An assessment of performance of bifurcations in thermal systems with new correlations for estimating Nusselt number and loss coefficient

Abstract

Branching networks are continuously being contemplated for use in compact heat transfer systems. They are constructed by connecting straight tubes through bifurcations. The present study investigates the thermo-hydraulic performance of three different bifurcation geometries, viz. symmetric Y-bifurcation, T-bifurcation and asymmetric Y-bifurcation. The velocity and temperature contours show a complementary nature wherein low velocity regions correspond to high temperatures and vice-versa. New quantitative parameters viz. relative secondary strength (E_s/t), velocity non-uniformity (δ_v), temperature non-uniformity (δ_t) and the performance index (ξ) are introduced to understand the effects of the geometry on the flow field and thermal performance. The T-bifurcation shows the best heat transfer performance, but also the worst hydraulic performance. On the other hand, the asymmetric Y-bifurcation shows the worst heat transfer performance, but the best hydraulic performance. Based on the values of the performance index, it is concluded that the performance of the asymmetric Y-bifurcation is better than that of the symmetric Y-bifurcation, which in turn performs better than the T-bifurcation. However, the non-uniform flow distribution in the asymmetric geometry may lead to issues when such bifurcations are connected in series and parallel to form a distribution network. A comparison of the pressure loss obtained by the CFD simulations with theoretical estimations shows the utility of the comprehensive numerical framework. New correlations are also proposed to determine the Nusselt number and loss coefficient as functions of the bifurcation angle and Reynolds number.