EFFECT OF JET TO PLATE SPACING ON FILM COOLING PERFORMANCE IN A COMBINED IMPINGEMENT AND FILM COOLING ARRANGEMENT

The influence of jet to plate spacing on conjugate heat transfer from a flat plate with combined impingement and film cooling has been studied both numerically and experimentally. The flow configuration consists of a matrix of impingement holes and multiple staggered rows of cylindrical film holes. One side of the plate is impinged with air jets, while the other side is exposed to hot mainstream as well as the vented air from the film cooling holes. The effect of increasing jet to plate ratio (H/D = 0.4, 1.2, and 2.0) on film cooling flow structure and heat transfer is analyzed under conjugate boundary conditions. The flow and temperature patterns are predicted in the entire computational domain; namely, the plenum chamber, film holes, and mainstream-coolant interaction zone. The mainstream Reynolds number is varied from 49,050 to 130,800 while keeping the jet Reynolds number constant at 825 and the blowing ratio ranging from 0.6 to 1.6. The velocity, turbulence, and temperature distributions at the exit of the film hole together define the film coverage on the interaction surface. The thermochromic liquid crystal (TLC) technique is used to evaluate the surface temperature of the plate and validate the computed values of the surface effectiveness. Both measured and computed effectiveness distributions are found to be comparable. Increase in the H/D values beyond a certain limit causes decrease in film effectiveness on the interaction surface, especially downstream of the film hole. The influence of H/D is noticed for all the three blowing ratios investigated, albeit to varying degree.