A NUMERICAL STUDY OF EFFECTS OF TURBULENT FLAME STRUCTURE ON DUCTED FLAME OSCILLATIONS

摘要

Confined premixed turbulent flame behind a bluff body is studied as a model of the afterburner of an aeroengine. Interaction between acoustic waves and unsteady heat release can lead to self-excited oscillations in such a system. A simple kinematic model of the phenomena was developed by Dowling [10] recently, who (1) reduced the combustion process to the propagation of an infinitely thin flame with a constant speed and (2) assumed that the heat release rate is proportional to the instantaneous flame surface area. The goal of this work is to investigate the accuracy of the above simplifications and to assess the role played by the flame structure. For these purposes, the so-called Flame Speed Closure (FSC) model developed and well validated for multi-dimensional computations of premixed turbulent combustion [11] was adapted to make it compatible with the kinematic model by Dowling. The key points of this approach are as follows. First, the self-similar approximation of density profiles across premixed turbulent flames is invoked. Second, the development of flame brush thickness is parameterized by the turbulent diffusion law. Third, a "flame speed surface" is located inside the flame brush using the method developed recently [16]. Thus, a kinematic G-equation is solved to determine the flame speed surface as done by Dowling [10], and the flame structure is reconstructed allowing the heat release rate to be integrated over the flame brush. A code for simulating the heat release rate dynamics in ducted flames due to oncoming one-dimensional flow oscillations was developed and simulations of selected cases were performed. The results indicate the importance of resolving the flame structure when modeling ducted flame oscillations.