  This paper reports on the investigation of combustion instabilities in a
Methane air nonpremixed half dump combustor for different flow Reynolds number
using Computational aero acoustic simulation. In order to simulate the flow
physics under turbulent flow conditions, Detached Eddy Simulation in
conjunction with generalized eddy dissipation model is adopted, while a CAA
formulation based on Lighthills acoustic analogy is used for computing the
acoustic field. It is observed that the unsteady pressure signals either
predominantly arise from the natural acoustic modes of the duct or the local
flow fluctuations in the vortex shedding process downstream of the dump plane,
giving rise to different dominant frequencies at different spatial locations at
lower Re and a single dominant lockedon frequency at higher Re. The
nondimensional numbers (Helmholtz and Strouhal numbers) are used to
characterize the duct acoustic modes from the vortex shedding modes. Under
reacting flow conditions, unsteady heat release and pressure oscillations are
monitored to compute Rayleigh index in order to verify if the instability is
driving or damping at different frequency levels. Moreover, the predicted
Helmholtz and Strouhal numbers are found to be in excellent agreement with the
experimental data available in open literature for wide range of Re.
