Confinement effects in heat transfer to a miniature compressible impinging air jet

The decreasing physical size of microchips accompanied by the increasing heat flux to be dissipated has led to the study of possible new innovative electronic cooling methods. Jet cooling has been successfully implemented in a variety of industrial applications and its capacity to maintain high heat transfer rates demonstrates its vast potential for incorporation into future cooling applications. The reduction in size of electronic components leads to the inevitability of jets used in this area being subjected to a degree of jet confinement. In this study the effects of confinement on the local heat transfer characteristics were experimentally investigated for a turbulent, fully developed, axisymmetric, compressible and submerged miniature air jet impinging normally onto an ohmically heated flat plate. The resulting surface temperature distribution was recorded via infrared thermography. Two 1mm diameter nozzles were examined, one confined and one unconfined. Tests were conducted for nozzle exit to impingement surface spacings of 1, 2, 4 and 6 jet diameters and for Reynolds numbers of 7,000 and 12,000 which corresponded to Mach numbers of 0.3 and 0.5. The heat transfer analysis accounts for compressibility effects by using the adiabatic wall temperature as the reference temperature in calculation of the heat transfer coefficients. Local heat transfer distributions are presented as a function of radial distance from the stagnation point. The results obtained indicate that confinement contributes to a flatter distribution of heat transfer over the impingement surface.