Reacting Nozzle Flow

The two limits, “frozen” and “equilibrium” flow share an important property: both are isentropic flows (if we ignore friction or heat losses). This is because, in the frozen case no chemical change during expansion), there are no rate processes at all occurring, the molecules preserving their identity all the way, while in the equilibrium case, in which reactions do occur, their rate is so high (compared to the expansion rate) that conditions adjust continuously to maintain equilibrium at the local pressure and enthalpy level, with the result that the whole process can be regarded as reversible (and hence isentropic). For any more realistic intermediate conditions, in which reactions may proceed at rates comparable to that of the expansion, these finite rate reactions produce an irreversibility, and a consequent entropy increase.