NUMERICAL PREDICTION OF SEMI-CONFINED JET IMPINGEMENT AND COMPARISON WITH EXPERIMENTAL DATA

SUMMARY The standard k-c eddy viscosity model of turbulence in conjunction with the logarithmic law of the wall has been applied to the prediction of a fblly developed turbulent axisymmetric jet impinging within a semi-conbed space. A single geometry with a Reynolds number of 20,000 and a nozzle-to-plate spacing of two diameters has been considered with inlet boundary conditions based on measured profiles of velocity and turbulence. Velocity, turbulence and heat transfer data have been obtained using laser-Doppler anemometry and liquid crystal thermography respectively. In the developing wall jet, numerical results of heat transfer compare to within 20% of experiment where isotropy prevails and the trends in turbulent kinetic energy are predicted. However, stagnation point heat transfer is overpredicted by about 300%, which is attributed directly to the turbulence model and inapplicability of the wall function. Jet impingment flows are frequently used in industrial practice for their high heat and mass transfer rates. Their employment is common but also diverse and typical applications include many heating, cooling and drying processes such as the manufacture of printed wiring boards, printing processes, production of foodstuffs, de-icing of aircraft wings and cooling of turbine aerofoils. The high heat transfer rates are especially needed to achieve short processing times for product quality or owing to temporal limitations of the process and/or for energy efficiency. The fluid dynamic structure of such processes is extremely complex and as such it is often reduced to that of understanding a single impinging jet, which will be turbulent except at very low Reynolds numbers. Even when the practical application is simplified, the necessary experimental rigs can be cumbersome and expensive, not to mention the time-consuming data acquisition, validation and analysis. Numerical simulations are an alternative to the experimental approach and can provide a fast and economic solution which will describe the flow or at least identify trends in the flow or heat transfer distribution. In this case the designer needs to be aware of the reliability and limitations of the numerical solutions for that particular geometry under investigation. An assessment of the model can only be obtained by comparison with experiment. Since numerical solutions are problem-dependent, there is a need for reliable experimental data specific to the jet impingement geometry to facilitate a direct assessment.