Numerical Simulations of Isothermal Flow in a Swirl Burner

In this paper, the non-reacting flow in a swirl burner is studied using large eddy simulation. The configuration consists of two unconfined coannular jets at a Reynolds number of 81,500. The flow is characterized by a Swirl number of 0.93. Two cases are studied in the paper differing with respect to the axial location of the inner pilot jet. It was observed in a companion experiment (Bender and Büchner, 2005, Proc. 12 Int. Cong. Sound and Vibration, Lisbon, Portugal) that when the inner jet is retracted the flow oscillations are considerably amplified. This is also found in the present simulations. Large-scale coherent structures rotating at a constant rate are observed when the inner jet is retracted. The rotation of the structures leads to vigorous oscillations in the velocity and pressure time signals recorded at selected points in the flow. In addition, the mean velocities, the turbulent fluctuations, and the frequency of the oscillations are in good agreement with the experiments. A conditional averaging procedure is used to perform a detailed analysis of the physics leading to the low-frequency oscillations.