Session: 04-20: Combustion Noise
Paper Number: 82480
82480 - Computation of Intrinsic Instability and Sound Generation From Autoignition Fronts
Two-stage sequential combustor architecture with a propagation-stabilized flame in the first stage and an autoignition-stabilized flame in the reheat stage offers efficient combustion of carbon-free fuels such as hydrogen. However, unsteady combustion processes, due to the interaction between the flame and the combustor acoustic field, are a major deterrent to the stable operation of combustion systems. While this undesirable phenomenon has been thoroughly studied for conventional single-stage combustors with propagation-stabilized flames, the mechanisms leading to unsteady combustion in a reheat system are not well understood. One particular mechanism that may lead to self-sustained flame oscillations involves modulation of the inlet reactant mixture by upstream traveling isentropic perturbations that are generated by the unsteady heat release rate in the ignition front. Previous high-resolution numerical modeling studies based on direct numerical simulations (DNS) have revealed that this ‘intrinsic’ interaction mechanism may result in an unstable feedback loop that manifests in strong oscillations of the ignition front location and the heat release rate.
In the present work, we first perform one-dimensional reactive Navier-Stokes and Euler computations of intrinsic thermoacoustic instabilities in a simplified one-dimensional reheat combustor configuration with an autoignition front. The governing equations are solved using a high-order finite difference scheme in combination with a Runge-Kutta time marching algorithm. Computations are performed for a lean vitiated hydrogen-air mixture at two different mean inlet temperatures and varying levels of acoustic reflection coefficients of the boundaries. The autoignition front exhibits self-excited thermoacoustic oscillations at a mean inlet temperature of 1000 K. These self-excited oscillations are observed even when the domain boundaries are fully non-reflecting and tend to be more unstable as the reflection from the boundaries increases. The corresponding Euler computations, which neglect the effect of viscosity and thermal/species diffusion, exhibit similar instability behavior with similar frequencies and damping rates.
In the second part of this work, we address the problem of computing the acoustic field generated by an unsteady autoignition front, which is an important element to understand and predict thermoacoustic instabilities. One of the main challenges in computing the sound field generated by an unsteady ignition front is the fact that the front significantly moves in response to small fluctuations in temperature and pressure. This harmonic flame motion causes local heat release rate and gas property fluctuations at any point close to the front, which act as sources of sound. In the present paper, we compute the spatio-temporal fluctuating acoustic pressure field generated by a one-dimensional unsteady ignition front by numerically solving the linearized Euler equations (LEE). The LEE framework computes the fluctuating pressure and velocity field for a given heat release rate source term. Two approaches are used to construct the source terms. In the first approach, the source terms are directly interpolated from the DNS to the LEE grid, and in the second approach the source terms are constructed using model profiles. The LEE predictions of the fluctuating pressure field obtained from both these approaches agree well with corresponding direct numerical simulations. The LEE based approach discussed in the present work can be used for efficient computation of acoustic transfer matrices of autoignition-stabilized flames. The results of the present work give useful insight and modeling capabilities to address the vital problem of prediction and control of thermoacoustic instabilities in the reheat stage of sequential combustion systems.
Presenting Author: Harish Subramanian Gopalakrishnan Norwegian University of Science and Technology
Presenting Author Biography: I am a PhD student at NTNU.
Authors:
Harish Subramanian Gopalakrishnan Norwegian University of Science and TechnologyAndrea Gruber Norwegian University of Science and Technology
Jonas Moeck Norwegian University of Science and Technology
Computation of Intrinsic Instability and Sound Generation From Autoignition Fronts
Paper Type
Technical Paper Publication