| Forward-extended double-blade sewage pumps are new type non-clogging pumps with high efficiency. The shape of the impeller is special, which is more and more popular in foreign research in recent years. Currently, the special sewage pumps with functions of chopping and stirring have been produced by Flygt(Sweden) and Vaughan(US), but there is almost no company researching and producing forward-extended double-blade sewage pump in China. The multifunctional chopper pumps and forward-extended double-blade sewage pump are still in the research and trial production stage, and the product reliability need to be further improved.This paper is funded by the National Science and Technology Support Project" key technology research and engineer application of typical centrifugal pumps(project No.2011BAF14B01). Based on the combination of numerical simulation and PIV experiment, the laws of internal flow in impellers of different radial clearance are explored. Hydraulic design and optimization on forward-extended double-blade sewage pumps of different specific speed were done to improve its efficiency, which converts to the method of design. Following are the main work and conclusions:(1) With a flow rate of 17 m3/h, head of 9m, rotating speed of 1450 r/min of forward-extended double-blade pump as the environmental study, we designed three semi-open impellers with lmm,2mm,3mm radial clearances. Steady numerical simulations were done on them, which were used to study what the influences are of radial clearance on the performance the characteristic, blade loading, and the internal flow field, and found:With the radial clearance increases, pump head, shaft power, efficiency are decreased, the entire curves shift downward; the blade load at inlet part increases, the smaller the blade load at oulet changes; impeller inlet static pressure increases, the overall pressure drop smaller, phenomenon of relatively speed stream line locally concentrated appeares in the impeller, and there is always a vortex at the radial gap.(2) At the same time, unsteady simulations were done on the three kinds of semi-open impeller, and 10 monitoring points were setted up in the impeller and volute, to research the influences of radial clearance on pump pressure pulsation, and found:the pressure pulsation frequency amplitudes of monitoring points decrease with increasing radial gap. With the increase of radial clearance, frequency and amplitude of the division were reduced, pressure pulsation at point P1 weakened slowly, and after N= 20 the pressure pulsation can weaken apparently.(3) PIV test was done and the relative velocity flow field distribution in the impeller was got. The pump’s flow rate is 17 m3/h, head is 9m, and rotating speed is 1450 r/min. From the analysis of the relative velocity flow field of the impeller in different conditions(Q/Qn=0.4,0.6,0.8,1.0,1.2,1.4,1.6), we tried to find the relationships of axial vortex and low speed area with flow rate, movement interference, radial gap and some other factors. We draw the following conclusion:By the performance characteristic test, the external characteristic curves are got and showed that with the increase of radial clearance, the head H falls, and the decreasing amplitude is increased geometrically, while power P reduces and pump efficiency η falls, and maximum efficiency point shifts to the small flow operating point.(4) Through analysis of relative velocity flow fields with different radial clearance in the same flow rate condition, we found that with the radial clearance increases, the low velocity zone and axial vortex decreases, but if the radial clearance is too large, there will be partial concentration phenomenon in the relative speed streamlines. Through analysis of relative velocity flow fields with the same radial clearance in different flow rate condition, we found that there are low-speed areas and axial vortex in the middle of the flow channel impeller nearby the working surfac, and with the flow rate increases, the low-speed region and the axial vortex gradually decreases until it disappears. Through analysis of relative velocity flow fields with different phases, we found that the main factor affecting the low-speed zone and axial flow vortex is flow rate, and the secondary factor is radial clearance, and the last is movement interference.(5) Hydraulic model optimization is carried out respectively on WQ800-40-132, WQ500-60-132 and WQ700-8-30. Five sets of external character datas are obtained by the optimized prototype tests. Thereby impeller inlet equivalent diameter coefficient Ko, impeller outlet diameter coefficient KD2 and impeller outlet width coefficient Kb2 are corrected which are more applicable to forward-extended double-blade sewage pump whose specific speed is between 90 and 300. |
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