Numerical investigation of heat transfer performance of an air heater with delta flow obstruction

Abstract

Air heaters are a vital component in many engineering systems, including room heating and agricultural dryers. The use of flow obstacles improves the convective heat transfer coefficient by interrupting the formation of a laminar sub-layer. In the previous studies reported, the thermal and frictional effects of various flow obstacle geometries were studied, but they are limited to non-inclined obstacles. The current research examines the thermal and hydraulic characteristics of air heaters with inclined and non-inclined delta shape obstacles using numerical techniques and compares the results with empirical predictions and literature data. The influence of flow rate and angle of attack (α) on Nusselt number (Nu) and friction factor (f) were evaluated using CFD for Reynolds numbers (Re) ranging from 5000 to 20,000 and for α ranging from 300 to 900. A rectangular duct (300 mm × 50 mm) with a constant heat flux of 800 W/m2 at the bottom surface of the air heater with delta obstructions of relative obstacle height (e/H) of 0.5 with a constant transverse (Pt/b) of 1.5 and longitudinal pitch (Pl/e) of 3 was considered. The Nu, f and Thermal Enhancement Factor (TEF) were calculated and investigated for various flow rates and attack angles. The predicted Nu and f values demonstrated a good agreement with empirical correlations, with an average variation of 5.68 percent and 7.2 percent, respectively. The CFD results were also consistent with earlier published experimental findings. The results indicate that using a delta obstruction enhances heat transfer rate considerably at the cost of increased pressure loss. The use of delta-shaped flow obstruction increases Nusselt number in the range of 71–162, with a 6–9.6 times increase in friction factor. A maximum TEF of 1.69 was obtained for the air heater with delta obstruction with α = 300 at Re = 5000.