Comparison of hydraulic performance between vertical shaft and shaft extension tubular pumping system

By keeping the blade and guide vane model unchanged, and the same length of the whole pumping system, in this paper, we simulated the hydraulic performance of the vertical shaft and shaft extension tubular pumping system with the commercial CFD software，and the experiment of vertical shaft tubular pumping system was done to verify the numerical calculation, the head and efficiency difference between model test and CFD was less than 1% under design flow condition, but large under undersign flow condition. By comparing the performance of independent impeller and pumping system of the vertical shaft, we found that the efficiency of independent impeller was high and the highest value was 92.09% at the design point, and the efficiency of pumping system was easily effected by hydraulic loss in the passage inlet, passage outlet and guide vane. The highest efficiency happened when the head was 2.69 m and reduced to 76.2% when the hydraulic loss in the passage inlet, passage outlet and guide vane was 0.46 m. The result showed that the hydraulic characteristic was different with the influence of different inlet and outlet conduit. The hydraulic loss of inlet conduit was less, but the victory distribution in the outlet section of the inlet conduit had a relationship with the characteristic of blade. The hydraulic loss of outlet conduit was large, and it affected the hydraulic performance of pumping system. The result of the performance indicated that the efficiency of the shaft extension pumping system was 5.3% lower than vertical shaft under 1.25 design flow condition, but was 4.5%, 3.5% higher under 1.0, 0.59 design flow condition. When only considering the hydraulic performance of the blade, the hydraulic performance of shaft extension tubular pump was slightly higher just under large flow condition. By analyzing the inlet velocity distributions, we found that when the motor position was in back, the water flow on the inlet of the impeller was near to the hub due to the water guide cone. When the motor position was in front, the water flow on the inlet of the impeller was near to the shroud due to the contraction of inlet conduit. When the flow trend to the hub, circumferential velocity did not change, axial velocity around the hub increased. As flow angle increased, attack angle and work ability decreased. Inlet flow angle around the rim decreased, attack angle and work ability increased. And blade with shaft extension pumping system had a slightly good performance under large and design flow condition, but not better than vertical shaft under very low flow condition. The difference of blade performance was mainly cause by the different inlet velocity distributions, and the characteristic of outlet conduit affected the pumping system performance. By comparing the hydraulic loss between shaft extension and straight outlet conduit, we obtained the following conclusions: 1) For the length of shaft extension outlet conduit was longer, the route loss was bigger, and the hydraulic loss was bigger than straight outlet conduit under large flow rate. 2) When the circulation on the inlet of the outlet conduit was bigger，the relatively spread angle was less, the local loss was much more less, and the total hydraulic loss was less than straight outlet conduit. 3)The hydraulic loss distribution of the outlet conduit was the mainly reason for the difference of the pumping station. The head of 1.27 m was the dividing line of the two type tubular pumping systems in this paper, the efficiency of vertical shaft was higher when the head was low, and vice versa. This paper provides useful reference on the selection of low head pumping station.