The Improvement Of Draft Tube Based On CFD Flow Calculation

The HL220-WJ-71turbine units of Tie Wu hydropower station in Qinghai province use the prismatic casting draft tube which has a clapboard.The purpose of adding the clapboard is to damage the vortex formation, reduce water losses and vibration; However, due to the presence of the clapboard, not only increases the friction loss,the water is also severely damage the clapboard.This article verifies the necessity of the presence of the clapboard through the CFD numerical simulation technology,and makes the performance comparison between the elbow draft tube and the prismatic draft tube,thus makes the reasonable transformation of the draft tube.After calculation,the performance comparison between the prismatic draft tube with the clapboard and the prismatic draft tube without clapboard is following:under the low flow, the internal flow of the prismatic draft tube without clapboard is very disorder, there are obvious vortex, secondary reflux and serious stagnation; while after adding the clapboard, the flow state has an improvement in some extent, although the clapboard will cause the hydraulic loss, but the efficiency of the turbine unit gets a significantly improvement. Under optimal and high flow, the flowing states of the two kinds of draft tubes are substantially the same, the velocity and pressure distribution is reasonable, the flowing state are stable, so the clapboard basically has no use to improve the flowing state. On the contrary, the clapboard increases the friction loss. Therefore, under optimal and high flow, adding the clapboard makes the efficiency entire unit has a slight downward trend.After making the comparison between the elbow draft tube and the prismatic draft tube, we can see that the efficiency of the unit is higher when using the elbow draft tube whether under low flow, optimal flow or high flow. So we can draw the conclusion that the internal flowing state is more reasonable, and the restore performance is better.