The experimental rig was established for the pumping performance investigation of a circularly lobed nozzle with a central plug and of the same nozzle without a central plug for comparison. Seven different cylindrical mixers were designed to match the circularly lobed nozzle. Results show that the central plug increases the pumping ratio of the secondary mass now rate to the primary mass flow rate about 60 70% over the same lobed nozzle without a central plug, and the total pressure prior to flow's entry into circularly lobed nozzle increased about 0.8-1.0% over the same lobed nozzle without a central plug. In order to know the mixing process between the primary and the secondary flows, in addition, a numerical procedure was developed to solve the incompressible Navier-Stokes equations in a curvilinear nonorthogonal coordinate. The numerical results show that there are great differences in the thermal mixing process, of the case with a central plug in which the low temperature contour lines in the exit cross-section of cylindrical mixer can reach the central line of the mixer, from the case without a central plug in which the elevated temperature contour lines in the exit cross-section of cylindrical mixer still occupy the central zone of the mixer. This was the fundamental reason why the thermal mixing efficiency of the lobed nozzle with a central plug was much more than that of without a central plug. Lastly, good agreements of temperature and axial velocity component in the exit cross-section of the mixer had been observed between the numerical results and the measured ones.