EXPERIMENTAL AND THEORETICAL STUDIES ON GAS CYCLONE SEPARATORS OPERATING AT HIGH EFFICIENCY

Motion of Dust Particles and Cyclone Collection Efficiency. It is shown that wall friction has negligible effect on particle motion, but dust particles on the wall move much more slowly than the surrounding gas, due to crowding. In the cylindrical section of a cyclone the dust traces out a stream line of the gas flow field near the wall. In the cone section, the centrifugal force has an upward component causing the dust to deviate slightly from gas stream lines. Measurements show that the residence time of dust particles is several times smaller than that of the gas.;Secondary Flow along the Cone Wall. A theory on the secondary flow along the cone wall was developed, based on a previous analysis. Good agreement with experimental data was obtained. The strength of this secondary flow near the cone apex was measured as a check of the theoretical development.;A similar analysis was made for the so-called short-circuit flow on the top of a cyclone which is known to carry some of the incoming dust directly to the overflow, causing a decrease in collection efficiency. The effect of this flow on efficiency was also studied.;Inlet Secondary Flow and the Formation of Dust Spirals. It is known that dust particles on a cyclone wall do not spread out evenly. Instead, they are concentrated into a narrow spiral band. Experiments show that the existence of this spiral is due to a secondary flow generated at the inlet and reinforced by turbulent wakes at the bottom of the inlet. This secondary flow is not supported by themain flow, however. The secondary flow velocity was found to be dependent on inlet geometry and is of order of 15% that of the main flow. In the cone section where this secondary flow has already died out for most cyclones, the spiral is kept from dispersing sideway by two opposite forces: the upward component of the centrifugal force and the fluid drag force which acts in the downward direction.;It is found that re-entrainment of particles in the region near the cone apex of a cyclone is a significant factor affecting collection efficiency of low and moderate efficiency cyclones (those which collect less than 80% of the incoming dust.) The use of an underflow which is recycled back to the cyclone is found to improve the collection efficiency of these cyclones by about 5%. For cyclones with efficiencies exceeding 90% the effect of re-entrainment is not important as most of the re-entrained particles will return to the cyclone wall.;Analysis of the Main Flow inside a Cyclone. A simplified theory was developed to give the flow field in gas cyclones. Good agreement with available data was obtained. In particular, the theory predicts correctly that the radial velocity is inwardly directed and is 2 order of magnitude smaller than the inlet velocity. An order of magnitude of the eddy viscosity was established, and the theory also accounts for the differences in vortex strengths for cyclones of different designs.;Fluidization of Solid Particles on Cyclone Walls. Solid particles collected on a cyclone wall undergo fluidization as a result of the excess kinetic energy imparted by the centrifugal force. Using simple kinetic theory for dense fluids it is shown that the particle volume fraction inside the dust spiral on a cyclone wall is of the same order as the emulsion phase in fluidized systems. The result was checked with experimental data and reasonable agreement was obtained.