A numerical analysis on flow field and heat transfer by interaction between a pair of vortices in rectangular channel flow

This paper is a numerical study of the effect of flow field and heat transfer created by interactions between a pair of vortices generated by a vortex generator in a rectangular channel flow. In order to analyze the vortices produced by the vortex generator, the pseudo-compressibility method is introduced into the Navier–Strokes (NS) equation of a three-dimensional unsteady, incompressible viscous flow. A two-layer k–ε turbulence model is used on the flat plate three-dimensional turbulence boundary to predict the turbulence characteristics of the vortices. The computational results accurately predict the vortex characteristics, which are related to Reynolds stress, turbulent kinetic energy, and flow field. Also, in the prediction of thermal boundary layers, skin friction characteristics, and heat transfers, the present results are reasonably close to the experimental results obtained by other researchers.     

气冷涡轮叶片传统热分析中热边界条件的影响

以MarkII气冷涡轮叶片为研究对象,考察了换热系数分布对叶片外表面温度分布的影响。首先分别以温度和换热系数实验值作为热边界条件验证了固体导热计算方法和CFD程序的有效性,然后分析了6种典型热边界条件下的叶片外表面温度分布,并与实验值进行对比。由分析结果可知:叶片外表面温度与换热系数紧密关联,经验关联式(层流、湍流)条件下预测得到的温度偏差较大,当换热系数由能考虑边界层转捩影响的CFD计算得到时,预测得到的温度与实验值较为接近。     

三维边界层内诱导横流失稳模态的感受性机理

边界层感受性问题是层流向湍流转捩的初始阶段,在转捩过程中起关键性作用,尤其是三维边界层流动.因此,研究三维边界层感受性问题对进一步理解层流向湍流转捩机理以及湍流成因具有重要的理论意义.采用数值方法研究自由来流湍流与三维壁面局部粗糙相互作用下三维边界层的感受性问题,确定是否能在三维边界层内寻找一种新的横流失稳模态;确定在何种条件下三维边界层内能诱导出定常、非定常的横流失稳模态;探索自由来流湍流的强度、展向波数和法向波数以及三维壁面局部粗糙的大小和结构类型等因素在自由来流湍流与三维壁面局部粗糙作用下三维边界层内被激发出的感受性过程中有何影响,并确定何种横流失稳模态在三维边界层感受性过程中占据何种地位.对自由来流湍流与三维壁面局部粗糙作用激发三维边界层内感受性问题的深入研究,将有助于完善流动稳定性与湍流理论,为层流向湍流转捩过程的预测与控制提供合理的理论依据.     

Experimental Investigation of the Effect of Multiple Synthetic Jets on Heat Transfer and Pressure Loss in Minichannels

Synthetic jets can enhance thermal performance by creating wall impingement, disrupting boundary layers, and enhancing mixing within a minichannel. This study investigates using multiple synthetic jets along the length of minichannels with an aqueous–glycol mixture. Using synthetic jets, an average heat transfer coefficient increase up to 81% was demonstrated. It was also shown that pressure loss can be reduced by 78% while maintaining equivalent heat transfer performance. The effects of momentum ratio, frequency, and bulk Graetz number on heat transfer and pressure loss is presented along with an empirical correlation that predicts the heat transfer enhancement.     

A theoretical study on the unsteady aerothermodynamics for attached flow models

The principle of the unsteady aerothermodynamics was theoretically investigated for the attached flow. Firstly, two simplified models with analytic solutions to the N-S equations were selected for the research, namely the compressible unsteady flows on the infinite flat plate with both time-varying wall velocity and time-varying wall temperature boundary conditions. The unsteady temperature field and the unsteady wall heat flux (heat flow) were analytically solved for the second model. Then, the interaction characteristic of the unsteady temperature field and the unsteady velocity field in the simplified models and the effects of the interaction on the transient wall heat transfer were studied by these two analytic solutions. The unsteady heat flux, which is governed by the energy equation, is directly related to the unsteady compression work and viscous dissipation which originates from the velocity field governed by the momentum equation. The main parameters and their roles in how the unsteady velocity field affects the unsteady heat flux were discussed for the simplified models. Lastly, the similarity criteria of the unsteady aerothermodynamics were derived based on the compressible boundary layer equations. Along with the Strouhal number Stu, the unsteadiness criterion of the velocity field, StT number, the unsteadiness criterion of the temperature field was proposed for the first time. Different from the traditional method used in unsteady aerodynamics which measures the flow unsteadiness only by the Stu number, present results show that the flow unsteadiness in unsteady aerothermodynamics should be comprehensively estimated by comparing the relative magnitudes of the temperature field unsteadiness criterion StT number with the coefficients of other terms in the dimensionless energy equation.     

封闭圆内开缝圆自然对流换热的振荡特性

本文通过数值计算探讨了封闭圆内开缝圆自然对流换热的振荡特性。数值计算以整个圆为计算区域,采用了非稳态的数学模型和具有QUICK差分格式的SIMPLE算法。在相同条件下计算结果和实验结果符合很好。数值结果显示, 当几何结构一定时,Rayleigh数Ra小于某个临界值时,流动和换热处于稳态,并且关于垂直中心线对称;Ra大于这个临界值时,流动和换热是振荡的,非对称的。数值实验还表明,流动和换热出现振荡时的临界Rayleigh数Rac与开缝圆的开缝度有关,且流动和换热的振荡会出现对称振荡和非对称振荡两种情形。     

对墙体不稳定传热反应系数法的应用研究

本文根据我国建筑结构的特点,从传热的角度考虑,将墙体结构按重量分成重、中、轻三类,用墙体非稳态传热理论和计算机模拟计算,对方程B(s)=0的根ai对反应系数的影响以及反应系数项数对传热计算的影响进行了分析,提出了在不同墙型条件下、传热反应系数项数的确定值,进一步完善了传热反应系数法。     

ENHANCEMENT OF FORCED CONVECTIVE HEAT TRANSFER IN NARROW CONCENTRIC ANNULI BY TURBULENCE PROMOTERS

Abstract

Forced convective heat transfer in a narrow concentric annulus was enhanced by turbulence promoters to improve the heat removal from a high-temperature gas-cooled reactor, a gas-cooled fusion reactor, and other narrow flow passages. The present experiments, which differed from those performed in conventional research, were carried out to examine the effect of turbulence promoters on the inner insulated wall opposite the outer smooth heated wall. This was achieved by changing the ratio of the pitch and the height P/ε, the ratio of the height and the space ε/ε¹, and the type of turbulence promoters used. Experimental results were examined for the local heat transfer coefficient distribution on the smooth outer tube, the average heat transfer coefficient, the friction factor, and the thermal performance. Five kinds of evaluations for thermal performance were carried out.P24     

交叉三角形波纹板流道在过渡流状态下的传热与阻力特性

交叉三角形波纹板流道中流动常处于过渡态。本文研究了交叉波纹板中的周期性完全发展流动及热传递。利用周期性降低几何流道的复杂性以及简化模拟对象。为了模拟这个拓扑结构中的过渡流,利用了已经验证的低雷诺数k-ε,湍流模型来说明流动中的湍流流动。得到了三维复杂计算区的温度、速度以及湍流场。计算了在恒壁温和恒热流密度两种边界条件下的摩擦系数和平均Nusselt数及其与雷诺数的关系。湍流中心从上层壁面的波纹处移向下层壁面的波纹处并逐步增强。     

Heat transfer in tubes with periodic draft under supercritical conditions

The temperature mode of a wall was studied experimentally for sub-and supercritical pressures of water in tubes with turbulence stimulators; these data were compared with similar results obtained for a smooth tube. An increase in heat transfer inside a turbulizing tube was revealed for a single-and two-phase water flows. Correlation dependencies for determination of heat transfer coefficient in a single-phase flow inside a tube with turbulence stimulators were obtained.     

Identification du coefficient de transfert thermique sur la surface externe d'un échangeur de chaleur à tubes et ailettes planes en fonction de l'humidité de l'air,en l'absence de condensation

The following study, which is rather oriented towards experimentation, shows the influence of the humidity content of air on heat transfer. This first article concerns heat transfer between the external fluid (moist air) and the internal fluid (water containing glycol, whose thermal behavior inside circular tubes is well-known) in a heat exchanger of the same type as those used in automotive air conditioning (horizontal copper tubes and plane aluminium fins), in the absence of condensation. The most difficult part of this experimental work is the measurement and control of the air humidity, since one has to make sure that the measurement incertainties are not significant compared to the precision of the calculation of the heat transfer coefficient. The conclusion is that, for this type of exchanger, the heat transfer coefficient decreases with air humidity in the absence of condensation (dry wall). Some correlations have been developed with respect to the relative air humidity. An analog experimental investigation, but this time carried out in the presence of condensation (partially or completely wetted wall), is about to be completed; the obtained results will be communicated later on.     

Heat and mass transfer on unsteady three-dimensional convective MHD flow past a porous plate moving in a parallel free stream

The unsteady three-dimensional convective flow of a viscous incompressible, electrically conducting fluid over a vertical, insulated, porous surface moving in a parallel free stream has been investigated, which flows belong to a separate class of problem of boundary layer theory. The effects of heat and mass transfer on this unsteady laminar flow past porous surface with transverse sinusoidal suction and oscillatory wall temperature have been analysed and discussed.     

Advanced Study of Unsteady Heat and Chemical Reaction with Ramped Wall and Slip Effect on a Viscous Fluid

This paper investigate the effect of slip boundary condition, thermal radiation, heat source, Dufour number,chemical reaction and viscous dissipation on heat and mass transfer of unsteady free convective MHD flow of a viscous fluid past through a vertical plate embedded in a porous media. Numerical results are obtained for solving the nonlinear governing momentum, energy and concentration equations with slip boundary condition, ramped wall temperature and ramped wall concentration on the surface of the vertical plate. The influence of emerging parameters on velocity,temperature and concentration fields are shown graphically.     

超临界R134a水平圆管内冷却换热模拟研究

采用增强壁面函数的标准k-ε模型对超临界R134a水平圆管内冷却换热进行了模拟研究.分析了管内不同截面上流体温度、速度和湍动能的分布情况及对应关系。研究了质量流量和浮升力对换热系数的影响。结果表明,流体速度随着温度的降低而减小,并且最大速度处对应着最高温度和最小湍动能.换热系数随着质量流量的增加而增大,其峰值出现在准临界温度附近。浮升力在似液体区的影响较大,对流体换热起到增强的效果。     

前缘曲率对三维边界层内被激发出非定常横流模态的影响研究

三维边界层感受性问题是三维边界层层流向湍流转捩的初始阶段,是实现三维边界层转捩预测与控制的关键环节.在高湍流度的环境下,非定常横流模态的失稳是导致三维边界层流动转捩的主要原因;但是,前缘曲率对三维边界层感受性机制作用的研究也是十分重要的课题之一.因此,本文采用直接数值模拟方法研究在自由来流湍流作用下具有不同椭圆形前缘三维（后掠翼平板）边界层内被激发出非定常横流模态的感受性机制;揭示不同椭圆形前缘曲率对三维边界层内被激发出非定常横流模态的扰动波波包传播速度、传播方向、分布规律、感受性系数以及分别提取获得一组扰动波的幅值、色散关系和增长率等关键因素的影响;建立在不同椭圆形前缘曲率情况下,三维边界层内被激发出非定常横流模态的感受性问题与自由来流湍流的强度和运动方向变化之间的内在联系;详细分析了不同强度各向异性的自由来流湍流在激发三维边界层感受性机制的物理过程中起着何种作用等.通过上述研究将有益于拓展和完善流动稳定性理论,为三维边界层内层流向湍流转捩的预测与控制提供依据.     

粒状物料常压吸附流化床冷冻干燥的传热研究

以粒状马铃薯为研究对象,在常压吸附流化床冷冻干燥的实验基础上,建立了吸附流化床内对流边界条件下的球坐标冻干模型,并采用变时间步长的有限差分法,求解了物料内部温度分布及升华界面的移动速率,与实验结果吻合较好,分析了有关因素对于冻干过程中传热及干燥过程的影响。     

螺旋折流片强化壳侧传热的四管模型数值模拟

针对螺旋折流片管壳式换热器的正方形布管方式,建立了相间套螺旋折流片的四管数学物理模型,利用FLUENT软件对该模型的流动与传热情况进行了数值模拟;并与光滑通道中及单管螺旋折流片模型的流动和传热结果进行了对比.结果显示旋向相反的相邻螺旋折流片所诱导的两股旋流通过相互作用可提高通道内流体流速,并有效地形成对相邻传热管外的斜向冲刷,这对于减薄边界层,促进近壁流体与主流区流体的动量和质量交换进而强化传热有明显的作用,算例显示其传热系数可比相同尺寸的光管通道中的情形提高约44%～57%.     

纳米流体对流换热系数增大机理

纳米流体流动换热能力优于传统流体介质.研究了纳米流体热物性的提升和热散射对其对流换热系数的影响.结果表明,纳米颗粒的加入,优化了介质的热物性,增大了导热系数,强化了纳米流体内颗粒、流体以及流道管壁碰撞和相互作用,同时加强了流体的混合脉动和湍流,从而增大了对流换热系数. 关键词： 纳米流体 换热系数 热散射     

低温试验冷库传热系数的测量研究

简述了对于低温冷库传热系数测量所采用的一般方法,采用稳态法对一低温试验冷库进行了测量实验,然后通过非稳态积分法对试验数据进行验证。针对冷风机电机功率较大的特点,本实验采用变频器将电机功率调小,从而成功得出该低温试验冷库的传热系数。并与理论计算的数据进行对比,找出存在差别的原因。     

Prediction of Liner Metal Temperature of an Aeroengine Combustor with Multi-Physics Scale-Resolving CFD

Computational Fluid Dynamics is a fundamental tool to simulate the flow field and the multi-physics nature of the phenomena involved in gas turbine combustors, supporting their design since the very preliminary phases. Standard steady state RANS turbulence models provide a reasonable prediction, despite some well-known limitations in reproducing the turbulent mixing in highly unsteady flows. Their affordable cost is ideal in the preliminary design steps, whereas, in the detailed phase of the design process, turbulence scale-resolving methods (such as LES or similar approaches) can be preferred to significantly improve the accuracy. Despite that, in dealing with multi-physics and multi-scale problems, as for Conjugate Heat Transfer (CHT) in presence of radiation, transient approaches are not always affordable and appropriate numerical treatments are necessary to properly account for the huge range of characteristics scales in space and time that occur when turbulence is resolved and heat conduction is simulated contextually. The present work describes an innovative methodology to perform CHT simulations accounting for multi-physics and multi-scale problems. Such methodology, named U-THERM3D, is applied for the metal temperature prediction of an annular aeroengine lean burn combustor. The theoretical formulations of the tool are described, together with its numerical implementation in the commercial CFD code ANSYS Fluent. The proposed approach is based on a time de-synchronization of the involved time dependent physics permitting to significantly speed up the calculation with respect to fully coupled strategy, preserving at the same time the effect of unsteady heat transfer on the final time averaged predicted metal temperature. The results of some preliminary assessment tests of its consistency and accuracy are reported before showing its exploitation on the real combustor. The results are compared against steady-state calculations and experimental data obtained by full annular tests at real scale conditions. The work confirms the importance of high-fidelity CFD approaches for the aerothermal prediction of liner metal temperature.