Abstract: The role of the central plug in promoting the thermal mixing between the primary and the secondary flows of the circularly lobed nozzle was assessed mainly by the numerical simulation.The fundamental equations were derived in the body fitted nonorthogonal coordinates from cylindrical coordinates for the circularly lobed exhaust ejector mixers with/without a plug in the center of the lobe.These equations were solved using SIMPLEC and a kind of complete pressure correction algorithm in the nonstaggered grids,with the velocity components in physical plane as solving variables and the contravariant velocity components as boundary velocities of the controlled volume.Better agreements have been observed between the numerical results and the experimental values.The numerical results show that there are great differences between the temperature contour in the outer section of the column mixer with the plug in which the low temperature contour lines can reach the center line of the mixer,and that in the outer section of column mixer without the plug in which the elevated temperature contour lines always lie in the central zone.This is the basic reason why the thermal mixing efficiency of the mixer with the plug is much more than that of the mixer without the plug.