Flow and heat transfer characteristics of an impinging jet with crossflow

Cheong, B.C.Y., 2002. Flow and heat transfer characteristics of an impinging jet with crossflow. PhD, Nottingham Trent University.

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A detailed study of confined single impinging jets with, and without, crossflow has been undertaken to determine the effects of near wall velocity, turbulence, temperature and fluctuating temperature on surface heat transfer. The turbulent incompressible jet discharged from a contoured nozzle of 100 mm diameter with uniform exit velocity and temperature profiles at Reynolds numbers of 20 000 and 40 000, and was subjected to an ambient crossflow at jet-to-crossflow velocity ratios (Uj/Uc) of 2, 3, 4 and 5. Non-dimensional nozzle-to-plate spacings (z/d) of 1.5 and 3 were considered and represent typical distances used in industrial cooling systems involving multiple impinging jets with crossflow.

New detailed experimental hydrodynamic and thermal flow field data have been obtained using the techniques of hot-wire anemometry and cold-wire thermometry. Smoke-flow visualisation has been employed to provide a qualitative insight into the nature of the complex flow field and to substantiate quantitative results. Full surface heat transfer coefficients and effectiveness have been determined using a heater mesh transient technique with double liquid crystals as the thermal indicator.

It has been demonstrated that surface heat transfer beneath a confined single impinging jet in crossflow varies sensitively with the near wall hydrodynamic and thermal characteristics. The impinging jets (with, and without, crossflow) at z/d =1.5 consistently yield greater magnitude and uniformity of surface heat transfer (compared to those at z/d = 3). For z/d = 1.5, the maximum average heat transfer with fine uniformity is achieved with a moderate strength crossflow (Uj/Uc=5 for Re = 40 000), through which turbulence is induced appreciably with a negligible loss in the momentum of the jet. Heat transfer falls substantially when the strength of crossflow becomes comparable to that of the impinging jet (Uj/U=2). For z/d = 3, optimum heat transfer condition exists when the impinging jet and the crossflow act with near equal intensity (Uj/Uc=4 for Re=40 000). A stronger crossflow would lead to a radical deterioration of the heat transfer performance. It was also revealed that jet deflection by the crossflow increases significantly at higher nozzle-to-plate spacing (z/d = 3) and Reynolds number (40 000).

Experimental near wall flow fields beneath a semi-confined axisymmetric impinging jets at z/d = 2 and 6.5 were compared with predictions from theoretical solutions. For z/d = 2, velocity profiles in the inviscid region of the flow can be predicted accurately using stagnation three-dimensional flow model. Prediction of boundary layer profiles using viscous flow model for an axisymmetric case is also reasonable provided that the flow remains laminar and dominated by the radial wall jet. For z/d = 6.5, the flow field is essentially turbulent and predictions using both the models are inappropriate.

Item Type: Thesis
Creators: Cheong, B.C.Y.
Date: 2002
ISBN: 9781369314564
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: 21 Sep 2020 13:54
Last Modified: 27 Jul 2023 11:06
URI: https://irep.ntu.ac.uk/id/eprint/40841

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