Investigation of thermal-hydraulic performance of circular, elliptical & mixed circular-elliptical tube bundles for two-phase cross-flow boiling

Abstract

Tube bundles are an integral part of industrial heat exchangers and many applications have boiling and two-phase flow over tube bundles. Due to their importance, there is a need to investigate tube bundles with different geometrical shapes to find optimum designs with lower pressure drop and higher heat transfer rates. In this paper, a numerical analysis is presented for two-phase flow boiling over tube bundles of 3 types – with only circular, only elliptical, and a combination of circular and elliptical tubes. 2D simulations are performed using Ansys Fluent for cross-flow of water over heated tubes with tube surfaces supplied with a constant heat flux. The simulations incorporate the effect of vapour bubble formation and movement from the tube surfaces and combine the solution of volume-averaged energy and momentum conservation equations with volume-of-fluid (VOF) based interface tracking. From the simulation results, the thermal and hydraulic performance of the tube bundles are compared in terms of the overall heat transfer coefficient (HTC) and pressure drop, and the interaction of vapour bubbles in the tube bundles are presented through vapour fraction contours for different values of heat flux, mass flux and pitch-to-diameter (P/D) ratio. Simulation results show that using elliptical tube bundle increases the heat transfer in the range of 3 % - 26 %, along with a reduction in the pressure drop in the range of 56 % - 94 %. Comparison of the combined heat transfer and pressure drop characteristics, quantified by using the ratio of HTC and pressure drop (H/P), show that the elliptical tube bundle has higher values in the range 3.0 - 4.25 times that of circular tube bundle for P/D = 1.25 and 8.5 – 18.4 for P/D = 1.95. However, to minimize cost, a mixed tube bundle with circular tubes at the bottom and elliptical tubes at the top can be a suitable alternative as it has H/P values ranging from 1.53 to 1.85 times that of the corresponding circular tube bundle.