Discussion About the Practice of Using a Heated Surface in Film Cooling Studies

It is found in many film-cooling experiments and computational analyses that a heated surface is employed to simulate the actual film-cooling condition with a cooling jet and a hot main flow. Considering that the dominant energy passage in turbine airfoil film cooling is always from the hot combustion gas flowing into the airfoil, employing a heated surface to simulate the actual film cooling condition does not provide the correct physics of the heat flow under an actual film cooling condition, and therefore, the results are questionable. The objective of this paper is to investigate the consequent results associated with the practice of employing a heated surface by comparing its result with actual conditions including a conjugate metal wall and internal cooling via a series of computational simulations. When the surface is heated, in some conditions, negative film cooling effectiveness can be found as a result of a higher surface temperature than the main gas stream temperature. This is unrealistic for an operational turbine system. The heated wall acts as an active heat source; as a result, the concept of using the adiabatic wall temperature (T_aw) as the driving temperature potential is no longer valid because an artificially created competing heat source is added into the system, and the heat transfer mechanism on the airfoil is not solely determined by T_aw. Heating the surface to simulate the film cooling boundary condition, although it does not provide correct physics, can provide the heat transfer coefficient value within 10–15% of the value calculated from the correct boundary conditions. Using a heated surface is only correct under one condition: when all the conditions are reversed, i.e. with a hot jet and cold main gas flow. The practice of using a jet flow with the same temperature of the hot gas (isoenergetic jet) to obtain the film heat transfer coefficient will result in about 20–25% discrepancy from the cooling jet case. The uniformly cooled wall cases fair better than heated cases because it provides correct physics in most part of the surface.