AN INVESTIGATION OF TREATING ADIABATIC WALL TEMPERATURE AS THE DRIVING TEMPERATURE IN FILM COOLING STUDIES

摘要

In film cooling heat transfer analysis, one of the core concepts is to deem film cooled adiabatic wall temperature (T_(aw)) as the driving potential for the actual heat flux over the film-cooled surface. Theoretically, the concept of treating T_(aw) as the driving temperature potential is drawn from compressible flow theory when viscous dissipation becomes the heat source near the wall and creates higher wall temperature than in the flowing gas. But in conditions where viscous dissipation is negligible, which is common in experiments under laboratory conditions, the heat source is not from near the wall but from the main hot gas stream; therefore, the concept of treating the adiabatic wall temperature as the driving potential is subjected to examination. To help investigate the role that T_(aw) plays, a series of computational simulations are conducted under typical film cooling conditions over a conjugate wall with internal flow cooling. The result and analysis support the validity of this concept to be used in the film cooling by showing that T_(aw) is indeed the driving temperature potential on the hypothetical zero wall thickness condition, ie. T_(aw) is always higher than T_w with underneath (or internal) cooling and the adiabatic film heat transfer coefficient (h_(af)) is always positive. However, in the conjugate wall cases, T_(aw)is not always higher than wall temperature (T_w), and therefore, T_(aw) does not always play the role as the driving potential. Reversed heat transfer through the airfoil wall from downstream to upstream is possible, and this reversed heat flow will make T_w > T_(aw) in the near injection hole region. Yet evidence supports that T_(aw) can be used to correctly predict the heat flux direction and always result in a positive adiabatic heat transfer coefficient (h_(af)). The results further suggest that two different test walls are recommended for conducting film cooling experiments: a low thermal conductivity material should be used for obtaining accurate T_(aw) and a relative high thermal conductivity material be used for conjugate cooling experiment. Insulating a high- conductivity wall will result in T_(aw) distribution that will not provide correct heat flux or h_(af) values near the injection hole.