多孔铝对流换热系数的反演分析

建立了多孔铝的一维瞬态对流换热模型,采用反演分析方法求得了多孔铝的体积对流换热系数.给出了多孔铝对流换热的无量纲准则关联式,并在较宽的孔结构范围内(孔隙率为60.0%～95.0%、孔径为2.5～6.0 mm)研究了孔结构对多孔铝换热系数的影响.结果表明:多孔铝的换热系数随着风速的增加而提高.在风速相同的条件下,换热系数随着孔径和孔隙率的减小而提高.在风机功率相同的条件下,换热系数随着孔隙率的增大而提高,在孔隙率约为85%时达到峰值.     

多孔泡沫金属相界面传热系数的研究

基于简单立方体模型并采用翅片理论分析得出了流体在多孔泡沫金属通道内层流强制对流时相界面传热系数的表达式,并在不同条件下研究了流体流速和孔隙结构对相界面传热系数的影响,结果表明,相界面传热系数随着流体流速的增大而增大,随着多孔泡沫金属孔密度的增大而增大,而随着多孔泡沫金属的孔隙率的增大先增大后减小。     

氮气在泡沫铜内的流动传热特性模拟研究

为探究氮气在多孔介质泡沫铜内的流动传热特性,应用FLUENT软件对低温制冷机冷头换热器中填充的泡沫铜进行数值模拟分析。研究了孔隙率、孔密度以及入口流速对流动、传热的影响规律,模拟结果表明,孔隙率较大、孔密度较小时,渗流性能较好,流体速度衰减较慢;随着孔隙率减小,流体压力损失增大,换热效果提高,但提高的趋势渐缓,因而可能存在压损与温降比值最小的最优孔隙率;孔密度增大,由无滑移壁面引起的压力损失增大,换热性能变差。     

多孔泡沫金属内层流强制对流传热的相界面温差分析

基于简单立方体模型,应用Brinkman-Forchheimer-extended Darcy流动模型并结合边界层理论,分析了边界恒热流条件下的多孔泡沫金属通道内层流强制对流时流体与多孔泡沫金属间的相界面温差,结果表明,相界面温差随着流体流速的增大而减小.随着多孔泡沫金属孔密度的增大而减小,而随着多孔泡沫金属孔隙率的增大而增大.     

基于二维Voronoi模型的多孔泡沫金属导热性能模拟研究

多孔金属材料作为新型功能材料具有密度低、强度高、导热性能优良等特性,应用前景广阔,受到越来越多的关注。多孔材料的有效导热系数与随机孔隙结构相关,仅用孔隙率不足以描述真实材料的孔隙结构。采用二维Voronoi模型,定义孔隙随机度S和孔隙率ε作为孔隙结构参数,通过调节核点位置偏移因子α和边宽系数β改变模型的随机度S和孔隙率ε,分析随机度S和孔隙率ε对相对有效导热系数k*的影响。结果表明,随机度和孔隙率同时影响多孔泡沫材料的有效导热系数,当随机度S一定时,随着孔隙率ε增大,材料的有效导热系数k*减小;当孔隙率ε一定时,随着随机度S的增大,有效导热系数k*减小。根据大样本的有限元数值模拟结果,拟合了有效导热系数由孔隙率和随机度组成的函数表达式。     

倾斜角度对封闭方腔内液体自然对流的影响

在多种倾斜角度下,对封闭腔内液体进行了数值模拟。重点分析了Ra数变化和倾斜角度对对流传热的影响。研究表明：随着Ra数的增大,换热由热传导为主转化为对流换热为主;倾斜角度对Nu数的变化影响很大,并且当倾斜角度处于90o～135o时,Nu数可以达到最大值。     

低温管路管外结霜过程的数值模拟研究

采用多孔介质法,构建了低温圆管壁面结霜的非稳态数学模型,对低温圆管壁面霜层的生长过程进行了数值模拟,研究了结霜过程的非稳态传热特性,并对壁面温度、管径以及相对湿度、气流速度等影响因素进行了分析,获得了霜层厚度、换热热流密度与霜层表面温度与各影响参数之间的依赖关系。结果表明:霜层厚度与密度的预测值与文献实验数据吻合良好;对流换热在结霜过程中起主导作用,总的热流密度随着霜层厚度的增加而减小;圆管壁面温度越低,圆管直径越大,相对湿度越高,霜层厚度越大;与强制对流条件相比,自然对流条件下形成的霜层更厚,表面温度更低。     

对流换热边界条件的多孔材料主动散热性能

通过推导2种不同换热边界条件下平板夹层多孔材料的散热指标, 研究了考虑对流换热因素的平板夹层多孔材料主动散热性能, 得到了影响材料散热性能的因素。分析了在确定的相对厚度下, 不同构型多孔材料的相对密度与散热指标的关系, 并得出正六边形构型的散热指数最大。随着相对厚度的增大, 最大散热指标和最优相对密度增大较快, 当相对厚度大于20时, 最大散热指标和最优相对密度变化较小并最终趋于定值。由上述结果可以得到相对应的最小质量, 随着最小质量的增大, 最大散热指标增大并最终趋于定值。在相同的最大散热指标下, 随着表面换热系数比值的增大, 最小质量逐渐减小。最后考虑承载因素对结构进行了优化分析, 正六边形构型的多孔材料具有明显的综合性能优势。      

粉末冶金法制备多孔金属钛免疫隔离材料

采用粉末冶金工艺制备出微米孔径且孔隙率较高的多孔钛免疫隔离材料，分析了压制压力、烧结温度等因素对多孔金属钛孔径及孔隙率的影响。试验结果表明，随着压制压力的提高，钛粉末烧结后平均孔径尺寸和孔隙率逐渐减小，孔隙分布和孔径尺寸较均一。烧结温度对多孔金属钛影响显著，随烧结温度的升高，平均孔径尺寸逐渐增大，孔隙率先略有上升后逐渐降低。     

泡沫材料冰蓄冷板融化过程的研究

建立了填充泡沫材料冰蓄冷板内冰融化过程的数学物理模型,该模型考虑了融化液态水自然对流的影响。分别数值模拟了填充开孔聚氨酯泡沫、泡沫铜的冰蓄冷板的融化过程,研究了泡沫材料冰蓄冷板融化过程的速率、温度分布、相界面移动等规律。进行了实验对比,验证分析了泡沫材料的孔隙参数对融化速率的影响。结果表明,填充低导热系数泡沫材料可有效延长冰蓄冷板的释冷时间,该时间随泡沫孔密度的减小而增加、随孔隙率的增大而略减。填充高导热系数泡沫材料可有效改善冰蓄冷板温度分布,可加快冰融化速率,该速率随着泡沫孔隙率的减少而增加、随孔密度的减少而略增。     

新型通孔泡沫铝的传热特性

用预制块法制成结构可控的通孔泡沫铝。在基本无对流条件下，由于高孔率孔隙中存在导热系数低的空气而具有好的隔热性能，这与孔隙率有关；在有对流的条件下，由于高比表面及相互连通的孔隙而使它具有良好的换热性能，则与孔隙结构及对流条件有关。     

The Simplified Analytical Models for Evaluating the Heat Transfer Performance of High-Porosity Metal Foams

The accurate evaluation of the specific surface area and the effective thermal conductivity (ETC) of high-porosity metal foams is an important prerequisite to study the mechanism of heat transfer enhancement. In this paper, the tetrakaidecahedron, concave triangular prism and equivalent tetrahedron were used to develop the geometry shapes of cell, ligament and node, respectively. The calculation model of the specific surface area characterized by porosity and pore density (PPI) was deduced by considering the shape characteristics. Based on the angle between the two-dimensional plane skeleton layer and the heat flow direction, the ETC analytical calculation model characterized by only the porosity in the three-dimensional space was deduced. Without any other fitting or experimental empirical parameters, these two models are only related to porosity and PPI, which are the most readily available parameters for the metal foams. The results of these models are consistent with the experimental and empirical data in other literature, indicating that these models are both versatile and accurate.     

Heat transfer from Ni–W tapes in liquid nitrogen at different orientations in the field of gravity

Ni–W tapes of the micrometric thickness are considered as the basis for the cost-effective manufacturing of coated conductors – the 2nd generation of high-temperature superconductor (HTS). Many HTS applications involve widely-available and inexpensive liquid nitrogen. The transition from superconducting to normal state may however occurs due to unexpected temperature fluctuations. In this case Ni–W tape is significantly heated by electrical current propagating through it. The amount of heat transferred from the tape to coolant is defined by heat transfer from the surface of tape to liquid nitrogen. The heat transfer, in turn, is strongly dependent on the tape orientation in the field of gravity. The present paper reports the experimental results on the heat transfer from Ni–W tape to a pool of liquid nitrogen. The heat transfer coefficients are quantified for three subsequent heat transfer regimes: natural convection of liquid nitrogen, nucleate boiling regime and film boiling. The dependence of heat transfer coefficient on inclination angle of the tape from vertical are experimentally clarified for each regime. The expression for the heat transfer coefficient at different inclination angles is derived for the case of nucleate boiling.     

单面波浪平板脉动热管的传热性能

对一种单面波浪平板脉动热管的传热性能进行了实验研究,分析在空气强制对流冷却条件下充液率、加热功率、倾角等因素对其传热性能的影响。研究结果表明,除0°倾角外,脉动热管的最佳充液率为20%～30%,倾斜角度对脉动热管传热性能的影响很小,但90°时相对最好。脉动热管在0°放置时其传热性能较差,在低充液率的情况下甚至丧失脉动效果,主要是工质回流不畅的原因,与平板脉动热管的槽道设计有很大关系。此外低加热功率时热管传热性能存在波动,有时甚至不能启动。     

多孔泡沫材料强化传热特性及场协同分析

实际生产生活中使用到的多孔泡沫材料通常都是非均质的,文章建立了多孔泡沫材料均质与非均质模型,结合场协同理论,从速度与温度梯度矢量的协同关系出发,分析了多孔泡沫材料内部单相流体对流强化换热的物理机制,研究了孔隙率、孔密度以及空气流速对流体顺流方向协同性能的影响。研究表明:场协同原理适用于分析多孔泡沫材料的强化传热机制;多孔泡沫材料孔隙中心与骨架后缘处的协同程度最好,骨架侧缘协同程度最差(协同角接近90°);非均质多孔泡沫材料孔壁附近协同程度较差,相同条件下全场平均场协同角比均质泡沫大;多孔泡沫材料越均匀全场协同情况越好,在相同流速、孔隙率和孔密度下,均质泡沫材料全场平均协同角余弦值可达非均质泡沫的1.2倍。计算结果表明,空气流速为3m/s时,孔隙率为0.8、0.85和0.9的多孔泡沫材料强化传热强度分别是普通平直翅片的3.3倍、1.9倍和1.2倍。该研究对新型散热器设计具有指导意义。     

Effect of a magnetic field on natural convection in an inclined half-annulus enclosure filled with Cu–water nanofluid using CVFEM

In this paper, the effect of a magnetic field on natural convection in a half-annulus enclosure with one wall under constant heat flux using control volume based finite element method. The fluid in the enclosure is a water-based nanofluid containing Cu nanoparticles. The effective thermal conductivity and viscosity of nanofluid are calculated using the Maxwell–Garnetts (MG) and Brinkman models, respectively. Numerical simulations were performed for different governing parameters namely the Hartmann number, Rayleigh number and inclination angle of enclosure. The results indicate that Hartmann number and the inclination angle of the enclosure can be control parameters at different Rayleigh number. In presence of magnetic field velocity field retarded and hence convection and Nusselt number decreases.     

Constructal multi-scale structure for maximal heat transfer density

Summary.  This paper presents a new concept for generating the multi-scale structure of a finite-size flow system that has maximum heat transfer density–maximum heat transfer rate installed in a fixed volume. Laminar forced convection and parallel isothermal blades fill the volume. The spacings between adjacent blades of progressively smaller scales are optimized based on constructal theory: the goal is maximum heat transfer density. The smaller blades are installed in the fresh-fluid regions that sandwich the tips of the boundary layers of longer blades. The overall pressure difference is constrained. As the number of length scales increases, the flow rate decreases and the volume averaged heat transfer density increases. There exists a smallest (cutoff) length scale below which heat transfer surfaces are no longer lined by distinct (slender) boundary layers. Multi-scale flow structures for maximum heat transfer rate density can be developed in an analogous fashion for natural convection. The constructal multi-scale algorithms are deduced from principles, unlike in fractal geometry where algorithms are assumed. Received January 3, 2003 Published online: June 12, 2003     

Aiding mixed convection through a vertical anisotropic porous channel with oblique principal axes

In this paper, an analytical study is carried out on the mixed convection in parallel-plate vertical porous channels with an anisotropic permeability whose principal axes are oriented in a direction which is oblique to the gravity vector. The channel walls are assumed to be isothermal and the flow fully developed at the entrance is upward, so that natural convection aids the forced flow. In the formulation of the problem, use is made of the generalized Brinkman-extended Darcy model which allows the no-slip boundary condition on solid wall, to be satisfied. The flow reversal and the limiting cases of low and high porosity media for natural and forced convection are considered. The effects of varying the anisotropic permeability ratio and the orientation angle of the principal axes on the flow and the heat transfer are investigated.     

Numerical Modelling of Flow Boiling Heat Transfer in Horizontal Metal‐Foam Tubes

The flow boiling heat transfer performance in horizontal metal‐foam tubes is numerically investigated based on the flow pattern map retrieved from experimental investigations. The flow pattern and velocity profile are generally governed by vapour quality and mass flow rate of the fluid. The porous media non‐equilibrium heat transfer model is employed for modelling both vapour and liquid phase zones. The modelling predictions have been compared with experimental results. The effects of metal‐foam morphological parameters, heat flux and mass flux on heat transfer have been examined. The numerical predictions show that the overall heat transfer coefficient of the metal‐foam filled tube increases with the relative density (1‐porosity), pore density (ppi), mass and heat flux.