Active Combustion Instability Control With Spinning

Proceedings of ASME TURBO EXPO 2002 June 3-6, 2002, Amsterdam, The Netherlands

CDI TOC

Proceedings of ASME Turbo Expo 2002 June 3–6, 2002, Amsterdam, The Netherlands

GT-2002-30042 GT-2002-30042

ACTIVE COMBUSTION INSTABILITY CONTROL WITH SPINNING VALVE ACTUATOR Prabir Barooah, Torger J. Anderson and Jeffrey M. Cohen United Technologies Research Center East Hartford, Connecticut, USA Abstract Active combustion control has been accomplished in many laboratory and real-world combustion systems by fuel modulation as the control input. The modulation is commonly achieved using reciprocating flow control devices. These demonstrations have been successful because the instabilities have been at relatively low frequencies (~200 Hz) or the scale of demonstration has been small enough to require very small levels of modulation. A number of real-world instabilities in gas turbine engines involve higher frequencies (200500 Hz) and attenuation requires the modulation of large fractions of the engine fuel flow rate (hundreds of pounds per hour). A spinning drum valve was built to modulate fuel for these applications. Tests showed that this device provided more than 30% flow modulation up to 800 Hz for liquid fuel flows of greater than 400 lbm/hr. This paper describes the performance of the valve in flow bench tests, openloop forcing and closed-loop instability control tests. The closed loop tests were done on a single-nozzle combustor rig which exhibited a limit-cycling instability at a frequency of ~280 Hz with an amplitude of ~7 psi. It also encounters an instability at 575 Hz under a different set up of the rig, though active control on that instability has not been investigated so far. The test results show that the spinning valve could be effectively used for active instability control, though the control algorithms need to be developed which will deal with or account for actuator phase drift / error. 1.0 Introduction As the environmental pressures to further reduce NOX, CO and smoke continue, combustion chambers in gas turbines have become more susceptible to combustion instabilities. The traditional design methodology for suppressing combustion instabilities is proving inadequate; therefore new techniques, such as active control approaches are sought.

Combustion instabilities are caused by the coupling of acoustic velocity/pressure fluctuations with unsteady heat release [ref 1]. One method of active control that has been widely used in recent times is modulation of fuel supply to the combustor. If the heat release fluctuation due to the modulating fuel supply is in opposite phase to the natural instability driven heat release fluctuation, the two will cancel each other out and instability will be suppressed. The simplest implementation of this method is the phase shifting controller, in which the fuel supply is modulated at a certain phase shift from the main harmonic of the pressure oscillations inside the combustor. One impediment to the successful application of such active control schemes is the lack of actuators capable of providing large modulation levels at frequencies in the range of these instabilities, i.e., in the 250 – 1000 Hz range. A rotary fuel modulation device, or “spinning valve” was developed which could modulate fuel with an amplitude of more than 30% of mean flow rate up-to 1 kHz. The valve was used for active combustion control in a NASA program in which the goal was the suppression of high-frequency longitudinal modes, which exist in higher-volumetric heat release aero engines. Two different instabilities are observed under different rig set-ups. One is a limit cycling instability at around 280 Hz and the other is a resonant peak at around 575 Hz. So far the spinning valve has been used in active control of only the 280 Hz instability, though it was developed with instabilities at higher frequencies in mind. This paper reports active control work for the 280-Hz instability using the spinning valve as the fuel modulation actuator. Section 1.2 describes the experimental facility set up for combustion instability control. Section 2 describes the operating principles of the spinning valve and the results of fuel modulation tests done in a cold-flow test loop. Section 3 briefly describes the phase shifting control algorithm, as it was applied in combustion control tests. A valve controller was employed to make the -1- 1

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spinning valve modulate fuel in phase with a controller command. Section 4 describes the valve controller development work. The results of active control tests with the spinning valve in the single nozzle combustor rig are described in section 5 and finally, section 6 summarizes the lessons learned from the exercise. 1.1 Nomenclature Inlet air pressure Inlet air temperature Fuel flow rate Air flow rate Unsteady pressure Unsteady combustor pressure Frequency Motor shaft Phase Desired motor shaft Phase Combustion pressure phase Control phase shift Feed line phase shift Command to valve Shaft position error PID controller gains Motor transfer function Valve controller transfer function

Test Variable Inlet air pressure, P3 (psia) Inlet air temperature, T3 (ºF) Fuel flow rate, Wf (lbm/hr) Air flow rate, Wa (lbm/sec) Unsteady Pressure Amplitude, P’comb (psi.) Mean fuel/air ratio Instability Frequency (Hz)

P3 T3 Wf Wa P' P'comb f

Mean Value 110 610 207 2.55 6.5 0.022 280

Table 1.2 - 1: Combustor operating conditions and uncontrolled instability characteristics for 300-Hz instability.

m d c




f

Vc e KP, KI, KD Gv Gc

1.2 Single-Nozzle Combustor As part of a NASA-sponsored active instability control program, a single-nozzle combustor rig was constructed to replicate combustion instabilities seen in full-scale aero engines [ref 2]. The active combustion control tests with the spinning valve were conducted using this combustor. Fig. 1.2 – 1 shows a schematic of the combustor test section assembly. Jet-A fuel was supplied to the injector by a highpressure fuel supply. The fuel injector was a prefilming, high-shear airblast injector that was traceable to full-scale engine hardware. The combustor operated at realistic engine conditions and produced ~4 MW at peak power. All the active control tests described in this paper were done at the operating conditions shown in table 1.2 – 1. These values are for a mid power condition.

Figure 1.2 - 1: Single-nozzle combustor assembly, showing choked inlet (bottom) and outlet (top) boundaries, fuel injector and combustor liner. Unsteady pressure measurement (PLA1C1) located at 1.865 inches downstream of combustor bulkhead. Dimensions shown are in inches. Dynamic measurements of the unsteady component of pressure were made at a number of locations within the combustor. The sensor labeled "PLA1C1" in Fig. 1.2-1 is used within this paper as a pressure representative of the unsteady combustor pressure. This sensor was mounted on the liner at 1.865 in. downstream of the combustor bulkhead. Data were acquired with a dSpace data acquisition system typically sampling at 5 kHz.

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Drum

From fuel supply

To injection orifice

Moog G413-404 brushless motor Seal

Figure 2.0 - 1: Schematic of the spinning valve concept, using a rotating drum with multiple holes, which align with exit holes in the case. A close tolerance was maintained to produce the maximum level of modulation and to minimize leakage. An even number of exit holes provided a pressure balance. at all frequencies to overcome motor losses, shaft seal Low-pass (2 kHz) anti-aliasing filters were installed friction and limited fluid friction in the valve on all channels. Fast response pressure chamber. measurements were also made in the fuel line The valve built for this program used a downstream and upstream of the fuel modulating Moog G413-404 motor, a readily available motor valve to measure modulation levels and check capable of meeting the speed, acceleration and steadiness of the fuel supply pressure, respectively. precision requirements. Runout and radial clearance The fuel flow rate was modulated with the spinning tolerances were better than 0.001 in., making it valve, which was controlled by a PC-based DSP possible to design for clearances that minimized (dSpace data acquisition/ control system, ds1003 leakage when the valve is closed. Twelve holes processor board) through a Moog T200 motor around the spool circumference allowed a fuel controller. modulation frequency of 1 kHz to be reached at 5000 rpm, well within the capability of this motor. 2.0 Spinning Valve Actuator The spinning valve design was based on a rotary concept rather than a conventional reciprocating-spool configuration in order to generate the maximum frequency response (Fig. 2.0 - 1). The concept used a rotating drum with twelve regularly spaced holes around the circumference, which aligned with holes in the surrounding housing to pass flow. By minimizing the clearance between the housing and the drum, leakage was reduced when the holes in the drum and housing were misaligned. Exit holes in the housing were radially opposed to balance pressure and minimize transverse loads. A design with two exit holes was chosen for this application to balance radial pressure loads on the shaft and provide sufficient flow capacity in a reasonably sized package. It should be noted that additional exit holes could have been built into the housing, reducing the valve size for a given flow capacity in exchange for added plumbing complexity. Additional manifold complexity can, however, introduce unexpected systems response characteristics due to the acoustics of the plumbing system. Unlike a reciprocating device, the upper frequency limit of the spinning valve was not due to the spool inertia or power required to accelerate it. A relatively low power was required to run this device

2.1 Fuel Flow Rate Modulation Authority The valve system and its associated plumbing were tested in a small Jet-A flow loop with a capacity similar to that required for the single nozzle combustor tests (~ 500 lbm/hr). The flow-loop facility configuration is shown in Fig. 2.1 – 1. The valve was tested with a downstream orifice with a flow capacity equal to that of the fuel nozzle. A bypass line upstream of the test valve could be opened to set the upstream supply pressure. An accumulator or pulse damper was installed upstream of the spinning valve to reduce pressure oscillations at the inlet. This device was pressurized to approximately 60% of the operating pressure at the valve inlet and, in lower modulation levels, could damp out almost all upstream oscillations. However, the modulation levels achieved using the spinning valve were so large that the pulse damper was less effective with this valve. The fuel reservoir could be pressurized with nitrogen to simulate operation at elevated fuel system pressures (such as would be encountered during combustion tests). The modulating valve must have a flow capacity approximately equal to or greater than the

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Fig. 2.1 - 1. Schematic of the cold flow test facility. A cooling system maintained a constant temperature despite heat added from pump work at high-pressure differentials. A high-pressure nitrogen system was used to pressurize the fuel system to realistic operating pressures. some error.Fig. 2.1 – 2 shows a sample time trace fuel injector flow capacity when open, and from a pressure transducer downstream of the valve substantially lower (or almost zero) than the orifice when it was running at 500 Hz. Flow modulation was flow capacity when closed, to allow the device to inferred from pressure oscillations by a simple effectively modulate the flow. The valve was analysis based on the momentum equation for required to have sufficient response at the frequencies incompressible mass flow through an orifice in terms expected in aeroengine instabilities, so it was of the pressure differential. From these basic designed to operate at frequencies up to at least 1 equations, it can be shown that kHz. It was designed to provide on/off flow modulation. The level of modulation could be wW f 1 w 'P reduced, if necessary, with the installation of a valve ˜ Eq. 2.1 - 1 Wf 2 'P bypass line. The performance metric used to evaluate this valve system was its ability to modulate the fuel flow This equation assumes that P remains at the point of injection: the fuel nozzle. It was approximately constant ( ( P)