Development of new-type S draft tube for low-head axial-flow water turbine.

An S-type tubular turbine is used for a low-head hydraulic turbine. The S-type draft tube plays an important role in the S-type tubular turbine. The S draft tube has two bend portions, first and second bends. Separation occurs here. The pressure recovery becomes low because of the separation. The paper proposes a new-type S draft tube which has the highest pressure recovery coefficient. One of the characteristics of the new draft tube is a twisted portion in the middle of the S draft tube. Asymmetrical secondary flow is generated in the twisted portion of the new S draft tube. The pressure recovery coefficient Cpo is largely improved because such asymmetrical secondary flow prevents the separation on the bend portion of the S draft tube. Such phenomena are assured by measuring the velocity distribution in the draft tube.