Abstract: The ordinary cyclone could only get the overflow of fine particles and the underflow of coarse particles in the once classification， difficult to achieve the requirements of fine classification of narrow particles. The double overflow pipe hydrocyclone was designed in order to obtain the fine particle size and narrow grain products in the once classification. The comparative study of the velocity field and pressure field in the cyclone， the granularity of field and separation performance of the single and double overflow pipe hydrocyclone were conducted by numerical simulation and experimental study. The numerical simulation results showed that the hydrocyclone with double overflow pipe structure could obtain three kinds of productions including internal overflow， external overflow and underflow after once separation. Compared with the single overflow pipe hydrocyclone， the tangential velocity and internal static pressure of the double overflow cyclone are higher， while the radial velocity， axial velocity and turbulent kinetic energy are smaller， indicating that the double overflow pipe structure can strengthen the separation process and improve the separation performance. The experimental results showed that compared with the single overflow pipe hydrocyclone， the underflow concentration of the double overflow pipe hydrocyclone decreases by 8.3 percentage points， and the yield of the underflow increases by 3.25 percentage points. The cumulative particle contents below 45 μm in the internal and external overflow product increases by 1.15 percentage points， and the comprehensive classification efficiency increases by 1.26 percentage points. The research results could provide a certain reference for the research and development of narrow-particle cyclone separation equipment and technology for multiple products.

Key words: high fineness classifying, double overflow pipe hydrocyclone, flow field characteristics, numerical simulation, compreshensive classification efficiency