Entropy generation analysis of turbulent flow in conical tubes with dimples: a numerical study

Abstract

Minimizing the entropy generation rate is one of the key performance indicators for enhancing the thermal design of heat exchangers. This paper introduces a comprehensive numerical entropy generation analysis of turbulent water flow inside—newly proposed—conical tubes with dimples subjected to a constant heat flux. The effect of different tube diameter ratios (DR = 1, 1.5, 2, 3, and 5) and flow modes (convergent and divergent tube configurations) on the thermal, viscous, and total entropy generation rates is investigated within Reynolds number (Re) range of 3 × 103–40 × 103 using ANSYS-Fluent package. Realizable k-ε (RKE) turbulence model is adopted in this study. A well-validated 3D model was adopted to estimate the dimensionless indices: Bejan number (Be), enhanced entropy generation ratio (Ns,en), and the irreversibility distribution ratio ($${\phi }_{\mathrm{s}}$$ ϕ s ) to characterize the entropy generation performance and to compare conical dimpled tubes to smooth ones. The results showed that total entropy production rate values for dimpled tube geometries are lower than those for the corresponding smooth ones, especially for convergent dimpled tubes. Convergent dimpled tubes with DR in the range of 1.5–3 achieved the lowest total entropy production values over the whole Re range, with an average value of 0.20 W K−1, as compared to an average value of 0.34 W K−1 for the smooth configurations. The average Ns,en values for dimpled convergent tubes with DR = 1.5–3 are 0.46 and 0.80 at Re = 3000 and 40,000, with reductions of 50.54% and 3.61% at both Re values, respectively. The study also showed that the entropy generation analysis could provide an effective tool to highlight the optimal design of tube heat exchangers based on the minimum entropy generation and the enhanced entropy generation ratio.