Thermal and hydraulic characteristics of an air heater with modified delta flow obstructions

Abstract

Air heaters are extensively used for various applications like space heating, drying of agricultural products etc. Air heaters are low efficient due to poor heat transfer properties of air. Flow obstructions enhances the heat transfer coefficient of air by disturbing the formation of the laminar sub-layer. In this work, the effect of a modified inclined delta obstruction (inclined in the direction of flow (α) and in the direction perpendicular to flow (β) on the thermal and hydraulic performance parameters of an air heater (Nusselt number-Nu, friction factor-f and Thermal Enhancement Factor-TEF). In this study initially, performance parameters of a smooth air heater (size: 30 mm × 50 mm × 300 mm, constant heat flux: 800 W/m2) are analysed using Computational Fluid Dynamics (CFD) at different air flow rates (Re = 5000,10,000,15,000 and 20,000). In the second stage, experiments are conducted on a bench-scale setup using the same conditions to validate the results obtained from CFD. Results from the CFD study were also validated by using empirical correlations (Dittus – Boelter equation and Modified Blasius equation). In third stage, the effect of orientation (α: orientation in the direction of flow, β: orientation in the direction perpendicular to flow) of the modified delta flow obstructions was studied using CFD. From the present study, it is observed that the thermo-hydraulic performance of modified delta obstruction is found to be superior to conventional delta flow obstructions. Modified delta obstructions enhance Nu by 4 to 6 times and increase pressure drop by 6 to 10 times in comparison to the air heater without obstruction. From the numerical study, it was observed that the modified delta flow obstruction improves the performance of the air heater in comparison with conventional delta obstruction and the maximum TEF of 1.99 was obtained at Re =5000, α = 300 and β = -450.