A multi-chamber model of combustion instabilities and its assessment using kilohertz laser diagnostics in a gas turbine model combustor

Abstract

A multi-chamber model for the combustion instabilities manifested in a gas turbine model combustor was developed. The proposed model was used to explain the dependencies of instability frequency on burner geometry and other flow parameters, some of which could not be reconciled with previous models. The new model was built upon the Helmholtz analysis of two connected resonators. The instability frequency as well as the complex pressure ratio between two chambers were predicted by solving ordinary differential equations. To assess the assumptions and predictions of the proposed model, the spectra and magnitude of the oscillations of pressure, heat release rate, and velocity were measured for four different operating conditions: rich (R1), lean (L1), stoichiometric (S1), and reduced flow rate (R2) with a kilohertz laser diagnostic system. These measurements reconfirmed that the instability is of Helmholtz type. A global equivalence ratio that is consistently greater than unity was identified to be an enabling factor for combustion instability. This is also in agreement with the predictions made by the proposed model. Furthermore, the model was shown to be able to predict the right trend of instability frequency when multiple parameters were changed. It is concluded that the current model is an improvement over previous models, because the acoustic coupling between different chambers of the burner was considered.

Study an Acoustic-Intrinsic Thermoacoustic Feedback on Swirling Flame Combustors

“…According to the previous work [25], we find that this unstable mode comes from the PA mode in terms of the Helmholtz resonator constructed by the plenum and swirl-tube. Chen and Driscoll [28] showed that the frequency of the Helmholtz resonator could be roughly calculated through…”

“…From previous work [25], we find that this unstable mode comes from the PA mode in terms of the Helmholtz resonator constructed by the plenum and swirl-tube. Chen and Driscoll [28] shows that the frequency of the Helmholtz resonator can As we are interested in the evolution mechanism from Helmholtz resonance to the thermoacoustic oscillation, we multiply the right hand side of Eq. (10) with a factor  to control the amplitude of the flame.…”

Time-Gated Single-Shot Picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS) for Equivalence-Ratio Measurements

“…[6][7][8][9][10][11] Knowledge of local equivalence ratios provides a means for researchers in the field of combustion modeling to explore the underlying physics that govern combustion instability and, hence, improve engine design. 12,13 Equivalence ratio determination with LIBS generally uses the ratio of emissions from two elements in the plasma: one found in the fuel (e.g., atomic emission from H or C) and one found in the oxidizer (e.g., atomic emission from O or N). For example, previous studies have shown that emission intensity ratios H (656 nm)/N (568 nm), H (656 nm)/O (777 nm), C (711 nm)/O (777 nm), and H (656 nm)/N (746 nm) all have a linear dependence on equivalence ratio for a range of fuel-to-air mixtures.…”

An experimental study of the global and local flame features created by thermoacoustic instability

“…Numerical and analytical work on thermoacoustic instability is also essential to The zero-dimensional model works with the assumption that: 1) the flow is onedimensional and incompressible, 2) the gas is inviscid and behaves as an ideal gas, 3) all variables in the control volume has negligible variations [8,60]. The governing equation for the model is [8],…”

“…The delay time  is determined by the method proposed in the work of Driscoll [60]. The equation is,…”