ARNOLD AIR FORCE BASE, TENN. – AEDC analysis team members have developed and demonstrated a first generation full frequency range screech analysis methodology for a re-heater on turbine engines to reduce instability when testing in the facilities at Arnold Air Force Base.
Innovative ideas like this one are common in daily work across AEDC and the U.S. Air Force, as the Air Force is known for its dedication in innovation and delivering war winning capabilities to the warfighter by inspiring and providing the necessary tools and support.
General Dave Goldfein, Air Force Chief of Staff, has commented on the importance of innovation in the Air Force.
“We are the service you rely on to push the limits of innovation,” Gen. Goldfein said. “It’s in our bloodline. We’ve faced challenges before and overcome them with ideas.”
One such idea is known as Screech Wave Analysis Methodology (SWAM), the technique created by Hale, Wesley D. Cothran, an AEDC programmer, and Kevin Sabo, of the Massachusetts Institute of Technology, uses sensors to provide integrated analysis.
“The methodology determines the underlying wave structure of combustion instabilities from surface mounted high response static pressure sensors,” Alan Hale, AEDC analyst, explained.
Screech is defined as an acoustic combustion instability that drives pressure oscillations higher than normal. More simply put, it is feedback generated by combustion, which in turn creates instability.
According to Hale, instability when testing is caused by feedback mechanisms between acoustics, hydrodynamic interactions and combustion.
“Instabilities grow when driving mechanisms are greater than damping until limit cycle operation is reached. For non-driven instabilities, screech frequencies correspond closely to combustor geometry natural modes,” he said.
So Hale and his colleagues utilized the existing high response static pressure sensors placed on the inside surface of combustor to detect screech pressure fluctuations.
“Typically, two or three sensors in a transverse plane and two or three sensors in a longitudinal plane, both data planes usually share a sensor, provide necessary information to analyze screech,” Hale said. “For cylindrical combustion geometries without a center body, good sensor arrangements in the transverse direction include two sensors 90 degrees apart with one of these sensors located top dead center and a third sensor located elsewhere on the cylinder surface 45 degrees from vertical or horizontal. Longitudinal sensor arrangements should be non-uniformly spaced and distributed to avoid instability nodes.”
“Because sensors are not typically located at the maximum pressure, no one sensor is expected to measure the maximum pressure over the period. This analysis is only concerned with longitudinal and transverse modes, since no sensors are located in the radial direction.”
Hale mentioned this is a practical methodology for analyzing multiple static pressure sensors simultaneously in order to determine the underlying simple or complex screech wave structure and the magnitude and location of maximum pressure over any screech period for low Mach number flows.
“Additionally, the methodology determines the direction of lumber [movement] for longitudinal modes; the direction of rotation for transverse modes; and the direction of helical rotation for complex modes,” he said.
The objective of this work is to provide cylindrical geometry analysts with a near real time view of the underlying wave structure of screech by having only two or three surface mounted static pressure sensors in the longitudinal and transverse directions. The full frequency range characterizes the ability to analyze simple longitudinal and transverse mode instabilities in isolation or in combination. The analysis methodology has also been extended to include complex helical type modes.
The methodology is currently being used to investigate facility generated screech at the Aerodynamic and Propulsion Test Unit. It provides test teams the analytical capability to identify and characterize both transverse as well as longitudinal combustion instabilities and acoustic screech behaviors in the test data, allowing for improvement strategies for combustion instabilities and undesirable acoustic behaviors to be developed.