Experimental analysis of effusion cooled combustor liners

The introduction of stricter and stricter Nox emissions standards in civil aero-engines is forcing all engine manufacturers to adopt Lean Burn concept as basic scheme for combustors. With this type of devices a high amount of air flow is admitted through the injection system (up to 70% of total air mass flow) in order to reduce and control primary zone fuel/air ratio. This has two consequences: first of all, for the sake of flame stabilization, primary air is injected with relevant swirl components producing very complex flow structures such as recirculations, vortex breakdown and precessing vortexes, which may deeply interact in the near wall region of the combustor liner. Secondly, a reduced amount of air is available for combustor liner cooling, pushing the adoption of high effectiveness cooling schemes. Among different cooling approaches, effusion or full-coverage film cooling, realized with the multi-perforation of the liner in single or double skin solutions, represents one of the first choices due to its high thermal effectiveness, low weight and a relatively easy manufacturability. Liner metal temperature is kept low by the combined protective effect of coolant film, heat removal inside holes and convection on the cold side. The evolution of film protection, which represents the dominant contribution to overall effectiveness, can be heavily influenced by the aforementioned swirl flow interaction with combustor walls. A large part of the activities and the achievements deriving from the Ph.D. course are collected on an experimental apparatus, developed within EU project LEMCOTEC, consisting of a non-reactive three sectors planar rig. The test model is characterized by a complete cooling system and three swirlers, replicating the geometry of a Avio Aero PERM (Partially Evaporated and Rapid Mixing) injector technology. 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. Particular attention is focused on the impact on the effusion system of the coolant injection angle, for this purpose three different liner configurations are tested. To deepen the comprehension and the knowledge on the thermal behaviour of the system, a supplementary investigation is carried out with the aim of estimating the heat transfer on the annulus side of the liner for the reference effusion geometry. To enhance the TRL (Technology Readiness Level) of experiments and to stress the capability of alternative manufacturing technologies, a new test rig is developed within EU project IMPACT-AE. It consists of a linear three sector combustor, whose main parts are manufactured using metal laser sintering technologies, equipped with an effusion cooling scheme on both the outer and inner liners. The same PERM injector concept is exploited to provide fuel atomization and flame stabilization. A large number of thermocouples are embedded into the combustor walls to monitor continuously the metal temperature. Such apparatus gives the opportunity of testing the key components of the combustion chamber in reactive conditions, achieving an important step toward their implementation in the engine.