N14A-T004 TITLE: Active Combustion Control (ACC) of Augmentor Dynamics

TECHNOLOGY AREAS: Air Platform

OBJECTIVE: Advance the innovation in active combustion control (ACC) technologies to develop active screech suppression system for thrust augmentors in high-performance gas turbine engines.

DESCRIPTION: Combustion instability, or screech, occurs in many combustion systems. Combustion instability is due to the complex physical coupling of the acoustic resonances in the combustion chamber with fluctuations in the heat release of the combustion process. In modern gas turbine afterburners, instability or screech modes typically occur in the range of frequencies from hundreds to thousands of hertz. Coupling can produce large pressure fluctuations that can be severe enough to damage engine hardware. Three stream engines and the integration of the augmentor, exhaust ducts, and nozzle for next-generation gas turbines will increase the desired range of operability for the augmentor and further challenge the ability to manage instabilities.

Historically, screech has been mitigated by two very different approaches; (1) passive combustion control, such as damping, and (2) altering the screech coupling or driving. In the damping approach, acoustic liners and resonators have been fashioned to absorb acoustic energy; liners, by their nature, are most effective on modes with frequencies greater than 1 kHz, whereby resonators can be tuned to suppress much lower frequency modes, less than 1 kHz. However, resonators are physically large (approximately 3 feet in diameter and 5 feet in length) and have a significant system weight penalty (approximately 30-40 pounds). Integration of screech liners or resonators with multi-stream engines also presents significant integration challenges for future systems.

Altering the coupling or driving for screech involves changing the aerodynamics or at least the fuel delivery to change the spatial or temporal characteristics of the heat release. This is often accomplished empirically since reliable analytical tools do not exist for this complex process. If such changes are needed late in the engine development program, this can be very costly to implement and difficult to retrofit. Another method of altering the heat release is to implement an active control system which will modulate the fuel or air sources depending on the operating condition and instability presence to alter the heat release.

ACC is an emerging art of regulating combustion performance using a dynamic hardware component that rapidly modifies combustion input. ACC relies on proper timing of software-controlled actuation rather than spatial changes to the combustor hardware as practiced currently in combustion instability problems. Since timing adjustment is simpler than the potential geometry modifications associated with conventional passive control, ACC could provide potential flexibility in performance while eliminating costly retrofit for combustor design modification. With continuing advances in electronics, ACC could be developed into a next-generation, paradigm-shifting technology for controlling unwanted combustion dynamics.

One of the main active control approaches is high bandwidth active control using the fuel, whereby fuel is modulated at the frequency of the instability using an actuator valve. The phase of the modulation is varied actively until sufficient heat release is out of phase with the instability which results in suppression of the instability. Active control methods have demonstrated excellent control of combustion instability in ground-based gas turbine systems, where weight and actuator power consumption are not significant factors. Development of actuators with sufficient driving capability is still an open research area.

Combustion in the augmentor is governed by many unsteady physical processes. Desired are new active control screech suppression technologies that target the physical processes in the afterburner. These new active control technologies should be developed such that they could easily be implemented in current gas turbine augmentors with little weight (less than 30-40 lbs) or cost consequence. These technologies should also address exhaust integration issues for next-generation systems without adversely impacting the proper functioning of the exhaust or the engine as a whole.

PHASE I: Identify, develop, and demonstrate the feasibility of an innovative concept using the proposed ACC for suppression of instabilities in a laboratory environment. Identify the experimental methodology to evaluate the influence of the technology on the magnitude and bandwidth of the instabilities observed in modern augmentors and address the feasibility for full-scale implementation.

PHASE II: Further develop the proposed concept and conduct extensive experimental evaluation of the active control technologies identified in Phase I. Assess the ability of the candidate technology to reduce the magnitude and bandwidth of screech instabilities that occur in modern augmentors. Perform a prototype demonstration of the active control suppression concept.

PHASE III: Collaboration with an original equipment manufacturer (OEM) of high-performance afterburners is recommended to ensure successful transition of technology concepts. Demonstrate a fully-functional, active control system for screech suppression and transition the approach to Naval Aviation platforms.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The technology has the true potential for dual-use applications by suppressing combustion instabilities in both military and civil gas turbine engines because the methodology should be applicable to main-burner combustion instabilities.

REFERENCES:
1. DeLaat, J.C., Kopasakis, G., Saus, J.R., Chang, C.T., and Wey, C. (2013). "Combustion Control for a Low-Emissions Aircraft Engine Combustor Prototype: Experimental Results," Journal of Propulsion & Power, Vol. 29 (4), pp. 991-1000.

2. Schadow, K.C., Gutmark, E. and Wilson, K. J. (1992). "Active Combustion Control in a Coaxial Dump Combustor," Combustion Science and Technology, Vol. 81, No 4-6, pp. 285-300.

3. Candel, S. (2002). "Combustion Dynamics and Control: Progress and Challenges," Proceedings of Combustion Institutes, Vol. 29, pp. 1-28.