Effect of wall surface roughness on condensation shock

Abstract

The effect of sand grain type surface roughness on an unseparated incipient condensation shock and its influence on the boundary layer has been studied numerically for wet steam supersonic flow in a Laval nozzle. In the vicinity of the condensation shock, the sudden jump in density and a reduction in the wall shear stress causes a rapid drop in skin friction. The pressure shows a mild adverse gradient in this region. The boundary-layer velocity profiles vary with roughness value at a constant inlet pressure. The boundary layer thickness increases with the increase in either the inlet pressure, or the size of the roughness elements. As the roughness height is varied from 1 μm to 1000 μm the boundary layer thickness increases typically by 33% whilst the peak value of pressure gradient drops down by 50%. The boundary layer thickness is found to be maximum for po = 80 kPa owing to secondary nucleation. The peak value of pressure gradient is noticed to be doubled as the flow medium changes from dry air to wet steam.