Heat and Mass Transfer in MHD Visco-Elastic Fluid Flow through a Porous Medium over a Stretching Sheet with Chemical Reaction

This paper presents the study of convective heat and mass transfer characteristics of an incompressible MHD visco-elastic fluid flow immersed in a porous medium over a stretching sheet with chemical reaction and thermal stratification effects. The resultant governing boundary layer equations are highly non-linear and coupled form of partial differential equations, and they have been solved by using fourth order Runge-Kutta integration scheme with Newton Raphson shooting method. Numerical computations are carried out for the non-dimensional physical parameters. Here a numerical has been carried out to study the effect of different physical parameters such as visco-elasticity, permeability of the porous medium, magnetic field, Grashof number, Schmidt number, heat source parameter and chemical reaction parameter on the flow, heat and mass transfer characteristics.

Effects of Solid Matrix and Porosity of Porous Medium on Heat Transfer of Marangoni Boundary Layer Flow Saturated with Power-Law Nanofluids

The effect of the solid matrix and porosity of the porous medium are first introduced to the study of power-law nanofluids, and the Marangoni boundary layer flow with heat generation is investigated. Two cases of solid matrix of porous medium including glass balls and aluminum foam are considered. The governing partial differential equations are simplified by dimensionless variables and similarity transformations, and are solved numerically by using a shooting method with the fourth-fifth-order Runge-Kutta integration technique. It is indicated that the increase of the porosity leads to the enhancement of heat transfer in the surface of the Marangoni boundary layer flow.

Modeling of Coupled Heat and Mass Transfer in a Trapezoidal Porous Bed on a Grid

We investigate heat and mass transfer in an isosceles trapezoidal cavity, filled with charcoal considered as a granular porous medium. The Darcy-Brinkman-Forchheimer flow model is coupled to the energy and mass equations with the assumption of non-thermal equilibrium. These equations are discretized by the finite volume method with an offset mesh and then solved by the line-by-line method of Thomas. The coupling between pressure and velocity is obtained by Semi-Implicit Method for Pressure Linked Equations. (SIMPLE) algorithm. The results show that the temperature in the cavity increases when the inclination angle of the sides walls decreases. The 15° inclination is selected as being able to offer better thermal performance in the cookstove combustion chamber.

MHD Heat and Mass Transfer of an Oscillatory Flow over a Vertical Permeable Plate in a Porous Medium with Chemical Reaction

The problem of magneto-hydro-dynamic (MHD) mass and heat transfer of an oscillatory fluid in two-dimensional viscous, electrically conducting over an infinite vertical permeable moving plate in a saturated porous medium with the presence of a transverse magnetic field and chemical reaction is analytically presented. The governing equations, momentum, energy, and concentration are solved. Various flow parameters effects on velocity, temperature and concentration fields are discussed. It is found that, the fluid velocity increases with increasing both the permeability and chemical reaction parameters. While, it increases with decreasing the magnetic field parameter. Furthermore, the concentration increases with increasing chemical reaction parameters.

Heat transfer in a porous saturated wavy channel with asymmetric convective boundary conditions

The viscous flow in a wavy channel with convective boundary conditions is investigated. The channel is filled with a porous viscous fluid. Two cases of equal and different external convection coefficients on the walls are taken into account. Effect of viscous dissipation is also considered. The governing equations are derived employing long wavelength and low Reynolds number approximations. Exact closed form solutions are obtained for the simplified equations. Important physical features for peristaltic flow caused by the wavy wave are pumping, trapping and heat transfer rate at the channel walls. These are discussed one by one in depth and detail through graphical illustrations. Special attention has been given to the effects of convective boundary conditions. The results show that for Bi1≠Bi2, there exists a critical value of Brinkman number Brc at which the temperatures of both the walls become equal. And, for Bi1>Bi2 and Br>Brc, the temperature of the cold wall exceeds the temperature of hot wall.

Establishment and Application of UFC-ACC Heat Transfer Coefficient Model

Based on medium plate runout table ultra-fast cooling( UFC)-accelerated cooling equipment( ACC) system,a heat transfer coefficient model was constructed. Firstly,according to the measured data,heat transfer coefficients under different roll speed and water volume were calculated by using an inverse heat conduction method. Secondly,a monofactorial heat transfer coefficient calculation formula was obtained. Finally,the heat transfer coefficient model based on medium plate runout table UFC-ACC system was constructed by intercept function,slope function,interaction influence function and linear or nonlinear influencing factors. The precision of these models was validated by comparing model prediction value with measured data,and the results were in good agreement with practical needs,and the average deviation was less than 5%.

Unsteady Heat Transfer of Viscous Incompressible Boundary Layer Fluid Flow through a Porous Plate with Induced Magnetic Field

Because of the great importance of thermal instability in nature, in chemical processes, in separation processes, in industrial applications as well as in geophysical and astrophysical engineering, the effect of thermal diffusion on the combined MHD heat transfer in an unsteady flow past a continuously moving semi-infinite vertical porous plate which is subjected to constant heat has been investigated numerically under the action of strong applied magnetic field taking into account the induced magnetic field. This study is performed for cooling problem with lighter and heavier particles. Numerical solutions for the velocity field, induced magnetic field as well as temperature distribution are obtained for associated parameters using the explicit finite difference method. The obtained results are also discussed with the help of graphs to observe effects of various parameters on the above mentioned quantities.

Research of Inorganic Heat Transfer Element Starting Proprerty

This article studies the wall temperature distribution of inorganic heat transfer element in different working conditions by experiments, and analyzes the impact of inclination angle, heating power, different kinds of cooling medium and different inlet temperature of cooling medium on the starting property of inorganic heat transfer element.

Joule Heating and Thermal Radiation Effects on MHD Boundary Layer Flow of a Nanofluid over an Exponentially Stretching Sheet in a Porous Medium

A numerical study on boundary layer flow behaviour, heat and mass transfer characteristics of a nanofluid over an exponentially stretching sheet in a porous medium is presented in this paper. The sheet is assumed to be permeable. The governing partial differential equations are transformed into coupled nonlinear ordinary differential equations by using suitable similarity transformations. The transformed equations are then solved numerically using the well known explicit finite difference scheme known as the Keller Box method. A detailed parametric study is performed to access the influence of the physical parameters on longitudinal velocity, temperature and nanoparticle volume fraction profiles as well as the local skin-friction coefficient, local Nusselt number and the local Sherwood number and then, the results are presented in both graphical and tabular forms.

Consideration of transient heat conduction in a semi-infinite medium using homotopy analysis method

In the current work, transient heat conduction in a semi-infinite medium is considered for its many applications in various heat fields. Here, the homotopy analysis method （HAM） is applied to solve this problem and analytical results are compared with those of the exact and integral methods results. The results show that the HAM can give much better approximations than the other approximate methods： Changes in heat fluxes and profiles of temperature are obtained at different times and positions for copper, iron and aluminum.

Peristaltic Transport of Magnetohydrodynamic Carreau Nanofluid with Heat and Mass Transfer inside Asymmetric Channel

In this work, the peristaltic motion of a nano non-Newtonian fluid which obeys Carreau model through a porous medium inside an asymmetric channel is investigated. The hall current effects with Joule heating and viscous dissipation are considered. The problem is modulated mathematically by a set of nonlinear partial differential equations which describe the conservation of mass, momentum, energy and concentration of nanoparticles. The non-dimensional form of these equations is simplified under the assumption of long wavelength and low Reynolds number, and then resulting equations of coupled nonlinear differential equations are tackled numerically with appropriate boundary conditions. Graphical results are presented for dimensionless velocity, temperature, concentration and pressure gradient in order to illustrate the variations of various parameters of this problem on these obtained solutions.

基于蒙特卡洛法的喷管中强非均匀性介质辐射换热问题求解加速策略

固体火箭发动机喷管内的辐射换热不可忽视,使用传统蒙特卡洛法模拟效率低下,而现有蒙特卡洛法加速策略研究多局限于辐射物性跨越1个数量级的情况,这与实际喷管中的强非均匀性特点不符。因此,辐射物性跨越2~3个数量级时,以零碰撞蒙特卡洛法为基准,对加权零碰撞蒙特卡洛法和分解蒙特卡洛法两种加速策略进行比较探究,分别考察了灰介质立方体、非灰介质圆柱体算例。结果表明:加权零碰撞蒙特卡洛法在零碰撞衰减系数取全局最大值的60%以上、分解蒙特卡洛法在控制介质辐射物性取全局最大值的3%以内时,二者均可在保证计算精度的前提下缩减计算用时;相较分解蒙特卡洛法,加权零碰撞蒙特卡洛法的加速性能更好,其在非灰算例中的效率提升最高可达14%。此外,还提出了一种改进的轴对称网格定位方法,在几种典型路径中计算用时可缩减43%以上。

中深层地热井地温影响规律及预测方法

中深层地热井初始地层温度、原始地温梯度是影响中深层地热井换热量计算的重要参数,在完井后短时间内所测得的井温并不能真实反映初始地层温度,也不能反映原始地温梯度。为准确地获取初始地层温度、原始地温梯度,通过分析测井数据发现由于钻探过程中钻井液循环致使井筒内部上下温差变小,导致未静井时获取的地温梯度偏低,需要增加静置时间才能保证所测初始地温的准确性和可靠性,但在实际工程中长时间增加钻孔静置时间是难以实现的。结果表明:通过对钻井液循环过程中传热分析可以获得钻柱内钻井液温度与地温梯度的关系式;在建立钻井液与地层瞬态传热解析解模型的基础上,计算出静置较短时间后井内钻井液温度,获得其与完井后所测试的井内钻井液温度的相关系数,可以快速地计算出原始地温梯度、初始地层温度。通过与文献中数据以及测井结果对比,验证了所提出方法的准确性,可为中深层地热井与岩层换热的分析提供准确数据,对中深层地热井换热性能研究具有实际意义。

多孔介质孔隙率对池沸腾传热性能影响机理的模拟研究

采用介观相变格子Boltzmann(lattice Boltzmann, LB)方法,在孔隙尺度下研究了多孔介质的孔隙率对池沸腾换热过程的影响,重点分析了不同孔隙率时气泡的运动过程,并对气泡在多孔介质中的典型状态进行了力平衡分析,进而探究了多孔介质孔隙率影响沸腾传热的机理.结果表明,与无多孔介质的平板表面相比,多孔材料能够有效地降低初始成核的壁面过热度,增强流体的扰动,并且能够显著提升临界热流密度(critical heat flux, CHF)值.在所研究的工况中,孔隙率ε=73.2%时,CHF值提升最大,约为平板的3.6倍,其余孔隙率的多孔介质最小也可将其CHF值提升至平板的2.3倍.研究发现,当孔隙率从97.7%开始逐渐减小时,CHF值逐渐增大,同时沸腾换热曲线向左上方移动,这是因为减小孔隙率能够增大有效换热面积,减小气泡成核的壁面过热度,从而强化沸腾换热.当孔隙率减小到ε=73.2%时,若继续减小孔隙率,热流密度将突然下降,沸腾传热性能显著降低.通过对沸腾过程中气泡的受力进行分析后发现,当孔隙率较小时,过小的孔隙直径显著增大了气泡的逸出阻力,降低了气泡的上升速度,延长了气泡脱离多孔介质的时间,且此时气泡会在蒸发动量力、接触压力以及摩擦力等的共同作用下聚集在加热器上表面,形成气膜,从而恶化沸腾传热.

基于中低温相变材料的管壳式储热单元传热性能数值分析

针对管壳式相变储热单元换热效率低的问题,设计一种带有径向矩形翅片的复合盐管壳式相变储热单元,进行数值模拟并与实验研究对比验证,然后选择适当的性能指标分析模拟结果。采用ANSYS FLUENT进行三维瞬态仿真,改变换热流体入口温度及储热单元翅片间距进行储热过程的数值计算,对相变材料温度变化、传热过程及熔化情况进行对比分析。结果表明:提高换热流体温度和缩小翅片间距是有效强化换热的手段。在本工作中,换热流体与相变材料复合盐CH_(3)COONa·3H_(2)O-KCl相变温差每增加5℃,相变材料熔化速率分别提升54.98%、 34.67%、 23.92%、 18.13%、 14.45%,潜热储热速率分别提升61.56%、 45.79%、 35.15%、27.04%、22.31%,速率提升效果均逐渐减弱。翅片间距每缩短10 mm,相变材料熔化速率分别提升32.37%、41.26%、38.66%,储热量随之减少6.40%、11.95%、6.55%,储能密度降低0.53%、10.97%、1.57%,实际应用中需综合换热能力、成本等方面问题选择合适的翅片间距。本工作可为实际工程中储热单元的设计优化提供理论支持。

Theoretical study on the effective thermal conductivity of silica aerogels based on a cross-aligned and cubic pore model

Aerogel nanoporous materials possess high porosity, high specific surface area, and extremely low density due to their unique nanoscale network structure. Moreover, their effective thermal conductivity is very low, making them a new type of lightweight and highly efficient nanoscale super-insulating material. However, prediction of their effective thermal conductivity is challenging due to their uneven pore size distribution. To investigate the internal heat transfer mechanism of aerogel nanoporous materials, this study constructed a cross-aligned and cubic pore model(CACPM) based on the actual pore arrangement of SiO_(2) aerogel. Based on the established CACPM, the effective thermal conductivity expression for the aerogel was derived by simultaneously considering gas-phase heat conduction, solid-phase heat conduction, and radiative heat transfer. The derived expression was then compared with available experimental data and the Wei structure model. The results indicate that, according to the model established in this study for the derived thermal conductivity formula of silica aerogel, for powdery silica aerogel under the conditions of T = 298 K, a_(2)= 0.85, D_(1)= 90 μm, ρ = 128 kg/m^(3), within the pressure range of 0–10^(5)Pa, the average deviation between the calculated values and experimental values is 10.51%. In the pressure range of 10^(3)–10^(4)Pa, the deviation between calculated values and experimental values is within 4%. Under these conditions, the model has certain reference value in engineering verification. This study also makes a certain contribution to the research of aerogel thermal conductivity heat transfer models and calculation formulae.

一种以熔盐为移热介质的氨合成塔及氨合成工艺

合成氨除用于生产化肥、化工产品,作为脱硝剂和制冷剂使用外,还可作为氢载体用于氢能储存,是未来清洁低碳燃料的重要形式。氨合成反应是一个可逆的放热反应,受反应平衡的限制,原料氢、氮气不能一次全部转化为氨产品,需要通过循环反应以提高产品收率。合成氨节能的关键在于在较低反应压力条件下提高反应单程转化率,现有氨合成塔可实现的单程转化率低、装置运行能耗高。以熔盐为移热介质的变温氨合成塔,可以实现不同床层高度的催化剂均总体处于最佳反应温度下操作。合成塔入口端高反应物浓度时高温,以提高反应速率;合成塔出口端高产物浓度时低温,以提高化学反应平衡转化率。以熔盐为移热介质的变温氨合成塔单程转化率高、节能降耗和经济效益显著,值得工程研究及推广。

从迪拜光热+光伏综合电站项目浅谈温度仪表新应用

迪拜光热+光伏综合电站项目是目前世界上集光能发电之大成,既有光伏发电又集成了光热发电,且光热发电涵盖了目前世界上仅有的2种光热发电形式——槽式、塔式,其是一个成体系的完整光能发电项目,极具代表性。通过对光热发电原理的解读,结合迪拜光热综合电站项目知晓光热发电的测温需求,通过工程案例论述了光热镜场传热介质的测温方案,以及测温技术应用的关键点,对测温技术在光热发电方面的应用有着积极的意义。

导热油和熔盐作为加热媒介的探讨分析

工业生产中加热媒介的选择需要考虑可控性、经济性和安全性,其中导热油和熔盐作为优异的加热媒介,如何选择则需要根据具体工艺条件确定。本文主要从可控性、经济性和安全性三个方面对导热油和熔盐进行对比分析,确定各自作为加热媒介的适用性。

液相导热油供热系统及其应用与设计

导热油供热系统是一种广泛应用于化工生产的供热形式。介绍了导热油的种类和特性以及使用导热油的注意事项,详细阐述了液相导热油供热系统的组成部件,设备及其功能和作用,导热油供热系统的流程。在对国内导热油供热系统供应商及现有运行案例进行了调研的基础上,基于其工作原理,叙述了导热油供热系统的应用操作要点和工程设计要点。