Feasibility of Controlling NOx Emissions from a Jet Flame by Acoustic Excitation

Abstract

This paper investigates the feasibility of using acoustic excitation and coaxial air to control NOx production from a jet flame and tries to scale the NOx emission index of the jet hydrogen flame in the laminar regime. Applying both coaxial air and excitations through coaxial nozzle at low frequencies are found to be effective in reducing the flame length and the NOx emissions. A simple scaling law for NOx emission index from hydrogen jet flames with no coaxial air in the laminar regime is developed. The scaling, based on the flame length, nozzle diameter and exit velocity, for the residence time in the laminar jet flames is derived. The normalized NOx emission index, EINOX normalized by the residence time, can be scaled as the 1/2 power of the global strain rate in the laminar regime that is identical with Chen and Driscoll's (1990) scaling in the turbulent regime. This scaling result implies that effects of the buoyancy vortices, which prevail in the laminar regime, on the NOX production in a laminar hydrogen jet flame take place only through the residence time. With low frequency excitation, close to the flame flickering frequency, the normalized EINOX data have a similar power dependence, about 0.6, on the global strain rate, (UF/dF). The effect of the coaxial air on EINOX comes through reduction of the flame length. The derived simple scaling is not suitable for the coaxial-air cases. Reduction of both EINOX and EICO using acoustic excitaion with a suitable combination of frequency and amplitude is possible for propane jet flame; however, no simple scaling law is yet obtained