Showing papers on "Stefan number published in 2007"

Analysis of Solid–Liquid Phase Change Under Pulsed Heating

Abstract: Solid/liquid phase change occurring in a rectangular container with and without metal foams subjected to periodic pulsed heating is investigated. Natural convection in the melt is considered. Volume-averaged mass and momentum equations are employed, with the Brinkman-Forchheimer extension to Darcy's law used to model the porous resistance. A local thermal nonequilibrium model, assuming equilibrium melting at the pore scale, is employed for energy transport through the metal foams and the interstitial phase change material (PCM). Separate volume-averaged energy equations for the foam and the PCM are written and are closed using a heat transfer coefficient. The enthalpy method is employed to account for phase change. The governing equations for the PCM without foam are derived from the porous medium equations. The governing equations are solved implicitly using a finite volume method on a fixed grid. The coupled effect of pulse width and natural convection in the melt is found to have a profound effect on the overall melting behavior. The influence of pulse width, Stefan number, and Rayleigh number on the temporal evolution of the melt front location and the melting rate for both the cases with and without metal foams is investigated.

Numerical simulation of control ablation by transpiration cooling

Abstract: A transient, one-dimensional numerical model is developed to describe the processes of transpiration cooling and ablation of the porous matrix used for the cooling. This model is based on the assumption of local thermal equilibrium. The problem of moving boundary due to ablation of the porous matrix is treated by the front-fixing method. This paper discusses the results of numerical simulations under different conditions and control parameters of ablation process. It was found that cooling effects and ablation processes are influenced by the coolant mass flow rate, the intensity of the heat flux, and the initial temperature at the start of transpiration cooling. In additional to the above three parameters, the Stefan number and the Biot number can also influence the transient cooling process, control ablative thickness of the porous plate by the reduction of ablative speed and duration, respectively.

Measurement of thermal conductivity of ice slurry made from solution by transient line heat-source technique (analytical discussion on influence of latent heat of fusion)

Abstract: We have been studying on ice slurry in a dynamic type ice storage system. The ice slurry has many good characteristics. The ice slurry can be made from a solution. When designing the ice storage system using this ice slurry, thermal conductivity of the ice slurry is essential. When thermal conductivity of the ice slurry made from a solution is measured by a transient line heat-source technique, a measurement value of thermal conductivity is affected by a latent heat of fusion of ice. Therefore, the thermal conductivity measured is apparent thermal conductivity. In this study, influences of Stefan number, initial concentration of the solution, initial solid fraction (initial IPF) and Fourier number on the thermal conductivity was analytically discussed to improve measurement accuracy of the thermal conductivity of ice slurry in the transient line heat-source technique.

Heat Transfer Model for Thin Solidified Material in Continuous Casting

Abstract: An asymptotic explicit numerical method was developed for the Stefan problem in which a series of solidification rates and boundary temperatures for the solidified material are given and the boundary heat flux is returned. A spectral method with several basis functions of a specialized shape in the solidification problem was adopted. Combined with multi-dimensional computational fluid dynamics methods for the liquid zone, this method is adequate for resolving the thin solidified material problem for a variety of continuous casting processes e.g. thin slab continuous casting, melt-spinning, twin roll casting, and edge-defined film-fed growth. The method is less expensive than conventional numerical methods and as accurate as a direct numerical approach such as the Finite Difference Method especially in the case of the Stefan number � 1 or in the case of variable material properties. [doi:10.2320/matertrans.MRA2007019]

Study on the freezing characteristics of soil surrounding the ground heat exchanger of ground coupled heat pump.

Abstract: The study on freezing characteristics of soil surrounding the GHE of GCHP (ground coupled heat pump) is performed based on the apparent heat capacity method. The influences of the soil water content, initial temperature, thermal diffusivity and Stefan number on soil temperature distribution and freezing velocity are discussed, the results indicate that the computed soil temperature, compared with unfreezing, is higher and thus the heat transfer resistance is smaller. This contributes to the decrease of GHE design length and system initial cost. It also proves that the increase of water content in soil is favourable to the design and operation of GCHP. Moreover, the increase of soil thermal diffusivity and the decrease of Stefan number can stay effectively the freezing velocity. These results may provide a preference for the application of GCHP in northern districts.

Moderate time scale finite amplitude analysis of large stefan number convection in a gravity modulated mushy layer during the solidification of binary alloys

Abstract: We investigate the convection amplitude in a binary alloy mushy layer subjected to a vibration body force that is collinear with the gravitational acceleration. The analysis shows that the convection amplitude decreases over time for all vibration frequencies tested. The analysis further reveals that as the vibration frequency increases, the convection amplitude subsequently decreases until a critical vibration frequency; at which the amplitude reaches the lowest value. Further increases in the vibration frequency increase the convection amplitude but gradually.