Effects of coflow velocity on the lift-off characteristics of autoignited jet flame in hot air coflow

Abstract

This paper aims to understand the effects of coflow velocity on the lift-off characteristics of autoignited jet flame and to develop a lift-off height model that considers such effects. A jet-in-coflow burner was used to conduct autoignition experiments. As the jet velocity increased, three distinctive flame types were observed: attached flame (AF), random spots (RS), and lifted flame (LF). The coflow velocity was found to have an interesting impact on the transition from one type to another. As the coflow velocity increases, the jet velocity needed to transition from AF to RS monotonically decreases, which aligns with our expectation. However, as the coflow velocity increases, the jet velocity required for RS to LF transition initially increases and then decreases. Such a shift implies that the dominant factor of the RS flame stabilization changes from the mixing to small-scale strain. The lift-off characteristics of RS under different coflow velocities were analyzed. With the increase of coflow velocity, the spots number increases non-monotonic, and the average lifetime decreases monotonically, further indicating that mixing and small-scale strain jointly determine the generation and extinction of RS. Moreover, the sensitivity of lift-off height to jet velocity is not only greater than that of coflow velocity but also affected by the coflow velocity. Therefore, considering the coflow velocity's effect, a modified mixing-strain model of RS was developed by improving the calculation of both strain rate and mixing time.