Proceedings of Combustion Institute - Canadian Section Spring Technical Meeting University of Windsor, Ontario May 12-15, 2014 Determination of thermo-acoustic energy transfer in a model gas turbine combustor using OH* chemiluminescence Maxwell G. Adams ∗ , Benjamin D. Geraedts † , Vincent Caux-Brisebois ‡ , Adam M. Steinberg § University of Toronto Institute for Aerospace Studies 4925 Dufferin Street, Toronto, Ontario, M3H 5T6, Canada 1. Introduction and Motivation Figure 1: Schematic of the combustor used in the experiments, with diagnos- tic fields of view indicated. The pursuit of reduced NO x emissions in terrestrial and aerospace gas turbine engines has led to the increased use of lean premixed (LP) combustion [1]. However, a major obstacle in designing LP combustors is that these systems are susceptible to thermo- acoustic oscillations. The flame, being an unsteady heat source and hence also an acoustic source, produces pressure fluctuations that, in turn, influence the heat release rate through a variety of possible coupling mechanisms [2, 3]. The thermo-acoustic en- ergy transfer (Θ) resulting from these coupled oscillations is described by the Rayleigh integral [4], Θ =  v  t γ − 1 γ 1 p ˙ q ′ p ′ dt dv =  v Ψdv (1) where ˙ q ′ and p ′ are the heat release and pressure fluctuations, respectively, and v is the combustor volume. When an appropriate phase is achieved between the flame’s heat release and the fluctuating acoustic field, Θ is positive and large-amplitude self-excited oscillations can develop. These pressure, heat release, and flow oscillations can reduce component lifetime and pose a threat to the structural integrity of the engine. There are many complex mechanisms through which coupled pressure and heat- release oscillations can develop. In many cases involving swirl-stabilized flames, it has been shown that large-amplitude limit-cycle thermo-acoustic oscillations are asso- ciated with the presence of a helical vortex core (HVC) that spirals around the burner centerline and precesses around the combustor at a fixed frequency [5–7]. Recent research has developed a method of calculating thermo-acoustic energy transfer in such situations based on data from high-repetition-rate stereoscopic particle image velocimetry (S-PIV) and OH planar laser induced fluorescence (PLIF) [8]. Because the HVC precession frequency is different from the thermo-acoustic frequency, under- standing the thermo-acoustic energy transfer processes required simultaneous consid- eration of both the phase in the thermo-acoustic cycle and the phase in the precession of the HVC around the combustor. It was found that periodic deformations of the HVC over the acoustic cycle gave rise to two distinct regions of thermo-acoustic coupling: an inner coupling region (ICR) that did not significantly contribute to the total energy transfer, and an outer coupling region (OCR) that was primarily responsible for thermo-acoustic energy transfer. However, the measurements required for the aforementioned analysis are difficult to implement, data intensive, and unfea- sible in practical high-pressure combustors. In particular, PIV and PLIF measurements require an external laser source, PIV measurements require seeding the flow and extensive data processing to produce velocity fields, and PLIF signals decrease at high pressure due to Doppler broadening. It therefore is desirable to establish a method for understanding and mapping thermo-acoustic energy transfer in complex flows without the need for these complex measurements. A more easily implemented technique for measuring the rate and distribution of heat release within a combustor is OH* chemiluminescence (CL) measurement. In this technique, spontaneous emissions of electronically excited OH radicals (OH*) are detected using an intensified CCD or CMOS camera. These emissions occur in high concentrations within the reaction ∗ Undergraduate student † Graduate student, Institute for Aerospace Studies ‡ Graduate student, Institute for Aerospace Studies § Assistant Professor, Institute for Aerospace Studies, corresponding author, steinberg@utias.utoronto.ca