The purpose of this study is to investigate the effects of coolant-to-mainstream density ratio on a real engine cooling scheme of a combustor liner composed of a slot injection and an effusion array with a central dilution hole. Measurements of heat transfer coefficient and adiabatic effectiveness were performed by means of steady-state thermochromic liquid crystals technique; experimental results were used to estimate, through a 1D thermal procedure, the Net Heat Flux Reduction and the overall effectiveness in realistic engine working conditions. To reproduce a representative value of combustor coolant-to-mainstream density ratio, tests were carried out feeding the cooling system with carbon dioxide, while air was used in the main channel; to highlight the effects of density ratio, tests were replicated using air both as coolant and as mainstream and results were compared. Experiments were carried out imposing values of effusion blowing and velocity ratios within a range of typical modern engine working conditions. 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; on the other hand, the adiabatic effectiveness is less sensitive to the cooling flow parameters, especially at the higher blowing/velocity ratios. 1. Introduction In the course of last years, the increase of performances for the gas turbine for aeronautics has been achieved by increasing the pressure ratio and the maximum cycle temperature. These working conditions are not bearable with the materials employed in the components exposed to high thermal loads; hence, the development of effective cooling schemes is fundamental to match the increasing trend of gas turbine operating temperature. On the other hand, the development of aeroengine combustor is driven also by the effort to reduce emissions, in order to meet stricter legislation requirements. To satisfy future ICAO standards concerning emissions, main engine manufacturers have been updating the design concept of combustors. Future aeroengines combustion devices will operate with very lean mixtures in the primary combustion zone, switching as much as possible to premixed flames. Whatever detailed design will be selected, the amount of air in the primary zone will grow significantly at the expense of liner cooling air, which thus will be reduced. Consequently, important attention must be paid to the appropriate design of the liner cooling system in order to optimize coolant consumption and guarantee an effective liner
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