Energy and Buildings, 17 (1991) 331-335 331 Research Communication The thermal behavior of a concrete 'finned' wall in a hot-arid zone Y. Etzion and E. Erell The Desert Architecture Unit, J. Blaustein Institute for Desert Research, Ben-Gurion University of the Negev, Sede-Boqer Campus, 84PPO (Israel) (Received February 17, 1990; accepted December 29, 1990; revised paper received May 17, 1991) Abstract The thermal behavior of an exposed concrete slab with a grid of truncated pyramids on its outer surface was found to differ considerably from that of a similar slab with a smooth outer surface, under hot-arid conditions. When the slab was placed ver- tically facing south (simulating a south-facing wall), the three-dimensional geometry of its exterior re- duced radiative heat gain. Placed horizontally, ra- diative heat gain increased. Painting the slabs white reduced the relative contribution of radiative heat gain, while the increased surface area resulted in the creation of a thick 'boundary' layer that affected the thermal behavior of the slab under certain conditions. Analytical description of these heat exchange processes has usually dealt with the general case of a wall (or ceiling) having the form of an infinite slab, thus simplifying the mathe- matical models. Where it is required to study more realistic conditions, numerical methods are used to provide computer simulations (Ak- bari et al. [1], for example). However, even though Oke [2] noted the effect of surface geometry in radiation exchange, and Gupta [3] described the effect of increased surface area on the thermal performance of a wall in Jais- almer, investigation of non-planar wall surfaces has so far been limited. A preliminary experiment was carried out to investigate the effects of altering the surface geometry of a building element on the heat flux through it. The basically planar surface form was replaced by a three-dimensional form, thus modifying both radiative and convective heat transfer between the surroundings and the building element, as well as the pattern of conductive heat flow inside the building material. 2. Experimental setup 1. Introduction The heat exchange processes occurring at the external surfaces of buildings have been studied extensively. Heat is transferred between the surface of the thermal mass and its en- vironment, mainly by radiation and convection, while the heat transfer taking place within the bulk of the material is purely conductive. As- suming that the ambient conditions are different from the internal ones and that they axe tran- sient in nature, the heat transfer in a wall having thermal mass and a finite resistance to heat flow consists of a fluctuating component (due to the thermal storage effect), superim- posed on the steady-state heat flow through it. Two concrete slabs were prepared, each one square meter in area, and having identical mass. Slab A, which served as the control, had a flat surface and a uniform thickness of 15 cm. Slab B had a 'finned' surface, created by an array of square truncated pyramids, 10 cm high and having a base of 8 cm × 8 cm and top of 6 cm × 6 cm. (This section was chosen in order to facilitate the extraction of the mold.) The pyramids were 2 cm apart at their bases, and the whole array projected from a 10-cm- thick slab (Fig. 1). Both slabs were insulated with 5-cm-thick expanded polystyrene board on all surfaces except the one being compared, and encased in a wooden frame 20 nun thick to facilitate handling. It is important to em- phasize that both slabs contained an identical amount of concrete and thus had the same thermal capacity, the only differences being in their geometry and their exposed surface area. 0378-7788/91/$3.50 © 1991 -- Elsevier Sequoia, Lausanne