Numerical Investigation on the Differences Between Temperature Ratio and Density Ratio in Film-Cooled Endwall

This paper presents a numerical investigation on the rationality and reliability of film cooling experiments, in which the realistic temperature ratio (TR) of mainstream to cooling air are substituted by the density ratio (DR) of ambient air to a foreign gas with high density. Today, advanced gas turbines operate at much higher temperatures than the allowable temperature of turbine component materials, which makes it very difficult to achieve real TR in the most of laboratory environments, because the real TRs are usually larger than 2.0. Therefore, the foreign gas (for example CO₂) with high density was widely used to obtain a proper density ratio for simulating the mixing process of cooling air with high temperature mainstream thereby. However, the TR effect on film cooling is not completely replaced by DR, because the influence factors of film cooling performances are not only DR, but also the thermal properties of cooling gas and mainstream, such as specific heat capacity, viscosity and conductivity. In this work, a film-cooled endwall is used as specimen, and DRs are controlled by two ways, i.e. changing mainstream temperatures, and using a foreign gas, respectively. The numerical results of film cooling performances at the same DR obtained by the two ways are analyzed and compared. The analysis reveals the difference of film cooling performances between TR and DR on the film-cooled endwall, and the comparison indicated that the errors caused by the substitution of foreign gas are acceptable, only when BR, DR, Re_∞ and T_∞ are all small, but when BR, DR or Re_∞ increases, the relative error cannot be neglected, and it may reach 30% in real running conditions.