Akira Nakayama

By Chen Yang, Huijin Xu and Akira Nakayama

A volumetric solar receiver receives the concentrated radiation generated by a large number of heliostats. Turbulent heat transfer occurs from the solid matrix to the air as it passes through the porous receiver. Such combined heat transfer within the receiver, including radiation, convection and conduction, is studied using a local thermal non-equilibrium model. Both the Rosseland approximation and the P1 model are applied to consider the radiative heat transfer through the solar receiver. Furthermore, the low Mach approximation is exploited to investigate the compressible flow through the receiver. Analytic solutions are obtained for the developments of air and ceramic temperatures as well as the pressure along the flow direction. Since the corresponding fluid and solid temperature variations generated under the Rosseland approximation agree fairly well with those based on the P1 model, the Rosseland approximation is used for further analysis. The results indicate that the pore diameter must be larger than its critical value to obtain high receiver efficiency. Moreover, it has been found that optimal pore diameter exists for achieving the maximum receiver efficiency under the equal pumping power. The solutions provide effective guidance for a novel volumetric solar receiver design of silicon carbide ceramic foam.

Part of the book: Foams

By Xiaohui Bai, Cunliang Liu and Akira Nakayama

In recent decades, many studies on flow and heat transfer in porous media have been conducted by researchers to take advantage of its high specific surface area and good heat transfer performance. What is more, the graded porous media have also drawn great attention since the graded arrangement enhances local heat transfer coefficient, so as to improve the overall heat transfer performance and temperature uniformity. The new forms of structure provide a new design of cooling system for some high-power heat sources, such as electronic components, compact heat exchangers, and hot components in aeroengine. In this chapter, the problems of channels filled with vertically graded porous media and axially graded porous media have been introduced, respectively. The flow and heat transfer characteristics of graded porous media have been studied based on porous media theory, considering the parameters of porosity, permeability, pore diameter, etc. The profiles of velocity and temperature vary with different porosity arrangements present. The maximum heat transfer coefficient was obtained for the case of high porosity at the center and low porosity near the wall. Furthermore, the possibility of application in the aeroengine cooling has been discussed.

Part of the book: Transport Perspectives for Porous Medium Applications