DENSITY RATIO EFFECTS ON THE COOLING PERFORMANCES OF A COMBUSTOR LINER COOLED BY A COMBINED SLOT/ EFFUSION SYSTEM

The aim of the present study is to investigate the effects of density ratio between coolant and mainflow on a real engine cooling scheme of a combustor liner. Measurements of heat transfer coefficient and adiabatic effectiveness were performed by means of a steady-state Thermochromic Liquid Crystals (TLC) technique; experimental results were used to estimate, through a 1D thermal procedure (Therm1d), the Net Heat Flux Reduction and the overall effectiveness in realistic engine working conditions. In order to reproduce a representative value of combustor coolant to mainstream density ratio, tests were carried out feeding the cooling system with carbon dioxide (CO2), while air was used in the main channel; to highlight the effects of density ratio and, as a consequence, to distinguish between the influence of blowing ratio and velocity ratio, tests were replicated using air both as coolant and mainstream and results were compared. The experimental analysis was performed on a test article replicating a slot injection and an effusion array with a central large dilution hole. Test section consists of a rectangular cross section duct and a flat perforated plate provided with 272 holes arranged in 29 staggered rows (d=1.65 mm, a = 30◦, L=d=5.5). Furthermore a dilution hole (D =18.75 mm) is located at the 14th row; both effusion and dilution holes are fed by a channel replicating a combustor annulus. The rig allows to control mainstream and coolant flow parameters, especially in terms of Reynolds number of mainstream and effusion holes. Located upstream the first effusion row, a 6.0 mm high slot ensures the protection of the very first region of the liner. Experiments were carried out imposing several values of effusion blowing and velocity ratios within a range of typical modern engine working conditions (BRe f f /VRe f f = 1.5; 3.0; 5.0; 7.0) and keeping constant slot flow parameters (BRsl ≈ 1.5). Results point out the influence of density ratio on film cooling performance, suggesting that velocity ratio is the driving parameter for the heat transfer phenomena; concerning the effectiveness, results show that the adiabatic effectiveness is less sensitive to the cooling flow parameters, especially at the higher blowing/velocity ratios.