Mixed convective laminar flow through non-circular channels heated at a constant heat flux

Abstract

Together with the need to improve the thermal efficiency, there is a continuous drive towards miniaturisation and compact heat exchangers using non-circular channels are commonly employed. In this study, mixed convective laminar flow through non-circular channels was numerically investigated using Ansys Fluent 2024R1. The hydraulic diameter of the channels was chosen as 4 mm, 8 mm, and 11.5 mm with aspect ratios (AR) of ½, 1, and 2, while the Reynolds number and heat flux ranges were 1000–1800 and 1–3 kW/m2, respectively. Special attention was given to the local Nusselt number trends, velocity and temperature profiles, and circulation strength along the channels. In general, the local Nusselt numbers increased with increasing hydraulic diameter and heat flux, and decreasing Reynolds number, due to increased buoyancy effects and circulation strength. While an increase in channel height enhanced the secondary flow velocity, an increased in channel width enabled the formation of a secondary vortex pair, which significantly enhanced the thermal performance. The relationship between the circulation strength and Nusselt number was identified, and the local Nusselt numbers increased with increasing heat flux and decreasing Reynolds number. At a fixed circulation strength, improved cross-sectional mixing led to higher Nusselt numbers. The Nusselt numbers increased with decreasing aspect ratio and overall AR = ½ provided the best heat transfer performance for mixed convective flow through non-circular channels. When comparing the single channels results to counterflow multi-channel heat exchangers, the results correlated very well, indicating potential savings in computational costs. HIGHLIGHTS • Decreasing the aspect ratios enhanced circulation strength and mixed convective heat transfer • The formation of secondary vortex pairs leads to increased heat transfer enhancement • Channel width is the key factor determining the formation of secondary vortex pairs • Heat transfer performance increases with increased cross-sectional mixing • Best heat transfer performance obtained using an aspect ratio of ½ • Single channel results can be translated to larger multi-channel systems