Autoignition flame transfer matrix: Analytical model versus large eddy simulations

Abstract

Modern gas turbines need to fulfil increasingly stringent emission targets on the one hand and exhibit outstanding operational and fuel flexibility on the other. Ansaldo Energia GT26 and GT36 gas turbine models address these requirements by employing a combustion system in which two lean premixed combustors are arranged in series. Due to the high inlet temperatures from the first stage, the second combustor stage predominantly relies on autoignition for flame stabilization. In this paper, the response of autoignition flames to temperature, pressure and velocity excitations is investigated. The gas turbine combustor geometry is represented by a backward-facing step. Based on the conservation equations an analytical model is derived by solving the linearized Rankine-Hugoniot conditions. This is a commonly used analytical approach to describe the relation of thermodynamic quantities up- and downstream of a propagation stabilized flame. In particular, the linearized Rankine-Hugoniot jump conditions are derived taking into account the presence of a moving discontinuity as well as upstream entropy inhomogeneities. The unsteady heat release rate of the flame is modelled as a linear superposition of flame transfer functions, accounting for velocity, pressure, and entropy disturbances, respectively. This results in a 3[Formula: see text]3 flame transfer matrix relating both primitive acoustic variables and the temperature fluctuations across the flame. The obtained analytical expression is compared to large eddy simulations with excellent agreement. A discussion about the contribution of the single terms to the modelling effort is provided, with a focus on autoignition flames.

Flame Transfer Functions of the Lean Burnout Zone of an RQL Combustion Chamber - Dynamic Response to Primary Zone and Mixing Port Velocity Fluctuations

“…The secondary zone can then be seen as a MISO (multiple input–single output) network system. It is assumed that the individual contributions of the two velocity fluctuations u u normal′ and u s normal′, on the unsteady heat release rate can be linearly superposed and quantified by individual FTFs (see also Innocenti et al 18 , Bothien et al, 19 and Gant et al 20 ). For the linear case, the heat release rate fluctuations can be expressed as two SISO (single input–single output) FTFs with normalFTF u , M and normalFTF s , M as the complex valued flame-transfer-functions of each input.…”