Why mass flow rate not increase above sonic velocity?
Answers
Density variations in the very high speed flows ( like the case of supersonic flow) is very high due to increased compressibility effects. From the flow continuity perspective, further acceleration( already supersonic to higher supersonic) shall require higher pressure decrease(associated with high density decrease,of course P=Rho*R*T). In order to provide such degree of density decrease(flow need to expand highly),we need to provide larger diverging areas. In molecular dynamics sense,slight increase in velocity in the very high speed flows requires high amount of energy transformation(appreciable enough) from the random KE of the molecules into the ordered KE of the molecules, thus causing greater pressure(associated with random KE of molecules) decrease hence associated large density decrease. This highly expandable flow can't be expanded in a constant area duct,instead we need to provide large diverging area ducts.
Hope this helps!!!