Stagnation Point Heat Transfer to an Axisymmetric Impinging Jet at Transition to Turbulence

A round air jet issuing from a long straight pipe and impinging perpendicularly onto a heated flat plate was investigated experimentally. The Reynolds number (Re) covered the fully laminar, transitional, and fully turbulent regimes—Re = 850–15,400. The main focus of this investigation was the transition regime, which occurred at Re = 2250–3010. Various measurements were recorded during the experiments using a hot-wire anemometer, an infrared thermometer, and a thermopile heat flux sensor; the mass transfer was measured using the naphthalene sublimation technique. The stagnation point heat transfer was correlated to the laminar and turbulent regimes in the form of the stagnation point Nusselt number, Nu₀ = CRe^mPr^0.4, where the exponent m = 0.50 and 0.55, respectively. The Nu₀–Re relationship exhibited nonmonotonic behavior (decrease) in the transition regime. Two counteracting mechanisms occur during transition—jet core shortening and an increase in velocity fluctuation; the former reduces Nu₀, whereas the latter increases it.