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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.
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.
The ultra-compact combustor (UCC) is an innovative combustor system alternative to traditional turbine engine combustors with the potential for engine efficiency improvements with a reduced volume. Historically, the UCC cavity had been configured such that highly centrifugally loaded combustion took place in a recessed circumferential cavity positioned around the outside diameter (OD) of the engine. One of the obstacles with this design was that the combustion products had to migrate radially across the span of a vane while being pushed downstream by a central core flow. This configuration proved difficult to produce a uniform temperature distribution at the first turbine rotor. The present study has taken a different spin on the implementation of circumferential combustion. Namely, it aims to combine the combustion and space saving benefits of the highly centrifugally loaded combustion of the UCC in a new combustor orientation that places the combustor axially upstream of the turbine versus radially outboard. An iterative design approach was used to computationally analyze this new geometry configuration with the goal of fitting within the casing of a JetCat P90RXi. This investigation revealed techniques for implementation of this concept including small-scale combustor centrifugal air loading development, maintaining combustor circumferential swirl, combustion stability, and fuel distribution are reported. The final combustor configuration was manufactured and experimentally tested, validating the computational results. Furthermore, dramatic improvements in the uniformity of the turbine inlet temperature profiles are revealed over historical UCC concepts.
Combustion at high G-loading offers the promise of higher flame speeds and shorter residence times. Ultra-Compact Combustors (UCC) make use of this phenomenon by injecting air and fuel into a circumferential cavity around the main core flow. Air is injected 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”) can be established. This paper investigates the flow within the combustion cavity to determine conditions that lead to the generation of higher g-loads and longer residence times. This is coupled with the desire to completely combust the fuel — ideally within the combustion cavity. These objectives have led to changes within the AFIT test setup to enable optical access into the primary combustion cavity. Particle Image Velocimetry (PIV), complemented by traditional high-speed video imagery, provided high-fidelity measurements of the velocity fields within the cavity. These experimental measurements 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 uniform distribution of air drivers. Lean cavity equivalence ratios at a high total airflow resulted in higher g-loads and complete combustion showing promise for utilizing the UCC as a main combustor.
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