Transition from bubbling to jetting in a co-axial air-water jet

TL;DR

This study experimentally investigates flow regimes in co-axial air-water jets and explains the transition from bubbling to jetting using linear stability theory, identifying a critical velocity ratio where flow behavior changes.

Contribution

It provides the first experimental analysis of flow regimes in co-axial air-water jets and links the transition to a stability theory-based explanation.

Findings

Existence of a critical velocity ratio $u_c$ for flow regime transition

Identification of bubbling and jetting regimes based on velocity ratio

Transition explained by change from absolute to convective instability

Abstract

In this Brief Communication we study experimentally the flow regimes that appear in co-axial air-water jets discharging into a stagnant air atmosphere and we propose a simple explanation for their occurrence based on linear, local, spatiotemporal stability theory. In addition to the existence of a periodic bubbling regime for low enough values of the water-to-air velocity ratio, $u=u_w/u_a$, our experiments revealed the presence of a jetting regime for velocity ratios higher than a critical one, $u_c$. In the bubbling regime, bubbles form periodically from the tip of an air ligament whose length increases with $u$. However, when $u> u_c$ a long, slender gas jet is observed inside the core of the liquid coflow. Since in the jetting regime the downstream variation of the flow field is slow, we performed a local, linear spatiotemporal stability analysis with uniform velocity profiles to model the flow field of the air-water jet. Similar to the transition from dripping to jetting in capillary liquid jets, the analysis shows that the change from the bubbling to the jetting regime can be understood in terms of the transition from an absolute to a convective instability.