J. R. Dawson

The effect of combustion instability on the structure of recirculation zones in confined swirling flames

ABSTRACT A study of the destabilising effect of self-exited bulk-mode oscillations on the shape and size of recirculation zones has been carried out in a swirl burner/furnace. Cycle resolved velocities were measured to examine the effect of the coupling of unsteady pressure and velocity on the orientation of recirculation zones for two different burner exit expansion planes; i) a sudden expansion and ii) a divergent expansion. The sudden expansion exhibited large amplitude axial velocity fluctuations, particularly in the swirling jet. By contrast, the tangential velocity field was found to be comparably insensitive to the unsteady velocity oscillations. Consequently, a disproportionate increase in axial momentum periodically reduced the level of swirl disrupting the recirculation zones. The effect was two fold: i) during decreasing unsteady pressure, a high axial strain field destabilised the central recirculation zone periodically lifting it out of the burner exit. During increasing unsteady pressure, a stable lobed-shaped recirculation zone was re-established and anchored in the burner exit; ii) concurrently, the acceleration of the swirling jet partially entrained flow from the outer recirculation zone. This resulted in an outer recirculation zone that was directionally unstable with regions of counter-rotating flow. By employing a divergent expansion to influence the formation of the recirculation zones, the mean unsteady pressure amplitudes were damped over the whole instability range seeing a 40% reduction between Φ = 0.6 to 0.7 and 50% between Φ = 0.75 to 0.9. The results illustrate how unsteady pressures in the order of less than 2% of the mean pressure have a significant effect on the amplitude of the velocity field. The results also suggest that flame dynamics are not a direct response to small amplitude pressure oscillations but rather a direct response to changes in the local flow velocity that is determined by the phase velocity at the burner exit.

Primary Pollutant Prediction from Integrated Thermofluid-Kinetic Pulse Combustor Models

A phenomenological model for pulsating combustion by predicting characteristics of primary pollutants NOx and CO, as well as minor combustion species via integration with a detailed kinetic mechanism (GRI-Mech2.11) and areduced kinetic model, both derived from the CHEMKIN suite is studied with the aim of improvement. Homogeneous methane-air mixtures have been modeled across a range of practical, fuel-lean operating conditions, and predictions of primary pollutants from both models have been compared with available data. The use of a more realistic oscillatory Nusselt number is necessary to provide agreement with the experimental finding of relative insensitivity of primary pollutant emissions to input power. Oscillatory characteristics of combustion species are generated and are consistent with published data. Optimum operating conditions of the combustor are predicted to be around an equivalence ratio of 0.74, where NOx and CO emissions of only a few parts per million occur. NOx and CO predictions using the reduced mechanism provide better agreement with experimental data than those using GRI-Mech2.11. This probably results from the construction of the reduced model from the detailed mechanisms in GRI Mech3.

THE EFFECT OF COMBUSTION INSTABILITY ON RECIRCULATION ZONES IN A SWIRL BURNER/FURNACE

The effect of changing the expansion plane geometry on combustion instability is discussed based on pressure, volumetric flowrates and global heat release measurements carried out in a swirl burner/furnace under combustion instability. The original expansion plane is modified by inserting a divergent exit in an effort to enhance the stability of the recirculation zones and passively damp the amplitude of the pressure oscillations. Results plotting the rms pressure amplitudes, prms, as a function of equivalence ratio, o, show that the modified expansion geometry reduced prms by as much as 40% between o = 0.6 to 0.7 and up to 50% between o = 0.75 to 0.85. The relative volumetric flowrates between the recirculation zones and the swirling jet are analysed over the limit cycle and show that significant variations in the structure of the recirculation zones occur over the limit-cycle. It is found that the reductions in prms and velocity amplitude are achieved by better stability of the recirculation zones and better preservation of their volumetric flowrates for the low-pressure part of the cycle. Evaluation of the Rayleigh index showed that the driving and damping regions were located near the proximity of the burner and downstream of the breakdown zone respectively. The passive of control of the recirculation zones is discussed and illustrates a simple, but effective technique that can be implemented alongside other control strategies.

AN INTEGRATED THERMO -FLUID, CHEMICAL KINETIC MODEL FOR PULSE COMBUSTORS

A simplified thermo -fluid model previously presented for an aerovalved pulse combustor has been integrated with a reduced reaction mechanism within the CHEMKIN suite comprised of 21 species and 17 reactions. Homogeneous methane and air reactant mixtures have been modelled in order to compare the predictive emissions performance (NOx and CO) against suitable experimental data -sets published in the literature . Both cons tant and oscillatory Nusselt number correlations were used to investigate the effects on the emissions and their trends. It was found that an oscillatory Nusselt number was required to provide agreement with experimental results that show relative insensit ivity of NO x emissions against input power. Good quantitative agree ment between predictions and published experiments for time averaged NO x and CO emissio ns is achieved in the sub -stoichiom etric operating range . The results indicate an optimum operating co ndition around equivalence ratio of about 0.74, where very low concentrations (<10ppm) of NO x and CO are predicted . NOMENCLATURE