The particle image velocimetry (PIV) system was used for the flow field tests of step diffuser at different flow rates

velocity distributions of axial cross section

resistance loss and cavitation were obtained.The results showed that outlet velocity distribution became uniform at the cross section with length-diameter ratio of 3—5 with varying inlet velocity between 8 m·s-1 and 24 m·s-1

and further changed from uniform into non-uniform for circular tube

while outlet velocity distribution of square tube still remained uniform.Fiber deflocculated better with the increase of inlet velocity

but pressure loss increased rapidly.Cavitation intensified after the expansion

so inlet velocity should not exceed the critical cavitation velocity.The test results indicated that step diffuser with square tube was more suitable for high-speed paper machine

and appropriate high-speed slurry

geometric shape and size were guarantee of quantitatively uniform and evenness good paper..