Heat Transfer Analysis of Different Storing Media Using Oil as Working Fluid

Abstract

Thermal analysis of heat transfer through different storing media using oil as working fluid is presented. The storing medium is solid material in spherical shape. Steel, glass, and pebbles are selected as storing media and oil is selected as working fluid. The physical model is a heat exchanger in cylindrical shape, which is packed with each of the selected storing medium. The heat transfer through the heat exchanger is assumed to be one dimensional along its height. The flow of the working fluid is an axial direction from the top to downward. The problem is governed by two partial differential equations for the working fluid (oil) and the storing medium. Finite difference method and Thomas algorithm solver are used to solve the couple of the two partial differential equations along with their associated initial and boundary conditions. The modified computer program is used to obtain the solution of transient temperature distribution of the storing medium and the working fluid. The amount of absorbed heat inside each of the storing medium is obtained. The effect of special operating parameters on the amount of absorbed heat inside the storing medium, such as aspect ratio (the ratio between diameter and length of the heat exchanger), storing media, mass velocity, the number of charging cycles, and void fraction, is discussed. Therefore, the dimensionless heat transfer coefficient parameter (Nusselt number, Nu) provides a measure of the convection and conduction heat transfer at the surface of storing medium when the working fluid (oil) flows over a solid surface of the medium. The numerical results of transient temperature profiles and the amount of absorbed heat inside the storing medium for each system with respect to the operating parameters and the heat exchanger characteristics are illustrated. The results show that steel storing medium is charging by four cycles while the pebble storing medium is charging by two cycles only, this due to the thermal and physical properties of these materials. The absorbed heat inside storing medium, which has aspect ratio equals one (diameter of the heat exchanger equals its length) is higher than others. Increasing mass velocity increases absorbed heat inside the storing medium and decreasing the charging time. Increasing void fraction decreases absorbed heat inside the storing media due to the smaller volume of absorbing medium. The amount of absorbed heat (at certain time) inside the steel > glass > pebble is due to the thermal conductivity of these materials.