Thermal gradient avoidance under stratified two-phase flow in DSG by capillar structures

Abstract
The benefits of substitution of thermal oil with water in two phase flow for electricity production in parabolic trough solar fields have been pointed out in several papers, [1]. The need of covering a wide range of flow masses to cope with variable solar isolation, means low mass flows, that provide an insufficient cooling of the horizontal pipes, when stratification occurs. The large differences between the heat transfer coefficients for wetted (liquid to solid) and dry segments (steam to solid) will lead to very large thermal stresses and absorber misalignment from the focal axis of the parabolic trough collector. Between the solutions to avoid the drawbacks just mentioned, the increase of the wetting perimeter by capillary forces is found. Capillary pressures, developed by internal porous coated or microgrooved pipes, lift up a thin liquid film, that rewets all the perimeter of the pipe and whose thermal resistance, because of boiling, is very low. In this paper, a model of the theoretical wetting film behaviour and its relation to the characteristic of the system (microgroove's depth, permeability of porous coatings,..) is shown. Experimental set up for measurements under temperature and pressure ranges for DSG conditions (31l°C, 100 bar) is shown.