Coupling of Evaporation and Thermocapillary Convection in a Liquid Layer with Mass and Heat Exchanging Interface

We propose and analyse a new model of thermocapillary convection with evaporation in a cavity subjected to horizontal temperature gradient, rather than the previously studied model without evaporation. The pure liquid layer with a top free surface in contact with its own vapour is considered in microgravity condition. The computing programme developed for simulating this model integrates the two-dimensional, time-dependent Navier-Stokes equations and energy equation by a second-order accurate projection method. We focus on the coupling of evaporation and thermocapillary convection by investigating the influence of evaporation Biot number and Marangoni number on the interracial mass and heat transfer. Three different regimes of the coupling mechanisms are found and explained from our numerical results.     

格子Boltzmann方法模拟自然对流作用下融化传热过程

建立自然对流作用下融化的格子Boltzmann双分布函数模型,根据非线性对流扩散方程的格子Boltzmann模型理论提出一个新的表征融化温度场的分布函数演化方程,并通过变松弛时间方法处理固液两相变热物性传热问题.应用模型对热传导融化及自然对流融化特别固液变热物的融化过程进行模拟.模拟结果与分析解、经典的关联式结果吻合较好,模型的正确性得到了验证.模拟结果表明,自然对流对融化传热过程有着重要的影响,此外固相热传导也对融化传热、融化速率及固液两相温度分布都有一定影响.     

Constructal Optimizations of Line-to-Line Vascular Channels with Turbulent Convection Heat Transfer

The multi-scale line-to-line vascular channels (LVCs) widely exist in nature because of their excellent transmission characteristics. In this paper, models of LVCs with turbulent convection heat transfer are established. Based on constructal theory and the entropy generation minimization principle, the constructal optimizations of LVCs with any order are conducted by taking the angles at bifurcations as the optimization variables. The heat flux on the channel wall per unit length is fixed and uniform. The areas occupied by vasculature and the total volumes of channels are fixed. The analytical expressions of the optimal angles, dimensionless total entropy generation rate and entropy generation number (EGN) of LVCs with any order versus dimensionless mass flow rate are obtained, respectively. The results indicate that the dimensionless total entropy generation rate of LVCs with any order can be significantly decreased by optimizing the angles of LVCs, which is significantly more when the order of LVCs is higher. As the dimensionless mass flow rate increases, the optimal angles of LVCs with any order remain unchanged first, then the optimal angles at the entrance (root) increase, and the other optimal angles decrease continuously and finally tend to the respective stable values. The optimal angles of LVCs continue to increase from the entrance to the outlet (crown), i.e., the LVCs with a certain order gradually spread out from the root to the crown. The dimensionless total entropy generation rate and EGN of LVCs first decrease and then increase with the growth of the dimensionless mass flow rate. There is optimal dimensionless mass flow rate, making the dimensionless total entropy generation rate and the EGN reach their respective minimums. The results obtained herein can provide some new theoretical guidelines of thermal design and management for the practical applications of LVCs.     

LED灯表面对流换热系数的测算研究

提出了一种在第三类边界条件下,根据LED灯瞬态温度场的变化规律,通过对其表面温度的实际数值测定来推算LED灯表面对流换热系数的快速测定方法。基于对流热平衡理论,设计了一种可以在较高表面换热强度条件下进行测试的装置,通过实验测定LED灯上下表面的温度,结合曲线拟合对实测数据进行数理分析,得到较宽范围内的表面对流换热系数。实验结果表明:该测试方法简单、实用,测试时间较短(实验准备与数据测定大约需要30 min),测试精度较高(数据拟合误差不高于0.2%),可靠性强,可以用于工程热设计等多种相关发热体表面对流换热系数的测定。     

Effect of External Induced Convection on Heat Transfer at Water Boiling

Physics of Atomic Nuclei - In this paper, we experimentally and theoretically study the effect of external induced convection on heat transfer from horizontal heaters under conditions of pool...     

轴向磁场对分离结晶熔体中热毛细对流的影响

为了了解磁场对分离结晶过程中熔体内热毛细对流的影响,利用有限差分法进行了数值模拟.假定熔体为不可压缩流体,熔体深径比为1,自由表面无因次宽度为0.1,研究了哈特曼数分别为0、25,50和75时的碲锌镉晶体生长过程.结果表明:轴向磁场能够抑制熔体内部的流动,并随磁场强度的增加,抑制作用增强.     

Experimental Investigation of Thermocapillary Convection in a Liquid Layer with Evaporating Interface

Thermocapillary convection coupling with the evaporation effect of evaporating liquids is studied experimentally. This study focused on an evaporation liquid layer in a rectangular cavity subjected to a horizontal temperature gradient when the top evaporating surface is open to air, while most previous works only studied pure thermocapillary convection without evaporation. Two liquids with different evaporating rates are used to study the coupling of evaporation and thermocapillary convection, and the interfacial temperature profiles for different temperature gradients are measured. The experimental results indicate evidently the influence of evaporation effect on the thermocapillary convection and interfacial temperature profiles. The steady multicellular flow and the oscillatory multicellular flow in the evaporation liquid layer are observed by using the particle-image-velocimetry method.     

Digital Interferometric Measurement of Forced Convection Heat Transfer in a Miniature Rectangular Channel

Investigations have been performed on convective heat transfer in water flowing through mini-channels using the non intrusive technique of laser interferometry coupled with digital image processing. Optical glass channels, fabricated with metallic heating surfaces, were studied using a Mach-Zehnder interferometer configuration. Fringe patterns captured using a high-sensitivity CCD camera were analyzed digitally based on a calculation method developed for the liquid medium. Results of parametric studies were compared and contrasted with relevant theoretical solutions from the literature. Indication of the onset of turbulence at Reynolds numbers smaller than the conventional transition Reynolds number for large channels has also been noticed in the experimental investigation.     

Influence of the Grain Boundaries on the Heat Transfer in Laser Ceramics

Grain boundaries play a key role in determining several key properties of polycrystalline laser ceramics. Heat transfer measurements at low temperature constitute a good tool to probe grain boundaries. We review the results of heat transfer measurements in polycrystalline Y₃Al₅O₁₂ garnets as well as Y₂O₃ and Lu₂O₃ sesquioxide materials obtained by self-energy-driven sintering of nano-particles. The average phonon mean free path in Y₃Al₅O₁₂ was found to be significantly larger than the average grain size and to scale with temperature as T ⁻² at low temperature. Existing models describing the interaction between phonons and grain boundaries are reviewed. Correct temperature dependence of the mean free path and order of magnitude of scattering rates were found by assuming the existence of a grain boundary layer having acoustic properties different from those of the bulk. A different temperature dependence of phonon mean free path was found for the sesquioxides and was ascribed to the stronger elastic anisotropy of these materials. The thermal resistance associated to the grain boundaries of laser ceramics was found to be lower than in other dense polycrystalline ceramic materials reported in the literature.     

Mixed Convection in Viscoelastic Boundary Layer Flow and Heat Transfer over a Stretching Sheet

An investigation is carried out on mixed convection boundary layer flow of an incompressible and electrically conducting viscoelastic fluid over a linearly stretching surface in which the heat transfer includes the effects of viscous dissipation, elastic deformation, thermal radiation, and non-uniform heat source/sink for two general types of non-isothermal boundary conditions. The governing partial differential equations for the fluid flow and temperature are reduced to a nonlinear system of ordinary differential equations which are solved analytically using the homotopy analysis method (HAM). Graphical and numerical demonstrations of the convergence of the HAM solutions are provided, and the effects of various parameters on the skin friction coefficient and wall heat transfer are tabulated. In addition, it is demonstrated that previously reported solutions of the thermal energy equation given in [1] do not converge at the boundary, and therefore, the boundary derivatives reported are not correct.     

不同金属材料在不同热源加热下的热传导研究

本文分别采用酒精灯火焰和CO_2激光器发射的波长为10.6μm的红外线对铜、铝、铁正入射加热,用DIS信息化系统测量各种情况下的温升情况。通过实验发现采用CO_2激光器发射的波长为10.6μm的红外线正入射加热时铝、铁出现反常温升现象,从理论上进行分析得到了正确的解释。     

Mixed Convection Heat and Mass Transfer in a Micropolar Fluid with Soret and Dufour Effects

A mathematical model for the steady, mixed convection heat and mass transfer along a semi-infinite vertical plate embedded in a micropolar fluid in the presence of Soret and Dufour effects is presented. The non-linear governing equations and their associated boundary conditions are initially cast into dimensionless forms using local similarity transformations. The resulting system of equations is then solved numerically using the Keller-box method. The numerical results are compared and found to be in good agreement with previously published results as special cases of the present investigation. The non-dimensional velocity, microrotation, temperature and concentration profiles are displayed graphically for different values of coupling number, Soret and Dufour numbers. In addition, the skin-friction coefficient, the Nusselt number and Sherwood number are shown in a tabular form.     

An Experimental Investigation of Forced Convection Heat Transfer from a Coiled Heat Exchanger Embedded in a Packed Bed

Forced convection heat transfer from a helically coiled heat exchanger embedded in a packed bed of spherical glass particles was investigated experimentally. With dry air at ambient pressure and temperature as a flowing fluid, the effect of particle size, helically coiled heat exchanger diameter, and position was studied for a wide range of Reynolds numbers. It was found that the particle diameter, the helically coiled heat exchanger diameter and position, and the air velocity are of great influence on the convective heat transfer between the helically coiled heat exchanger and air. Results indicated that the heat transfer coefficient increased with increasing the air velocity, increasing helically coiled heat exchanger diameter, and decreasing the particle size. The highest heat transfer coefficients were obtained with the packed-bed particle size of 16 mm and heat exchanger coil diameter of 9.525 mm (1/4 inch) at a Reynolds number range of 1,536 to 4,134 for all used coil positions in the conducted tests. A dimensionless correlation was proposed for Nusselt number as a function of Reynolds number, particle size, coil size, and coil position.     

高能激光破岩的传热学特性研究

高能激光破岩中激光与岩石相互作用的过程,是一个复杂的非稳定的传热学问题。利用能量守恒原理,得到了激光破岩的传热方程,并对其相变过程中固液相变界面、液气界面上的焓、导热系数和比热等热物性参数进行了近拟简化,从而建立了激光破岩温度场分析的物理模型。利用Galerkin方法,对激光破岩的温度场模型进行了数值分析求解和实例计算。     

金属凝固过程界面换热系数的Tikhnonv正则化辨识

金属凝固过程中,铸件与铸型间的界面换热系数是数值模拟所必需的边界条件之一.充分考虑非线性瞬态热传导过程测温数据的误差特点,根据Tikhonov正则化理论,构造合适的正则化泛函,利用Arcangeli准则和Morozov偏差原理确定正则参数,采用灵敏度系数和Newton-Raphson迭代法求解该泛函.具体的案例分析表明,该方法克服了热传导反问题的不适定性,能够保证辨识结果的稳定性和精度.与其他方法相比,随着测温误差的增大,该方法具有较高的辨识精度.     

金属凝固过程界面换热系数的Tikhonov正则化辨识

金属凝固过程中,铸件与铸型间的界面换热系数是数值模拟所必需的边界条件之一.充分考虑非线性瞬态热传导过程测温数据的误差特点,根据Tikhonov正则化理论,构造合适的正则化泛函,利用Arcangeli准则和Morozov偏差原理确定正则参数,采用灵敏度系数和Newton-Raphson迭代法求解该泛函.具体的案例分析表明,该方法克服了热传导反问题的不适定性,能够保证辨识结果的稳定性和精度.与其他方法相比,随着测温误差的增大,该方法具有较高的辨识精度.     

Experimental Investigation on Pressure Drop and Heat Transfer Characteristics of Copper Metal Foam Heat Sink

Abstract

The effect of the cooling performance of a copper metal foam heat sink under buoyancy-induced convection is investigated in this work. Experiments are conducted on copper metal foam of 61.3% porosity with 20 pores per inch. The pressure drop experiment is carried out to find the permeability and foam coefficient of the porous media. It is found that the property of porous media changes by changing the angle of inclination of the porous media from a horizontal to a vertical position while keeping the orientation and porosity the same. The Hazen-Dupuit Darcy model is used to curve-fit the longitudinal global pressure drop versus the average fluid speed data from an isothermal steady-flow experiment across the test section of the porous medium. The study concludes that the permeability and foam coefficient for copper foam is found to be 1.11 × 10^?7 m² and 79.9 m^?1, respectively. The heat transfer study shows that the thermal performance of copper metal foam is 35–40% higher than the conventional aluminum metal heat sink under an actual conventional mode.     

三维方腔内竖直平板混合对流换热的数值研究

本文建立了三维方腔内竖直板混合对流换热的数学模型,探讨Re数,进风口位置、竖直板位置对混合对流换热的影响.结果表明:随Re数增加,竖直板下部的平均Nu2下降,而总体平均Nu1增加.随送风口高度增加,竖直板对流换热量增加.在竖直板位置L1/H=0.5～1.5的范围内,竖直板处于中间位置L1/H=1.0时,竖直板混合对流换...