Abstract: The straight pipe length of the bottom outlet is the important section. In order to find out the influence law of the straight pipe length of the bottom outlet on the separation performance of the hydrocyclone， the influences of different insertion depths of the underflow straight section on the pressure field， velocity field and turbulent kinetic energy of the cyclone were studied by numerical simulation and experiment. The simulation results showed that with the increase of the absolute value of straight pipe inserted into the depth， the static pressure， tangential velocity and turbulent kinetic energy of flow field appears varying degrees of increase， which could effectively increase the centrifugal strength and strengthen the separation effect. While the radial velocity decrease with the increase of the absolute value of straight pipe inserted into the depth， and the decrease of the radial velocity was beneficial to the improvement of the accuracy of classification. The test results showed that with the feeding pressure of 0.1 MPa， the feeding concentration of 10% and the median particle size of 16.75 μm， the underflow concentration times increase from 4.61 times to 6.48 times and the median particle size of the underflow product increases from 49.32 μm to 65.88 μm with the straight pipe length of the underflow port increasing from 0 mm to 80 mm. The median particle size of the overflow product increased from 8.57 μm to 21.16 μm， the fineness of the overflow product became larger， the separation particle size increased， and the comprehensive classification efficiency was increased by 13.85 percentage points compared with the traditional hydrocyclone.

Key words: straight pipe length of the bottom outlet, numerical simulation, multiple of concentration, separation particle size, comprehensive classification efficiency