CFD Modelling of Thermoacoustic Oscillations Inside an Atmospheric Test Rig Generated by a DLN Burner

The ENEL Produzione Research Centre of Pisa is deeply involved in the study of flame instabilities which could arise in particular operating conditions in the gas turbine equipped with Dry Low NOx (DLN) lean premixed combustors. An atmospheric pressure test facility, named TAO (Turbogas ad Accesso Ottico), is presently used to test, under scaled conditions, the onset of self-sustained thermoacoustic instabilities during the operation of a typical DLN burner. To support this activity, Computation Fluid Dynamic (CFD) analyses are carried out by means of KIEN, an in-house Reynold Averaged Navier Stokes (RANS) code, for simulating 3D instationary reactive flows. A 3D geometrical model, extending from the air plenum upstream the burner up to the end of the long exhaust tube, is adopted. The great extension of the calculation domain, coupled with the long real time required for the spontaneous onset of oscillations, could became computationally very onerous for Large Eddy Simulation (LES) codes, which require a strong control on the cell maximum size throughout the entire domain, while it can be acceptable for an instationary RANS code, which could use a coarser grid. This allows the simulation of long real transients with overnight runs. Starting from uniform no-motion conditions, the same fluctuating behaviour, detected during TAO experiments, spontaneously onsets in the numerical simulations too. The experimental and numerical frequencies are nearly the same and the amplitude of the pressure oscillations is very close. Supported by the congruence with experimental available data, computed results are utilized to get a more detailed description of the evolution of many thermofluiddynamics quantities during these instabilities. A wide sample of information that can be derived directly from the output of the instationary RANS code or by postprocessing of its results is provided.Copyright © 2004 by ASME