Date of Award

2016

Degree Type

Thesis

Degree Name

Doctor of Philosophy

School

School of Engineering

First Advisor

Dr Yasir Al-Abdeli

Second Advisor

Dr Ferdinando Guzzomi

Abstract

Convective heat transfer and drying processes are found in industrial applications from gas turbine blade cooling to drying of food products and paper. In many instances, these processes rely on either a single or an array of fluid jets which impinge onto a surface. Traditionally, non-swirling impinging jets have been used, but interest surrounds possible advantages from imposing swirl into these jets to further promote heat and mass transfer at the surface. The challenge of resolving this question is that including swirl further complicates fluid-surface interactions. Studies are faced with the complexity of flow behaviour, the need for intricate measurement techniques and jets which seamlessly transition from non-swirling to swirling with well-defined boundary conditions. To better understand the nature of turbulent jet impingement with, and without, swirl requires carefully designed experiments covering parameters believed to affect the magnitude and uniformity of heat transfer.

This research investigated, experimentally and numerically, incompressible turbulent impinging air jets using aerodynamically derived swirl. The aim was to elucidate the effects of different parameters on fluid flow and surface heat transfer characteristics. Measurements of mean velocity and turbulence, surface pressure and temperatures were done using Constant Temperature Anemometry, integrating micro-manometer (pressure) tappings and steady-state heated thin foil technique via infrared thermography. Imaging for flow visualisations was also done. Numerical simulations were performed using ANSYS Fluent (version 14.5). Test conditions investigated encompassed a range of Reynolds numbers (Re = 11,600 – 35,000),

swirl numbers (S = 0 – 1.05) and nozzle-to-plate distances (H = 1D – 6D). Results show that the use of low-to-medium swirl numbers (S = 0.27 – 0.45) is found to improve heat transfer (Nu) in the impingement region compared to non-swirling (S = 0) jets over H ≤ 4D, with little improvement in spatial Nu uniformity. When S further increases, significant enhancement in Nu occurs only at near-field impingement (H ≤ 2D), regardless of the impingement area (footprint). At H ≥ 4D, a significantly low but more uniform radial profile of Nu is obtained. Results conclude the effect of swirl on the heat transfer characteristics is a complex relationship, which depends on the Reynolds number and nozzleto- plate distance. Whilst high swirl can lead to significant improvements in heat transfer, this is not necessarily always the case. It appears that there exist a threshold impingement distance and a transitional swirl number (dependent on Re) over which the effect of swirl on field and turbulence at different swirl numbers and nozzle-to-plate distances, with flow recirculation in near-field impingement (H = 2D) and non-swirl like at far-field (H = 6D). The occurrence of peak heat transfers at different swirl numbers is largely correlated with swirl induced turbulence characteristics near the impingement surface. Increase in Reynolds number augments the magnitude of Cp and heat transfer. For a given S, flow field and heat transfer distributions are found to be largely independent of Re.

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