[en] A hybrid numerical method is developed to solve one-dimensional phase change problems with the mushy zone. This hybrid numerical method involves the control volume formulation for the space domain and the Laplace transform technique for the time domain. In the present study, nonlinear terms are linearized by using the Taylor series approximation. The growth of the mushy zone is unknown a priori and is predicted by using the least squares concept. To show the efficiency of the present numerical method, various comparative examples are illustrated. It can be seen that excellent agreement is observed between present numerical results and those of early works

[en] A numerical model is presented, based on the local volume averaging formulation of transport phenomena in porous media, for simulating meltwater infiltration into unsaturated, frozen soil. With the defined flow and freezing boundary conditions at the snow-soil interface, using the concept of a surface local averaging volume, the time variation in profiles of temperature, liquid/ice content, infiltration/percolation rates, and rate of phase change in upper soil layers are predicted. In addition to a parametric analysis, model estimates of infiltration are compared with quantities calculated from field measurements of soil moisture changes and temperature during snow cover ablation, showing a reasonable agreement

[en] Buoyancy-driven flows in enclosures play a vital role in many engineering applications such as double glazing, ventilation of rooms, nuclear reactor insulation, solar energy collection, cooling of electronic components, and crystal growth in liquids. Here, numerical study on buoyancy-driven laminar flow in an inclined square enclosure heated from one side and cooled from the adjacent side is conducted using finite difference methods. The effect of inclination angle on fluid flow and heat transfer is investigated by varying the angle of inclination between 0 degree and 360degree, and the results are presented in the form of streamlines and isotherms for different inclination angles and Rayleigh numbers. On the basis of the numerical data, the authors determine the critical values of the inclination angle at which the rate of the transfer within the enclosure is either maximum or minimum

[en] This work deals with the problem of transient conjugated forced convection heat transfer in turbulent pipe flows. The external surface of the pipe over a finite heated section is subjected to either uniform heat flux or uniform wall temperature. The governing parameters identified in this work are the Reynolds number Re, the wall-to-fluid conductivity ratio K, the wall-to-fluid diffusivity ratio A, the dimensionless wall thickness Δ, and the Prandtl number Pr. A modified low-Re κ-ε turbulent model is adopted to solve for the fully developed velocity and eddy viscosity distributions. Predicted results show that effects of wall conduction and wall heat capacity have a significant impact on the unsteady heat transfer, especially in the early transient period

[en] Combined free and forced convection in vertical annular passages is important in the design of coolant channels for power transformers, nuclear reactors, double-pipe heat exchangers, certain types of catalytic converters, and modern electronic equipment. Although most such equipment is designed to operate in a turbulent flow regime, laminar flow and heat transfer become important when the equipment operates under reduced power or during accidental pump failure. This paper presents a numerical study of laminar, fully developed mixed convection in vertical eccentric annular ducts. The equations governing the velocity and temperature are solved on a body-conforming grid by using a finite-volume technique. The effects of radius ratio, eccentricity, and Rayleigh number on the friction factor and the Nusselt number are discussed. The buoyancy forces significantly increase both friction and heat transfer. The effect of buoyancy is stronger for configuration with larger eccentricities

[en] Magnetohydrodynamic mixed convection flow about a vertical flat plate embedded in a porous medium is considered. The effect of the magnetic field strength on the local Nusselt number and local wall shear stress is presented. The non-Darcian model including both the inertial and boundary effects is used. A particular transformation for the governing equations is adopted to cover the whole mixed convection regime within two finite limits. Appreciable effects of the magnetic field strength on the local Nusselt number as well as on the local wall shear stress in the mixed convection regime are found

[en] A numerical investigation is presented of steady and unsteady heat transfer in axially and readily diluted nuclear fuel rods. The transient performance is assumed to follow a sudden and complete loss of coolant starting from steady state operation. Steady state conditions are obtained from solving numerically a conjugate conduction problem in the fuel rod and a turbulent forced convection problem in the coolant section. To model turbulence, the mixing length model is used. Dilution is accomplished by adding high thermal property materials, either axially or radially, to the original fuel rods with the intention of increasing the time delay before melting of the reactor in case of loss of coolant. The effects of the amount, distribution, and material of added diluent on steady and unsteady heat transfer are studied. Results indicate that axial dilution has negligible influence on the thermal performance of the reactor. Radial dilution, however, holds great promise and shows a reduction in the maximum wall and fuel temperature sunder steady operation and substantial increase in the time delay before melting under transient conditions. The value of this time delay increases as the amount of added diluent is increased. Moreover, the distribution of the added diluent is shown to have small effects on the steady and unsteady performance of the reactor, while the type of diluent material is found to affect the transient performance only

[en] Mixed convection flow in tubes is encountered in many engineering applications, such as solar collectors, nuclear reactors, and compact heat exchangers. Here, a numerical investigation has been conducted in order to determine the effects of variable properties on the flow pattern and heat transfer performances in laminar developing ascending flow with mixed convection for two cases: in case 1 the fluid is heated, and in case 2 it is cooled. Calculations are performed for air at various Grashof numbers with a fixed entrance Reynolds number of 500 using both the Boussinesq approximation (constant-property model) and a variable-property model. In the latter case, the fluid viscosity and thermal conductivity are allowed to vary with absolute temperature according to simple power laws, while the density varies linearly with the temperature, and the heat capacity is assumed to be constant. The comparison between constant- and variable-property models shows a substantial difference in the temperature and velocity fields when the Grashof number |Gr| is increased. The friction factor is seen to be underpredicted by the Boussinesq approximation when the fluid is heated (case 1), while it is overpredicted for the cooling case (case 2). However, the effects on the heat transfer performance remain negligible except for cases with reverse flow. On the whole, the variable-property model predicts flow reversal at lower values of |Gr|, especially for flows with opposing buoyancy forces. The deviation in results is associated to the difference between the fluid bulk and the wall temperature

[en] The countercurrent flow of gas and water in a short horizontal pipe is studied numerically with a two-phase flow model. It is observed that the onset of flooding cannot be predicted at low liquid flow rates using conventional one-dimensional equations. The conventional equations yield the same underestimated results as the Taitel-Dukler criterion. Utilizing physical reasoning, improved equations have been derived. The basic idea is that the distribution of the phase velocities should not be treated as uniform in the cross-sectional area occupied by phases but transverse dependencies for the velocities should be allowed. By comparing measurement data and calculated results, it is shown that flooding transition can be predicted accurately with these equations

[en] In the chemical vapor deposition process, the objective is to grow a crystal film as fast as possible, while at the same time ensuring its chemical uniformity and crystal graphic perfection. This paper studies the possible enhancement of diffusion-controlled mass transfer by rotating a cylindrical substrate, which is used in the manufacture of superconductor materials. To preserve the free-stream properties, a numerical technique bearing a strong conservation property is described. The solutions are validated against previously published data on the drag and lift coefficients for a rotating cylinder. The numerical inaccuracy associated with the use of the conventional weak conservation form of the Navier-Stokes equations in polar coordinates is shown to be around 15%. It is found that rotation has minor influence on the average mass transfer rate and it generally increases the uniformity of the transport process