Injection angle plays a major role in swirl flow dynamics

Swirling flows are prevalent across nature and technology, from hurricanes to hot gas mixtures in the combustor of a rocket engine. The highly nonlinear system of swirling flow, with a large number of complex, dynamic structures that develop, can provide methods of dynamic stabilization for processes like gas combustion. But many of its fundamental mechanisms have yet to be fully characterized.

Work reported in Physics of Fluids establishes basic structures of internal swirling flows over a broad range of operating conditions, identifying and quantifying the mechanisms dominating flow evolution and instability mode selection.

The authors use the Galerkin finite element analysis to systematically study the effects of Reynolds number and swirl level on flow dynamics for a cylindrical chamber with tangential entry. They emphasized detailed flow structures and evolution, including the central recirculating zone (CRZ) driven by centrifugal force, and associated instability waves from sharp velocity changes and flow rotations. Notably, CRZs were characterized by a set of logarithm-linear functions of Reynolds number and swirl level. “That means that the behavior of the CRZ is predictable and can be described by simple correlations,” said co-author Yanxing Wang.

Specifically, the results link distinct flow states, which depend on injection angle, from the perspective of evolving instability waves to the CRZ. When the swirl level decreases below a critical value, the basic axisymmetric flow loses integrity and instability waves develop. The number of vortex cores, described by the azimuthal wave number, primarily determines swirl level. A decrease in the swirl level leads to a smaller wave number, suggesting that vortex breakdown could be considered as the limiting case of azimuthal instabilities for a wave number of zero. If true, the finding could offer a mechanism for connecting different flow states with a unified theory.

Source: “Central recirculation zones and instability waves in internal swirling flows with an annular entry,” by Yanxing Wang and Vigor Yang, Physics of Fluids (2018). The article can be accessed at https://doi.org/10.1063/1.5000967 .