Forced convection heat transfer around large two-phase bubbles condensing in an immiscible liquid

In this investigation, an equation is theoretically developed to predict the Nusselt number and hence the collapse rate of large spherical two-phase bubbles condensing in quiescent immiscible liquid. Heat transfer from the thin film of condensate and in the wake is determined. Theoretical prediction and experimental data show satisfactory agreement. A parametric study is also carried out for different parameters affecting the collapse rate.     

MHD mixed convection flow and heat transfer in a porous medium

The effects of a steady two-dimensional laminar MHD mixed convection flow and heat transfer against a heated vertical semi-infinite permeable surface in a porous medium are discussed. The coupled nonlinear partial differential equations describing the conservation of mass, momentum, and energy are solved by a perturbation technique. The results are presented to illustrate the influence of Hartmann number (M), Prandtl number (Pr), permeability parameter (K _p ), suction/blowing parameter (f _w ), heat generation/absorption coefficient (?), and mixed convection or buoyancy parameter (γ). The effects of different parameters on the velocity and temperature as well as the skin friction and wall heat transfer are discussed with the help of figures.     

Forced convection heat transfer from porous heat sinks placed in partially open cavity: Some case studies

In this present study, the forced convection heat transfer from aluminum foam heat sinks with 10, 20, 40 PPI pore density placed in a discrete form in a partially open cavity were experimentally investigated. Air was used as working fluid. The uniform heat flux was applied to 3 × 3 array of foam heat sinks horizontally mounted in the cavity. The experimental studies were performed for the 3363–9743 range of Reynolds number and the 2.7 x 10⁶ and 7.5 x 10⁶ range of modified Grashof number. The effects of the Reynolds number, the modified Grashof number and the pore density of foam heat sink on the heat transfer were investigated. The results obtained were compared with the results obtained without foam heat sink cases. In addition, the most heated elements within the cavity were identified and solution proposals were presented. In addition, the most heated elements within the cavity were identified and solution proposals were presented.     

Visualization of transient natural convection heat transfer from a vertical rectangular fin

Experiments on thermal visualization of transient natural convection from short vertical rectangular fins were conducted using the technique of laser holographic interferometry. A sequence of infinite-fringe interferograms recorded for the heating regime of an aluminum fin demonstrates the effect of fin base heating on local convection coefficients and reveals alternating and oscillatory buoyancydriven flows similar to those over the top surface of heated horizontal plates. The effect of fin base heating results in greater uniformity of the local heat transfer coefficient along the fin. It also significantly reduces the steady-state heat transfer coefficients of short vertical fins compared to their transient values. Hence, the use of steady-state solutions for the design of short vertical fins operating in transient conditions may not introduce as much error as was previously thought.     

Soret and Dufour effects on MHD non-Darcian mixed convection heat and mass transfer over a stretching sheet with non-uniform heat source/sink

An analysis has been carried out to study the effects of thermal-diffusion and diffusion-thermo on non-Darcian mixed convection heat and mass transfer of an incompressible, electrically conducting fluid over a stretching sheet embedded in a porous medium in the presence of an external magnetic field and non-uniform heat source/sink. Similarity transformations are used to convert highly non-linear partial differential equations into ordinary differential equations. Similarity equations are then solved numerically using shooting algorithm with Runge-Kutta-Fehlberg scheme over the entire range of physical parameters. The effects of various physical parameters on the dimensionless velocity, temperature and concentration profiles are depicted graphically. Present results are compared with previously published work on various special cases of the problem and the results are found to be in very good agreement. Numerical results for local skin-friction, local Nusselt number and local Sherwood number are tabulated for different physical parameters.     

Natural and forced convective heat transfer on slender cylinders

This paper presents an experimental study of free and forced convective heat transfer along vertical slender cylinders. The local heat transfer coefficient is determined from the measurement of the surface temperature distribution performed by quantitative infrared thermography. It is found that the convective heat transfer is strongly dependent on the cylinder curvature and misalignment with the flow. The effect of proximity of two cylinders is emphasized in the case of forced convection. Correlations are proposed for the two types of convection. It is worth noting that circumstances exist where the turbulent heat transfer in free convection can be of the same order of magnitude as for laminar forced convection. The outcome of the study demonstrates the suitability of quantitative infrared thermography to solve complex problems and to provide a deeper understanding of the heat transfer on slender cylinders.     

Mathematical modelling of heat and mass transfer under conditions of mixed convection in rectangular region with heat source and heat-conducting walls

Numerical modelling of a conjugate convective-conduction heat transfer in a rectangular region with a heat-release source was carried out in the presence of forced flow and mass exchange. The distributions of thermal and hydrodynamic characteristics, which describe the specific peculiarities of flow regimes under study, were obtained. The mutual influence of forced and free-convective flow was analysed. The scales of the effect of determining dimensionless complexes (Gr, Br, Re) on flow regimes were established. The evolution of analysed process was shown.     

毛细管内液氮的自然对流换热数值计算分析

主要探讨了毛细管管径以及倾角对其内的加热丝与液氮的换热效果的影响。应用FLUENT软件对0、30、60、90倾角下管径为1.2mm和2.0mm的毛细管内的加热丝与液氮的换热情况进行了三维数值模拟,得到了管内液氮的速度、温度以及加热丝的温度分布情况。数值计算结果与实验结果吻合的比较好。计算结果表明倾角为30°和60°的换热效果最好,大管径下的换热情况要比小管径的换热效果好。     

Microscale heat transfer in a free jet against a plane surface

A new two-layer model has been proposed to study microscale heat transfer associated with a developing flow boundary layer. As an example, a cold, microscale film of liquid impinging on an isothermal hot, horizontal surface has been investigated. The boundary layer is divided into two regions: a micro layer at microscale away from the surface and a macro layer at macroscale away from the surface. An approximate solution for the velocity and temperature distributions in the flow along the horizontal surface is developed, which exploits the hydrodynamic similarity solution for microscale film flow. The approximate solution may provide a valuable basis for assessing microscale flow and heat transfer in more complex settings.     

Forced transverse vibrations of a submerged tensioned string

For a submerged cable, the classical equation of a vibrating string must be modified to account for added mass and damping effects. This can be done with the aid of Morison's formulae. The resulting equation is non-linear. It is solved for the case when one end of the string is forced into sinusoidal transverse motion. The first harmonic approximations for both resonant and non-resonant responses are obtained by the method of matched asymptotic expansions under the assumption that the non-dimensional damping coefficient is small.     

纳米流体对流换热机理分析

考虑在纳米流体中纳米颗粒做布朗运动引起的对流换热, 基于纳米颗粒在纳米流体中遵循分形分布, 本文得到纳米流体对流换热的机理模型. 本解析模型没有增加新的经验常数, 从该模型发现纳米流体池沸腾热流密度是温度、纳米颗粒的平均直径、 纳米颗粒的浓度、纳米颗粒的分形维数、沸腾表面活化穴的分形维数、基本液体的物理特性的函数. 对不同的纳米颗粒浓度和不同的纳米颗粒平均直径与不同的实验数据进行了比较, 模型预测的结果与实验结果相吻合. 所得的解析模型可以更深刻地揭示纳米流体对流换热的物理机理.     

射流冲击泡沫金属流动液膜传热的实验研究

薄液膜蒸发由于其优良的传热特性而被广泛应用于工业领域。在流动液膜上表面覆盖铜质泡沫金属,并耦合空气射流冲击,能够进一步强化传热。多孔泡沫金属提供的毛细驱动力能够有效控制流动液膜的厚度以避免干涸,同时多孔材料特殊的固体骨架构造可以扩大固液、气液传热面积。为了研究射流冲击条件下多孔介质覆盖流动液膜的传热特性,本文通过实验方法,对包括液膜流速Vf、空气射流速度Va、液膜厚度δf和多孔介质孔隙率ε在内的影响因素进行分析,研究并对比这些因素对加热壁面温度Tw、表面传热系数hw以及传热系数提升率的影响。     

竖直恒温面自然对流边界层不稳定性研究

在无扰动、随机式扰动以及正弦式扰动下,通过对竖直恒温面处状态Ra为1.328×10^9、Pr为6.24的自然对流进行模拟,探索了热边界层的不稳定性和共振强化自然对流换热。结果表明:(1)竖直自然对流边界层上游位置的随机式扰动对热边界层的影响主要体现在稳定阶段;(2)该状态下的竖直自然对流边界层的特征频率为15 067,且相比于无扰动状态,频率为15 067的正弦式扰动能在竖直恒温面处提高5.15%的换热量;(3)在竖直自然对流边界层上游位置加入特征频率的正弦式扰动,竖直恒温面处的局部努塞尔数Nu均出现明显波动,且波动随着边界层高度的增加而增大。     

Targeted heat transfer augmentation in circular tubes using a corona jet

Natural convection heat transfer can be noticeably enhanced by corona wind in tubes and channels. A corona-induced secondary flow may be generated in tubes with no major changes in the geometry, or causing any noise or vibration. In this investigation, it is shown that the eccentric configuration of a wire electrode inside a tube forms a local jet along the eccentricity direction, which impinges on the tube wall and improves the local heat transfer. Since the direction of the corona jet is determined by the eccentricity direction of the electrode, the jet may be oriented properly to target the desired spots.     

Effects of chemical reaction on combined heat and mass transfer by laminar mixed convection flow from vertical surface with induced magnetic field and radiation

The elevated temperature electromagnetic materials production system in chemical engineering requires increasingly more refined theoretical and computational models for describing multiple, simultaneous thermophysical effects. Motivated by this application, the present paper addresses heat and mass transfer in a chemically reacting laminar mixed convection flow from a vertical sheet with inducedmagnetic field. The governing equations of the flow are solved analytically using a perturbation technique. The influences of various established parameters on the flow, induced magnetic field, and heat and mass transfer are studied graphically in the present analysis. Finally, we also obtained expressions for shear stress, current density and Nusselt number, and discussed the results through tables.     

微槽道热管阵列散热器的传热性能实验研究

本文介绍了平板微热管阵列散热的工作原理,制作了U型微槽道热管阵列散热器,通过与无微槽道热管阵列散热器传热性能的实验对比,在平板微热管阵列、与水平面夹角为45°的微槽道热管阵列和U型微槽道热管阵列热通量及均温性的测试基础上,得到在蒸发段温度≤58℃时,U型平板微槽道热管具有良好的传热性能,在热流密度≥72 W/cm~2的情况下,弯折角度越小,传热性能越好。实验表明:U型微槽道平板热管阵列具有良好的均温性、热响应性及传热性能;微槽道能强化热管阵列的传热效果;在一定热流密度下,弯折角度越小,传热性能越好。     

封闭腔内纳米流体强化自然对流换热的数值模拟

采用F luen软件对封闭腔内Cu-H2O纳米流体强化自然对流换热进行了数值模拟,重点分析Cu纳米粒子添加量和Gr数对换热性能的影响,并解释其换热机理。研究结果表明:在水基液中加入Cu纳米粒子可以显著提高基液的自然对流换热特性。对于一给定的Gr数,随着纳米粒子质量分数的增加,纳米流体的速度组成部分增加,纳米流体质量分数越大,x方向和y方向的速度峰越大,因此加速了流体中能量传输。另一方面,随着Gr数的增加,流线图中旋涡逐渐变大,流线间强度增加,说明换热效果逐渐增强。当Gr数较小时,传热主要是由热壁和冷壁之间的热传导引起的,随着Gr数的增大,换热逐渐变为由对流换热占主导地位。