Experimental investigation of thermal-hydraulic characteristics of finned oval tube bundles in cross-flow arrangements

Abstract

Enhancement of heat transfer using finned circular tubes arranged in counter-flow configurations are accompanied by high pressure losses. To obviate these problems, finned oval tubes are introduced which can enhance heat transfer while keeping pressure losses minimum. In this paper, different configurations of finned oval tubes which are called tube bundles are experimentally investigated using a thermal camera and a smoke wind tunnel (1800 <Re∞<8200). Given that thermal performance and hydraulic behavior of the tube bundles have reciprocal effects on each other, flow patterns, shedding behavior, and temperature distribution on the finned oval tubes in presence of a heat flux are visualized experimentally for the first time in order to monitor wake zones and their corresponding sizes and impacts on the heat transfer performance of finned oval tube bundles. Effects of diagonal tube pitch and angle of tubes arranged in the staggered configurations on the flow regime, heat transfer rate and pressure loss are studied and the Chilton–Colburn J-factor and friction coefficient f are investigated. Following these investigations, three distinct flow patterns are identified. Based on the results, recirculation regions formed behind the central tube and the dead zone between upper tubes imposes negative effects on the flow behavior leading to a bad thermal performance. Based on the results, for the studied Reynolds numbers and arrangements, smaller tube diagonal pitches are the most desirable in staggered configurations of finned oval tubes. Nusselt number is almost 25% higher for smaller tube pitches in relation to the larger one. Bundles with smaller tube pitches have also higher efficiency indices. Accordingly, at all configurations, finned oval tube bundles perform better at low and high Reynolds numbers from the efficiency index point of view. Therefore, it is better to use these kinds of tubes at low Reynolds numbers in the industry.