Characteristics of liftoff, blowout and instability in nonpremixed jet flames with NH3/CH4 mixture fuels

Author：Zheng, J. R., Tang, F., Chung, S. H., Hu, L. H.

Journal：Proceedings of the Combustion Institute

DOI:  10.1016/j.proci.2024.105591

Keywords: Ammonia-menthane mixture, Nonpremixed jet flame, Lifted flame, Blowout, Flame instability, laminar burning velocity, diffusion flames, height behavior, schmidt number, lifted flames, coflow jets, stabilization, methane, Thermodynamics, Energy & Fuels, Engineering

Abstract: 

The effect of ammonia addition on the characteristics of liftoff, blowout, and instability is investigated experimentally in nonpremixed jet flames with NH3/CH4 mixture fuels by the varying mole fraction of ammonia (X-NH3). Both laminar and turbulent lifted flames are observed and the lifted flame for X-NH3 = 0.3 has a transition from laminar to turbulent lifted flame, when the jet flow is in the laminar-to-turbulent transition regime. The results demonstrate that, for X-NH3 <= 0.3, only turbulent lifted flame exists and the liftoff height increases linearly with the fuel jet velocity U-0, as well as the ammonia mole fraction. A satisfactory linear relationship between nondimensional turbulent lifted flame height and nondimensional flow velocity can be obtained, based on the turbulent intensity theory. For X-NH3 > 0.3, only laminar lifted flame exists and the liftoff height increases reasonably linearly with U-0, which is stabilized in the jet developing region. Both the critical liftoff (U-LO) and blowout (U-BO) velocities decrease with the increase in X-NH3. When U-LO is scaled with laminar burning velocity S-L, U-LO/S-L is insensitive to X-NH3 for X-NH3 <= 0.3, having U-LO/S-L approximate to 50, while it decreases linearly for X-NH3 > 0.3. While U-BO/S-L decreases linearly with the increase in X-NH3. These critical velocities show that no flame can be stabilized for X-NH3 > 0.5. The oscillation frequency of laminar nozzle-attached flame for CH4/NH3 mixture fuel slightly increases with X-NH3, while the critical Froude number (Fr) for the transition from sinuous to varicose modes of oscillation increases with ammonia addition. The flame oscillation frequency can be characterized by the Strouhal number St having a power law relationship of St proportional to (1/Fr)(0.46).