An analysis of laminar forced convection film boiling

A study of forced convection film boiling on a horizontal isothermal plate is made using a treatment based on boundary layer theory. For the vapour, the velocity and temperature distributions are obtained from equations of motion and energy. For the liquid layer, the Howarth approximation for flow over a plate is used. Then, by employing interface matching conditions, the significance of the rhμ ratio between the vapour and the liquid on the variation of Nusselt number is examined.     

管内进口段对流换热过程不可逆性的数值模拟

The irreversibility due to laminar forced convection in the entrance region of a duct was studied by means of numerical simulation. Local entropy generation distributions and total entropy generation were obtained based on the entropy generation equation. The effect of Reynolds number on the irreversibility was studied for constant wall temperature and heat flux. The results showed that the irreversibility in the entrance region was different from that in the fully developed region. Reynolds number had a strong effect on the irreversibility in the entrance region. The entropy generation caused by temperature difference was relatively dominant over that caused by viscous flow. The irreversibility at constant wall temperature was different from that at constant wall heat flux.     

Analysis of laminar flow forced convection heat transfer with uniform heating in the entrance region of a circular tube

A new integral or boundary-layer solution for laminar flow heat transfer in the combined entrance region of a circular tube is presented for the case of constant wall heat flux. The solution is based on the hydrodynamic inlet-filled region concept originally proposed by Ishizawa (1966) and later adopted by Mohanty and Asthana (1978) to hydrodynamically developing flow through a circular tube. Unlike available boundary-layer solutions, the new analysis provides results which join smoothly and asymptotically to the fully developed values. Results for the Nusselt number were found to agree favorably with available numerical solutions.     

Heat transfer in turbulent forced convection between parallel-plates

The thermal entry region heat transfer due to turbulent forced convection of a Newtonian fluid inside a parallel-plate duct maintained at constant uniform temperature is solved analytically and the local Nusselt numbers are determined with a high degree of accuracy up to the region very close to the inlet. The calculated local Nusselt numbers are compared with those obtained from a recently proposed empirical correlation.     

Effects of stagnant and dispersion conductivities on non-darcian forced convection in square Packed-Sphere channels

This paper presents a numerical and experimental investigation of the effects of stagnant and dispersion conductivities on non-Darcian forced convection in square packed-sphere channels. The theoretical prediction of Nusselt number are found to be in good agreement with the experimental data. A larger near-wall damping of stagnant conductivity is found for the present water-steel medium than for that described by the mixing rule based on the volume fraction. A nonlinear Peclet number dependence, Pe^λ and λ = 0,88, for dispersion conductivity is found to induce better agreement between the theoretical and experimental results especially for the cases of high Peclet number and high D_c/d.     

Numerical simulation of salt water electrolysis in parallel-plate electrode channel under forced convection

A steady-state numerical model for multi-ion parallel-plate electrode (PPE) system is developed to investigate the concentration and electric distribution in the process of salt water electrolysis under forced convection. The finite-element method is used to solve this multi-ion transport model coupled with ionic diffusion, convection and ionic migration. The flow field and the electrode current's effect on the concentration distribution are investigated. The comparison between the diffusion-transported current and the migration-transported current demonstrates that the current is almost fully transported by the ionic migration, with the result that the diffusion-transported current can be neglected and the control equation of potential is simplified. For comparison, a hypothetical model with uncoupled situation of concentration and electric field is taken into consideration. The variation of electric field strength near the cathode can reach 25% of the value of the uncoupled model in the condition of u₀ = 0.01 m/s. Compared with this, the variation of ionic electro-conductivity shows a reverse tendency.     

Transient forced convection in the entrance region of porous concentric annuli

The problem of transient laminar forced convection in the entrance region of a porous concentric annulus with developing thermal boundary layer is solved numerically by the finite difference technique. The hydrodynamic behaviour of the flow is assumed to be steady and fully developed, and both Darcian and non-Darcian effects on the flow are considered.     

Transient forced convection in the entrance region of a porous tube

The problem of transient forced convection in the entrance region of a cylindrical porous medium with developing thermal boundary layer is investigated. The hydrodynamic behaviour of the flow is assumed to be steady and fully developed, and both Darcian and non-Darcian effects on the flow are considered. Thermal transients are created by a step change in the temperature of the tube wall. The effect of different fluid and solid-matrix parameters on Nusselt number, thermal entrance length, total heat absorbed and mixing cup temperature are considered.     

Electromagnetic control of electroplating of a cylinder in forced convection

Continuous electrodeposition on a cylindrical cathode, e.g. electrodeposition of gold on electrical connectors, is usually characterized by an undesirable non-homogeneity of the deposit thickness. This has been observed in industrial applications. Numerical simulations have shown very good agreement with observations. This paper deals with the possibility of improving the homogeneity of the deposit thickness by a magnetic field that is parallel to the axis of the cylinder. The electromagnetic volume force generated by such a magnetic field may set up a swirling motion around the cylinder. By controlling the force density it is possible to control the thickness of the diffusive layer and consequently the mass transfer. The magnetic field can be optimized with respect to strength, spatial extent and variation with time. It is shown that a strong alternating magnetic field of low frequency gives a nearly homogeneous deposit.     

Prandtl number effect on combined free and forced laminar convection in the thermal entrance region of a horizontal tube

This note presents the Prandtl number effect on combined free and forced laminar convection in the thermal entrance region of a horizontal tube by a numerical vorticity-velocity method. The tube wall is heated with a uniform wall heat flux. Vorticity-velocity governing equations in cylindrical coordinates and an evaluation of boundary vorticity on the tube wall are presented along with a numerical method of solution. Local Nusselt number variations are shown for Pr = 100 and 10 and compared with the existing data for Pr → ∞. It is observed that the large Prandtl number assumption is valid for Pr = 10 within a difference of 4% at Ra = 1 × 10⁵. The numerical results are compared against the available experimental data with good agreement.     

Conditions of the Thermal Explosion under Forced Convection of a Reacting Mixture

Conditions of origination of the thermal explosion in a cylindrical chemical reactor equipped by a certain number of symmetrically located stirrers ensuring forced convection of the reacting mixture are considered in the approximation of an infinite Peclet number and under the assumption of laminar motion of the fluid. The critical value of the thermal explosion parameter (Frank-Kamenetskii parameter) is found as a function of the number of stirrers and the distance between the centerlines of the stirrers and reactor. As the number of stirrers increases, the calculations predict a lower probability of the thermal explosion, and the critical value of the parameter can severalfold exceed its classical value. It is found that the thermal explosion parameter substantially depends on the location of the stirrers, responsible for heat removal from the reactor. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 5, pp. 49–54, September–October, 2005.     

Heat and Mass Transfer Analogy Studies of Binary Liquid Mixture in Comparison with Eteanol-n-Heptane Evaporating to Air

An expenmental and theoretical study of heat and mass transfer analogy and a comparison of that to a binary liquid mixture evaporation is presented. Common organic solvents, ethanol and n-heptane, were used to form an alcohol - hydrocarbon mixture. Studies were carried out in a horizontal rectangular channel having air flow velocities of 0.2 - 0.9 m/s. Heat transfer coefficients were measured with a copper plate resistor and mass transfer coefficients with a square pool. The heat and mass transfer analogy is presented for a system having two evaporating compounds with a fixed value of air flow and verified by measuring and comparing mass transfer coefficients for distilled water with air flow velocities of 0.2 - 0.9 mls. An illustrative example of the use of the theory for industrial ventilation is presented.     

Heat and Mass Transfer Analogy Studies of Binary Liquid Mixture in Comparison with Eteanol-n-Heptane Evaporating to Air

ABSTRACT

An expenmental and theoretical study of heat and mass transfer analogy and a comparison of that to a binary liquid mixture evaporation is presented. Common organic solvents, ethanol and n-heptane, were used to form an alcohol - hydrocarbon mixture. Studies were carried out in a horizontal rectangular channel having air flow velocities of 0.2 – 0.9 m/s. Heat transfer coefficients were measured with a copper plate resistor and mass transfer coefficients with a square pool. The heat and mass transfer analogy is presented for a system having two evaporating compounds with a fixed value of air flow and verified by measuring and comparing mass transfer coefficients for distilled water with air flow velocities of 0.2 - 0.9 mls. An illustrative example of the use of the theory for industrial ventilation is presented.     

有体积热源的矩形空腔内层流自然对流的数值模拟

对具有内热源方腔的稳态层流耦合自然对流换热进行了三维的数值模拟,采用的模拟代码基于连续介质计算力学的开源库OpenFoam,解决了自然对流换热与固体传热的耦合问题.Ra数的变化从105到109.对外壁面为常温、方腔内充满含体积热源流体的自然对流计算结果表明,温度场、速度场与非耦合的工况有很大差异.     

Combined buoyancy effects of thermal and mass diffusion on laminar forced convection in the thermal entrance region of horizontal square channels

Heat and mass transfer in laminar mixed convection in the thermal entrance region of horizontal square channels is investigated by using the vorticity-velocity formulation of the Naiver-Stokes equations. The numerical results, including the developments of temperature and concentration contours, Nu_z and friction coefficient ratios, fRe / (fRe)₀, are presented for an air-water system. The effects of bottom wall temperature and the relative humidity on the momentum, heat and mass transfer in the flow are examined in detail. Results show that the influences of the evaporation of the water vapor along the wetted wall on the heat and mass transfer and the ratio fRe / (fRe)₀ are rather substantial.     

表面辐射对部分填充吸湿性多孔介质的封闭腔体内热湿耦合传递的影响

根据多孔介质热质传递原理, 基于有限元的方法数值分析了具有表面热辐射的部分填充吸湿性多孔介质的封闭腔体内部自然对流流动及热湿耦合传递过程, 探讨了表面发射率、Rayleigh数和Darcy数等参数对封闭腔体内部自然对流流动及热湿耦合传递过程的影响, 研究结果表明, 壁面热辐射的作用可以提高多孔介质内部的温度, 而且随着表面发射率的增大, 多孔介质内部的水分逐步向其右上角迁移和聚集。另外, Darcy数、多孔介质与空气的热导率比对方腔内部多孔介质的热量传递和水分迁移影响较小。     

Non-darcy forced convective heat transfer in a channel embedded in a non-newtonian inelastic fluid-saturated porous medium

The present analysis investigates non-Darcy forced convective heat transfer in a channel confined by two parallel walls subjected to uniform heat flux in a highly porous medium saturated with a non-Newtonian power-law fluid. Extensive numerical integrations have been carried out utilizing the Brinkman-Forchheimer extension of the Darcy model in order to study the effects of pseudoplasticity, and Brinkman and Forchheimer terms on the heat transfer characteristics.     

Natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium

This article deals with natural convection heat and moisture transfer with thermal radiation in a cavity partially filled with hygroscopic porous medium. The governing equations for the momentum and heat transfer in both free fluid and hygroscopic porous medium and moisture content transfer in hygroscopic porous medium were solved by the finite element method. The radiative heat transfer is calculated by making use of the radiosity of the surfaces that are assumed to be grey. Comparisons with experimental and numerical results in the literature have been carried out. Effects of thermal radiation and Rayleigh number on natural convection and heat transfer in both free fluid and porous medium and moisture content transfer in porous medium were analyzed. It was found that surface thermal radiation can significantly change the temperature and moisture content fields in the regions of free flow and porous medium. The mean temperature at the interface decreases, the temperature and moisture content gradients are created on the upper two corners of the porous medium region, and the moisture content in the porous medium decreases in the porous medium as Ra increases.     

Numerical and experimental analysis of forced convection in rib-roughened channels with moisture-permeable walls

Ribbed features can promote mixing and improve convective heat and mass transfer in channels. This can be beneficial in certain exchangers, such as Energy Recovery Ventilators (ERVs), which employ moisture-permeable membranes to transfer both latent and sensible heat. In this work, a computational fluid dynamics model with experimental validation was used to study angled rib mixing features in compact channels. Results show that, relative to a smooth channel, the ribs increase the channel Sherwood and Nusselt numbers by a larger fraction than the corresponding increase in friction factor. For a typical commercial grade ERV, total effectiveness can be improved by over 10% for an equal pressure drop by adding ribs and slightly increasing the channel height. A custom test stand and rib-forming technique were developed to validate the simulation predictions. The experiments confirm the net benefit of ribbed channels and agree with the simulation results within experimental uncertainty.