat-transfer rates were derived from the temperature data. A simplified analytical technique, using the boundary condition of the third kind in transient heat transfer, was used to predict the theoretical heat transfer rates for two cases, the first considering that the heat transfer coefficient is a convective heat transfer coefficient, and the second considering that heat transfer coefficient is the sum of the convective and radiative heat transfer coefficients. The experimental heat-transfer rates were compared with the predictions for two cases, and a very good agreement was obtained.     

Freezing of Water in a Slab with Boundary Conditions of the Third Kind

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Total heat transfer coefficients for canned foods during sterilization

An analytical mathematical model for determining the total heat transfer coefficient of a cylindrically shaped canned food subjected to sterilization was developed. There is a need to determine these coefficients in a simple and accurate form for process heat transfer analysis and energy optimization. In the mathematical modelling, a new technique for heat sterilization conduction problem was used by considering the boundary condition of the third kind in transient heat conduction. The temperature data at the centres of the cylindrically shaped cans were obtained in the experimental investigation at the medium temperatures of 115 and 121°C and were used to determine the total heat transfer coefficients of the individual canned products. The total heat transfer coefficient for an individual canned product increased with increasing medium temperature. The results of this study shows that the present analytical model is a simple tool for determining the total heat transfer coefficients for the individual canned products.     

采集凝固热热泵技术凝固及换热性能的理论分析

介绍了一种新型的采集凝固热热泵技术,建立了凝固热采集装置中层流流动水在常壁温条件下的管内凝固问题的数学模型。采用准稳态近似方法进行了凝固特性的分析,使用当量平均表面换热系数和潜热显热比两个参数讨论了冰层对换热性能的影响。计算结果与所得结论对凝固热采集装置的设计和热泵系统性能的改善有一定指导作用。     

Development of new correlations for forced convection heat transfer during cooling of products

This paper presents the new, simple but powerful effective Nusselt–Reynolds correlations for estimating the effective convective heat transfer coefficients of spherical and cylindrical products cooled in water and air flows. In this respect, both experimental and theoretical works were obtained. In the experimental case, several spherical and cylindrical products, namely, tomatoes, pears and cucumbers were cooled in water and air flow and their centre temperature variations were measured. In the theoretical case, the effective convective heat transfer coefficients for the individual spherical and cylindrical products were determined using the centre temperature data in the present approach including Dincer's models. Therefore, the new Nusselt–Reynolds correlations were developed using the effective convective heat transfer coefficient values and a general diagram of Nu/Pr^1/3 against Reynolds number was drawn. This study indicates that the present effective Nu–Re correlations are capable of estimating the effective convective heat transfer coefficients of any spherical and cylindrical shaped products exposed to water and air cooling in practical applications in a simple and accurate manner.     

Improvement of the pulsed photothermal technique for the measurement of the convective heat transfer coefficient

<正>The present study concerns the measurement of the convective heat transfer coefficient on the solid-fluid interface by the pulsed photothermal method.This non-intrusive technique is apphed for the measurement of the local heat transfer coefficients in cooling of a rectangular slab that simulates an electronic component.The heat transfer coefficient is deduced from the evolution of the transient temperature induced by a sudden deposit of a luminous energy on the front face of the slab.In order to draw up the heat transfer cartography by a non-destructive tool, the infrared thermography has been used.Two inverse techniques for the identification of the heat transfer coefficient are presented here.The first one is based on the assumption that heat transfer coefficient remains constant during the pulsed experiment,and the second one considered it variable in space and time.The temporal and spatial evolutions are expressed as a constant heat transfer coefficient(h_0)multiplied by a function of time and space f(x,t).The function f is deduced from the resolution of the conjugated convection-conduction problem,by a control volume technique for the case of thermally thick sample.The results are given for different air velocities and deflection angles of the flow.     

R1234ze(E)与R134a在水平双侧强化管外的凝结换热对比实验

搭建了水平单管降膜蒸发试验台,以第四代制冷剂R1234ze(E)和第三代制冷剂R134a作为工质,在新型水平双侧强化管管外分别进行了改变管内水速、热流密度和冷凝温度条件的凝结换热实验。使用Wilson-Gnielinski图解法计算得到管内表面传热系数h_i,进一步采用热阻分离法分离出两种制冷剂的管外表面传热系数,并分析了管内冷却水水速、冷凝温度和壁面过冷度的变化对其换热性能的影响。实验结果表明:同根实验管下不同制冷剂凝结换热性能的差异与制冷剂物性与强化管结构之间的匹配特性有关,实验管型下,R1234ze(E)的管外凝结换热性能高于R134a。     

微通道内流动沸腾特性研究

对国内外微通道流动和换热的研究实验作了总结,阐述了影响微通道换热系数的因素,如热流密度、过热度和干度等.对去离子水在内径为0.65 mm、长为102 mm的圆形管道内流动沸腾换热进行了实验研究,得到了局部换热系数随干度的变化关系,进而根据换热系数的变化趋势讨论了饱和流动沸腾区微通道内主导的换热机制.结果表明:从换热系数随干度的变化关系很难判定主导的换热机制;将实验数据与已发表的预测关联式进行了比较,发现大多关联式都失效,说明基于常规理论的模型不再适用于微通道.     

台阶型旋转热管充液量的研究

建立了台阶型旋转热管最佳充液量的理论分析模型，探讨了旋转热管的转速，几何尺寸，工作温度及传输功率对最佳充液量的影响规律，可为工程应用中不同工况下台阶型旋转热管最佳充液量的确定提供理论参考依据，同时，由该模型对冷凝段的传热系数进行了预测，并将预测值与实验值进行了对比分析，图8参4。     

Modelling the heat and mass transfer analysis during oil-frying of cylindrical sausages

An analytical and experimental study was performed on the analysis of the heat and mass transfer within cylindrically shaped sausages during deep-oil frying. Experiments were conducted to determine temperature distribution at the centre of an individual sample and the changes in the weight, moisture and oil contents of the samples. The proposed approach was used to estimate these parameters theoretically. Good agreement was found between the experimental and theoretical results. The results of this study indicates that the proposed approach is capable of analysing the heat and mass transfer during frying of the cylindrical sample.     

熔盐与过冷沸腾水传热特性实验研究

为研究熔盐蒸汽发生器的传热特性,以Solar Salt二元硝酸盐为工质,对熔盐与过冷沸腾水在蒸汽发生器中的传热规律进行实验研究.实验结果表明:熔盐传热系数随着水侧与熔盐的质量流速的增大而增大,随着水侧压力的升高而减小;当水侧在过冷水状态时,熔盐传热系数随着熔盐入口温度的升高而增大;当水侧在过冷沸腾状态时,熔盐传热系数随...     

管内插入物单相流体强化传热的实验研究

采用管内插入物增强锅炉烟管的放热系数是行之有效的手段,它不影响换热器的机械强度,便于拆装、检修和更换。在设计烟管锅炉,特别是改造烟管锅炉中,它是最有希望的强化传热方式之一。本文介绍了内插物强化传热实验装置的设计调试,及其一种内插件——螺旋线圈强化传热的实验研究和有关理论探讨。     

低温烟气凝结换热及对天然气热效率的影响实验研究

为研究烟气露点附近及以下的低温烟气对流凝结换热规律与烟气换热对天然气利用热效率的影响,建立了烟气在翅片管换热器内对流凝结换热实验系统,研究了不同烟气温度、水蒸气含量对烟气凝结换热的影响,得出了烟气凝结换热实验准则关联式,分析对比了天然气利用热效率实验值与理论值。实验结果表明:当被加热水温度为23℃,烟气出口温度为73℃,比烟气露点53℃高20℃时已开始冷凝;过量空气系数1.3,烟温由54.1~73.4℃降到28.7~57.8℃,被加热水进口温度21~25℃的条件下,烟气中水蒸气质量含量降低了27%~76%,潜热换热占总换热51%~63%;天然气利用低热值热效率实验值比理论值高1.04%~8.74%。为低温烟气对流凝结换热规律研究与烟气余热回收利用技术开发及应用提供理论依据和数据资料。     

A study on latent heat storage exchangers with the high‐temperature phase‐change material

This paper presents a theoretical analysis and an experimental test on a shell‐and‐tube latent heat storage exchanger. The heat exchanger is used to recover high‐temperature waste heat from industrial furnaces and off‐peak electricity. It can also be integrated into a renewable energy system as an energy storage component. A mathematical model describing the unsteady freezing problem coupled with forced convection is solved numerically to predict the performance of the heat exchanger. It provides the basis for an optimum design of the heat exchanger. The experimental study on the heat exchanger is carried out under various operating conditions. Effects of various parameters, such as the inlet temperature, the mass flow rate, the thickness of the phase‐change material and the length of the pipes, on the heat transfer performance of the unit are discussed combined with theoretical prediction. The criterion for analyzing and evaluating the performance of heat exchanger is also proposed. Copyright © 2001 John Wiley & Sons, Ltd.     

塑料管单效溴化锂吸收式制冷机系统设计研究

进行溴化锂吸收式制冷机塑料换热装置及制冷机系统的结构设计,塑料换热装置的传热管采用聚四氟乙烯塑料管,传热管布置为阿基米德螺线状盘管,将实验传热系数与理论计算传热系数进行比较。比较结果表明：冷凝器、蒸发器实验传热系数低于理论计算值,吸收器的实验传热系数高于理论计算值。     

空气冷却式膜蒸馏构型的传热分析

在膜蒸馏的不同构型中,直接采用环境空气作为冷却媒介的空气冷却式构型很大程度上简化了系统配置。在强化传热的条件下,其跨膜通量与水冷构型接近。对空气冷却式膜蒸馏构型的传热过程进行理论分析,并通过量化分析各参数对膜蒸馏传热性能的影响,构建综合的传热模型。引入关联热阻系数这一概念,用以量化空气冷却的参数对膜蒸馏过程总传热系数的抑制作用。通过模拟计算研究了冷凝板导热系数、空气流速、冷凝板肋化系数、料液温度等参数对膜蒸馏传热性能的影响,并分析和量化多参数对关联热阻系数的综合影响。结果表明冷凝板导热系数、空气流速、冷凝板肋化系数是影响关联热阻系数的重要因素,各参数对膜蒸馏传热性能的综合影响得以量化。以上研究为后续传质模型的研究提供了指导。     

Numerical study of convective heat transfer of nanofluids in a circular tube two-phase model versus single-phase model

Laminar convective heat transfer of nanofluids in a circular tube under constant wall temperature condition is studied numerically using a CFD¹ approach. Single-phase and two-phase models have been used for prediction of temperature, flow field, and calculation of heat transfer coefficient. Effects of some important parameters such as nanoparticle sources, nanoparticle volume fraction and nanofluid Peclet number on heat transfer rate have been investigated. The results of CFD simulation based on two-phase model were used for comparison with single-phase model, theoretical models and experimental data. Results have shown that heat transfer coefficient clearly increases with an increase in particle concentration. Also the heat transfer enhancement increases with Peclet number. Two-phase model shows better agreement with experimental measurements. For Cu/Water nanofluid with 0.2% concentration, the average relative error between experimental data and CFD results based on single-phase model was 16% while for two-phase model was 8%. Based on the results of the simulation it was concluded that the two-phase approach gives better predictions for heat transfer rate compared to the single-phase model.     

Determination of heat transfer coefficients for cylindrical products exposed to forced-air cooling

An analytical model for estimation of transient heat transfer coefficients in forced-air precooling experiments of cylindrically shaped grapes, using a lumped capacitance approach were addressed and investigated. In order to determine transient heat transfer coefficients, the centre transient temperature measurements during forced-air precooling were used. Experiments involved cooling individual grapes in air flow without water losses. The individual grapes were instrumented with several interior thermocouples for measuring the centre transient temperature response during cooling. The transient values of the heat transfer coefficient history for five different air velocities were found to be about 21–40 W/m² K. These values were in good agreement with the values predicted using well-known Nusselt-Reynolds empirical correlation for forced convection. The present technique has the capability of determining transient heat transfer coefficients in a single transient experiment.     

Effect of pressure on heat transfer coefficient at the metal/mold interface of A356 aluminum alloy

The aim of this paper is to correlate interfacial heat transfer coefficient (IHTC) to applied external pressure, in which IHTC at the interface between A356 aluminum alloy and metallic mold during the solidification of casting under different pressures were obtained using the inverse heat conduction problem (IHCP) method. The method covers the expedient of comparing theoretical and experimental thermal histories. Temperature profiles obtained from thermocouples were used in a finite difference heat flow program to estimate the transient heat transfer coefficients. The new simple formula was presented for correlation between external pressure and heat transfer coefficient. Acceptable agreement with data in literature shows the accuracy of the proposed formula.     

六缸柴油机冷却系统流动与传热的数值模拟研究

冷却水的流动与传热直接影响柴油机的冷却效率、高温零件的热负荷、整机的热量分配和能量利用。在冷却系统传热计算时，利用流固耦合的方法，较为准确地确定了缸体水套的传热边界条件。采用CFD商用软件STAR—CD对直列六缸柴油机的冷却系统的流动与传热进行三维数值模拟，给出了整机冷却水套内冷却液的流场、换热系数及压力场分布，为柴油机冷却水腔的优化设计提供了理论依据。