Heat Transfer of Confined Impinging Jet onto Spherically Concave Surface with Piston Cooling Application.

Abstract

This paper describes a detailed experimental investigation of heat transfer for confined impinging jet onto spherically concave surface with piston cooling application. The experimental conditions involved several nominal Reynolds numbers renging from 17 500 to 43 200 with five different reciprocating frequencies, namely 0, 0.833, 1.25, 1.67 and 2.08 Hz. It resulted in the pulsating number, which represented the ratio of reciprocating force to inertial force effect, varying from 0 to 1.85E-4. Along the spherically concave heating surface the evolution of flow structure from separation point into laminar and then turbulent annular flows was demonstrated by examining the variations of power index suffixed to Reynolds number in the non-reciprocating Nusselt number correlations. The typical effects of flow reciprocation on heat transfer were then illustrated by examining the distributions of temporal Nusselt number variation along the heating surface. When test section reciprocated, the Nusselt numbers at measurement locations periodically oscillated and the amplitude of such temporal Nusselt number variation was mostly pronounced at the stagnation point and became location and Reynolds number dependent. At the stagnation point, there were coupling effects of Reynolds and pulsating numbers on heat transfer and, in general, the heat transfer level increased with the increase of Reynolds or pulsating numbers while kept one of these two flow parameters unchanged. At Reynolds and pulsating numbers of 40 000 and 8.41E-5, the reciprocating Nusselt number value at the stagnation point could be increased up to a level about 3.2 times of the non-reciprocating level. Less reciprocating effects were found in the spherically concave heating area ; and reciprocation reduced the time-averaged heat transfer to the levels abut 80%-95% of non-reciprocating heat transfer values after flow passed the concave surface. As the reciprocating force significantly modified the heat transfer from non-reciprocating situation, it needed to account for the effect of reciprocating motion on the heat transfer within the coolant channels of piston in order to achieve the optimum design of cooling system.