Discussions of some myths and concerned practices of film cooling research

This paper focuses on discussing four myths and concerned conventional practices of film cooling research guided by a series of computational simulations. The issues that have been discussed include: (a) the film cooling effectiveness (φ) is given a constant (0.6) to calculate the heat flux ratio HFR=q″/qo″ between film-cooled and no-film cases; (b) the adiabatic wall temperature is the driving temperature of film cooling; (c) the adiabatic film heat transfer coefficient can be obtained from an isoenergetic film experiment (Tj = Tg); (d) using a heated surface can provide a simplified approach to simulate the film-cooling condition. The result shows that the adiabatic wall temperature is not always the driving temperature (i.e. Taw is not always larger than Tw), but Taw does act well as the reference temperature in correctly predicting the heat flux direction and giving an always positive adiabatic film heat transfer coefficient. Using a constant value for φ is questionable and may lead to un-realistic or false HFR. The conventional equation of HFR was not theoretically exact and can lead to an error of up to 20%. A revised HFR equation is provided in this paper. The dominant energy passage in turbine airfoil film cooling is always from hot combustion gas flows into the airfoil; therefore, employing a heated wall as a boundary condition with hotter main stream flow and cold film injection to simulate the actual film cooling condition is found to be fundamentally questionable. A conjugate simulation that includes wall heat transfer and internal flow cooling shows that reversed heat transfer from blade to gas that gives a negative HFR is possible, due to the heat conduction from the downstream hotter region into the near film hole cooler region.