Measurements of Discharge Coefficients in Film Cooling

Measurements of discharge coefficients for several film cooling configurations having hole length-to-diameter ratios of 2.3, 4.6, 6.6, and 7.0 are presented. Recently, it was documented that the velocity distributions over the hole exit plane vary significantly with changes in hole length-to-diameter ratio. This paper documents the effects of such variations on coolant discharge coefficients. Due to the short holes, injection in engines is with a substantial amount of coolant departing the upstream portions of the hole exit plane. This results in a higher rate of momentum exchange with the free stream at that location than for longer holes, which permits more uniform exit flows. Discharge coefficient measurements are discussed in terms of this distribution of velocity. This paper also documents the effects of the hole supply plenum geometry on discharge coefficients. When the coolant flow is delivered to the holes with significant momentum either in the direction of the free stream or opposite to that direction, significant changes in discharge coefficient values are observed.