Influence of Strouhal number on pulsating methane–air coflow jet diffusion flames

Abstract

Four periodically time-varying methane–air laminar coflow jet diffusion flames, each forced by pulsating the fuel jet's exit velocity U _j sinusoidally with a different modulation frequency w _j and with a 50% amplitude variation, have been computed. Combustion of methane has been modeled by using a chemical mechanism with 15 species and 42 reactions, and the solution of the unsteady Navier–Stokes equations has been obtained numerically by using a modified vorticity-velocity formulation in the limit of low Mach number. The effect of w _j on temperature and chemistry has been studied in detail. Three different regimes are found depending on the flame's Strouhal number S = aw _j /U _j , with a denoting the fuel jet radius. For small Strouhal number (S = 0.1), the modulation introduces a perturbation that travels very far downstream, and certain variables oscillate at the frequency imposed by the fuel jet modulation. As the Strouhal number grows, the nondimensional frequency approaches the natural frequency of oscillation of the flickering flame (S ≃ 0.2). A coupling with the pulsation frequency enhances the effect of the imposed modulation and a vigorous pinch-off is observed for S = 0.25 and S = 0.5. Larger values of S confine the oscillation to the jet's near-exit region, and the effects of the pulsation are reduced to small wiggles in the temperature and concentration values. Temperature and species mass fractions change appreciably near the jet centerline, where variations of over 2 % for the temperature and 15 % and 40 % for the CO and OH mass fractions, respectively, are found. Transverse to the jet movement, however, the variations almost disappear at radial distances on the order of the fuel jet radius, indicating a fast damping of the oscillation in the spanwise direction.

keywords:

• pulsating flames
• full chemistry
• maximum flame temperature
• CO oxidation
• flame pinch-off

Acknowledgements

The authors gratefully acknowledge financial support from the following sources: the Spanish Ministry of Science and Innovation through the José Castillejo Mobility Program for Young Researchers (MSS) and Project #ENE 2005-09-190-C04-01 (AL); the US Department of Energy Office of Basic Energy Sciences under Grant #DE-FG02-88ER13966 (MDS and BAVB); the US National Science Foundation under Grant #CBET-0828802 (MDS and BAVB); and the US Air Force Office of Scientific Research under Grant #FA9550-06-1-0018 (MDS and BAVB). The authors also gratefully acknowledge helpful conversations with Forman A. Williams.

This work is dedicated to the memory of Dulce N. Morillo-Martí n.