Gas turbine combustors are often susceptible to self-excited oscillations, which lead to unacceptable levels of pressure, velocity, and heat release fluctuations. Although instabilities can occur in systems with locally constant equivalence ratio, it is very important to take into account the influence of equivalence ratio fluctuations, which are generated in the fuel air mixer in the unstable case. These fluctuations are convected into the flame and lead to an additional mechanism for the generation of heat release fluctuations. Moreover, entropy waves are produced in the flame, which travel through the combustor and generate additional pressure waves during the acceleration of the flow at the combustor exit. To date, available theories use the physically unrealistic assumption that the equivalence ratio waves as well as the entropy waves are convected downstream without any spatial dispersion due to the combustor aerodynamics. An analytical approach is presented, which allows us to take the spatial dispersion into consideration. For that purpose, the response of the burner and the combustor to an equivalence ratio impulse or an entropy impulse is calculated using the Laplace transformation and a more general transfer function for harmonic waves is derived. The obtained expression has three parameters, which represent the influence of the burner or the combustor aerodynamics, respectively. This equation can be used in numerical codes, which represent the combustion system through a network of acoustic multiports, if the equivalence ratio and the entropy are added to the vector of variables considered. The parameters required for the dynamic combustor model can be deduced from a detailed CFD analysis of the combustor flow in case of the application of the theory to a particular combustor design. As an example, a simple model combustor is used to demonstrate the application of the theory. It is highlighted how the spatial dispersion of the equivalence ratio and entropy fluctuations can be included in the stability analysis. The calculated examples reveal that the influence of both variables on the generation of instabilities is highly overpredicted if the spatial dispersion is not taken into account. Furthermore, it can be deduced from the study that burner and combustor designs with a wide range of convective time scales have advantages with respect to the stability of the combustor.

No abstract

No abstract

No abstract

Flame flashback from the combustion chamber into the mixing zone is one of the inherent problems of lean premixed combustion and essentially determines the reliability of low NOx burners. Generally, flashback can be initiated by one of the following four phenomena: flashback due to the conditions in the boundary layer, flashback due to turbulent flame propagation in the core flow, flashback induced by combustion instabilities and flashback caused by combustion induced vortex breakdown. In this study, flashback in a swirling tubular flow was investigated. In order to draw maximum benefit from the tests with respect to the application in gas turbines, the radial distribution of the axial and circumferential momentum in the tube was selected such that the typical character of a flow in mixing zones of premix burners without centerbody was obtained. A single burner test rig has been designed to provoke flashback with the preheating temperature, the equivalence ratio and the mean flow rate being the influencing parameters. The flame position within the mixing section is detected by a special optical flame sensor array, which allows the control of the experiment and furthermore the triggering of the measurement techniques. The burning velocity of the fuel has been varied by using natural gas or hydrogen. The characteristics of the flashback, the unsteady swirling flow during the flame propagation, the flame dynamics and the reaction zones have been investigated by applying high-speed video recordings, the laser Doppler anemometry and the laser induced fluorescence. The presented results show that a combustion induced vortex breakdown is the dominating mechanism of the observed flashback. This mechanism is very sensitive to the momentum distribution in the vortex core. By adding axial momentum around the mixing tube axis, the circumferential velocity gradient is reduced and flashback can be prevented.

Imaging of OH* or CH* chemiluminescence with intensified cameras is often employed for the determination of heat release in premixed flames. Proportionalitx is commonly assumed, hut in the turbulent case this assumption is not justified. Substantial deviations from proportionality are observed, which are due to turbulence-chemistry interactions.In this study a model based correction method is presented to obtain a better approximation of the spatially resolved heat release rate of lean turbulent flames from OH* measurements. The correction method uses a statistical strain rate model to account for the turbulence influence. The strain rate model is evaluated with time-resolved velocity measurements of the turbulent flow. Additionally, one-dimensional simulations of strained counterflow flames are peiformed to consider the nonlinear effect of turbulence on chemiluminescence intensities. A detailed reaction mechanism, which includes all relevant chemiluminescence reactions and deactivation processes, is used. The result of the simulations is a lookup table of the ratio between heat release rate and OH* intensity with strain rate as parameter. This lookup table is linked with the statistical strain rate model to obtain a correction factor which accounts for the nonlinear relationships between OH* intensity, heat release rate, and strain rate. The factor is then used to correct measured OH* intensities to obtain the local heat release rate. The corrected intensities are compared to heat release distributions which are measured with an alternative method. For all investigated flames in the lean, partially premixed regime the corrected OH* intensities are in very good agreement with the heat release rate distributions of the flames.1.2 Purpose of the Study. The nonlinear influence of turbulence on chemiluminescence intensities has to be considered to Journal of Engineering for Gas Turbines and Power

A novel method for the simulation of combustion instabilities in annular combustors is presented. It is based on the idea to solve the equations governing the acoustics in the time domain and couple them to a model for the heat release in the flames. The linear wave equation describing the temporal and spatial evolution of the pressure fluctuations is implemented in a finite element code. Providing high flexibility, this code in principle allows both the computational domain to be of arbitrary shape and the mean flow to be included. This yields applicability to realistic technical combustors. The fluctuating heat release acting as a volume source appears as a source term in the equation to be solved. Employing a time-lag model, the heat release rate at each individual burner is related to the velocity in the corresponding burner at an earlier time. As saturation also is considered, a nonlinearity is introduced into the system. Starting the simulation from a random initial perturbation with suitable values for the parameters of the heat release model, a self-excited instability is induced, leading to a finite-amplitude limit cycle oscillation. The feasibility of the approach is demonstrated with three-dimensional simulations of a simple model annular combustor. The effect of the model parameters and of axial mean flow on the stability and the shape of the excited modes is shown.

No abstract