Unsteady Flow Modelling in Plate-Fin Heat Exchanger Channels

Abstract

A methodology using Computational Fluid Dynamics (CFD) was developed to predict the flow and heat transfer performance of a single two dimensional sinusoidal channel of a Heat Exchanger (HE) at a Reynolds number (Re) range of 5 ≤ Re ≤ 500. The impact of different modelling assumptions was thoroughly evaluated which has not has been done in detail before. Two computational domains were used: a single period sinusoidal channel for fully periodic flow predictions and finite length channel consisting of 6 sinusoidal channel periods. Mesh and time independence was achieved for both domains whilst results with periodic domain were compared to numerical results in the literature. Laminar, k-ε and k-ω SST predictions were assessed throughout the Reynolds range with unsteady flow onset detected at Re ≈ 200 using laminar and k-ω SST models. The impact of different accuracy numerical discretisation schemes is assessed throughout the Re range and it was found that second order accuracy schemes should be used to fully capture the unsteady flow. Comparison between open-source CFD package OpenFOAM and Ansys was Fluent was performed and agreement was ‘ found.

Unforced Rayleigh instability of an immersed liquid jet

“…The study of liquid jet breakup is motivated by practical applications such as ink-jet printing, DNA sampling or dosing consumer products [1][2][3]. Besides the applicative side of jet instability, one may find that liquid jets can be used to highlight the influence of physical macroscopic properties such as viscosity, density, interfacial tension or elasticity in interfacial phenomena.For example, the relaxation time of weakly elastic polymer solutions, can be determined via R.O.J.E.R, a device specifically created that makes use of jet dynamics and filament thinning in the vicinity of drop detachment [4,5]. The density contrast between an injected fluid and its surrounding medium is still a subject of interest, considering that when immersed, the force of gravity is reduced by a factor proportional to the density difference between the two fluids [6].The injection of a fluid in a certain industrial process might be subject to many perturbations, therefor the breakup process will be dependent of the superposition of allexcited modes.…”