A multi-scale flow investigation of energy losses in a large Francis turbine

Complex internal flow in hydraulic turbine leads to energy loss, reducing efficiency. In order to examine flow and energy losses in the turbine components under various operating conditions, this work used multi-scale modeling and numerical simulation of a large Francis turbine using the entropy production approach. According to the results, the spiral casing shows very little loss in entropy production, however the draft tube and runner regions suffer the most. The main causes of hydraulic losses are wall shear (EPWS) and turbulent dissipation (EPTD). At constant guide vane opening, increased water head significantly reduces EPTD in vane and runner regions. However, at constant head, expanded guide vane opening raises overall entropy production, especially at low head (0.78H<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf>). This is due to low-pressure vortex expansion from water flow impact on the runner blades, substantially increasing EPTD and thereby reducing hydraulic turbine efficiency.