Impingement Cooling Experimental Investigation Using Different Heating Elements

The aim of this work is to study heat transfer due to a single submerged impinging jet by using different experimental techniques, thus to provide cross-validation of the various employed methods. In particular, steady-state technique is applied, and target wall temperature is evaluated thanks to thermochromic liquid crystals. To allow heat transfer to take place, electrically heated film elements are applied to the jet target surface. Both a smooth Inconel sheet (arithmetic average roughness 1.2 mu m) and a fine stainless steel mesh (arithmetic average roughness 23 mu m) have been used as heating elements, with the aim to evaluate their ability to provide a uniform and controllable heat flux condition, as well as to determine the effect of surface roughness on heat transfer. Impingement hole characteristics include nozzle diameter Dj=12.5 mm, nozzle length to diameter ratio L/D-j=0.4 and nozzle-to target surface distance to diameter ratio H/D-j=2. The performances of the different techniques are evaluated on a flat target surface, and the coolant is evacuated from the impingement cavity by two extraction holes with a diameter of 12.5 mm each, located on the target surface itself. The Inconel heating element is cut in correspondence of such holes, in order to allow coolant extraction; on the contrary, the permeability of stainless steel mesh allows to lay it above the holes without cutting it, thus providing a more uniform heat flux. Jet Reynolds number up to 40000 are investigated, and results are presented in terms of Nusselt number distributions and area averaged values. Results obtained with the two heating elements show that surface roughness has a significant effect on heat transfer, the steel mesh coated surface providing up to 15.6% higher area averaged Nusselt values with respect to the Inconel coated one. The application of a thin non conducting film over the mesh reduces surface roughness and causes Inconel and mesh to provide coherent results, thus providing a validation for the second method