Power plants – Combustion products used as motive fluid – Process
Reexamination Certificate
2002-12-03
2003-08-05
Kim, Ted (Department: 3746)
Power plants
Combustion products used as motive fluid
Process
C060S746000, C060S039281, C060S039760, C431S001000
Reexamination Certificate
active
06601393
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to air breathing engines and more particularly to an active combustion control device for an air breathing engine combustor. In more particularity, the present invention relates to a device that applies active combustion control technology to advanced propulsion devices and closed-loop fuel injection at sub-harmonic frequencies of the instability frequency of the combustor. Also, the present invention relates to active combustion control with a combination of open loop fuel injection and closed loop fuel injection.
2. Description of the Related Art
Many propulsion systems, such as those used in various tactical missile systems, involve an enclosed combustor. There are two basic methods for controlling combustion dynamics in a combustion system: passive control and active control. As the name suggests, passive control refers to a system that incorporates certain design features and characteristics to reduce dynamic pressure oscillations. Active control, on the other hand, incorporates a sensor to detect, e.g., pressure fluctuations and to provide a feedback signal which, when suitably processed by a controller, provides an input signal to a control device. The control device in turn operates to reduce the dynamic pressure oscillations.
The combustion characteristics of an enclosed combustor, including flammability limits, instability, and efficiency are closely related to the interaction between shear flow dynamics of the fuel and air flow at the inlet and acoustic modes of the combustor. Strong interaction, between the acoustic modes of the combustor and the airflow dynamics may lead to highly unstable combustion. Specifically, unstable combustion may occur when the acoustic modes of the combustor match the instability modes of the airflow. For such conditions, the shedding of the airflow vortices upstream of the combustor tends to excite acoustic resonances in the combustion chamber, which subsequently cause the shedding of more coherent energetic vortices at the resonant frequency. The continued presence of such vortices provides a substantial contribution to the instability of the combustion process. For a more thorough discussion, please refer to U.S. Pat. No. 5,361,710 issued to Gutmark et al. on Nov. 8, 1994, which is incorporated herein by reference.
In a jet of fluid that exits from a conduit to a surrounding medium of another fluid, sudden increase of the mass-flow leads to formation of well-defined vortices that dominate the boundary between the jet fluid and the surrounding fluid. Because these vortices help transport chunks of fluid over a large distance, the rate of turbulent mixing between the two fluids is closely linked to the dynamics of these vortices. One way to manipulate the dynamics of vortices is to modulate periodically the instantaneous mass-flux of the jet.
In combustion devices, actuators can be used to enhance combustion performance such as efficiency improvement, pollutant reduction, flammability extension, and instability suppression. Combustion apparatuses, which use actuators, have been disclosed in U.S. Pat. No. 5,428,951 issued to Wilson et al. on Jul. 4, 1995, which is incorporated herein by reference. The '951 Patent discloses several active control devices, including loudspeakers to modify the pressure field of the system or to obtain gaseous fuel flow modulations, pulsed gas jets aligned across a rearward facing step, adjustable inlets for time-variant change of the inlet area of a combustor, and solenoid-type fuel injectors for controlled unsteady addition of secondary fuel into the main combustion zone.
The periodic shedding of vortices produced in highly sheared gas flows has been recognized as a source of substantial acoustic energy for many years. For example, experimental studies have demonstrated that vortex shedding from gas flow restrictors disposed in large, segmented, solid propellant rocket motors couples with the combustion chamber acoustics to generate substantial acoustic pressures. The maximum acoustic energies are produced when the vortex shedding frequency matches one of the acoustic resonances of the combustor. It has been demonstrated that locating the restrictors near a velocity antinode generated the maximum acoustic pressures in a solid propellant rocket motor, with a highly sheared flow occurring at the grain transition boundary in boost/sustain type tactical solid propellant rocket motors.
An apparatus and method for controlling pressure oscillations caused by vortex shedding is disclosed is in U.S. Pat. No. 4,760,695 issued to Brown, et al. on Aug. 2, 1988. The '695 patent discloses an apparatus and method for controlling pressure oscillations caused by vortex shedding. Vortex shedding can lead to excessive thrust oscillations and motor vibrations, having a detrimental effect on performance. This is achieved by restricting the grain transition boundary or combustor inlet at the sudden expansion geometry, such that the gas flow separates upstream and produces a vena contracta downstream of the restriction, which combine to preclude the formation of acoustic pressure instabilities in the flowing gas stream. Such an inlet restriction also inhibits the feedback of acoustic pressure to the point of upstream gas flow separation, thereby preventing the formation of organized oscillations. The '695 patent does not present a method or apparatus, which attempts to control pressure oscillations in a combustor by using sub-harmonic frequencies of the instability frequency of the combustor.
While there has been a renewed interest on active combustion control (ACC) stemming from increasingly restrictive requirements on gas turbine pollution, many of the earlier studies on ACC were motivated by the desire to improve combustion performance in rockets, ramjets, and afterburners. Past studies on active combustion control (ACC) have shown that it is possible to enhance combustion performance through fast-response closed-loop feedback control as described in McManus, K. R., Poinsot, T., and Candel, S. “A Review of Active Control of Combustion Instabilities,”
Prog. In Energy and Comb. Sci
., Vol. 19, 1993, pp. 1-29. The scope of earlier investigations, however, often remained relatively basic in nature making it difficult to transition such research results into a practical system. For a more detailed explanation, please refer to Yu, K. et al. “An Experimental Study on Actively Controlled Dump Combustors”, NATO Active Control Symposium (Braunschweig, Germany, May 8-12, 2000), which is incorporated herein by reference.
Some of the previous studies in this area include instability suppression, efficiency improvement, flammability limit extension, and pollutant reduction. These studies have opened up the opportunity to study more practical issues related to potential implementation of ACC in real systems. Current research has studied the possibility of applying the active combustion control technology to advanced propulsion devices.
SUMMARY OF THE INVENTION
A preferred embodiment of the present invention involves a closed-loop liquid-fueled active control technique, which is applied in a dump combustor to enhance its combustion performance. The method and apparatus of the current invention incorporate the requirement of critical fuel flux, effects of fuel droplet size on control, and novel controller concepts. The critical fuel flux is dependent on the fuel droplet size and initial magnitude of the instabilities. When the fuel droplet size, D
0
, is reduced in the controlled injection, the control efficiency for heat flux actuation increases significantly. Upon analysis, an exponential dependency on the droplet size is determined. For a moderate droplet Reynold number, the amplitude of
Parr Timothy
Schadow Klaus C.
Wilson Kenneth J.
Yu Kenneth H.
Foster Laura R.
Kim Ted
The United States of America as represented by the Secretary of
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