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Title: Assessment of computational strategies for modelling in-line tube banks
Author: West, Alastair Peter
ISNI:       0000 0004 2737 7908
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2013
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This thesis provides an assessment of various computational strategies for modelling the turbulent flow and heat transfer around in-line tube banks. The research has direct application to the heat exchanger of an Advanced Gas-cooled Reactor (AGR). The suitability and accuracy of different Computational Fluid Dynamic (CFD) techniques were investigated first on generic square in-line tube banks where experimental data are available. The assumption of flow periodicity in all three Cartesian directions is initially investigated whereby the domain size was varied. Wall-resolved Large Eddy Simulations (LES) predict an increasing flow asymmetry with decreasing tube spacing. Two dimensional (2D) and three dimensional (3D) Unsteady Reynolds Averaged Navier-Stokes (URANS) models were simulated at the tube spacing known to be close to the flow pattern transition from symmetric to asymmetric. Marked differences were observed between the flow pattern predicted by turbulence models resolving the boundary layer and those that rely on wall functions. Ultimately, an improved understanding of the flow physics and heat transfer mechanisms encountered within in-line tube banks was gained. The assumption of flow periodicity was then removed and the effects of confining walls were investigated by reproducing experimental conditions. The correct pressure forces and heat transfer around the central tubes could only be accurately predicted when the walls in the crossflow direction were modelled. The inclusion of walls in the spanwise direction gave rise to small flow asymmetries which have been reported on similarly-spaced in-line tube banks. The latter half of the thesis focuses on the reasons for the enhanced thermal mixing and 3D secondary flow patterns observed in the in-line section of the AGR heat exchanger. A wall-resolved periodic LES was conducted at the lower Reynolds number of 11,000 along with URANS calculations of the full experimental conditions at both Reynolds numbers 11,000 and 66,000. These calculations required the use of High Performance Computing (HPC) facilities. Large 3D secondary flow structures were predicted that produced the same level of crossflow temperature drifting as that reported experimentally. Multiple upward and downward flow paths were observed which qualitatively explained why the experimental temperature profiles reported at different spanwise locations indicated multiple spirals (or secondary vortices). Quantification of the levels of thermal diffusion were investigated using both decaying temperature spikes and blanked tube platens. Thus the CFD provided recommendations about the thermal diffusivity assumptions used by the AGR heat exchanger code.
Supervisor: Launder, Brian; Iacovides, Hector Sponsor: EDF Energy ; EPSRC
Qualification Name: Thesis (Eng.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: in-line tube banks ; URANS ; LES ; heat exchangers