Nonlinear combustion instability analysis of a bluff body burner based on the flame describing function

Lean premixed combustion is a common form of combustion organization in power equipment and propulsion systems. In order to understand the dynamic characteristics of lean premixed flame and predict and control its combustion instability, it is necessary to obtain its flame describing function (FDF). Based on the open source CFD toolbox, OpenFOAM, the dynamic K-equation model, and the finite rate Partially Stirred Reactor (PaSR) model were used to perform large eddy simulations (LES) of lean premixed combustion, and the response of the unsteady heat release rate to single-frequency harmonic disturbances was studied. The response of the unsteady heat release rate was characterized by the FDF, and the response of the unsteady heat release rate to the two-frequency harmonic disturbance was studied. The results show that the quantitative heat release rate response and flame dynamics have very proper accuracy. In the single-frequency harmonic disturbance, as the forcing frequency increases, the curling behavior of the flame surface and the instantaneous vortex structure change; the nonlinear kinematics effect is manifested by the entrainment of the vortex. At lower forcing frequencies, the heat release response changes linearly with the increase of forcing amplitude; at intermediate frequencies, the heat release response exhibits obvious nonlinear behavior; at high frequencies, the heat release response to amplitude changes decreases. The introduction of the second harmonic disturbance will significantly reduce the response range of the total heat release rate and make the combustion more stable.

Three-state switching cell boost converter using H-inf controller

This paper presents the modeling and control of a non-minimum phase dc-dc boost converter based on the three - state switching cells. In any double stage grid-connected system the converter forms an interface between the photovoltaic source and the inverter. As the control and regulation of the converter output is a vital part of penetration of renewable to grid, therefore, this paper had attempted the control of a converter topology that can reduce the current stress across its switches. But the system becomes highly unstable and complex which has been validated by predicting the limit cycle with a describing function. The Controller design is implemented after reducing the complexity of the system using the Model order reduction principle. H-inf controller being robust in nature is applied for stable and regulated output.

A random signal generator by using analog electronics

The describing function theory is a powerful mathematical tool to predict oscillations in non-linear dynamical systems. This theory is here invoked to design a random signal generator and realized by using analog electronic elements. Then, and according to experimental results, histograms of the resultant signal are shown along with the generated signal in the time domain. Finally, the proposed electronic circuit is simple and cheap to construct.