Effects of variable partial premixing on turbulent jet flame structure

Characteristic of partially premixed turbulent flames are investigated using a burner design that allows for a variation in the level of premixing between fuel and air. In this study, initially nonpremixed flames with a coaxial injection of air at variable positions are considered. Velocity and mixing characteristics of coannular streams have been measured, which show a region of enhanced turbulence and mixing in the shear region, and also indicate an increased partial premixing with increasing inner tube recess (Δx/D(i)). The ratio of the inner and outer jet velocities (flow rates) affects the flame characteristics, as the overall fuel-air ratio increases with an increase in the inner jet velocity. Inner jet air also assists in entrainment of surrounding air. These effects lead to a decrease in the flame height as the ratio of the inner and outer jet velocities increases. Through enhanced fuel air mixing via inner tube traverse, flame radiation decreases with increasing Δx/D(i); however, a similar change in Δx/D(i) has a minor effect on the flame height. The lift-off height has a linear dependence on the velocity ratio (U(1)/U(o)), while the slope becomes smaller with increasing Δx/D(i). Flame stability characteristics have also been obtained, which indicates that the region of the stable flame operation is extended when Δx/D(i) is increased beyond a certain limiting value. These observations suggest a method of controlling both the flame length and flame stability via variations of U(i)/U(o) and Δx/D(i), respectively.