Structure and generation mechanism of blue whirls

Abstract

Blue whirls are newly discovered blue rotating flames transitioned from fire whirls generated on liquid fuel surfaces. Compared with the traditional fire whirls, blue whirls have a distinct flame structure and are almost soot-free. Studies of the blue whirl are expected to promote the development of clean combustion theory and technology. In this paper, we numerically and theoretically investigate the lifting process of fire whirls and blue whirls, and then discuss their structure and transition mechanisms. In contrast to the explanations in the literature, it is found that blue whirls can be created without the occurrence of bubble-type vortex breakdown in the rotating flow. The blue whirl can be considered as a special form of lifted fire whirl with a considerable liftoff distance and without being blown out. According to the simulation, both blue whirls and fire whirls have the typical triple-flame structure. The unique structure of blue whirls is closely related to the thermal expansion of gas and the restriction of the radial flow by the axis. The theoretical solution of the unified lifting curve for fire whirls and blue whirls is obtained, which is qualitatively in agreement with the numerical result. There are 4 different branches in the lifting curve, two of which are stable, respectively representing fire whirls with small liftoff distances, and blue whirls with larger liftoff distances. The unstable branches between the stable branches lead to non-continuous transitions between fire whirls and blue whirls, as observed in the experiment. The current study explains the existing physical phenomenon and makes valuable predictions that may guide future experiments and simulations.