Comparison of Two-Phase Electronic Cooling Using Free Jets and Sprays

The performances of free jets and sprays were compared experimentally in cooling a 12.7 x 12.7 mm(2) chip in order to ascertain the effects of key parameters on cooling performance and to develop correlations, for critical heat flux (CHF) which are applicable to dielectric coolants. Increasing liquid flow rate and subcooling increased CHF for both cooling schemes. At high subcooling, comparable CHF values were attained with both for equal flow rates. However, spray cooling produced much greater CHF at low subcooling than did jet cooling. This phenomenon was found to be closely related to the hydrodynamic structure of the liquid film deposited upon the chip surface. In jet cooling, the film (wall jet), being anchored to the surface only at the impingement zone, was separated from the surface during vigorous boiling due to the momentum of vapor normal to the surface. The individual spray drops were more effective at securing liquid film contact with the surface at low subcooling, which delayed CHF relative to jet cooling with the same flow rate. This paper also discusses practical concerns associated with implementation of each cooling scheme including system reliability and the risk associated with premature CHF during chip power transients.