Inter‐particle contact heat transfer in soil systems at moderate temperatures

An inter‐particle contact heat transfer model for evaluating soil thermal conductivity is analysed with respect to soils, representing different textural classes, exposed to moderate temperatures ranging from 15 to 30°C. This model is a combination of a self‐consistent approximation model, enhanced with an inter‐particle contact heat transfer correction coefficient. For dry and saturated soils, this coefficient is defined as a ratio of a soil harmonic mean thermal conductivity of solid and fluid (air or water) phases, to the average thermal conductivity of soil solid grains. For unsaturated soils, we assume a linear interpolation of the correction coefficient between absolutely dry and saturated states, with a Kersten function (Ke) as a proportional factor. The strongest impact of the correction coefficient (maximum reduction of heat transfer) is observed for coarse soils below a critical value of saturation degree (S_r‐cr–corresponds to Ke?0) followed by medium and fine soils. For S_r>S_rcr, the reduction of heat transfer gradually diminishes as S_r approaches 1 (i.e. saturated state). Soil texture, soil specific surface area, porosity and mineralogical composition (particularly quartz content) are important factors influencing the heat transfer correction coefficient. Their influence appears to be more substantial at the lower half of the wetness range (S_r<0.5). Simulation results from the new enhanced model closely follow experimental data.     

A study of heat transfer in high‐performance hydrogen bell‐type annealing furnaces

The mechanisms of heat transfer in high‐performance hydrogen bell‐type annealing furnaces are discussed in this paper. It examines two important parameters, the convective heat transfer coefficient and the equivalent radial thermal conductivity, which have large effects on the overall heat transfer in the furnace. The calculated annealing curves are in good agreement with experimental data. © 2001 Scripta Technica, Heat Trans Asian Res, 30(8): 615–623, 2001     

Effect of pressure and temperature on cluster and particle heat transfer in a pressurized circulating fluidized bed

The present work reports the influence of pressure and bed temperature on particle‐to‐wall heat transfer in a pressurized circulating fluidized bed (PCFB). The particle convection heat transfer plays a dominant role in determining the bed‐to‐wall heat transfer coefficient. So far, no information is reported on the effect of pressure and bed temperature on particle‐to‐wall heat transfer in a PCFB in the published literature. The present investigation reports some information in this direction. The effect of system pressure and bed temperature are investigated to study their influence on cluster and particle heat transfer. The particle convection heat transfer coefficient increases with system pressure and bed temperature due to higher cluster thermal conductivity. The increase in particle concentration (suspension density) results in greater cluster solid fraction and also the particle concentration near the wall is enhanced. This results in higher cluster and particle convection heat transfer between the bed and the wall. Higher particle convection heat transfer coefficient results in enhanced heat transfer between the bed and the wall. The results will also help to understand the bed‐to‐wall heat transfer mechanism in a better way in a PCFB. Copyright © 2001 John Wiley & Sons, Ltd.     

Convective heat transfer of horizontal spherical particle layer heated from below and cooled from above Part II: Effect of thickness of particle layer

Convection heat transfer and pressure drop measurements were performed with a rectangular duct, having a cooled upper and a heated lower surface, which was packed with spherical particles. Air was used as the test fluid and four kinds of spherical particles having different diameters and thermal conductivities were used as the packing materials. The ratio of the diameter of the spherical particle to the distance between the cooled and heated surfaces, d/H, was varied from 0.173 to 1. The thermal conductivity of the particle layer was also measured under the still air condition. The thermal conductivity of the particle layer was not affected by the value of d/H. In the case of the one-stage arrangement of spherical particles (d/H = 1), the flow resistance took on a remarkably small value compared with the flow resistance of a homogeneous spherical particle layer. Moreover, the flow resistance of the particle layer formed with some layers of particles could be predicted by combining the flow characteristics of the one-stage particle layer and that of the homogeneous spherical particle layer. The heat transfer coefficient of the particle layer was larger than that of turbulent air flow on a flat plate. At a constant superficial air velocity, there existed a value of d/H which gave a maximum value of the average heat transfer coefficient. Nondimensional heat transfer correlation equations were derived in terms of parameters expressing the average characteristics of the spherical particle layers. © 1998 Scripta Technica, Heat Trans Jpn Res, 26(3): 176–192, 1997     

Convective‐radiative fins with simultaneous variation of thermal conductivity,heat transfer coefficient,and surface emissivity with temperature

This paper is a numerical study of thermal performance of a convective‐radiative fin with simultaneous variation of thermal conductivity, heat transfer coefficient, and surface emissivity with temperature. The convective heat transfer is assumed to be a power function of the local temperature between the fin and the ambient which allows simulation of different convection mechanisms such as natural convection (laminar and turbulent), boiling, etc. The thermal conductivity and the surface emissivity are treated as linear functions of the local temperature between the fin and the ambient which provide a satisfactory representation of the thermal property variations of most fin materials. The thermal performance is governed by seven parameters, namely, convection–conduction parameter Nc, radiation–conduction parameter Nr, thermal conductivity parameter A, emissivity parameter B, the exponent n associated with convective heat transfer coefficient, and the two temperature ratios, θ_a and θ_s, that characterize the temperatures of convection and radiation sinks. The effect of these parameters on the temperature distribution and fin heat transfer rate are illustrated and the results interpreted in physical terms. Compared with the constant properties model, the fin heat transfer rate can be underestimated or overestimated considerably depending on the values of the governing parameters. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20408     

Enhancement of the cubic cell soil thermal conductivity model

An analysis of measured soil thermal conductivity (λ) data for temperatures (T) varying from 5 to 90°C, was conducted with respect to four soil moisture content domains, i.e. residual, transitory meniscus, micro/macro porous capillary, superfluous. It was shown that each domain has a specific behaviour of λ vs soil moisture content (θ). For example, λ varies insignificantly with θ and T at very low moisture contents (residual moisture domain). In the transitory meniscus and micro/macro porous capillary domains, the relation λ (θ) shows in general a nonlinear behaviour, which is difficult to model, particularly at high T. A sensitivity analysis applied to the Gori (1983) model for dry soil showed better predictions when the model was restricted to the use of the first term only (dependent on soil porosity and thermal conductivity of air). Two linear λ approximations have been tested, across the second domain (from a critical θ to the permanent wilting point) and across the second and third domains (from a critical θ to field capacity). The enhanced model has been tested against soil λ data measured at moderate and high T. The numerical results show considerably improved predictions in the first three soil moisture domains. The first linear λ interpolation shows better agreement with experimental data for T up to 65°C, while the second interpolation was much more beneficial at higher T. The original Gori model gives generally the best predictions in the superfluous domain. Copyright © 2002 John Wiley & Sons, Ltd.     

Numerical study of radiation‐convection heat transfer

In this study, the authors attempted to introduce a simulation technique for radiation‐convection heat transfer in the high‐temperature fields of industrial furnaces, boilers, and gas turbine combustors. The convection effect was analyzed by a differential equation, but the radiation effect was analyzed by an integral equation. Thus, it was not easy to arrange both effects using the same type of equations. Then, the authors introduced the zone method and Monte Carlo method for the integral equation of the radiation effect and the finite difference method for the differential equation of the convection effect. A three‐dimensional analysis of the high‐temperature furnace was performed by this simulation technique to obtain its temperature distribution. Furthermore, another radiation‐convection heat transfer analysis in the low‐temperature living room was performed by the same technique. Finally, the authors tried to develop a computer software for radiation‐convection heat transfer and described their idea of software construction for the above. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(5): 391–407, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10042     

Transient heat transfer in a U-tube borehole heat exchanger

A transient heat transfer model has been development for a thermal response test (TRT) on a vertical borehole with a U-tube. Vertical borehole heat exchangers are frequently coupled to ground source heat pumps, which heat and cool buildings. The model provides an analytical solution for the vertical temperature profiles of the circulating fluid through the U-tube, and the temperature distribution in the ground. The model is verified with data sets from a laboratory sandbox and field TRTs, as well as a previously reported numerical solution. Unlike previous analytical models, the vertical profiles for the circulating fluid are generated by the model without any assumption of their functional form.     

涂层导热系数变化对换热器传热的影响

采用换热器效能法,对不同种类换热器进行校核计算.研究换热工质为气气、气液、液液时,不同种类换热器的换热性能随涂层导热系数的变化趋势.结果表明,各种换热器的总传热系数皆随涂层导热系数的增加而增大,且在不同低温侧流体雷诺数下增加程度不同,雷诺数越大,增加程度越大.设定涂层厚度为200.000 μm,涂层导热热阻所占比例随着...     

Mathematical Model and Its Application of Radial Effective Thermal Conductivity for Coil Heat Transfer in HPH Furnace

Temperature uniformity of steel coils in High Performance Hydrogen bell-type annealing furnace has a significant effect on their quality and production. The hot rolled coil can be considered as a periodically laminated material composed of steel layers and interface layers in radial direction. A new formula for the radial effective thermal conductivity has been proposed, which is based on surface characteristic, strip thickness and compressive stress of the rolled coil. Furthermore, it has been used to develop a heat transfer mathematical model for steel coils in the HPH furnace. The calculated annealing curves using this mathematical model are in good agreement with the experimental data.     

A water heater using very high‐temperature storage and variable thermal contact resistance

This paper describes the role of thermal contact resistance in a high‐temperature sensible heat storage water heater using cast iron as a storage material. An experimental set‐up consisting of a cast iron cylinder and a stainless‐steel tube running through its centre was fabricated and tested. The experimental data were compared with a theoretical model. It was observed that the variation in thermal contact resistance between the cast‐iron blocks and the tube with temperature plays a dominant role in extracting the heat at a reasonably constant temperature. The contact resistance between two contacting surfaces was modelled as the composite of two parallel resistances: one due to the points where two surfaces contact each other and the other due to a gap between the surfaces, which is often air filled. An approximate contact resistance prediction was obtained by assuming the resistance due to the air gap modulated by a correction factor which accounts for the contacting surface area. Based on the results from the experimental set‐up and theoretical modelling, a prototype storage water heater using cast‐iron blocks as the storage material was designed, fabricated and tested. Copyright © 2001 John Wiley & Sons, Ltd.     

开孔式SK型直接接触换热器混合与换热特性研究

为了进一步强化流体在SK型静态混合器中换热效率,对SK型静态混合器进行3,6和9孔的开孔设计,并将开孔式SK型静态混合器应用于直接接触蒸发器中,以高沸点导热油作为直接接触换热器中的连续相,以低沸点的有机工质作为直接接触换热器的分散相,运用ANSYS软件进行数值模拟,研究30,50和70℃3种低温差情况下,不同开孔数量对流动和换热特性的影响。结果表明：3孔式SK型静态混合器的混合特性最稳定,在3种温差下混合效果分别是传统式SK型静态混合器的1.5,3.3和1.6倍;湍流强度分别为传统式SK型静态混合器的3.3,3.8和1.3倍。在SK型静态混合器上合理的开孔可以提高气态工质在直接接触换热器内的停留时间,从而使传热得到强化,达到了节约能源的目的。     

Modelling approaches to predicting thermal conductivity of soils at high temperatures

Three predictive models are tested against soil thermal conductivity data at high temperatures: two (deV‐1 and deV‐2) are modifications of the original de Vries model, and the third is the Gori model. Model deV‐1 results correlated very closely for 30, 50 and 70°C, and moderately close for 90°C. Generally, this model does not require the use of a mass transfer enhancement factor. The Gori model results correlated well at 30 and 50°C. Model deV‐2 results correlate well above field capacity, and provide the best predictions at full dryness. Model deV‐1 is recommended for computer modelling of heat and moisture movement in soils at high temperatures. Copyright © 2000 John Wiley & Sons, Ltd.     

An experimental study of heat transfer in an open thermosyphon

An experimental study was performed on heat transfer of an open thermosyphon with constant wall heat flux. Water and aqueous glycerin were used as working fluids. The experimental range of modified Rayleigh number was 1 × 10³ < Ra_m < 3 × 10⁵. The average and local heat transfer coefficients, vertical temperature distributions of the tube wall and fluid at the centerline of the tube, and temperature fluctuations of the fluid were measured. Flow patterns were observed by adding tracer powder to the fluid. Fluid velocities were measured by laser Doppler velocimeter. Experimental results indicate that, for a water thermosyphon, there are three regimes where different heat transfer characteristics and flow patterns occur. For 1 × 10³ < Ra_m < 3 × 10³, the flow was laminar and the thermal boundary layer reached the center of the tube. Heat was exchanged between the wall and descending flow. Wall temperature increased in the downward direction. For 4 × 10³ < Ra_m < 3 × 10⁴, no turbulence was observed in the flow and the thermal boundary layer was localized in the vicinity of the wall. The wall temperature increased upward. For 3 × 10⁴ < Ra_m < 3 × 10⁵, flow was considerably disturbed by the mixing of upward and downward flow in the upper part of the tube. However, the flow was laminar in the lower part of the tube. Reduction of the flow rate induced by the flow mixing at high Ra_m can be one of the major causes of the deterioration of heat transfer from Lighthill's theory. © 2001 Scripta Technica, Heat Trans Asian Res, 30(4): 301–312, 2001     

Thermal contact resistance between an element and a housing in an aluminum electrolytic capacitor

Cooling performance of an aluminum electrolytic capacitor, whose element pressed on the inner bottom surface of the capacitor housing in order to decrease thermal contact resistance between the element and the housing, was measured and improved. It was found that a thermally conductive elastomer decreases this contact resistance. The elastomer with a thickness ranging from 0.5 to 1.5 mm was inserted between the bottom surface of the capacitor element and the plain metal surface. Experimental measurements show that the contact resistance with the 0.5‐mm‐thick elastomer is smaller than that without an elastomer. A simple analysis was also developed for predicting the thermal contact resistance with different elastomer thicknesses, and these predicted resistances agree reasonably well with the experimental measurements. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(3): 268–277, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10090     

A finite line‐source model for boreholes in geothermal heat exchangers

An analytical solution of the transient temperature response in a semi‐infinite medium with a line source of finite length has been derived, which is a more appropriate model for boreholes in geothermal heat exchangers, especially for their long‐duration operation. The steady‐state temperature distribution has also been obtained as a limit of this solution. An erratic approach to this problem that appears in some handbooks and textbooks is indicated. Two representative steady‐state borehole wall temperatures, the middle point temperature and the integral mean temperature, are defined. Differences between them are compared, and concise expressions for both are presented for engineering applications. On this basis the influence of the annual imbalance between heating and cooling loads of the geothermal heat exchangers is discussed regarding their long‐term performance. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 558–567, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10057     

换热器结垢工况下换热系数变化的分析研究

介绍了换热器污垢热阻的数学模型，包括污垢沉积模型和剥蚀模型。分析了换热器结垢工况下的换热系数的变化，重点研究了时间、流体雷诺数Re和流体—污垢界面温度Ts对换熟系数K的影响，以及在结垢诱导期内换热系数K的变化。得到了冷却水流速与污垢热阻之间的关系式，界面温度Ts与污垢热阻和换热系数之间关系的示意图，并得出了诱导期内的换热系数K大于结垢过程的其他四个阶段的结论。最后，阐述了分析结果对工程的实际指导意义。     

壳管式相变蓄热器传热效率研究

设计了一套定量测试不同工况下壳管式相变蓄热器传热效率装置。采用壳管式相变储热,石蜡填充入壳管间,管内通入冷、热载流体,模拟吸热放热过程。测试发现:相同入口条件下,单位时间传热量随入口水温增加呈线性增加;管内载流体流量加大有助于提高传热水平,15~60 L/h流量内单位时间传热量增速随流量增加放缓;不同材质传热管单位时间传热量变化并不明显,表明管道热阻在相变蓄热器总热阻中所占份额较小;相同工况下的蓄热过程,热载流体由下向上流动传热形式明显优于由上向下管排形式;尝试在封装相变材料中添加金属网状结构,强化相变材料内部热传导速率,对比发现相同工况下相变材料中添加金属网状结构,可提高10%~15%左右传热量。     

Fundamental heat transfer mechanism between bed‐to‐membrane water‐walls in circulating fluidized bed combustors

In the present work, the fundamental mechanism between bed‐to‐membrane water‐walls in the riser column of a circulating fluidized bed (CFB) combustor is presented. The bed‐to‐membrane water‐wall heat transfer depends on the contributions of particle heat transfer, dispersed phase heat transfer and radiation heat transfer. The fundamental mechanism of particle heat transfer and the effect of fraction of wall exposed to clusters and gas gap thickness between cluster and wall on particle heat transfer coefficient and bed‐to‐wall heat transfer coefficient are investigated. The influence of operating parameters like cross‐sectional average volumetric solids concentration and bed temperature on particle and bed‐to‐wall heat transfer are also reported. The present work contributes some fundamental information on particle heat transfer mechanism, which is responsible for increasing the bed‐to‐wall heat transfer coefficient (apart from dispersed phase convection and radiation heat transfer). The details on particle heat transfer mechanism will enable to understand the basic heat transfer phenomena between bed‐to‐membrane water‐walls in circulating fluidized bed combustors in a detailed way, which in turn will aid for better design of CFB combustor units. The particle heat transfer mechanism is significantly influenced by the fraction of wall exposed to clusters and gas gap thickness between clusters and wall. Copyright © 2003 John Wiley & Sons, Ltd.     

Direct‐contact heat exchange between fluidizing particles and a heat transfer surface in a fluidized bed: Temperature visualization of fluidizing particles

Heat conduction during contact between a heat transfer surface and fluidizing particles, a phenomenon which is one of the effective heat transfer mechanisms in a gas–solid fluidized bed, has been empirically investigated. The temperature profile of the fluidizing particles during the contact period is visualized with the aid of an infrared imager. The visualization reveals that the particles have been considerably heated in the thermal boundary layer on the heat transfer surface before contact. Based on the visualized temperature profile of the particles, the contact conductance between a fluidizing particle and the heat transfer surface is estimated by an in inverse analysis. Using the evaluated contact conductance, the contributions of the conductive heat transfer to the total heat transfer are also evaluated. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 165–181, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10027