Stefan number

About: Stefan number is a research topic. Over the lifetime, 482 publications have been published within this topic receiving 32056 citations.

Phase change with multiple fronts in cylindrical systems using the boundary element method

Abstract: In this paper, multiple front phase change problems in one-dimensional cylindrical systems are investigated. The objective is to develop a numerical solution using the BEM for freezing and melting problems involving multiple moving fronts. Multiple moving phase fronts arise when the phase change material (PCM) is subjected to alternate driving temperatures that cause the surface temperature of the PCM to change back and forth across the phase change temperature. This kind of problem is highly nonlinear at the phase fronts that separate alternate liquid and solid layers with different properties. Fully implicit time discretization is applied to ensure numerically stable results. Numerical results are presented for a cylindrical problem with the inner surface subjected to a convective environment where the temperature changes between values above and below the freeze temperature of the PCM. This condition could occur in ice thermal storage systems. The numerical behaviour of the creation and collapse of the moving fronts is investigated by changing the Stefan number, Biot number, initial temperature, the cycling time length and the time step size.

Studies on the inward spherical solidification of a phase change material dispersed with macro particles

Abstract: This paper investigates numerically the solidification of a phase change material (PCM) dispersed with high conductivity macro particles inside a spherical container. The formulation takes into account the addition of particles invoking an effective thermal conductivity model. A case study has been made comparing the experimental results available in the open literature for the solidification of a pure PCM case (without particles) to investigate the influence of particles. The results show that the addition of particles between 10 and 50% by volume, enhances the heat transfer rate by about 13.5 and 59% respectively. Parametric studies have been carried out to investigate the influence of relevant dimensionless numbers such as Biot number (Bi) and Stefan number (Ste) on the solidification characteristics of the PCM for different particle fractions. The role of particles was found to be significant at lower Ste compared to Bi. It has been concluded that the effect of particle fraction on the solidification is more compared to that of particle-PCM thermal conductivity ratio. It was found that there is no restriction on the choice of particle material for a given PCM, as long as the particle-PCM thermal conductivity ratio considered remain larger than 5.

Fluctuation-dissipation analysis of nonequilibrium thermal transport at the hydrate dissociation interface.

Abstract: Herein, a nonequilibrium molecular-dynamics simulation in an NVE ensemble was performed to investigate the spontaneous dissociation of methane-hydrate when it came in contact with liquid water. The nonequilibrium in the interface region is linked to the dissociation process of the hydrate near the interface according to the Onsager's hypothesis. The simulated thickness of the interface was found to be close to the acoustic phonon mean path of methane hydrate and agreed with the reference value. The normalized heat flow autocorrelation function was introduced to study fluctuation–dissipation in terms of the thickness and moving velocity of the interface and the Stefan number. This helped to clearly identify three distinct hydrate-decomposition regimes dominated by sensible heat, latent heat and an intrinsically unstable lattice framework. It was found that the fluctuation–dissipation theory could express the nonequilibrium nature in the front two stages before the threshold was reached, and the dissociation rate increased in the latter stage; this was different from the case of thermal-driven dissociation. The Stefan number decreased rapidly with dissociation in the initial stage and then fluctuated in the intermediate stage; this was analogous to the fluctuation characteristics of the heat flow autocorrelation function. The Stefan number effect shows that thermal dissipation drives the hydrate dissociation and correlates fluctuation to the nonequilibrium nature. It was also found that a small Stefan number was enough to break up the residual hydrate soon after the threshold was achieved. The transient interfacial thermal resistance of the interfacial region was obtained as a typical value in the range of 10−7–10−9 m2 K W−1. This justifies that fluctuation–dissipation exists in the nonequilibrium process of hydrate dissociation either in terms of heat flux, as observed in this study, or the diffusion of guest molecules, as reported in other studies.