Heat transfer under impinging jets at very close jet-to-target spacing

Many jet impingement heat transfer applications have very low jet-to-target spacings. Some examples are electronics cooling, gas turbine blade cooling and thermal processing of glass. Face-averaged measurements were made on an isothermal test plate using a steady-state method, and confirmed using transient tests. Steady-state heat transfer distributions on a uniform heat flux test surface were imaged with thermochromic liquid crystals. The liquid crystal measurements were verified directly for a case where the uniform heat flux test surface produces a very nearly isothermal temperature distribution. Overall uncertainty in the measurement of face-averaged heat transfer coefficients was no more than $pm$5%. Local heat transfer coefficients were measured with overall uncertainties less than $pm$6%.;A 50% coverage turbulence generator in the exit plane of the jet increased face-averaged heat transfer coefficients by up to 2.5 times the open jet case at constant pumping power. Heat transfer under slot-turbulated jets exhibits two regimes based on jet-to-target separation, when compared at constant pumping power. In the near-field, heat transfer increases with decreasing jet-to-target spacings. In the very-near-field, the heat transfer decreases with aspect ratio. The division between near and very-near behavior depends on both the non-dimensional jet-to-target distance, $z/L$, and turbulator slot width, $delta$. A correlation for each region that collapses face-averaged data under slot-turbulated jets is presented, along with local measurements of the heat transfer.;The flow under an impinging square jet confined by the impingement surface and jet nozzle plate exhibits highly complex three-dimensional flow patterns, including three-dimensional separation and recirculation. Laser-sheet and surface flow visualization for a square, confined jet impinging at $z/L$ = 0.5 and $Resb{L}$ = 65,000, along with laser doppler velocimetry measurements, can explain the main features of the heat transfer distribution.;The present facility has many characteristics desired for rapid testing. Rapid prototyping and heat transfer testing of various jet nozzle exit shapes illustrated the potential of these techniques for thermal design. Total time from jet orifice plate design to measured heat transfer distribution was as little as 3 hours. Distributions of heat transfer under these jets are presented, and compared with the results from the square jet.