Numerical Studies on the Effect of Different Obstruction Geometries on Performance of Rectangular Channel Air Heaters

Abstract

Air heaters have poor thermal efficiency. Flow obstructions improve the efficiency of the air heater by enhancing heat transfer and disrupting the formation of the laminar sub-layer. In the present study, a rectangular channel of aspect ratio 6 and obstructions of various geometry (triangular, rectangular, and arc (or semi-circular)) are used with a longitudinal pitch (Pl/e) of 4, transverse pitch (Pt/b) of 2 ratios for Reynolds Number (Re) from 5000 to 20000. Various performance parameters such as Nusselt Number (Nu), Friction factor (f), and Thermal Enhancement Factor (TEF) were analyzed for the above conditions using Computational Fluid Dynamics (CFD) to compare the thermohydraulic performance of air heaters with various obstruction geometries. Maximum TEF of 1.27, 1.08, and 1.05 are obtained at Re = 5000 for delta, rectangular, and arc flow obstructions, respectively. Delta flow obstructions are superior to arc and rectangular shapes for the investigated range of Reynolds numbers.

Keywords

• air heater
• thermal enhancement factor
• artificial roughness
• flow obstruction
• turbulence