Experimental investigation of convection enhancement by inlet swirl in a bayonet-type heat exchanger

A high temperature air heater (HTAH) is necessary for achieving the high combustion efficiencies requirement for commercial Magnetohydrodynamic (MHD) power generation plants. Recent developments in ceramic composite materials have shown great promise in making a recuperative HTAH possible. Given the high cost of HTAH ceramic composite materials it becomes necessary to maximize the heat transfer across the HTAH with respect to the pumping power. A bayonet-type is a unique heat exchanger tube configuration which has several advantages over a straight heat exchanger tube for use in the high temperature, highly corrosive MHD combustion exhaust. This thesis investigated the potential of convection enhancement by inlet swirl in a bayonet-type heat exchanger tube. An unheated flow visualization experiment was conducted to investigate the swirl characteristics of various inlet swirl techniques in order to obtain the greatest overall swirl component. A swirl injection header was used for the swirl technique, with the air injected at a 22° angle with respect to the radial direction. An electric tube furnace was used for heat transfer studies. Reynolds number investigated in the heat transfer studies ranged from 6900 to 55400. Overall heat transfer enhancement of 40% was consistently achieved for the entire range of turbulent Reynolds numbers investigated. In one test configuration, a 180% increase in the local Nusselt number was achieved. Further research should look into: additional swirl techniques for heat transfer studies such as curved-bladed vane swirlers or block swirlers. Effort should be made to investigate a higher range of Reynolds numbers.