Effect of inlet and outlet boundary conditions on swirling flows

Numerical simulations are conducted for both three-dimensional, turbulent flow in a multi-channel swirler and axisymmetric, isothermal, turbulent flow in combustion chambers using the standard κ−ε turbulence model. Calculations are first carried out for three-dimensional, isothermal and turbulent flow inside the swirler channels in order to derive the velocity profiles of both air and gas at the swirler outlets, which are used as inlet boundary conditions of the model combustor and can also be used in future studies for different combustors with the same type of swirler. In order to study the sensitivity of swirling flow inside the chamber to the inlet and outlet boundary conditions, different inlet velocity profiles and outlet boundary conditions are also employed. The results show that in the cases considered, the flow behaviour in the chamber is not very sensitive to the actual shape of the inlet velocity profiles provided the averages of the inlet axial, radial and azimuthal velocity components are separately preserved. Other conditions being equal, we find that the swirling flow performance in the combustor depends not only on the inlet swirl number, but also strongly on the relative magnitude of the radial velocity component at inlet and introduce a new dimensionless number N_r, analogous to the swirl number, to measure the relative importance of this quantity. Outlet boundary conditions have some influence near the outlet, but nearly no effect further upstream for the cases investigated.