Heat transfer enhancement in solar air heater having multi-arc shape artificial roughness with gap

Abstract

Energy in various forms has played an increasingly important role in world-wide economic progress and industrialization. Sunlight available freely as a direct and perennial source of energy provides a non-polluting reservoir of fuel. The simplest and the most efficient ways to utilize solar energy are to convert it into thermal energy for heating applications by using solar collectors. The thermal efficiency of solar air heaters in comparison of solar water heaters has been found to be generally poor because of their inherently low heat transfer capability between the absorber plate and air. Various methods to improve heat transfer in thermal system have been developed extensively. The majority of commercially available enhancement techniques are passive ones. The use of artificial roughness on a surface is an effective technique to enhance heat transfer to fluid flowing in ducts. In this research work, a new technique of heat transfer enhancement is proposed, which contains an artificial roughness in the form of multi arc shape ribs with gap on the inner side surface of the absorber plate. The artificial roughness on the absorber plates having three different pitch ratios (15, 20 and 25). The heat transfer enhancement with newly designed artificial roughness is compared with smooth absorber plate. The maximum heat transfer enhancement is observed for the Relative Roughness Pitch (P/e) of 15 and 51% increase in heat transfer with respect to smooth plate on account of increase in friction factor of 40%. It has been observed that the new passive enhancement technique proposed in this work shows substantial increase in heat transfer rate over smooth plate.