2.3.1. Profile loss
This is due to the formation of boundary layers on the blade surfaces. The fluid in the boundary layer is subject to viscous forces which slow it down and increase its entropy relative to the flow in the mainstream.
This results in the steam downstream of the moving blades having a higher entropy than the steam upstream. At a given flow rate this would result in a larger pressure drop across the blades than there would be in isentropic flow. Since the pressures in a turbine are fixed, the flow rate becomes less than isentropic and the entropy of the exhaust steam greater than isentropic. This means the heat drop obtained across the stage is less than that for an isentropic expansion and is therefore less efficient.
Profile loss is a boundary layer phenomenon and is therefore subject to factors that influence boundary layer development. These factors are Reynolds Number, Surface Roughness, Exit Mach Number and Trailing Edge Thickness.