Morphological characteristics of non-vertical turbulent jet flames: Experimental investigation and analytical model

Abstract

Non-vertical turbulent non-premixed jet flame occur in combustion systems and by accidental release of fuels caused fires. Understanding the combustion dynamics of these asymmetric diffusion jet flames is needed to characterize the flame morphology and entrainment, which is a foundation for further detailed analysis of the physics and implications. This paper investigates experimentally and theoretically the flame morphological characteristics of non-vertical turbulent jets at both positive- and negative inclined angles in a systematic way. A total of 168 experimental cases were considered involving various initial fuel flow rates, inclined angles and nozzle diameters. Dimensional analysis was performed taking into account the controlling parameters of the physical mechanisms, namely the momentum flux, the fuel mass flow rate, the flame buoyancy, the stoichiometric ratio and the jet initial inclined angle, which together determine a characteristic length scale and a characteristic volumetric flow rate. These two characteristic parameters provide successful non-dimensional correlations for the location of the flame tip. A new integral model was also developed physically considering the momentum and continuity equations along the trajectory to predict the flame tip, as well as the lowest point for negative inclined jets. By comparing the experimental correlations with numerical prediction, the effective air entrainment coefficients were derived for different jet initial inclined angles, which showed little change with inclined angle from positive to negative, but the constant relating the average values to the integrals of the (Gaussian) radial profiles decreased as the initial angle decreased from positive to negative. This change is related to the variation of the profile properties normal to the trajectory as well as to mass detrainment laterally escaping from the jet flame especially for negative inclined angles. Finally, the local Richardson number and the flame lowest point for negative inclined angles was numerically predicted and compared well with the experimental results.