Thermal diffusion effects and vortex-flame interactions in hydrogen jet diffusion flames

Abstract

The structure of hydrogen jet diffusion flames has been investigated using coherent anti-Stokes Rama scattering (CARS) to obtain temperature profiles in both steady laminar flames and during the interaction of an induced fuel-side vortex with the diffusion flame sheet. The experimental profiles are compared with the results of direct numerical simulations (DNS) of the diffusion flames. The accuracy of the CARS system was evaluated by comparing experimental temperature measurements from hydrogen-air flames produced with a Hencken burner, with adiabatic flame temperatures found using the NASA Lewis equilibrium code. The CARS measurements and equilibrium code calculation are in excellent agreement. Temperature profiles were then obtained in steady, laminar hydrogen (H2), hydrogen-nitrogen (H2/N2), and hydrogen-nitrogen-helium (H2/N2/He) jet diffusion flames. Thermal diffusion has been found to be very significant in these hydrogen jet diffusion flames and has therefore been incorporated into our DNS model. The experimental and computational results for the steady flames show good quantitative agreement in terms of peak temperature and flame location when thermal diffusion is included in the model. CARS was also used to measure radial temperature profiles at several axial positions and at one specific time during vortex-flame interactions in the H2/N2 and H2/N2/He flames. It has been predicted by Katta and Roquemore [Combust. Flame 100:61–70 (1995)] that the local flame temperature in nonunity Lewis number flames can depart significantly from the steady-state temperature during vortex-flame interactions. The CARS and DNS model results support this prediction and show similar temperature departures from the steady-state solution. The high mole fraction and temperature gradients that are created during vortex-flame interactions probably increase the importance of thermal diffusion in these flames.