Influence of shape factor and notch design of transverse trapezoidal shaped rib on thermal performance of solar air heater-a CFD study

Abstract

Solar air heaters generally exhibit lower levels of thermal performance due to thermal losses and poor heat transfer characteristics of air. One of the commonly used techniques for performance improvement is to enhance turbulence levels in the flowing air stream. This paper presents the computational fluid dynamics analysis on the influence of trapezoidal rib shape factor and notch on the overall thermal behavior of solar air heater at constant heat flux conditions. The simulation is conducted for different Reynolds number varying from 11000 to 25000. The rib shape factor is varied as 1.71-8.85 for a fixed rib height value of 1.4 mm. The results reveal that lower values of shape factor improve heat transfer by about 13.1% for the rib shape factor of 2.42 while lower values of shape factor produce increased pressure drop by about 31% on an average for the rib shape factor of 1.71. The presence of notches create additional peak Nusselt number locations which however is negated by the presence of entrapped smaller eddies which effectively result in marginal improvement in heat transfer relative to the notchless rib irrespective of the geometry of notch. It is concluded that the suitable design modification of notches is essential to overcome the hot spots in order to take advantage of the concept of notch on ribs to achieve improved thermal characteristics.