Experimental Investigations of Effusion Cooling Systems for Lean Burn Aero-Engine Combustors

Legislation limits concerning polluting emissions, for civil aircraft engines, are expected to become even more stringent in the future. To meet these targets, especially in terms of NOx, it is required to maintain the temperature in the combustion zone as low as possible. Lean burn swirl stabilized combustors represent the key technology to reduce NOx emissions. The high amount of air admitted through a lean-burn injection system is characterized by very complex flow structures such as recirculations, vortex breakdown and processing vortex core, that may deeply interact in the near wall region of the combustor liner. This interaction and its effects on the local cooling performance make the design of the cooling systems very challenging. In addition, since up to 70% of the overall air mass flow is utilized for fuel preparation and the initiation of lean combustion, the amount of air available for combustor liner cooling has to be strongly reduced with respect to the traditional diffusive combustor architectures. State-of-the-art of liner cooling technology for modern combustors is represented by the effusion cooling. Effusion cooling is a very efficient cooling strategy based on the use of multi-perforated liners, where metal temperature is lowered by the combined protective effect of coolant film and heat removal through forced convection inside each hole. Beyond that, multiperforated liners act also as passive devices to mitigate thermoacoustic phenomena which is one of the main concern regarding lean combustors operability. A large part of the activities and the achievements deriving from the Ph.D. course are collected in the present study, that deals with two experimental campaigns on effusion cooling schemes designed for aero-engine combustor liner applications. In the first part of the current research, an experimental survey has been performed for the evaluation of thermal performance, in terms of overall and adiabatic effectiveness, of seven multi-perforated planar plates representative of a portion of combustor liner, with uniform mainstream conditions. Effusion geometries were tested imposing 6 blowing ratios in the range 0.5-5, two values of density ratio and two level of mainstream turbulence. Concerning the geometrical features, different porosity levels have been considered: such values are obtained both increasing the hole diameter and pattern spacing. Then, the effect of hole inclination and aspect ratio pattern shape have been tested to assess the impact of typical cooling system features. The analysis of the data points out the impact of the main geometrical and fluid dynamics parameters on the thermal performance, proposing a possible thermal optimization strategy that seems to be promising also from the acoustic damping requirements. Results represent a wide experimental database relevant for the design of an high efficiency effusion cooling systems, even though the survey leaves the impact of the swirled gas flow on thermal performance an open issue. To enhance the TRL (Technology Readiness Level) of experiments, a planar sector test rig equipped with three AVIO Aero PERM (Partially Evaporated and Rapid Mixing) injector systems and working at atmospheric conditions has been considered in the second part of the work. The test rig allowed to reproduce a representative flow field on the gas side and to test the complete liner cooling scheme composed by a slot system, that reproduced the exhaust dome cooling mass flow, and an effusion array. The final aim of the study is the experimental characterization of the flow field and the measurement of cooling performance in terms of heat transfer coefficient and adiabatic effectiveness due to the interaction of the swirling flow coming out from the injectors and the cooling scheme. Tests were carried out imposing several realistic operating conditions, especially in terms of reduced mass flow rate and pressure drop across swirlers and effusion cooling holes.