The Effect of Irreversibility on the Capacity of a Turbine Blade Row

Abstract

If the assumption is made that the process followed by the expansion of a gas in a converging blade row is isentropic, thermodynamic analysis leads to the result that the attainment of maximum mass flow, for a given flow area and predetermined inlet pressure and temperature, occurs when sonic velocity is reached at the throat. If, however, as in practice, the expansion proceeds with internal frictional losses, and if certain common assumptions are made regarding these losses, then, assuming adiabatic flow, thermodynamic analysis shows that the maximum flow will occur at a Mach number less than unity. If this paradox is not noticed, it may lead to an appreciable underestimate of the mass flow passed by a blade row for a given set of inlet conditions. This paper examines the relationship between blade row losses and boundary layer characteristics in an attempt to rationalize the calculation of the capacity of a turbine as modified by irreversibility.