Combustion Instability Modelling and Control
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Recent papers in Combustion Instability Modelling and Control
The article analyzes the different nature of conflicts that have occurred in Africa since the end of Cold War. A special attention is given to the role of external factors in the process of conflict evolution and the escalation of... more
The article analyzes the different nature of conflicts that have occurred in Africa since the end of Cold War. A special attention is given to the role of external factors in the process of conflict evolution and the escalation of violence on the African continent. In effect, this paper demonstrates through a critical examination of the meaning of proxy war, zone of influence or provocation of instability as well as its strategies and an analysis of its employment by the United States and China, France etc. in Africa. The new potential confrontation between the United States and China as in Sudan, France in its former colonies is not only based on a clash of world views about the structure and nature of international relations and security but largely over the control of strategically vital energy resources based in Africa. The authors concludes that this ultimately creates permanent tensions or bitter conflicts between the actors and African populations as a factor that have negative impacts on the peace and stability of the continent. According to the context of superpower conflict strategies, this paper critically examines, zone of influence, provoking of instability or proxy war as a viable national strategy of nuclear armed great powers in advancing and/or defending their global national interests in a bipolar/multipolar international system. During the Cold War, war by proxy was a key strategy of indirect conflict between the United States and the Soviet Union. The purpose of these proxy wars was to either maintain or change the balance of power between the superpowers/great powers in conflict areas outside the central front in Europe. Within the condition of Mutual Assured Destruction (MAD), both the United States and the Soviet Union sought to avoid direct confrontation between their conventional military forces in regional conflicts out of fear that it would escalate to an all out nuclear war. In this condition, both powers engaged minor powers rather than each other directly. This entailed limited , indirect war via proxy forces to minimize the threat of direct confrontation between the superpowers for fear of escalation. Close to two decades after the fall of the Berlin Wall there has been very little discussion about proxy wars between major international powers that possess nuclear capabilities. The Soviet Union no longer exists and Russia is not the existential threat to the United States that the Soviet Union once was. The international focus has shifted towards Western state intervention in small local conflicts and away from Cold War strategies under the umbrella of the concept of Peace Support Operations. The United States and the Soviet Union used foreign governments and international organizations as proxies, such as during the United Nations operation in the Congo in 1960 and the Angolan Civil war in 1975, to influence and alter the outcome of a local conflict to suit its national interests and alter the regional balance of power.
The present paper presents a methodology to account for local mean-flow effects on thermo-acoustic instabilities to improve typical thermo-acoustic calculations generally performed under the zero-Mach number assumption. A 3D FEM model of... more
The present paper presents a methodology to account for local mean-flow effects on thermo-acoustic instabilities to improve typical thermo-acoustic calculations generally performed under the zero-Mach number assumption. A 3D FEM model of a simplified combustor is solved in COMSOL Multiphysics with the pressure acoustics module. The Helmholtz equation is used to model the combustor and the classical k-τ model for the Flame Transfer Function (FTF) is adopted. In order to account for local non-zero Mach number effects in the burner region, the burner is replaced with its transfer matrix (BTM), computed through the aero-acoustics module considering an assigned mean-flow, and which implicitly takes into account the mentioned effects. The obtained matrix is inserted in the FEM model of the simplified combustor. The BTM ability to represent local mean-flow effects and the impact on the resonant frequencies and their growth rate is then evaluated comparing the results with those provided by an in-house 1D code solving the linearized Navier-Stokes equations in the presence of a mean flow.
The present work describes the study and the development of a control system using fuzzy logic for active control of a Rijke tube type pulse combustor installed at National Institute of Space Research, in Cachoeira Paulista, SP, Brazil. A... more
The present work describes the study and the development of a control system using fuzzy logic for active control of a Rijke tube type pulse combustor installed at National Institute of Space Research, in Cachoeira Paulista, SP, Brazil.
A study on the existing control types for pulse combustion was conducted. A simulation model for the Rijke tube, based on the Matlab and Simulink programs, was proposed. Specific program blocks were developed for this study. A control diagram was developed for the control system.
The membership functions and the inference rules of the fuzzy controller were developed. The results have shown that the controller converges in 0.2 s, for a processing delay time of 2 ms, and in 0.8 s for a processing delay time of 4 ms, from the starting pulse.
A study on the existing control types for pulse combustion was conducted. A simulation model for the Rijke tube, based on the Matlab and Simulink programs, was proposed. Specific program blocks were developed for this study. A control diagram was developed for the control system.
The membership functions and the inference rules of the fuzzy controller were developed. The results have shown that the controller converges in 0.2 s, for a processing delay time of 2 ms, and in 0.8 s for a processing delay time of 4 ms, from the starting pulse.
Air-fuel ratio (AFR) is a crucial parameter for combustion controls in internal combustion engines. An incorrect AFR metering for reciprocating internal combustion engine causes high toxic gases emissions formulation, serious fuel... more
Air-fuel ratio (AFR) is a crucial parameter for combustion controls in internal combustion engines. An incorrect AFR metering for reciprocating internal combustion engine causes high toxic gases emissions formulation, serious fuel consumption problems and unbearable combustion noise and combustion deterioration. Traditionally, the AFR is obtained by direct measurement of intake air and the fuel either injected into the combustion chamber or pre-mixed at the carburetor. However, the accurate AFR obtained from direct measurement is difficult due to measuring equipments resolution prone to errors. This paper describes a method for accurate determination of air-fuel ratio based on exhaust emission gas analysis as an additional tool used to be validated the conventional direct air fuel flow rates measurement. This method explains all the possible parameters that may affect the accuracy of air-fuel ratio measurement which includes the instrument error, ambient conditions, the assumed water-gas shift reaction constant, the humidity of the atmospheric air and the inclusion of nitrogen in the air-fuel ratio model. Results show that four essential exhaust gas emission concentrations, namely carbon monoxide (CO), carbon dioxide (CO2), oxygen (O2) and unburn hydrocarbons (HCs) are adequate for obtaining an accurate air-fuel ratio. The fuel type and the range of parameters that may affect the accuracy of air-fuel ratio are properly defined. This paper will also present experimental results of a bi-fuel natural gas spark ignition engine to be compared with computational results. The results of this investigation will be used to develop a new dedicated natural gas engine. Copyright © 2004 SAE International.
http://papers.sae.org/2004-01-0640/
http://papers.sae.org/2004-01-0640/
O presente trabalho analisa um sistema de controle de lógica difusa para um combustor pulsante tipo tubo de Rijke. Durante o desenvolvimento do mesmo, realizou-se um estudo dos tipos de controle existentes para a combustão pulsante e foi... more
O presente trabalho analisa um sistema de controle de lógica difusa para um combustor pulsante tipo tubo de Rijke. Durante o desenvolvimento do mesmo, realizou-se um estudo dos tipos de controle existentes para a combustão pulsante e foi proposto um modelo de simulação a ser utilizado pelo pacote Matlab e Simulink. Uma planta de controle foi utilizada para o desenvolvimento do sistema. Um controlador difuso foi desenvolvido e suas funções de pertinência e regras de inferência foram estabelecidas. A simulação realizada mostrou que a lógica difusa no controle de instabilidades de combustão é viável. Através dos resultados pôde-se inferir que o controle respondeu aos impulsos colocados de maneira eficiente e conforme o desejado. Verificou-se que o controle precisou de aproximadamente 0,2 s para elevar a pressão interna do tubo de 30 mbar para 90 mbar, considerando-se um atraso total de 2 ms. Estudaram-se os efeitos da variação do atraso e verificou-se que, mesmo levando-se um maior tempo para se conseguir a convergência, esta ocorre, não influenciando no desempenho geral. O controlador envia um sinal de pressão em fase com o sinal de pressão interno ao tubo, através dos alto-falantes, quando se deseja aumentar a pressão de oscilação. Quando se deseja diminuir a onda de pressão interna ao tubo, o controlador envia um sinal de controle defasado 180o ao sinal.
A method for predicting the onset of acoustically driven combustion instabilities in gas turbine combustor is examined. The basic idea is that the governing equations of the acoustic waves can be coupled with a flame heat release model... more
A method for predicting the onset of acoustically driven combustion instabilities in gas turbine combustor is examined. The basic idea is that the governing equations of the acoustic waves can be coupled with a flame heat release model and solved in the frequency domain. The paper shows that a complex eigenvalue problem is obtained that can be solved numerically by implementing the governing equations in a finite element code. This procedure allows one to identify the frequencies at which thermoacoustic instabilities are expected and the growth rate of the pressure oscillations, at the onset of instability, when the hypothesis of linear behaviour of the acoustic waves can be applied.
The method can be applied virtually to any three dimensional geometry, provided the necessary computational resources that are, anyway, much less than those required by Computational Fluid Dynamics (CFD) methods proposed for analysing the combustion chamber under instability condition. Furthermore, in comparison with the “lumped” approach that characterize popular Acoustics Networks, the proposed method allows one for much more flexibility in defining the geometry of the combustion chamber.
The paper shows that different types of heat release laws, for instance, heat release concentrated in a flame sheet as well as distributed in a larger domain, can be adopted. Moreover, experimentally or numerically determined flame transfer functions, giving the response of heat release to acoustic velocity fluctuations, can be incorporated in the model.
To establish proof of concept, the method is validated at the beginning against simple test cases taken from literature. Over the frequency range considered, frequencies and growth rates both of stable and unstable eigenmodes are accurately evaluated. Then the method is applied to a much more complex annular combustor geometry in order to evaluate frequencies and growth rates of the unstable modes and to show how the variation of the parameters of the heat release law can influence the transition to instability
The method can be applied virtually to any three dimensional geometry, provided the necessary computational resources that are, anyway, much less than those required by Computational Fluid Dynamics (CFD) methods proposed for analysing the combustion chamber under instability condition. Furthermore, in comparison with the “lumped” approach that characterize popular Acoustics Networks, the proposed method allows one for much more flexibility in defining the geometry of the combustion chamber.
The paper shows that different types of heat release laws, for instance, heat release concentrated in a flame sheet as well as distributed in a larger domain, can be adopted. Moreover, experimentally or numerically determined flame transfer functions, giving the response of heat release to acoustic velocity fluctuations, can be incorporated in the model.
To establish proof of concept, the method is validated at the beginning against simple test cases taken from literature. Over the frequency range considered, frequencies and growth rates both of stable and unstable eigenmodes are accurately evaluated. Then the method is applied to a much more complex annular combustor geometry in order to evaluate frequencies and growth rates of the unstable modes and to show how the variation of the parameters of the heat release law can influence the transition to instability
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