AN EXPERIMENTAL INVESTIGATION OF FILM-COOLING HEAT-TRANSFER COEFFICIENTS USING THE MASS HEAT ANALOGY

An experimental investigation has been carried out to determine the he at/mass transfer coefficient downstream of a two-dimensinoal, normal, film cooling injection slot. The plate downstream of the slot is porou s, and air contaminated with propane is bled through it. By measuring the propane concentration very close to the wall using a flame ionizat ion detector, mass transfer measurements are conducted for film coolin g mass flow ratios ranging from 0 to 0.5. The mass transfer coefficien ts are calculated using a wall function correction formula, which corr ects the measurements for displacement from the surface, and are then related directly to corresponding heat transfer coefficients using the mass/heat analogy. The validity of the method and the wall function c orrection formula are checked by examining the case with zero film coo lant injection, a situation analogous to the well-known turbulent boun dary layer mass/heat transfer with impermeable/unheated starting lengt h. Good agreement with predicted data is obtained for this experiment. For film cooling with low mass flow ratios, heat transfer coefficient s close to those of a conventional turbulent boundary layer are obtain ed. At high values of mass flow ratios quite different trends are obse rved, reflecting the important effect of the separation bubble, which is present just downstream of the injection slot.