Improving the performance of air-cooled condensers by using metal foams

Abstract

This research investigates the implementation of porous metals particularly aluminum foam as an enhanced surface to improve heat transfer on the air side of air-cooled, tubular heat exchangers. The target application for these heat exchangers is the condensers for geothermal power plants in remote locations of Australia where water for wet cooling is scarce. Traditional practices for liquid-to-air or vapor-to-air heat exchangers rely predominantly on finned surfaces to enhance heat transfer rate. Heat transfer enhancement in these designs is achieved via an increased surface area of simple geometries. Recent advancement in manufacturing and availability of porous materials make them possible to be utilized in thermal exchange equipment to improve efficiency and compactness.This work describes heat exchanger tubes based on circular cylinders with aluminum foam covering on the outer surface. Thermo-hydraulic performances are evaluated experimentally in a cross-flow using low-speed wind tunnel. The tests are performed on single cylinders, single row arrays, and multi-row tube bundles in both aligned and staggered configurations subjected to airflow of 0.5 to 5.0 m/s. This range of air velocities is chosen as it encompasses vertical flow regimes occur inside typical cooling towers. The effects of foam layer thickness, transversal and longitudinal tube pitches, foam-to-tube bonding methods, and tube bank patterns, are collectively investigated. The results are compared to baseline data obtained from conventional, annular finned tubes and tube bundles tested under the same conditions.The overall conclusions are drawn on heat transfer and pressure drop as two main comparison parameters. Experimental results on these parameters confirm the trend of numerical findings of a previous in-house study, although with varying degrees of differences on their magnitude. In all finned-versus foam-surface comparisons, under the same testing conditions and using specimens of similar dimensions, the latter shows convincing benefits on heat transfer/pressure drop ratios over the full range of chosen airflows. Within this range, the maximum relative advantage of the foam-covered heat exchangers over the finned type is observed at the midrange of airflow between 2.0 and 2.5 m/s. This result is both desirable and opportune because these designated airflows coincide with the top end velocity found inside most natural-draft cooling towers. Beyond this range of airflow up to 5.0 m/s, a typical range at the lower end of induced-or forced-draft cooling towers, the relative merit of heat transfer/pressure drop is rapidly degraded. However, in a situation where the increased heat transfer is under demand and compactness is not a critical factor, the pressure drop can be reduced satisfactorily by choosing a suitable transversal pitch within the tube bundle. Under this reduced blockage condition, metal foam heat exchanger bundles tend to behave as a group of an individual tube; therefore, retain higher relative benefit in terms of the sum...