Flow visualisation of semi-confined jet impingement

Ashforth-Frost, S., 1994. Flow visualisation of semi-confined jet impingement. PhD, Nottingham Trent University.

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The effect of near wall velocity and turbulence on surface heat transfer within the stagnation region of a semi-confined impinging jet has been studied. The single turbulent incompressible fully developed jet impinged perpendicularly onto a smooth surface.

New qualitative and quantitative experimental data of near wall velocity and Reynolds stress components have been obtained, using full field flow visualisation and laser-Doppler anemometry, at a Reynolds number of 20000 and nozzle to plate spacing (z/d) of two diameters, supported by some additional data for z/d=4 and z/d=6. The distribution of heat transfer coefficient has been obtained using a transient wall heating technique with liquid crystals as the temperature indicator. The heat transfer data has corroborated and extended existing data for the semi-confined case.

The influence of the semi-confinement and impingement plates on flow and heat transfer has been quantified. Both have the effect of extending the jet potential core. Heat transfer rates of typically 80% of the unconfined impinging jet are achieved with semi-confinement, which severely limits entrainment.

The experimental results have provided clear evidence of turbulent motions in the stagnation region which have a direct influence on heat transfer; the locations of the maxima in the turbulence components of the Reynolds stresses have been shown to coincide with the heat transfer maxima.

An instantaneous stagnation point was observed when z/d≥6 which was attributed to periodicity of the flow due to coherent turbulent structures generated at the jet exit.

The k-e turbulence model, available in the commercial software PHOENICS, has been shown to predict the correct trends in mean axial and radial velocities, and in turbulent kinetic energy, k, downstream of the stagnation region (r/d≥1). Within the stagnation region, computed values of k were as large as nine times the experimental values. The predicted heat transfer was within 20% in the wall jet region where isotropy prevails and the magnitude of k compared well with experiment, but at the stagnation point where the flow has been shown to be anisotropic, the heat transfer was overpredicted by ~300%.

High average heat transfer rates are obtained when the impingement plate is placed within, and just at the end of, the potential core of the jet. These spacings, preferably without semi-confinement, are recommended for optimal performance not taking into account the required pumping power. At larger nozzle to plate spacings, jet impingement heat transfer becomes less effective.

Item Type: Thesis
Creators: Ashforth-Frost, S.
Date: 1994
ISBN: 9781369325492
Rights: This copy of the thesis has been supplied for the purpose of research or private study under the condition that anyone who consults it is understood to recognise that its copyright rests with its author and that no quotation from the thesis, and no information derived from it, may be published without proper acknowledgement.
Divisions: Schools > School of Science and Technology
Record created by: Linda Sullivan
Date Added: 05 Jul 2021 15:34
Last Modified: 20 Mar 2024 15:44
URI: https://irep.ntu.ac.uk/id/eprint/43326

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