流体绕流不同方向振动圆柱换热特性数值分析

基于Fluent动网格及UDF编程技术对二维流场中振动圆柱的换热特性进行了研究,分析了来流速度及圆柱振动方向对其换热特性的影响。数值计算结果表明,相比于静止圆柱换热,当圆柱的最大振动速度远大于来流速度时,其振动强化传热效果显著;随着流速的增大,振动圆柱的强化传热效果降低,当流速为圆柱最大振动速度的4倍时,圆柱传热效果仅增强1.2%。另外,对相等流速下的圆柱沿不同方向的振动换热特性研究表明,振动方向对圆柱强化换热的影响不显著。     

Unsteady compressible boundary layer flow on the stagnation line of an infinite swept cylinder

Summary Numerical solutions of flow and heat transfer process on the unsteady flow of a compressible viscous fluid with variable gas properties in the vicinity of the stagnation line of an infinite swept cylinder are presented. Results are given for the case where the unsteady temperature field is produced by (i) a sudden change in the wall temperature (enthalpy) as the impulsive motion is started and (ii) a sudden change in the free-stream velocity. Solutions for the simultaneous development of the thermal and momentum boundary layers are obtained by using quasilinearization technique with an implicit finite difference scheme. Attention is given to the transient phenomenon from the initial flow to the final steady-state distribution. Results are presented for the skin friction and heat transfer coefficients as well as for the velocity and enthalpy profiles. The effects of wall enthalpy parameter, sweep parameter, fluid properties and transpiration cooling on the heat transfer and skin friction are considered.     

Effect of heat transfer on compressible boundary layer flow over a circular cylinder

Summary The effect of heat transfer on the steady laminar compressible boundary layer flow past a horizontal circular cylinder has been studied. The resulting coupled nonlinear partial differential equations have been solved numerically using a very efficient finite-difference scheme. Specific results are given for the boundary layer separation parameter and the skin friction and heat transfer coefficients.     

内燃机燃烧测试分析系统的设计与开发

开发一套基于PC机的内燃机燃烧测试分析系统,并对气缸压力的采集,数据的匀化、光顺处理,动态上止点位置的确定等做介绍。利用测量的气缸压力曲线和能量守恒原理,在LabVIEW软件平台上编写出内燃机燃烧放热率计算程序,研究主要经验参数对放热率及缸内平均温度的影响。结果表明:采用Woschni传热公式计算的放热率曲线值高于采用Eichelberg传热公式和Sitkei传热公式计算的放热率曲线值。扫气系数φs的变化对气缸内平均温度有显著影响,φs=0.99时的缸内平均温度明显高于φs=0.95时的缸内平均温度,两者最高温度相差81.4 K。     

A review of free,forced and mixed convection in a saturated porous annulus

A computational study of heat transfer in a liquid-saturated porous annulus with a heated inner wall and a cold outer wall is reported here. Results are presented for heat transfer rates from the inner cylinder for a wide range of parameters which characterize free, forced and mixed convective regimes of flow. Both horizontal and vertical annulii are included in the study. Heat transfer results have also been obtained from a non-Darcy model for flow and these are compared to the Darcy solutions. Non-Darcy effects are taken to arise from inertial and viscous effects in the fluid phase and the variation of porosity near the solid wall. The approach of heat transfer results of an annulus with a large radius ratio towards those of an isolated circular cylinder is discussed. Limited data on transient convection is also presented in this work.     

Self-oscillations of nonisothermal flow of viscous liquid in a channel

The process of emergence of self-oscillations of a flow of viscous liquid in a channel under conditions of heat transfer to the cold channel wall is considered. The parameters of liquid and heat transfer, at which self-oscillations may exist, are obtained, as well as the dependences of the period and amplitude of self-oscillations on the parameters of the system. The behavior of the system is investigated in the vicinity of the critical point determined in the curve of the dependence of liquid flow rate on pressure difference. A generalizing scheme of operation is given for a number of self-oscillatory systems, and it is demonstrated that their behavior in the vicinity of the critical point is described by the same equations.     

Numerical study on characteristics of flow and thermal fields of offset cylinder arrays in the middle Reynolds number range

In order to offer a detailed explanation for heat transfer enhancement caused by the instability, a computational model for offset cylinder arrays was established by using a compound grid system. The fully implicit forms of continuous equation, unsteady Navier–Stokes equations and energy equation were finite-differenced. The numerical results demonstrate that cylinder row pitch plays an important role on the flow effects, which enhance or deteriorate the heat transfer on cylinder surfaces. Among these flow effects, the acceleration effect and the instability effect are the main factors for enhancing the downstream cylinders. Moreover, heat transfer enhancement caused by the flow instability is an effective way to reach the aim of reducing flow resistance and promoting heat transfer.     

Numerical simulation of reactive polymer flow during rotational molding using smoothed particle hydrodynamics method and experimental verification

A numerical study of flow behavior and the heat transfer process of reactive polymer in reactive rotational molding systems is carried out using the Smoothed Particle Hydrodynamics method (SPH). The flow of reactive polymer during rotational molding inside a horizontal rotating cylinder is modeled as the slightly compressible viscous fluid flow and with free surface. These simulations show the influence of process parameters on the flow of polymer. Especially, the influence of the change of viscosity on the flow, due to the chemical reactions, is simulated by using an adapted viscosity expression. For a constant rotational speed, several flow regimes are observed as follows: the fluid remains at the bottom of the mold with the formation of a thin layer on the surface if the viscosity of reactive polymer is very low (less than 850 Pa.s), the presence of cascades is shown (after the point of maximum height) if the viscosity is higher (850 to 980 Pa.s), and the fluid moves at the same speed as the mold if the viscosity is sufficiently high (approximately 1230 Pa.s). The heat transfer between the mold and reactive polymer is also simulated. The results of SPH simulations are then compared to the experimental results conducted on polyurethane (TPU) system based on 1,6-hexamethylene diisocyanate (HDI, Sigma Aldrich, France), polyethylene glycol (PEG 1000) as macrodiol, 1,3-propanediol (PDO), and dibutyl tin dilaurate (DBTDL).     

Conjugate Heat and Mass Transfer under Conditions of Motion of a Viscous Incompressible Liquid in an Open Rectangular Cavity and Wall Melting

Numerical simulation is performed of heat transfer and hydrodynamics under conditions of motion of a viscous incompressible liquid in an open rectangular cavity with allowance made for the process of melting of its walls. Temperature distributions for the solid and liquid phases, as well as the velocity fields and the contours of craters being formed, are obtained at different moments of time. The effect of the model parameters and of the liquid flow mode on the pattern of motion and heat transfer of liquid in the cavity is studied.     

Buoyancy effects on the laminar boundary layer heat transfer along vertically moving cylinders

Abstract

The effects of buoyancy forces on the laminar boundary layer flow and heat transfer along vertically moving cylinders are analyzed for the cases of prescribed surface temperature and prescribed wall heat flux in power of streamwise distance. Local similarity solutions are obtained to show the effects of buoyancy parameters and the transverse curvature of the cylinder on the surface friction and heat transfer rate.     

Homotopy analysis method for chemical reaction and thermophoresis effects on heat and mass transfer for mhd hiemenz flow over a porous wedge in the presence of heat radiation

An analytical technique, namely, the homotopy analysis method, is applied to analyze the effect of chemical reaction and thermophoresis particle deposition on the MHD mixed convective heat and mass transfer for a Hiemenz flow over a porous wedge in the presence of heat radiation. The fluid is assumed to be viscous and incompressible. Analytical and numerical calculations are carried out for different values of dimensionless parameters, and an analysis of the results obtained shows that the flow field is influenced appreciably by the buoyancy ratio as well as by the thermal diffusion and suction/injection parameters. The effects of these parameters on the process characteristics are investigated methodically, and typical results are illustrated. An explicit, totally analytical, and uniformly valid solution is derived which agrees well with numerical results.     

Unsteady mixed convection flow of a thermomicropolar fluid on a long thin vertical cylinder

The unsteady laminar mixed convection boundary layer flow of a thermomicropolar fluid over a long thin vertical cylinder has been studied when the free stream velocity varies with time. The coupled nonlinear partial differential equations with three independent variables governing the flow have been solved numerically using an implicit finite difference scheme in combination with the quasilinearization technique. The results show that the buoyancy, curvature and suction parameters, in general, enhance the skin friction, heat transfer and gradient of microrotation, but the effect of injection is just opposite. The skin friction and heat transfer for the micropolar fluid are considerably less than those for the Newtonian fluids. The effect of microrotation parameter is appreciable only on the microrotation gradient. The effect of the Prandtl number is appreciable on the skin friction, heat transfer and gradient of microtation.     

制冷用大容量交叉式电磁四通换向阀加工工艺研究

制冷用大容量交叉式电磁四通换向阀作为大型风冷热泵中央空调机组的核心部件,在系统中起着冷热切换、热气化霜的作用,其实际工作的可靠性是工程技术人员关注的焦点。本文围绕其使用的可靠性要求,着重介绍了筒体时效和内磨加工,活塞热处理和外磨加工的加工工艺;最后给出了合理的加工流程和工艺参数。     

Mixed convection in vertical internal flow of a micropolar fluid

The theory of micropolar fluids due to Eringen is used to formulate a set of equations for the flow and heat transfer characteristics of the combined convection micropolar flow in vertical channels. It is found that the microstructure and substructure parameters have significant effects on the flow and thermal fields. By making the Newtonian solvent more and more micropolar, it is possible to obtain drag reduction as well as reduced heat transfer characteristics.     

Transient free convection flow past an impulsively started isothermal vertical cylinder with mass flux

This paper concern with the laminar flows, which arise in fluids due to the interaction of the force of gravity and density differences, caused by temperature differences and material or phase constitution for both air and water. A solution of laminar boundary layer equations has been obtained for the transient free convective flow past an impulsively started semi-infinite isothermal vertical cylinder with uniform mass flux. The solutions of the dimensionless, unsteady, coupled and non-linear governing partial differential equations are obtained by a more accurate, unconditionally stable and fast converging implicit finite difference scheme. The results show many interesting effects on velocity, temperature and concentration profiles and local as well as average shear stress, rate of heat and mass transfer. It is observed that there is a rise in the velocity due to the presence of mass diffusion.     

Thermodynamic analysis of free convection film condensation on an elliptical cylinder

Abstract

This paper aims to perform thermodynamic analysis of saturated vapor flowing slowly onto and condensing on an elliptical cylinder. This is the first approach to investigate how the geometric parameter‐ellipticity and surface tension affect local entropy‐generation rate during film‐wise condensation heat transfer process. The results observe that entropy generation decreases with decreasing ellipticity. It indicates that the entropy generation number is nearly unaffected by surface tension forces at small ellipticity like e ≤ 0.7, but somewhat influenced at large ellipticity for the whole perimeter. From the second law point of view, local entropy generation increases with ellipticity as local heat transfer coefficient does. Furthermore, the entropy‐generation rate due to gravity‐driven film flow friction is proportional to Brinkman group parameter. The irreversibility ratio indicates that film flow friction irreversibility starts to dominate over heat transfer irreversibility in the lower half of streamwise length for higher values of Brinkman group parameter (Br/T = 1).     

A study of induced magnetic field with chemically reacting and radiating fluid past a vertical permeable plate

The influence of thermal radiation and chemical reaction on the steady MHD heat and mass transfer by a mixed convective flow of a viscous, incompressible, electrically conducting Newtonian fluid (an optically thin gray gas) past a vertical permeable plate was investigated with account for the induced magnetic field. The similarity solutions of the transformed nondimensional governing equations are obtained by the series solution technique. The influence of numerous parameters on the process characteristics is studied.     

An experimental investigation of heat transfer in a pebble bed

An experimental investigation is performed of heat transfer under conditions of longitudinal flow of water moving in a bed of glass pebbles past a flat heated wall. Experiments involving the flow of single-phase liquid are performed in the ranges of variation of filtration velocities from 6 to 60 mm/s and of heat fluxes from 40 to 140 kW/m²; in the case of wall boiling, the experiments are performed in the ranges of variation of filtration velocities from 2 to 50 mm/s and of heat fluxes from 27 to 86 kW/m². The temperature distribution is measured over the height of the heated wall and over the cross section (depth) of the channel at the outlet from the pebble bed. The experimental data are processed for single-phase flow using numerical optimization techniques. The values of the coefficient of “turbulent” thermal conductivity in the pebble bed are obtained as a function of the process parameters     

Heat and mass transfer on a cylinder surface in cross flow under supersaturated frosting conditions

In this paper a semi-empirical model describing heat and mass transfer on a cylinder surface in humid air cross flow under supersaturated frosting conditions is presented. The lack of psychrometric data in the supersaturated zone of the psychrometric chart has historically impeded the ability of researchers to accurately predict heat and mass transfer in supersaturated air. The work described in this paper has been partially made possible by developing a systematic procedure to compute the properties of supersaturated moist air, especially in the low temperature zone of the psychrometric chart. Development of such a capability will allow us to predict the amount of frost collected on a coil, the frost deposition and coil heat transfer rates, frost thickness and frost surface temperature, and other important coil frost parameters under supersaturated conditions.     

The effect of variable properties on the helical flow and heat transfer of power law fluids

Summary Laminar flow and forced convection heat transfer of the time independent non–Newtonian fluid obeying power law stress-strain relation have been investigated numerically in the annular space between two coaxial rotating cylinders. The problem is considered when the inner cylinder rotates about the common axis with constant angular velocity and the outer cylinder is at rest. The viscosity of the fluid and thermal conductivity are assumed to vary with the temperature. The outer surface of the annulus is considered to be adiabatic, while the inner surface has a uniform temperature. The tangential and axial momentum equations and energy equation have been solved iteratively by using a finite difference method. For the steady fully developed flow, the velocity distributions, temperature profiles, the volumetric flow rate, torque and the average Nusselt number have been obtained for different values of the radius ratio and model parameters.