The Use of Foreign Gas to Simulate the Effects of Density Ratios in Film Cooling

Abstract

This paper provides a unique experimental assessment of the use of a foreign gas to vary the injection-to-mainstream density ratio in film-cooling experiments. It is widely recognized that it is important to scale both the velocity and density field in such tests in order that they will be representative of conditions in the gas turbine. Where it is difficult to achieve the temperature required for the correct injection-to-mainstream density ratio, some experimental techniques resort to the use of a heavier foreign gas to simulate the colder injection flow. In the experiment reported, the Isentropic Light Piston Tunnel has provided an environment in which a desired injection-to-mainstream density ratio can be achieved both by varying the temperature of the two airstreams and by using a heavier injection gas, carbon dioxide. Direct heat transfer measurements, which do not require the use of a mass transfer analogy, are taken. Use of the superposition model of film cooling allows the results to be interpreted easily to yield heat transfer parameters all obtained under isothermal wall boundary conditions. Comparison of results for air and carbon dioxide injection show that with this approach, it is possible to use a foreign injection gas to simulate the required injection-to-freestream density ratio.