Numerical and experimental study of scaling criteria accuracy for film cooling using CO2 coolant

Abstract

To have a better understand of film cooling technique, many experiments to investigate parameters affecting the film cooling effectiveness and heat transfer have been conducted. In this kind of experiments, different sorts of coolant (CO2, N2 and etc.) are often applied to match coolant-to-mainstream properties or use some measuring techniques. However, the application of those coolant probably leads to a similarity problem, as the properties of them are different from air coolant in real engine conditions.This study investigates the accuracy of six criteria for the film cooling effectiveness scaling which changes the coolant from air to CO2 based on both experimental and numerical method, to promote the similarity to air coolant of film cooling experiments using CO2 as coolant. The study conducted on the pressure side of 1st stage stator from E3 aero engine. The mainstream inlet pressure and temperature of the experiment was 112kPa and 312K, respectively. Numerical simulations were conducted by solving the Reynolds Averaged Naiver-Stokes (RANS) equations and Re-Normalization Group (RNG) k-ε turbulence model. The effect of CO2 coolant on pressure side film cooling effectiveness was initially investigated, followed by the analysis of the accuracy for six scaling criteria. Finally, reasons leading to the deviation of scaling was studied.Both numerical and experimental results indicate that the application of CO2 coolant leads to an obvious reduction of ηAPS at relatively low coolant mass flow rate. That effect almost disappears at high coolant mass flow rate. In the experiment, η with the variation in VR has the highest Pearson correlation coefficient (Rp) between air and CO2 coolant at almost all measuring points on pressure side, illustrating VR criterion has the highest accuracy in the scaling which the coolant is changed from air to CO2. The cooling effectiveness deviation of VR criterion is lower than 3% at all coolant CO2 percentage(obtained from CFD results). The low specific heat capacity of CO2 will lead to a relatively faster decay of injection temperature compared with air, producing an adverse influence on cooling effectiveness. The high density of CO2 can increase attachment ability of injections(especially on mid-span of the blade) and form a similar cooling film state to the air coolant at higher coolant mass flow rate. The results of this study can be used to promote the accuracy of the experiment using CO2 as coolant at relatively low temperature conditions in terms of film cooling effectiveness and injection state(attachment or separation).