Unsteady flow structures of a counter-rotating swirl cup

Swirl can form Central Toroidal Recirculation Zones (CTRZ) in the center of flow region and can cause a negative axial pressure to be set up in the vicinity of the axis, reducing the length of flame, stabilizing combustion and enhancing the mix of fuel/air. Understanding the characteristics of swirling flow, especially the dynamics of Large Coherent Structures (LCS) associated with swirl, are very important for achieving suppression of combustion instability and obtaining low-NOx emissions. The present study investigated the flow field characteristics of a counter-rotating swirl cup in a real model combustor of rectangular geometry under isothermal condition, using PIV. Two-dimension PIV is adopted as the diagnostic technique to obtain the velocity vector field in the plane along to the axis of the test section. The experiment was aimed at understanding the swirl flow field characteristics down stream the swirl cup. Apart from obtaining the mean and turbulent velocity data, the structure and length scales of the CTRZ obtained from PIV measurement are also described.