Why Film Cooling?

A technology of particular interest in the Constellation Universities Institutes Project (CUIP) is film cooling for thrust chambers and rocket motor nozzles. The viability and advancement of this technology depends on understanding the complex heat transfer and mixing processes that occur near walls. The focus of our research is on near-wall mixing processes and heat transfer for thrust chamber film cooling.
Film cooling is a ubiquitous technology used to manage heat transfer between hot, reacting gasses and structural components in engines. In large rocket applications, film cooling of the thrust chamber is often required. A very high film cooling effectiveness (close to 1) must be maintained for rocket applications because of the extreme adiabatic wall temperatures expected in this environment. Even a local reduction in film cooling effectiveness could have a significant impact on durability. Advances in film cooling methods and models will help propulsion system developers achieve the aggressive durability and performance objectives required by the NASA moon-Mars Initiative.
Although near-wall mixing is critical in determining heat transfer performance in film cooling configurations, the details of this process are poorly understood. As a result, detailed transport models in film cooling configurations are largely unavailable and empirical correlations are often used. Since the scaling laws used to develop these correlations are incomplete, the agreement of these empirical correlations with actual test data is often poor. For example, the slot Reynolds number, blowing ratio, and temperature ratio have been identified as important parameters governing the film cooling performance; however, these correlations tend to fail near the slot and at high blowing ratios. Also, the mixing performance has been shown to be quite sensitive to the fine details of the injection geometry. Careful experiments and detailed measurements are required for understanding near-wall mixing dynamics to refine correlations and improve detailed CFD models used in film cooling applications.