Flow visualisation of impinging air and water jets

Rüdel, U., 2002. Flow visualisation of impinging air and water jets. PhD, Nottingham Trent University.

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Abstract

A detailed study of a single jet impinging orthogonally onto a flat surface has been undertaken for air and water to obtain qualitative and quantitative information of heat transfer and fluid flow characteristics in impinging air and water jets over a Prandtl number range of 0.71 ≤ Pr ≤ 8. Reynolds numbers of 2000, 10,000 and 20,000 were considered at a nozzle-to-plate spacing of z/d=2. New experimental data have been obtained using flow visualisation techniques for the mean velocity and turbulence characteristics including Reynolds shear stresses. Particular attention was paid to the near wall characteristics which dominate the heat transfer process. By comparing the flow field data with local heat transfer characteristics (obtained using liquid crystals) an increased physical understanding of the mechanism of jet impingement heat transfer has been attained.

At turbulent Reynolds numbers and all Prandtl numbers, mean and turbulent flow characteristics showed close agreement. The finding of negligible fluid property effects is continued for the flow along the centreline and along the impingement plate. The shear layer that surrounds the core of the approaching jet was identified as the origin of vortical structures, which impinge on the developing wall jet. In due course wavy streamlines between the convected vortices and the impingement surface were observed.

The effects of kinematic viscosity on the flow field are more apparent at the lowest Reynolds number. Quantitative differences in the mean and turbulent intensity profiles normal and parallel to the wall were found in the mixing layer, in close proximity to the wall and further away, in the free stream. The water jet spreads more rapidly than the air jet due to higher levels of shear leading to more entrainment and faster decay of the axial velocity. After impingement the air jet develops a thinner boundary layer. As the flow returns to isotropic behaviour levels of turbulent shear stresses increase rapidly to similar levels as observed in the water jets. For Re=2,000, the wall jets of both fluids are less stable than at higher Reynolds numbers. The peak in Reynolds shear stress moves away from the stagnation point as the Reynolds number is increased.

Generalised correlations for optimised jet impingement heat transfer which have been derived based on gaseous jets project trends in heat transfer coefficients satisfactorily for higher Prandtl numbers (water jets) at turbulent Reynolds numbers. This is reflected in identical qualitative trends of the radial heat transfer profiles where a primary peak was observed at r/d?0.55 and a secondary peak at r/d≈1.8. This secondary peak in heat transfer coincides with the location of the maxima in the shear stress and turbulence intensity and is attributed to the transition from laminar to turbulent flow.

The Finite Element software package ANSYS/FLOTRAN in conjunction with the Standard k- ? turbulence model has been assessed for the prediction of jet impingement flows. The inherent strong point of the Finite Element technique is its meshing capabilities which was demonstrated by creating 'wedge-shaped' near wall elements to satisfy near-wall model criteria. Despite the inability of the eddy-viscosity model to predict anisotropic flows in the stagnation region, near the wall the mean velocity was over-predicted to within 19%.

Item Type:	Thesis
Creators:	Rüdel, U.
Date:	2002
ISBN:	9781369313895
Identifiers:	Number Type PQ10183108 Other
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 The Nottingham Trent University 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:	18 Sep 2020 08:01
Last Modified:	19 Jul 2023 09:36
URI:	https://irep.ntu.ac.uk/id/eprint/40784

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