POEs Based on Highly Porous Cellular Materials

Abstract

Study of boiling processes in porous structures is an urgent task because the use of such structures is associated with the possibility of achieving extremely high levels of heat transfer. Analysis of the evaporating liquid inside a heated porous structure [1] shows that a necessary condition for the enhancement of heat transfer is a high hydraulic permeability of the porous structure for a vapor–liquid coolant flow. Most fully these requirements are satisfied by highly porous cellular materials (HPCMs), produced by metallization of polyurethane foam. The structural composition of HPCMs is important for understanding the characteristic features of heat and mass transfer intensification. In the case of HPCMs the cells are virtually identical and represent polyhedrons with cavities having a close-to-prolate ellipsoid of rotation (eccentricity of 1.1–1.3), the major axes of the cells being oriented in the direction of the polyurethane foam (Fig. 10.1a). Stacking of the HPCM cells is close to dense packing—each cell has on average 12 nearest neighbors; the average edge length is 0.4 of the average diameter of the cell. The HPCM cells are interconnected via the ‘windows’, the maximum size of which is 0.6 of the cell diameter. The HPCM structure is a matrix of the triangular bridges, which form the edge of cell faces; the bridges are secured with their ends in the nodes, i.e., each node has four bridges and belongs to four cells and the bridges belong to three cells. Such a connection provides a high rigidity and hydraulic permeability of HPCMs. Currently these porous structures are made of all the metals and alloys used in technology, the cell diameters range from 0.4 to 5 mm and porosity—from 80 up to 98 %.