Aerodynamic and thermo-acoustic stability analysis in a super-steady reactive flow

Abstract

Aerodynamic and thermo-acoustic stability were systematically analyzed to reveal the underlying cause of the super-steady performance found in a strong rotating reactive flow, which is achieved using a stratified vortex-tube combustor (SVC) employing methanol as fuel. Results show that the SVC possesses a wide stability limit and weak pressure fluctuation with a uniform flame. The lean stability limit is always smaller than 0.2 and the amplitude of pressure fluctuation is generally within 2 kPa, indicating a super-steady combustion process. The large tangential velocity produces a large centrifugal force that suppresses the turbulent motion to promote aero-dynamic stability, yielding a small Froude number. The delay in the mixing process is the crucial reason for the generation of the stratified distribution of species and the triple-flame structure. The large transport fluxes of the species and enthalpy promote the reaction and ignition processes to intensify the combustion, which produces a large density gradient to promote thermo-acoustic stability. The large tangential velocity and density gradient result in a large Richardson number, suggesting the laminarization of the fluid. The momentum flux and its fluctuation decrease distinctly downstream of the vortex-flow because of the strong momentum exchange and transfer in the vortex flow, which weakens the pressure fluctuation in the post-flame zone. The intensified combustion can also increase the burning velocity, and promotes its matching with the flow field. The reduced momentum flux and the increased burning velocity are responsible for good thermo-acoustic stability.