fraction. The variation of mean void fraction e with D0/Ds is also demonstrated by the data of Beavers et al. (1973) and Clark & Beasley (1984). Flow characteristics in porous media have been studied by several researchers. Whitaker (1966) and Greenkorn (1981) demonstrated that the nonlinear behavior of flow in porous media was due to internal effects in the flow. Ergun (1952) developed a nonlinear model for flow in terms of a friction factor. This was later modified by MacDonald et al. (1979) to include geometry-dependent parameters. The heat transfer coefficient in packed beds has been extensively studied. Gamson et al. (1943) and Galloway & Sage (1970) developed correlations to calculate the overall heat transfer coefficient. Yagi & Kunii (1957) combined the effect of thermal conduction between particles and radiation into a stagnant effective thermal conductivity at no net fluid flow. Packed bed energy storage models can be generally divided into single and separate phase models for one- or two-dimensional cases. A summary of studies on modelling of packed beds for thermal energy storage using sensible heat can be found in Beasley & Clark (1984). In the area of latent heat thermal storage, several studies have been conducted on melting and freezing of PCM. Longwell (1958) used a graphical method for obtaining a numerical solution to Stefan-type problems involving a moving boundary which can be described in terms of our space coordinate. Murray & Landis (1959) improved the previous methods for the solution of one-dimensional heat conduction problems with melting and freezing. Sutherland & Grosh (1961) considered superheating and subcooling in calculating the temperature distribution before phase change occurs, during the transient process of phase change, and after steady-state conditions. Tao (1967) developed a numerical method and graphs of generalized solutions for the moving interface problem of freezing a saturated liquid inside a cylinder or a sphere. Solomon (1980) developed simple equations for the evaluation of the melting time of phase change material having a slab shape and a convective boundary condition. Marianowski & Maru (1977) studied latent heat thermal energy storage systems above 450°C. They placed emphasis on the choice of the salts. Lou (1983) proposed to add some nucleating agents to the salts to improve their poor nucleating properties which result in supercooling of the liquid salt hydrate prior to freezing. Torab & Beasley (1985) employed a finite difference method to study the dynamic response of a packed bed of encapsulated PCM. Ji (1986) developed two transient models for both sensible and latent thermal energy storage. Both models involve modeling of the phase change material as a conduction problem with sensible and latent heat energy storages and include consideration of the temperature gradients in both phases. Torab & Chang (1988) used a one-dimensional model to stimulate the performance of high temperature thermal energy storage for space power systems. Analysis and Modeling The model presented in this study is based on the constant temperature approach. The following coupled set of equations are the governing equations representing the temperature changes of the fluid and PCM. These equations are one-dimensional and for separate phases, i.e. they differentiate between temperatures of fluid and PCM at each point (Torab & Beasley, 1985). For fluid,
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