Measurement and Analysis of Flame Transfer Function in a Sector Combustor Under High Pressure Conditions

Abstract

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. A flame transfer function describes the change in the rate of heat release in response to perturbations in the inlet flow as a function of frequency. It is a quantitative assessment of the susceptibility of combustion to disturbances. The resulting fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result. Flame transfer functions for LPP combustion are poorly understood at present but are crucial for predicting combustion oscillations. This paper describes an experiment designed to measure the flame transfer function of a simple combustor incorporating realistic components. Tests were conducted initially on this combustor at atmospheric pressure (1.2 bar and 550 K) to make an early demonstration of the combustion system. The test rig consisted of a plenum chamber with an inline siren, followed by a single LPP premixer/duct and a combustion chamber with a silencer to prevent natural instabilities. The siren was used to induce variable frequency pressure/acoustic signals into the air approaching the combustor. Both unsteady pressure and heat release measurements were undertaken. There was good coherence between the pressure and heat release signals. At each test frequency, two unsteady pressure measurements in the plenum were used to calculate the acoustic waves in this chamber and hence estimate the mass-flow perturbation at the fuel injection point inside the LPP duct. The flame transfer function relating the heat release perturbation to this mass flow was found as a function of frequency. The same combustor hardware and associated instrumentation were then used for the high pressure (15 bar and 800 K) tests. Flame transfer function measurements were taken at three combustion conditions that simulated the staging point conditions (Idle, Approach and Take-off) of a large turbofan gas turbine. There was good coherence between pressure and heat release signals at Idle, indicating a close relationship between acoustic and heat release processes. Problems were encountered at high frequencies for the Approach and Take-off conditions, but the flame transfer function for the Idle case had very good qualitative agreement with the atmospheric-pressure tests. The flame transfer functions calculated here could be used directly for predicting combustion oscillations in gas turbine using the same LPP duct at the same operating conditions. More importantly they can guide work to produce a general analytical model.

Modelling of Circumferential Modal Coupling Due to Helmholtz Resonators

“…These regions are assumed to be axisymmetric and so each circumferential mode can be treated independently. For combustion, the energy flux is increased by the rate of heat input, Q. Typically,Q is found by specifying the value of the mean temperature after combustion.Q is given by a flame transfer function which may come from an analytical model (see [10]), from CFD calculations (see [11,12]) or from experiments (see [13]). A simple model is to takê…”

Efficient retrieval of the thermo‐acoustic flame transfer function from a linearized CFD simulation of a turbulent flame

“…This allows to solely fluctuate the equivalence ratio, which is for most combustion systems the principal cause for inducing heat release rate perturbations [13,14]. The flame transfer function between the change of the volume-integrated heat release rate with a change in the equivalence ratio is determined in several transient simulations, using impulse excitation in the equivalence ratio.…”

Thermoacoustic combustion instability of propane‐oxy‐combustion with CO2dilution: Experimental analysis

“…4,5 The onset of thermoacoustic instability was first postulated by Rayleigh 4 in 1878 for a mass of air vibrating in a cylinder bounded by a piston as follows: "If heat be given to the air at the moment of greatest condensation or to be taken from it at the moment of greatest rarefaction, the vibration is encouraged." 13 The acoustic velocity upstream of the flame is widely used as such parameter since velocity modulation upstream of the flame will result in heat release fluctuation by changing variables like equivalence ratio, swirl number, and vortex shedding process. In combustion systems, therefore, heat addition or removal to fluid can encourage vibrations within the fluid when there is coupling between the heat release and the pressure fluctuations in the system.…”

“…The most important aspect of developing an FTF, therefore, is the identification of an acoustic quantity that incorporates the driving parameters for heat release fluctuations (parameters like equivalence ratio fluctuation). 13 The acoustic velocity upstream of the flame is widely used as such parameter since velocity modulation upstream of the flame will result in heat release fluctuation by changing variables like equivalence ratio, swirl number, and vortex shedding process. 14 FTF…”