NATURAL CONVECTIVE HEAT TRANSFER IN AN ASYMMETRIC GREENHOUSE-TYPE SOLAR STILL--EFFECT OF ANGLE OF INCLINATION

Abstract

The natural convective flow and heat transfer in air inside an asymmetric, greenhouse-type solar still is studied numerically. The actual configuration consists of two top glass covers, perpendicular to each other and forming angles of 30° and 60° with respect to the horizontal, along with a blackened basin at the bottom where saline water evaporates. The Rayleigh number varies over a wide range of values during a 24-hour cycle of operation, but most of the time the values are high enough so that turbulent flow conditions prevail. Here the interest is in the heat transfer problem and the arising flow structure in the still, and two-dimensional computations are performed using a stream function-vorticity formulation along with a low Reynolds turbulence model. A curvilinear coordinate system is used, with a grid that is made orthogonal to all solid boundaries for most of their length. A multicellular flow pattern arises in the core of the still, depending on the value of the Rayleigh number (here Ra = 10 7 - 10 10 based on the horizontal dimension of the still), with thin boundary layers forming along the top covers and the bottom surface. Alternative configurations are investigated by changing the angle of inclination of the main condensation surface at the top cover. Detailed comparisons are made for a reduced slope of 15° for that surface, and by examining flow patterns and heat transfer rates the relative merits of each of the angles within the chosen range of Ra are examined.