Numerical computations of the lobed mixer were conducted to study the effects of interfacial contact area and streamwise vortices on the pumping ratio of lobed mixer. The numerical results showed that the greater interfacial contact area of primary flow and second flow caused the better performance of the lobed mixer

and the pumping ratio grew linearly with the increase of lobe perimeter ratio. The same effects were found in streamwise vortices

but the pumping ratio grew sharply and then gently with the increase of non-dimensional streamwise vortices intensity; and an index relationship was found between them. In case no flow separation occurred

the streamwise vortices angular kinetic energy of nozzle outlet increased with the increase of lobe angle

and there was a parabolic relationship between them. Moreover

the increase of lobe diffusion angle not only can increase the vorticity of streamwise vortices

but also enlarge the streamwise vortices distribution area.