Flow and heat transfer in a turbulent boundary layer through skewed and pitched jets

Abstract

A study was carried out of longitudinal vortices in a low speed turbulent boundary layer due to small jets. The jet was pitched at 45 deg and skewed between 0 and 120 deg. The effects of jet velocity ratio (0 < Ay < 3.0) and skew angle (0 < 0 < 120 deg) were addressed. Mass-averaged Navier-Stokes equations were solved using computational fluid dynamics and heat transfer. Turbulence closure was achieved using k-e model. Three types of vortex production were noted at various jet velocity ratios. When a longitudinal vortex is positioned at the edge or in the outer region of the boundary layer, optimal effects of the flow and heat transfer control were achieved. When the vortex lay deeply in the boundary layer, the effects on heat transfer control could be detrimental. When the jet penetrated the boundary layer, the favorable effects of the vortex on control were reduced due to the larger distance away from the surface. Significant improvements of vortex strength and heat transfer were seen between skew angle 0 < $ < 45 deg, although in 45<0<120 deg the improvements were marginal. Nomenclaturekinetic energy Nu = Nusselt number, h • D/\ Nu -averaged Nusselt number, l/S$ s Nu -djc dz q = heat flux on the wall, W/m 2 5 = surface area T = temperature T f = cross-stream temperature 7} = jet exit temperature T s = wall temperature U, V, W = velocity components V = flow velocity in vector form V e = cross-stream velocity Vj = jet exit velocity x, y, z = Cartesian coordinates a = jet pitch angle 6 = oncoming boundary-layer thickness e = turbulence dissipation rate f = bulk viscosity 0 = jet skew angle X = thermal conductivity of air \j = jet velocity ratio, Vj/V e I* = viscosity p -density a t = turbulent Prandtl number T = shear stress S x = vorticity level on x plane, d/V e [(dW/dy)/-(dV/dz)]

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