Flow Patterns and Heat Transfer in a Cylindrical Annulus under 1g and low-g Conditions: Theory and Simulation

A dielectric fluid is confined in a stationary vertical cylindrical annulus. A temperature difference is applied between the two cylinders, as well as an alternating electric potential. This configuration creates an active force called dielectrophoretic force, which acts as a thermal buoyancy force. Different axial gravity intensities are considered, so that two thermal buoyancies will affect the flow: the thermoelectric buoyancy intervenes in the radial direction and the Archimedean buoyancy acts in the axial direction. Linear stability analysis and direct numerical simulation are performed following experimental research that has been performed during parabolic flight campaigns.     

圆柱型翅片管换热器变工况传热性能模拟与分析

基于分排参数模型,本文建立了圆柱型翅片管换热器的性能仿真计算模型,对换热器的传热性能进行计算,并进行实验验证。结果表明:换热量的平均相对误差最大,为6.31%;出风干球温度的平均相对误差最小,为0.61%。计算所得各性能参数与实验值吻合良好。根据仿真模型,对不同制冷工况下的换热器进行计算,研究了循环风量、水质量流量、进风干球温度以及进水温度的变化对换热性能的影响。分析换热器的变工况特性可预测其非设计工况下的换热性能,并为换热器的运行工况调节提供依据。     

1 000 MW燃煤机组不同负荷锅炉传热特性

以1 000 MW超超临界塔式锅炉为研究对象,建立了锅侧和炉侧的三维耦合传热计算模型。基于锅炉的设计参数,采用该模型计算了锅炉传热过程。对超超临界锅炉不同负荷条件进行数值模拟,分别选取锅炉的最大连续蒸发量（boiler maximum continuous rating,BMCR）工况、75% BMCR工况和50% BMCR工况来考察机组在调峰时锅炉传热规律。通过对炉侧炉膛壁面、炉膛内部、炉膛水平截面温度和热流分布以及水冷壁管壁和工质温度分布的分析,发现BMCR工况、75% BMCR工况的温度和热流在炉膛内的变化趋势一致,而50% BMCR工况的温度变化缺乏一般性规律,并且在螺旋水冷壁区域容易出现传热恶化现象。     

Bistability of Heat Transfer of Viscous Liquid under Conditions of Flow in a Channel

A model of heat transfer of viscous liquid flowing under the effect of a pressure drop in a pipe surrounded by a medium of a lower temperature is treated. It is shown that, if the viscosity of the liquid is an exponential function of temperature and if dissipative heat release is negligibly small, a bistable behavior of the system is possible. In this case, transitions between cold and hot flows are jumplike. The liquid and channel parameters are found at which bistability may be observed.     

Heat Transfer in a Turbulent Flow of Gas in a Tube under Conditions of Resonance Oscillation of the Flow Rate

A prediction study is made into the heat transfer in a turbulent flow of gas (air) in a narrow tube with superimposed resonance oscillation. The model of turbulent transfer includes the effect of nonstationarity on the turbulent stress and heat flux. The finite difference method is used to solve the equations of motion and energy. The distribution of the flow rate and pressure along the tube is found by way of numerical solution of a set of cross section-averaged nonstationary equations of motion, continuity, and energy. The effect of the process parameters (Reynolds number, dimensionless oscillation frequency, thermal boundary conditions on the wall) on the period-average heat transfer and heat flux to the wall is analyzed.     

Flow and Heat Transfer Analysis in a Deformable Channel

Journal of Engineering Physics and Thermophysics - An analytical solution of the Navier–Stokes equations in a semi-infinite rectangular channel with porous and uniformly expanding or...     

水平细通道内CO_2流动沸腾换热流态特性实验研究

本文对水平细通道内CO_2流动沸腾换热过程中流态及其转变特性进行理论分析和可视化实验研究。根据可视化实验结果,更新了CO_2在低蒸发温度下的理论流动状态预测模型。实验工况为:热流密度(7. 5～30 k W/m2)、质量流率(50～600kg/(m2·s))、饱和温度(-40～0℃)、干度(0～1)、内径(1. 5 mm)。理论分析表明:质量流率对换热过程中经历的流态形式有决定性作用,热流密度对环状流-干涸区域、干涸区域-雾状流边界转变曲线影响较大,饱和温度对流态转变具有重要影响。可视化研究表明:基于理论流态图对于CO_2在细通道内流动沸腾换热的流态能够较好的预测,也能反映不同工况下流态的变化趋势,但理论流态图对干涸区域和雾状流区域预测偏差较大;在实验数据的基础上,增加了液气黏度比的无量纲因子,并提出一种新的临界热流密度预测模型。在考虑质量流率和热流密度影响的情况下,根据更新后临界热流密度预测模型和实验数据,引入沸腾数Bo对理论流态图中环状流-干涸区域、干涸区域-雾状流及间歇流/弹状流-环状流边界转变曲线进行了更新,可视化研究获得的流态数据中89. 4%符合更新后的CO_2理论流态预测模型。     

Thermomagnetic Convection as a Tool for Heat and Mass Transfer Control in Nanosize Materials Under Microgravity Conditions

Magnetic colloids are relatively new man-made nanomaterials whose magnetic susceptibility is several orders of magnitude larger than that of natural substances. Experiments conducted in magnetic fluids show that strengthening or weakening of thermal convection in colloids is dictated by a competition between the gravitational and magnetic mechanisms as well as by the effect of the fluid density stratification due to gravitational sedimentation of magnetic particles and their aggregates. Therefore experiments in microgravity conditions are required to eliminate gravitational sedimentation. This will enable an accurate investigation of convection in magnetic fluids and unambiguous study of the interaction of a magnetic field with a magnetopolarized medium. Such experiments are also needed to perform an accurate measurement of fluid’s transport coefficients.     

Unsteady Heat Transfer in HeII in Cylindrical and Spherical Geometries

The problem of evolution of strong heat pulses in superfluid helium interacting with quantum vortices induced by these pulses is investigated numerically on the basis of equations of hydrodynamics of superfluid turbulence. The “discontinuity decay” method (Godunov method) is used for the calculations. In order to study nonlinear effects, the initial equations are expanded to the second order in the amplitudes of pulses. The influence of the main flow parameters—heat flux and rest time—on temperature evolution is reported. The numerical investigations are carried out at the bath temperature of 1.4 K. The obtained results are compared with experimental data.      

Heat Transfer under Conditions of Longitudinal Laminar Flow past Chaotically Arranged Cylinders

An analytical investigation is performed of the variation of intensity of heat transfer in a system of parallel heat-releasing cylinders subjected to longitudinal laminar flow during transition from an ordered to chaotic structure.     

Simulation of Heat and Mass Transfer in a Cylindrical Evaporator of a Loop Heat Pipe with a Rectangular Interface

High Temperature - The stationary heat and mass transfer in a cylindrical evaporator of a loop heat pipe with a thermal interface in the form of a rectangular parallelepiped and a heat source...     

Numerical Simulation of Heat Transfer under Conditions of Turbulent Separated Flow in Tube Banks

The separated flow and heat transfer in an in-line bank of round tubes are calculated using a multiblock computational algorithm and the semi-empirical Menter and Spalart-Allmaras models of turbulence. An ingenious procedure is suggested for the correction of the mass-average temperature. Analysis is made of the effect of viscosity on the vortex structure and heat transfer from a remote cylinder within the bank.     

Pyroelectric Transducers for the Measurement of Heat Transfer under Conditions of Hypersonic Flow

Pyroelectric detectors have been fabricated from polycrystalline samples of barium titanate. These detectors were mounted in a model for tests in a hypersonic wind tunnel and calibrated. The method of calibration is discussed and tables of calibration data are included. Thermocouples were mounted in the model to evaluate the detectors. The detectors were tested and results of the tests are discussed. Advantages of pyroelectric detectors are presented and typical output traces from both the pyroelectric detectors and the thermocouples are shown.     

Numerical Simulation of Unsteady-State Heat Transfer under Conditions of Laminar Transverse Flow past a Circular Cylinder

Two-dimensional Navier-Stokes and energy equations are used to perform a numerical calculation of the parameters of unsteady-state flow and heat transfer under conditions of laminar transverse flow of a viscous incompressible fluid past a circular cylinder and at Re = 140. The process of formation of Karman vortex street is treated, as well as the cyclic heating of the near and far wakes behind the heated cylinder. Special attention is given to analysis of the behavior of the integral characteristics of flow and heat transfer, and the distributions of pressure, friction and heat-transfer coefficients on the cylinder surface, as well as to correlations of the parameters of flow and temperature at selected points in the vicinity of the cylinder. Evaluation is made of the effect of the factor of unsteadiness on the increment of power and heat loads in the process of flow past the cylinder.