Mass transfer cooling is an evolving solution to the aerodynamic heating problem. Mass transfer cooling is a type of active cooling technique. The field of mass transfer cooling can be further classified into ‘transpiration cooling’ and ‘film cooling’. In transpiration cooling, coolant mass is injected into the boundary layer from a porous surface of the vehicle so that the coolant comes out as a continuous mass steam and not as an individual jet. Typical transpiration cooling method is shown in the Fig. 39.5. The film cooling technique uses discrete hole injection of coolant to come out of a large number of jets, as shown in Fig. 39.6. Liquid, chemically inert or active gas can be used as coolant in the case of mass transfer cooling.

Fig.39.6 Film cooling system.