Abstract: The aerodynamic characteristics of sudden expansion region between the prediffuser exit and the head of the flametube in relation to its geometric parameters are investigated with the purpose of the annular combustor optimization.The mass flow ratio,total pressure loss of outer and inner annulus,and whole combustor are measured with different total pressure probes and flowmeters,respectively.Flow velocity profiles in the asymmetric sudden expansion region are measured by Laser Doppler anemometry.Experemental results of flow field total pressure loss and flow ratio for two types of combustor and seven configuration sectors show that two eddies occur in the sudden expansion regions;the prediffuser geometric parameters,such as deflection angle;relative sudden gap L/h and inlet Mach number,have an effect on the airflow patterns,drag coefficient and mass flow ratio.