No abstract

Ultra-Compact combustion presents a novel solution to address the demand for increasingly compact, efficient, and low weight aircraft gas turbine engine propulsion systems. An Ultra-Compact Combustor (UCC) operates by diverting a portion of the compressor exit flow into a cavity about the engine outer diameter. Injection into the cavity can be done at an angle in order to induce bulk circumferential swirl. Swirl velocities in the cavity then impart a centrifugal load of approximately 1000g0. This high-g UCC concept has been investigated by The Air Force Institute of Technology with the goal of incorporating a common upstream flow source to distribute the simulated compressor exit flow into separate core and combustion cavity flow paths. Experimental results from this test rig are presented, with particular emphasis on establishing the design flow split through the diffuser into the circumferential cavity. The implementation of a core channel plate was instrumental in control of the mass flow splits. Computational Fluid Dynamics (CFD) supplement the experiments and enable a more detailed understanding of the interactions within the diffuser and the interactions between the air injection jets and the fuel jets. A range of cavity equivalence ratios was studied and combustion within the cavity was shown to be a strong function of cavity loading, which was in turn a function of the total mass flow. Varying the orientation of the channel plate with respect to guide-vane leading edges caused a change in the core flow development which then had a secondary effect of aiding the combustion process within the cavity.

A gas turbine combustion process subjected to high levels of centrifugal acceleration has demonstrated the potential for increased flame speeds and shorter residence times. Ultracompact combustors (UCC) invoke the high-g phenomenon by introducing air and fuel into a circumferential cavity which is recessed radially outboard with respect to the primary axial core flow. Upstream air is directed tangentially into the combustion cavity to induce bulk circumferential swirl. Swirl velocities in the cavity produce a centrifugal load on the flow that is typically expressed in terms of gravitational acceleration or g-loading. The Air Force Institute of Technology (AFIT) has developed an experimental facility in which g-loads up to 2000 times the earth’s gravitational field (“2000 g’s”) have been demonstrated. In this study, the flow within the combustion cavity is examined to determine factors and conditions which invoke responses in cavity g-loads. The AFIT experiment was modified to enable optical access into the primary combustion cavity. The techniques of particle image velocimetry (PIV) and particle streak emission velocimetry (PSEV) provided high-fidelity measurements of the velocity fields within the cavity. The experimental data were compared to a set of computational fluid dynamics (CFD) solutions. Improved cavity air and fuel injection schemes were evaluated over a range of air flows and equivalence ratios. Increased combustion stability was attained by providing a uniform distribution of cavity air drivers. Lean cavity equivalence ratios at a high total airflow resulted in higher g-loads and more complete combustion, thereby showing promise for utilization of the UCC as a main combustor.