Modeling and Measurements of Heat Transfer Phenomena in Two-Phase PbSn Alloy Solidification in an External Magnetic Field

The main aim of this work is to study numerically the influence of an external magnetic field on the solidification processes of two-component materials. Based on the continuum model of two-phase flow a mathematical model for the directional solidification of a binary alloy in a magnetic field is presented. The model includes mass, momentum, energy and species mass conservation equations written in compressible form and additional relationships describing the temperature-solute coupling. The geometry under study is a cylindrical mold with adiabatic walls and cooled bottom. The macroscale transport in the solidification of alloys is governed by the progress of the two-phase mushy zone, which is treated by means of a porous medium approach. The volume fraction of liquid and solid phases, respectively, is calculated from a 2D approximation of the phase diagram. The results of calculation are compared with experimental data.     

Optimal operation of alloy material in solidification processes with inverse heat transfer

A transient nonlinear inverse heat transfer problem arising from alloy solidification processes is considered. In practice, the solidus and liquidus interface motions and thus the mushy zone thicknesses are pre-given to control the material quality. To achieve the desired front motions, the required time-dependent boundary conditions have to be predicted on both mold sides simultaneously. In this study, the enthalpy method is used for the derivation of governing equations. Hence, the inverse problem will be solved only in a single spatial and temporal domain. The conjugate gradient method with adjoint equation is applied for the resulting minimization problem. The method is applied as comparison for pure material with other previous studies. Then, alloy material with different front velocities is set up to investigate the solidification process. The obtained results show a close agreement between the desired and computed front motions and mushy zone thickness.     

A three-dimensional simulation of coupled turbulent flow and macroscopic solidification heat transfer for continuous slab casters

A numerical investigation was conducted for exploring the steady state transport phenomena of turbulent flow, heat transfer and macroscopic solidification in a continuous stainless steel slab caster. The numerical model is based on a generalized transport equation applicable to all the three regions, namely liquid, mushy and solid, which exist in a slab caster. The turbulence effects on the transport equations were taken into account using a low-Reynolds number k- turbulence model. The solidification of molten steel was modeled through the implementation of the popular enthalpy-porosity technique. A control volume based finite-difference scheme was used to solve the modeled equations on a staggered grid arrangement. A series of simulations was carried out to investigate the effects of the casting speed, the delivered superheat and the immersion depth of the twin-ported submerged entry nozzle (SEN) on the velocity and temperature distributions and on the extent of the solidified and mushy regions on the narrow and broad faces of the caster. In the absence of any known experimental data related to velocity profiles in a slab caster, the numerical predictions of the solidified profile on a caster's narrow face were compared with limited experimental data and a good agreement was found.     

Analysis of heat transfer coefficients and evaporation in a solar still

The dependence of the glass cover temperature, the individual heat transfer coefficients, the overall upward heat flow factor, the fraction of upward heat flow utilized for evaporation, and the rate of water evaporation on the basic parameters has been studied. A semi-empirical equation for estimation of the glass cover temperature has recently been proposed by the authors. An analysis has been made of the capability of the new method to compute accurately the glass cover temperature, the overall upward heat flow factor, the rate of water evaporation, the fraction of upward heat flow utilized for evaporation over an extensive number of combinations of the basic parameters.     

Suppression of natural convection heat transfer coefficients in an attic shaped enclosure

The use of convection suppression devices has been widely discussed in the literature as a means of reducing natural convection heat loss from enclosed spaces. In this study the use of a single baffle was examined as a possible low cost means of suppressing heat loss by natural convection in an attic shaped enclosure.     

Enhancement of heat transfer coefficients by actuation against an impinging jet

Recent technological developments have lead to significant increase in the generated heat by electronic and optical components. The removal of high heat fluxes can be successfully treated by several methods, e.g. impinging jets. Further improvement is offered by incorporating arrays of jets or causing jets to pulsate. The research reported herein introduces a new method which is based on actuation of a slab against a two dimensional steady, impinging, laminar, liquid micro-jet. This leads to enhanced heat transfer in the wall region of the jet. An experimental setup which included a piezoelectric (PZT) actuator, a dedicated silicon chip and a steady, slot, impinging jet, was assembled. Using a high speed infrared (IR) radiometer, the cooling process of the chip was recorded and the heat transfer enhancement values were determined for normalized actuation amplitudes, Reynolds and Strouhal numbers in the ranges of 0.45 < δ < 0.75, 756 < Re < 1260 and 0 < St < 0.052, respectively. It was experimentally found that heat transfer coefficients were enhanced by up to 34%.     

Self-similar solidification of an alloy from a cooled boundary

The self-similar solidification process of an alloy from a cooled boundary is studied on the basis of two models with a planar front and mushy layer. Approximate and exact analytical solutions of the process, which demonstrate unusual dynamics near the point of constitutional supercooling, are found. The rate of solidification and front position of the solid/mush boundary (parabolic growth rate constant) are expressed in an explicit form in the case of slow dynamics of this boundary. The theory under consideration is in a good agreement with experimental and numerical studies carried out by Huppert and Worster for ice growing from aqueous salt solutions.     

Experimental determination of natural convection heat transfer coefficients in an attic shaped enclosure

Heat transfer by natural convection in triangular enclosures is an area of significant importance in applications such as the design of greenhouses, attics and solar water heaters. However, given its significance to these areas it has not been widely examined. In this study, the natural convection heat transfer coefficients for air in an attic shaped enclosure were determined for Grashof Numbers over the range of 10⁷ to 10⁹. It was found that the measured heat transfer coefficients could be predicted to within 5% by Ridouane and Campo's [E.H. Ridouane, A. Campo, Experimental-based correlations for the characterization of free convection of air inside isosceles triangular cavities with variable apex angles, Experimental Heat Transfer 18 (2) (2005) 81–86] equation (Eq. (1)) for natural convection in a triangular enclosure previously developed for Grashof Numbers in the range of 10⁵ to 10⁶.

equation(1)

Nu=0.286A−0.286Gr1/4.$Nu=0.286A^{-0.286}G{r}^{1/4}\text{.}$

A simulation study of heat and mass transfer in a honeycombed rotary desiccant dehumidifier

A one-dimensional coupled heat and mass transfer model, which is expected for use in designing and manufacturing of a honeycombed rotary desiccant wheel, is presented in this paper. The mathematical model has been validated using a real desiccant wheel, and the calculation results are in reasonable agreement with the experimental data. Based on this model, the temperature and humidity profiles in the wheel during both the dehumidification and the regeneration processes are analyzed and verified by experimental data. The numerical results indicate that in the regeneration process a hump curve of air humidity ratio along the channel exists all the time. In the regeneration process the hump of air humidity ratio moves from the duct entrance to the duct exit and increases gradually until the hump reaches the duct exit, where the hump will drop subsequently. The effects of velocity of regeneration air V_reg inlet temperature of regeneration air T_reg and velocity of process air V_ad on the hump moving speed are investigated. To improve the performance of desiccant wheel, it is essential to accelerate the hump moving from the duct entrance to the duct exit as soon as possible.     

Interfacial heat transfer during microdroplet evaporation on a laser heated surface

A comprehensive experimental and numerical investigation on water microdroplet impingement and evaporation is presented from the standpoint of phase-change cooling technologies. The study investigates microdroplet impact and evaporation on a laser heated surface, outlining the experimental and numerical conditions necessary to quantify the interfacial thermal conductance (G) of liquid-metal interfaces during two-phase flow. To do this, continuum-level numerical simulations are conducted in parallel with experimental measurements facilitating high-speed photography and in-situ time-domain thermoreflectance (TDTR). During microdroplet evaporation on laser heated Al thin-films at room temperature, an effective interfacial thermal conductance of G_eff = 6.4 ± 0.4 MW/m² is measured with TDTR. This effective interfacial thermal conductance (G_eff) is interpreted as the high-frequency (ac) interfacial heat transfer coefficient measured at the microdroplet/Al interface. Also on a laser heated surface, fractal-like condensation patterns form on the Al surface surrounding the evaporating microdroplet. This is due to the temperature gradient in the Al surface layer and cyclic vapor/air convection patterns outside the contact line. Laser heating, however, does not significantly increase the evaporation rate beyond that expected for microdroplet evaporation on isothermal Al thin-film surfaces.     

Pressure loss and forced convective heat transfer in an annulus filled with aluminum foam

This experimental study investigates non-Darcy flow and heat transfer in an annulus with high porosity aluminum foams to attain the miniaturization of thermal systems. The local wall temperature distribution, inlet and outlet pressures, and temperatures and heat transfer coefficient were measured for heat flux of 13.6–31.4 kW/m². The results show that aluminum foam enhances heat transfer from a surface compared with that of laminar flow in a clear annulus. Correlations for the friction factor and the Nusselt number are proposed and used for design of thermal applications.     

Convective heat transfer coefficients evaluation for a portable forced air tunnel

The objective of this work was the determination of the convective heat transfer coefficient in order to evaluate an experimental portable forced-air freezing tunnel and work on comparative studies with air exhausting and blowing. The heat transfer coefficients of the cooling air and the product were analyzed during freezing process. Convective coefficient results were higher for air exhaustion than air blowing in every part of the batch, except for the upper layer of products, where the cooling air of the chamber was directly in contact with the product. These results, with the temperature analysis obtained, indicated that the air circulation around the samples, as well as the heat transfer, improved with the use of the portable system, and had better results for the exhaustion compared to the blowing process.     

An experimental study of heat transfer in oscillating flow through a channel filled with an aluminum foam

An experimental study has been conducted on the heat transfer of oscillating flow through a channel filled with aluminum foam subjected to a constant wall heat flux. The surface temperature distribution on the wall, velocity of flow through porous channel and pressure drop across the test section were measured. The characteristics of pressure drop, the effects of the dimensionless amplitude of displacement and dimensionless frequency of oscillating flow on heat transfer in porous channel were analyzed. The results revealed that the heat transfer in oscillating flow is significantly enhanced by employing porous media in a plate channel. The cycle-averaged local Nusselt number increases with both the kinetic Reynolds number Re_ω and the dimensionless amplitude of flow displacement A₀. The length-averaged Nusselt number is effectively increased by increasing the kinetic Reynolds number from 178 to 874 for A₀ = 3.1-4.1. Based on the experimental data, a correlation equation of the length-averaged Nusselt number with the dimensionless parameters of Re_ω and A₀ is obtained for a porous channel with L/D_h = 3.     

Boundary layer flow and heat transfer on a continuous accelerated sheet extruded in an ambient micropolar fluid

An analysis is presented for the boundary layer flow and heattransfer on a continuous accelerated sheet extruded in a stationary ambient micropolar fluid. The governing non-linear differential equations have been solved numerically using implicit finite difference method. Numerical results explaining the effects of various parameters associated with the problem are discussed. Comparison results obtained for a Newtonian fluid reveals that the microelements present in the fluid reduce the velocity and frictional drag and cool the boundary. Larger acceleration is accompanied by larger skin-friction and heat transfer coefficients. In addition, varying the prescribed power law constant for the surface temperature affects the mechanism of heat transfer. Melt-spinning, polymer and glass industries and cooling of extruded melting plates are practical applications of this problem.     

Mathematical model for turbulent flow,heat transfer,and solidification

A steady‐state, two‐dimensional numerical model has been used to describe coupled liquid steel's turbulent flow and heat transfer with solidification for Fe‐C binary alloy in a crystallizer of inverse casting. The solid‐liquid phase change phenomena have been modeled by using continuum formulations and considering the mushy zone as porous media. The turbulence flow in the crystallizer has been accounted for using a modified version of the low‐Reynolds‐number κ?ε turbulence model. The flow pattern in the liquid zone and the temperature distribution in the solid, mushy, and liquid regions have been predicted. The numerical analysis indicates that the residence time of the mother sheet in the crystallizer is one of the key parameters. The effects of some other main parameters on the solidification behavior have also been studied, such as the thickness and the initial temperature of the mother sheet, and the superheat degree of liquid steel. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(7): 582–592, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10112     

Interfacial suction effects of a porous layer on filmwise condensation heat transfer enhancement

Considering the liquid transverse suction effect at the porous layer interface, a mathematical model was presented to investigate the influence of the porous layer characteristic parameters on condensation heat transfer. The results revealed that the enhancement ratio increased with the increase of the porous layer thickness and permeability. The effective thermal conductivity of the porous layer was, however, of little significance for condensation heat transfer enhancement. Also, the enhancement mechanism was analyzed by comparing the thermal resistances within the external condensate film and the porous layer. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(7): 568–577, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10058     

Study and modeling of heat transfer during the solidification of semi-crystalline polymers

Semi-crystalline polymers are materials whose behavior during their cooling is difficult to model because of the strong coupling between the crystallization, heat transfer, pressure and shear. Thanks to two original apparatus we study solidification of such a polymer without shear. Firstly the comparison between experimental results and a numerical model will permit to validate crystallization kinetic for cooling rate reachable by DSC. The second experiment makes it possible to analyze solidification for high cooling rate, corresponding to some manufacturing processes. It appears that crystallization has an influence on the thermal contact resistance.     

A numerical model for the analyses of heat transfer and leakages in a rotary air preheater

Air preheaters make a considerable contribution to the improved overall efficiency of fossil-fuel-fired power plants. In this study we used a combination of fluid dynamics and a newly developed three-dimensional numerical model for heat transfer as the basis for a theoretical analysis of a rotary air preheater. The model enables studies of the flue-gas flow through the preheater and the adjoining channels as well as the regenerative heat transfer and the resulting temperature distribution in the matrix of the preheater. Special attention was focused on the influences of leakages on the flue-gas parameters in the preheater. The numerical analysis and the experimental results showed an obvious dependence of the flue-gas parameters on various seal settings. Based on the results a method for online monitoring of the tightness of the radial seals is proposed.