Impingement pressure characteristics of swirling and non-swirling turbulent jets

Abstract

This paper experimentally investigates the effects of swirl on the impingement surface pressure for an incompressible, turbulent, swirling impinging air jet. The swirl flow is generated aerodynamically, where the nozzle can achieve a seamless progression from non-swirling to high swirling flows. Hotwire anemometer is used to measure velocity components. A digital micromanometer with flush-mounted pressure taps on the impingement plate is used to measure static pressures on the impingement surface. The effect of swirl number (S), nozzle-to-plate distance (H) and Reynolds number (Re) on the pressure distribution is examined for S=0–1.05, H=1D–6D and Re=11,600, 24,600 and 35,000.For low swirl flows, the coefficient of pressure (Cp) shows a non-swirl like behaviour with maxima at the stagnation point. For medium-to-high swirls, maximum Cp shifts radially outward from the stagnation point and becomes relatively flat with increasing S. The stagnation pressure reduces nonlinearly with increasing swirl intensity and follows a quadratic relationship for a given Re. For any S, the pressure distribution is found to be independent of Re for low swirl numbers (up to S=0.3), but it varies up to r/D=2 for larger swirl numbers. A negative Cp (flow separation) occurs near the stagnation region for H=1D, however, vanishes at larger H. For very high swirl number (S=1.05) and at H=1D, three different regions are recognised on the impingement surface from the stagnation point: a rotating, reversed inward flow at r/D⩽0.5, a transition and less stable region at 0.5<r/D⩽0.75 and an outward flow with stronger tangential component at r/D⩾1.0. These surface pressure results may significantly affect the heat transfer characteristics as well as wall shear stresses for future impinging jet studies.