The non-reaction flow field generated by a single axial swirler was simulated by using FLUENT software. The variation laws of dimensions and strength of central toroidal recirculation zone (CTRZ) with the change of swirler structures were analyzed

and an empirical formula for the length of CTRZ was obtained. The computed results were compared with experiment measurements

and the comparisons of the results indicate that the empirical formula can calculate the length of CTRZ correctly with an error lower than 5%; the length of CTRZ decreases with the increase of swirl number and expansion ratio; the width of CTRZ increases with the increase of expansion ratio; the swirler has a complex impact on backflow ratio of the center plane of vorticity; there is a critical angle of 43° for Venturi; at both sides of the critical angle the CTRZs are quite different.