الفهرس | Only 14 pages are availabe for public view |
Abstract The problem of active control of thermoacoustic instability that appears in the context of NDx control is approached by understanding the basic mechanism of thermo acoustic instability in combustion systems, obtaining models that describe the behavior of the combustion system and designing the required stabilizing compensator using the developed dynamic models. Experimental verifications of Rayleigh’s criterion, which is the first attempt to interpret the thennoacoustic instability, are reviewed and analyzed. Rigorous theoretical derivations and analytical verifications of Rayleigh’s criterion are reviewed and extended to include the effect of mass addition in three dimensions with general boundary conditions. Combustion modifications for NDx reduction are shown to be represented by multiple heat and mass sources. A model that shows how the multiple heat and mass sources can lead to thermo acoustic instability has been developed. The model is used to show the conditions under which the instability can occur in a boiler. A model that describes the response of the class of laminar premixed flames to an oscillating flow field has been developed from first principles. The developed flame dynamics model, has been shown to verify the sensitive time lag model. Then the flame dynamics was transformed into a transfer function that is suitable for controller design. Using Galerkin method, the longitudinal acoustic field in a duct combustor has been modeled in a form of a finite dimensional dynamic model. Using the acoustics dynamics and the flame dynamics model, the thermoacoustic instability is shown to occur due to an unstable feedback between the unsteady heat release and the acoustic field. The developed feedback model is used to predict the growth rate and the frequency of the pressure oscillations at the onset of instability in a previously published experiment. A rigorous active control methodology for the class of laminar premixed combustors has been implemented for active control of thermo acoustic instability . Using this methodology, a control system which consists of a microphone, a loudspeaker and a compensator, has been designed for stabilizing a laminar premixed combustor. Compensator design is shown to depend mainly on the relative positions of the sensor and the actuator with respect to the acoustic field and the heat source. |