Showing papers on "Stefan number published in 2006"

Experimental and Numerical analysis of melting of PCM inside a spherical capsule

Abstract: The aim of this study is to analyze heat transfer performance of a spherical capsule using paraffin wax as PCM placed in a convective environment during the melting process. In the melting process, hot water enters the test vessel and flows over the spherical capsule which gets heated resulting in the melting of PCM. Experiments were carried out with heat transfer fluid temperature ranging from 70 to 82 °C for a capsule of radius 49 mm. A model for phase change using enthalpy method and finite difference formulation has been developed. The predicted results are compared with the experimental results and are found to be in good agreement. The model was then used to investigate the effect of capsule radius and the Stefan number on the thermal performance of the capsule i.e. the instantaneous heat flux, molten fraction and time for complete melting. Among different radii of the capsules analyzed, it is found that the shorter time for complete melting and higher time averaged heat flux occurs in the capsules of smaller radii and for higher Stefan number.

Heat transfer and parametric studies of an encapsulated phase change material based cool thermal energy storage system

Abstract: This work investigates the transient behaviour of a phase change material based cool thermal energy storage (CTES) system comprised of a cylindrical storage tank filled with encapsulated phase change materials (PCMs) in spherical container integrated with an ethylene glycol chiller plant. A simulation program was developed to evaluate the temperature histories of the heat transfer fluid (HTF) and the phase change material at any axial location during the charging period. The results of the model were validated by comparison with experimental results of temperature profiles of HTF and PCM. The model was also used to investigate the effect of porosity, Stanton number, Stefan number and Peclet number on CTES system performance. The results showed that increase in porosity contributes to a higher rate of energy storage. However, for a given geometry and heat transfer coefficient, the mass of PCM charged in the unit decreases as the increase in porosity. The St number as well as the Ste number is also influential in the performance of the unit. The model is a convenient and more suitable method to determine the heat transfer characteristics of CTES system. The results reported are much useful for designing CTES system.

Nonlinear oscillatory convection in mushy layers

Abstract: We study the problem of nonlinear development of oscillatory convective instability in a two-dimensional mushy layer during solidification of a binary mixture. We adopt the near-eutectic limit, making the problem analytically tractable using standard perturbation techniques. We consider also a distinguished limit of large Stefan number, which allows a destabilization of the system to an oscillatory mode of convection. We find that either travelling waves or standing waves can be supercritically stable, depending strongly on the sensitivity of permeability of the mushy layer to variations in the local solid fraction: mushy-layer systems with relatively weak sensitivity are more likely to select travelling waves rather than standing waves in the nonlinear regime. Furthermore, the decrease in permeability is found to promote the subcritical, and hence more unstable, primary oscillatory states. In addition to mapping out the location of different stable oscillatory patterns in the available parameter space, we give the detailed spatio-temporal structure of the corresponding thermal, flow and solid-fraction fields within the mushy layer, as well as the local bulk composition in the resulting eutectic solid.

A comparison of an enthalpy and a temperature method for melting problems on composite domains

Abstract: In optical rewritable recording media, such as the Blu-ray Disc, amorphous marks are formed on a crystalline background of a phase-change layer, by means of short, high power laser pulses. It is of great importance to understand the mark formation pro- cess, in order to improve this data storage concept. The recording layer is part of a grooved multi-layered geometry, consisting of a variety of materials of which the material proper- ties are assumed to be constant per layer, but may differ by various orders of magnitude in different layers. The melting stage of the mark formation process requires the inclusion of latent heat. In this study a comparison is made of numerical techniques for resolving the associated Stefan problem. The considered methods have been adapted to be applicable to multi-layers.

A Numerical Investigation of Ablation and Transpiration Cooling Using the Local Thermal Non-equilibrium Model

Abstract: This paper presents a simplified, transient and local thermal non-equilibrium model to simulate the cooling processes of ablation and transpiration. The errors caused by the assumption of the local thermal equilibrium are discussed, and the performance of ablation and transpiration cooling is investigated by the local thermal non-equilibrium model. The discussion and investigation indicate that in the ablation and transpiration cooling process, the effect of the thermal non-equilibrium and the variation of thermal properties should be considered, and the Biot number, Stefan number, heat flux as well as coolant mass flow rate are the important parameters to control the thickness loss due to material ablation.

Heat transfer performance of paraffin as a phase change material(PCM) in a concentric annulus

Abstract: In order to the investigate heat transfer enhancement in paraffin side,a heat transfer experimental setup was constructed.Heat transfer performance of thermal storage processes of paraffin PCM was studied using a constant temperature heat source as the heating medium.The relationships between Stefan number,thermal storage time,paraffin temperature and radial distance were also investigated.The result shows that heat transfer effect with a finned tube under conditions of heat conduction is better than that under conditions of convection.With heating medium temperatures of 65?℃,70?℃ and 75?℃,when comparing the finned tube with the plain tube,thermal storage times are shortened by 77.5%,69.2% and 56.3% respectively,demonstrating the beneficial effect of the fins on the heat transfer enhancement in the paraffin.In addition,the thermal storage time and the lapse rate of phase interface vary monotonically with Stefan number.

Experimental Research on Heat Transfer Characteristics in Microencapsulated Phase Change Material Suspensions

Abstract: The research progress of microencapsulated phase change material suspensions is introduced.The experimental system of laminar heat transfer characteristics in microencapsulated phase change material suspensions is established.The laminar forced convection heat transfer of the suspensions in a circular tube under heating condition with constant heat flux density is tested and analyzed.The key factors affecting the heat transfer enhancement are Reynolds number and volumetric fraction of micro-capsule,and the influence of Stefan number is negligible.The correct Nusselt number for the laminar heat transfer of the suspensions in a circular tube is higher than that for water by a factor of 2.5 to 3.0,and the correct Nusselt number increases with increasing Reynolds number and volumetric fraction of micro-capsule.

Nonlinear oscillatory convection in mushy layers

Abstract: We study the problem of nonlinear development of oscillatory convective instability in a two-dimensional mushy layer during solidification of a binary mixture We adopt the near-eutectic limit, making the problem analytically tractable using standard perturbation techniques We consider also a distinguished limit of large Stefan number, which allows a destabilization of the system to an oscillatory mode of convection We find that either travelling waves or standing waves can be supercritically stable, depending strongly on the sensitivity of permeability of the mushy layer to variations in the local solid fraction: mushy-layer systems with relatively weak sensitivity are more likely to select travelling waves rather than standing waves in the nonlinear regime Furthermore, the decrease in permeability is found to promote the subcritical, and hence more unstable, primary oscillatory states In addition to mapping out the location of different stable oscillatory patterns in the available parameter space, we give the detailed spatio-temporal structure of the corresponding thermal, flow and solid-fraction fields within the mushy layer, as well as the local bulk composition in the resulting eutectic solid