水基磁性流体池沸腾传热强化的实验研究

通过水和水基磁性流体池沸腾传热的对比实验,确定了水基磁性流体强化沸腾传热的效果,并进行了机理分析。实验结果显示,热流密度相同时,水基磁性流体的沸腾换热系数比水至少增强2倍,施加磁场可进一步强化沸腾传热,增强倍数可超过5倍。通过分析磁场对磁性流体中沸腾汽泡的影响,认为施加磁场有使汽泡脱离直径减小,生长速度加快和脱离频率增加的作用。     

水基磁性流体竖直加热棒下的池沸腾传热实验研究

研究了磁性微粒浓度、外加磁场对水基磁性流体在竖直加热棒加热情况下的沸腾传热影响;实验结果显示,磁性微粒浓度和外加磁场对磁性流体的沸腾换热有很大影响：对于中低浓度的磁性流体,存在一个最优的磁粒浓度,在该浓度下沸腾传热的强化效果最显著,施加磁场时,该结论仍然成立。施加磁场能强化磁性流体的沸腾传热。     

带格架四棒束超临界水流动传热数值分析

棒束内超临界水流动传热是超临界水堆堆芯热工水力研究的重要内容,但对其认识还十分有限。本文针对四棒束内超临界水的流动传热现象开展数值模拟,特别分析了定位格架对棒束通道内流动和传热的影响。结果表明,采用SSG湍流模型计算所得到的棒束壁面温度和实验结果吻合良好,定位格架的存在影响下游流体的速度分布,显著提高格架下游的传热特性,交混系数有大幅上升,使得加热棒周向壁面温度分布更加平均,最高温度出现位置发生改变。     

10 mm垂直单管通道内超临界水传热弱化现象的实验与数值分析

超临界压力下的流体因拟临界点附近物性的剧烈变化,形成了非常奇特的传热现象。因流体密度突变,在低流量下会引起强烈的浮升力作用,对超临界流体的流动和传热均有极大影响。本工作通过实验获得10 mm单管内传热弱化现象的实验数据,并采用改进的低雷诺数湍流模型,使用数值方法模拟该传热弱化现象。计算结果表明,不同于以往传统的模型会高估壁面温度,改进的低雷诺数湍流模型能较好预测实验结果。数值模拟结果还揭示了浮升力对湍流剪切应力和速度分布的影响,进而引起传热弱化和传热恢复。     

Influence of Transverse Magnetic Field on Heat Transport Characteristics of Potassium Heat Pipe

The influence of a transverse magnetic field on the heat transport characteristics of a potassium heat pipe was experimentally studied in the range of field strength 0~0.6 T. The wick was constituted of a multilayer mesh screen, and the adiabatic section, to which the magnetic field was applied, was made up of a concentric double-wall rectangular tube, with the inner wall completely separating the vapor and liquid flows.

The magnetic field was applied perpendicularly to the heat pipe, upon which the axial temperature distribution of the heat pipe was observed to be affected, and the heat transport rate to be reduced with increasing field strength.

The effect of the magnetic field on the heat transport rate is analyzed in terms of the liquid pumping ability of the wick and of the magnetohydrodynamic (MHD) effect on the liquid flow through the wick. The MHD effect on the flow through wick is shown to be expressible by a formula similar to that for flow between parallel plates.

The heat transport rate measured in magnetic field are compared with values calculated assuming that the wick pumping ability was not influenced by the magnetic field but that it was the MHD effect on the liquid flow through the wick that affected the heat transport. The calculated results well explained the experimental data.     

摇摆对圆管内层流温度分布的影响

对摇摆条件下的层流流体进行理论分析。推导出了摇摆条件下圆管内层流流体的速度和温度分布。分析了摇摆运动对流体温度的影响。摇摆条件下,流体温度呈周期性波动,波动周期为摇摆周期的1/2。在管壁附近,温度梯度很大。摇摆运动引起的附加力可改变圆管内的速度分布,进而改变管壁处的温度梯度增强传热。     

Heat transfer analysis for unsteady MHD flow past a non-isothermal stretching surface

An analysis is made for the unsteady two-dimensional magneto-hydrodynamic flow of an incompressible viscous and electrically conducting fluid over a stretching surface having a variable and general form of surface temperature which removes the restrictions of the particular forms of prescribed surface temperature. Similarity solutions for the transformed governing equations are obtained. The transformed boundary layer equations are solved numerically for some values of the involved parameters, namely the unsteadiness parameter, magnetic parameter, the temperature exponent parameters. The features of the flow and heat transfer characteristics for different values of the governing parameters are analysed and discussed. It is found that the fluid velocity and temperature decrease for increasing unsteadiness parameter. Fluid velocity decreases with the increasing values of the Hartman number resulting an increase in the temperature field in steady as well in unsteady case. It is observed that the variation of the sheet temperature in respect of distance and time has analogous effects both on the free surface temperature and on the heat transfer rate (Nusselt number) at the sheet.     

Effect of Gas Introduction on Temperature Distribution for Tube Flow with Internal Heat Sources

An experimental study has been conducted to investigate the effect of gas introduction on the heat transfer characteristics for turbulent flow of a heat generating liquid in an adiabatic tube 20 mm in inside diameter. Heat generation within the fluid was brought about by passing an alternating current through the working fluid, which was an aqueous solution of sodium chloride mixed with air bubbles. The superficial liquid Reynolds number ranged 3,700–11,000. The quality was varied from 2.6×10^?5 to 3.3×l0^?3. Measurements were made of the temperature distributions in the fluid as well as on the tube wall. The experimental results were compared with theoretical analyses.

In bubbly flow; the introduction of air into liquid brought forth a flat temperature distribution due to a considerable increase of turbulence and a saddle-shaped void distribution, which had a maximum near the tube wall. In slug flow, however, the void distribution changed to a dome-shaped profile with a maximum at the tube center and the rate of heat generation was higher near the wall than in the center region, resulting in a steep temperature distribution.     

Numerical Simulation of Fluid Flow and Heat Transfer in a DC Non-Transferred Arc Plasma Torch Operating Under Laminar and Turbulent Conditions

In this work, a magnetic fluid dynamics (MHD) model is used to simulate the electromagnetic field, heat transfer and fluid flow in a DC non-transferred arc plasma torch under laminar and turbulent conditions. The electric current density, temperature and velocity distributions in the torch are obtained through the coupled iterative calculation about the electromagnetic equations described in a magnetic vector potential format and the modified fluid dynamics equations. The fluid-solid coupled calculation method is applied to guarantee the continuity of the electric current and heat transfer at the interface between the electrodes and fluid. The predicted location of the anodic arc root attachment and the arc voltage of the torch are consistent with corresponding experimental results. Through a specific analysis of the influence of mass flow rates and electric current on the torch outlet parameters, the total thermal efficiency, thermal loss of each part, and the laws of the variation of outlet parameters with the variation of mass flow rates and electric current was obtained. It is found that operation under a laminar condition with a limited area of the anode could increase the total thermal efficiency of the torch.     

Heat Transfer Performance of High Temperature and High Velocity Hydrogen Flow inside Circle Tube

In order to investigate the thermo-hydraulic characteristics of working fluids in the coolant channel of nuclear thermal propulsion reactors, the flow and heat transfer performance of high temperature and high velocity hydrogen in the circle tube was studied by the numerical calculation method. Comparing with the experimental data, it is found that the pressure-based coupled algorithm, SST k-ω turbulence model and hydrogen property model are reasonable and feasible to simulate the flow and heat transfer performance of hydrogen at high temperature and high velocity. The calculated values agree well with the experimental data, and the numerical simulation model is correct. Based on the analysis of flow and temperature field of the base case, the effects of thermal parameters on the flow and heat transfer performance of hydrogen were also studied. The increasing inlet mass flow rate enhances heat transfer performance and the increasing heat flux weakens it. The methods and results can provide some references and guidance for the study of the flow and heat transfer performance of gaseous fluid under high temperature and high heat flux, and thermal design and simulation of nuclear thermal propulsion reactor.     

Magnetic Influence on Natural and Marangoni Convections in a Zone of Metal Melted by High Heat Flux

In readiness for utilization as material for the first wall of fusion reactors that will simultaneously generate high heat flux and a high magnetic field, the heat transfer and melting behavior of stainless steel has been numerically analyzed applying the finite difference method. Envisaging the application of a heat flux of 2.34 kW/cm² axially on an 8 mm thick, 60 mm diameter stainless steel disk under an axial magnetic field of intensity varied parametrically, the analysis clarifies the effect of differences in applied magnetic field intensity on the configuration of the melted metal zone boundary, on the flow pattern of convective circulation generated within the same zone, and on the radial temperature distribution across the zone. The analysis is performed both for the cases of natural convection alone taken into account and of combination with Marangoni convection. As a result, it is shown that, assuming steady state, the surface flow velocity at the point of interest varies with the applied magnetic field intensity approximately in inverse proportion to the square of Hartmann number in the case of natural convection alone, and that the same applies to the case of combination with Marangoni convection, though with greater deviation from the foregoing analytical result toward higher magnetic field intensities. It is also shown that the assumption of steady state (adopted in deriving the above relation between surface flow velocity and Hartmann number) becomes valid after the lapse of a short time after the start of heat flux application.     

高温、高流速氢气在圆管内流动换热特性研究

为探究工质在核热推进反应堆冷却剂通道内的热工水力行为，基于数值计算方法，开展了圆管内高温、高流速氢气流动换热特性研究。通过与实验数据对比发现，采用压力基耦合算法、SST k-ω湍流模型以及物性模型进行高温、高流速氢气流动换热特性数值模拟是合理可行的，计算值与实验值符合较好，计算模型选择正确。在分析基础工况流场与温度场的基础上，还研究了热工参数对氢气管内流动换热特性的影响，结果表明，随质量流量的增大换热效果增强，随热流密度的增大换热效果变差。研究方法与结果可为高温、高热流密度环境下气体工质流动换热特性研究、核热推进反应堆的热工设计与仿真模拟提供参考。     

不同运行工况下蒸汽发生器热工水力稳态特性数值研究

基于相似模化理论建立了蒸汽发生器一、二回路流体及传热管流 固耦合传热的单元管三维物理模型,对大亚湾核电厂蒸汽发生器不同工况下的热工水力稳态特性进行了数值模拟研究。采用热相变模型描述二回路汽液两相流动与换热、流-固耦合模型描述一回路冷却剂借助U型管与二回路流体换热。数值计算结果表明：满负荷运行时,传热管内壁温度变化趋势与一次侧流体基本一致,外壁温度与二次侧流体温度变化趋势相同;截面平均含汽率沿传热管高度的升高呈上升趋势,出口质量含汽率与大亚湾核电厂实际运行参数相符;随负荷降低一回路出口温度基本不变,二回路出口温度升高,质量含汽率及传热系数下降,平均传热系数与Rohsenow经验关联式的计算结果基本吻合。     

以R134a为工质的乏燃料池非能动冷却热管实验研究

随着分离式热管不断被提出用于核电站非能动余热排出方案中,开展针对大尺度分离式热管的换热性能的实验研究变得日益迫切。为此,本文开展了以R134a为工质的304不锈钢材质的分离式热管传热特性实验研究,获得了热管整体换热性能、蒸发段内部温度分布特性,以及热源温度和冷凝段外风速对热管工作温度、换热量、换热系数和循环流量的影响。热管蒸发段内R134a经历过冷、两相和过热状态,其中两相区域较长,达6.6 m,因而具有较好的换热能力,在所研究的工况下换热量最高达21 kW。参数敏感性分析表明,热源入口温度和冷凝段风速的增大能促进热管的换热性能,特别是热源入口温度的影响更显著。冷凝段风速较小时,其对换热量的影响较为显著,然而随空气速度的增加,影响降低。此外,依据试验数据拟合得到了换热量与冷热源温差的经验关系式,能在工程应用中快速预测热管的性能。     

Radiation effects on heat and mass transfer in MHD stagnation-point flow over a permeable flat plate with thermal convective surface boundary condition,temperature dependent viscosity and thermal conductivity

We study a steady laminar 2-D MHD viscous incompressible flow over a permeable flat plate with thermal convective boundary condition and radiation effects. The viscosity and thermal conductivity of fluid are assumed to vary linearly with temperature. Similarity representation of the governing partial differential equations is obtained via group method. Similarity equations are then solved numerically by implicit finite difference technique. Effects of convective heat transfer parameter (b), radiation parameter (R,) magnetic field parameter (M), the thermal conductivity parameter (S), suction parameter (fw), Prandtl number (Pr) and Schmidt number (Sc) on the dimensionless axial velocity, temperature, concentration, wall temperature, the rate of heat transfer and the rate of mass transfer are investigated. Good agreement is found between the numerical results of the present paper with published result for special case.     

Heat transfer in liquid metals

This article is a survey of basic work on heat transfer between solid surfaces and flowing liquefied metal that has been reported by both Soviet and foreign authors. The results of experiments on heat transfer to liquid metals are analyzed for cases where flow took place in both long and short tubes, flat channels, over bundles of rods and over a plate along their longitudinal directions, across cylinders, with free convection and condensation of liquid metal vapors. The effect of additives on the rate of heat transfer is examined. Questions that arise in connection with boiling of liquid metals are discussed, as are problems of wetting effects on fluid friction and on heat transfer rate. Equations for calculation of heat transfer rates are given.     

带定位格架的类三角形堆芯通道超临界水传热试验研究

针对带定位格架的超临界水冷堆堆芯垂直上升类三角形子通道,开展超临界水的流动传热试验研究。反应堆堆芯类三角形子通道棒束直径为8 mm、栅距比为1.4,试验参数范围为:热流密度q=200~600 kW/m2、压力P=23~28 MPa、质量流速G=700~1300 kg/(m2·s)。分析了热流密度、压力和质量流速等热工参数对超临界水传热特性的影响。试验结果表明:定位格架处质量流速升高,流体扰动性增强,换热系数提升显著;在超临界压力下,提高压力会导致内壁温度上升,换热系数峰值降低;过高的热流密度会导致换热系数峰值降低,适当减小热流密度可提高换热性能;提高质量流速会导致内壁温度降低,换热系数峰值上升,能够显著提高换热性能。压力变化对定位格架区域传热特性影响较小,适当提升压力可提高系统安全性。      

超临界压力水冷包层方案第一壁的热与应力分析

针对超临界水冷包层中第一壁的运行工况,利用数值计算软件ANSYS中CFX和Workbench两个模块对第一壁结构中的固体域和流体域进行数值分析研究。对比矩形管道和圆形管道内传热及热应力分布发现,矩形管道四个角域强化了壁面流体和主流流体的动量和热量的交换,使传热性能优于圆形管道,而四个角域的存在也造成了该处的应力集中,使结构材料的最大应力明显高于圆形管道。进一步研究冷却剂流向和冷却管道几何结构参数对第一壁结构温度场和应力场的影响发现,在ITER运行工况下,冷却剂流向影响很小,增大冷却管道直径和减小冷却管道最小壁厚均能改善第一壁结构材料中的最高温度,而这两个几何结构参数对第一壁应力的影响较为复杂。     

Hydromagnetic free convection effects on the oscillatory flow of an electrically conducting rarefied gas past an infinite vertical porous plate

Unsteady two-dimensional free convection flow of an electrically conducting, viscous, incompressible rarefied gas, past an infinite vertical porous plate in the presence of a transverse magnetic field is studied. The freestream velocity oscillates in time about a constant mean, while the suction velocity, normal to the porous plate, is constant. The magnetic Reynolds number of the flow is not taken to be small enough, so that the induced magnetic field is not negligible. The plate temperature is constant and the difference between the temperature of the plate and the freestream is moderately large causing the free convection currents. The flow field is described by a nonlinear coupled system of equations subjected to the first-order velocity slip and temperature jump boundary conditions. With viscous dissipative heat and Joule heating taken into account, approximate solutions of the problem are obtained for the velocity, temperature and induced magnetic field, as well as, for the related to them quantities of the skin friction, rate of the heat transfer and electric current density.     

Evaluation of Heat Transfer Augmentation in a Nanofluid-Cooled Microchannel Heat Sink

Present investigation deals with appraising heat transfer enhancement of single phase microchannel heat sink (MCHS) by ultra fine Cu particle incorporation in base coolant fluid. The particle diameter is of nanometer size and base fluid in combination of nanoparticles is called nanofluid. Governing equations for fluid flow and heat transfer are based on well established “porous medium model” and accordingly, modified Darcy equation and two-equation model are employed. Appropriate equations for both fluid flow and heat transfer are derived and cast into dimensionless form. Velocity profile is obtained analytically and in order to solve conjugate heat transfer problem a combined analytical–numerical approach is employed. For heat transfer analysis, thermal dispersion model is adopted and latest proposed model for effective thermal conductivity – which considers the salient effect of interfacial shells between particles and base fluid – is integrated into model. The effects of dispersed particles concentration, thermal dispersion coefficient and Reynolds number are investigated on thermal fields and on thermal performance of MCHS. Additionally, the impact of turbulent heat transfer on heat transfer enhancement is considered.