Experimental study on CH4/NH3 turbulent lifted flames with the variations in air coflow velocity and fuel nozzle diameter

Abstract

Many environmental problems caused by the significant increase in carbon emissions are receiving widely attention, while developing ammonia-containing fuel combustion technology is an essential and feasible strategy. In this paper, an experimental study and theoretical analysis of CH₄/NH₃ jet flame in air coflow were carried out to investigate the effects of air coflow velocity and fuel nozzle diameter variations on the combustion characteristics of ammonia-blended conventional fuels. With the increase of ammonia concentration, the flame chemical luminosity of blended fuels gradually decreased, and the flame root transitioned from bright blue to dark orange. The nozzle diameter and air coflow velocity significantly affected whether the CH₄/NH₃ jet attached flame can develop into a lifted flame. Meanwhile, three trends and developmental stages of liftoff height with increasing jet velocity were identified for CH₄/NH₃ turbulent flame, which could be attributed to the ammonia concentration and the flow regime of fuel stream, respectively. The experimental results indicated that the blowoff limit and steady burning region decreased gradually with the increase of ammonia mixing ratio and air coflow velocity, which were positively correlated with the nozzle diameter. Based on the premixed flame model and large-scale mixing model, flammability limit range and relative dilution effect of the fuel mixture were found as important factors in predicting the liftoff height. The results indicated that the modified models could estimate the liftoff height better. In addition, the impact mechanism of air coflow velocity on the blowoff limit in two prediction models was discussed, and a more accurate prediction of the blowoff velocity was achieved by introducing the correction factor k.