Computational simulation of air-side heat transfer and pressure drop performance in staggered mannered twisted oval tube bundle operating in crossflow

Abstract

With a high demand for reduced energy consumption, twisted oval tube (TWT) heat exchangers are increasingly convenient for industrial applications. After a comprehensive literature survey, it was found that the literature so far discussed the counterflow efficiency of such heat exchangers, unfortunately, there is minimal emphasis on the thermohydraulic behaviour of the staggered TWT bundle operating in air-water cross-flows. And in particular, there are almost no computational studies to explore the thermal-hydraulic behaviour of staggered TWT bundles operating in crossflow. Further investigations with design optimisation are therefore required for efficient and cost-effective operation. In this study, a three-dimensional (3D) computational fluid dynamics (CFD) model is developed to analyse the air-side heat transfer and pressure drop performances in TWT bundle with staggered layout operating in air-water crossflow (600 < Re < 17,000). The Realizable k-ϵ model is used to evaluate the turbulence effect of the 3D fluid flow. The simulation results is firstly validated against the measured results and then the effects of geometrical parameters on the overall performance were analysed, such as ratio of outer major to outer minor axis (A/B = 1.491, 2.063 and 3.034), twisted pitch length (S = 0.2, 0.3 and 0.4 m), transverse tube pitch, (S1 = 0.033, 0.038 and 0.043 m), longitudinal tube pitch (S2 = 0.050, 0.065 and 0.080 m), diagonal tube pitch (S2/= 0.0527, 0.0671 and 0.0827 m), longitudinal tube rows (Z = 4, 6 and 8), and the number of TWTs (tn = 6, 9 and 12). The air-side overall heat transfer coefficient, h, the Nusselt number, Nu, and pressure drop, ΔP, were observed to be positively correlated to the Reynolds number, Re. As the Re increased, both Nu and ΔP increased with the increase of A/B, S2, and S2/, Z, respectively. However, both Nu and ΔP decreased with the geometric increase in S and S1, respectively. Finally, a generalised Nu correlation (independent of tn) is developed and validated against a recent measurement, offering a useful reference for the design of such heat exchangers.