Hydraulic and heat transfer characteristics in corridor-shaped air-cushion surge chambers in hydropower systems

The large aspect ratio of a corridor-shaped air cushion surge chamber in hydropower systems results in special hydraulic and heat transfer characteristics that differ from those of cylindrical shapes. The complexities of inflow jet and outflow vortex phenomena at the throttle orifice, along with the thermal energy exchange across the water-air interface during load variations, continue to be areas of limited understanding. The hydraulic and heat transfer processes during the load variation conditions were simulated precisely using the volume of fluid model to address the above knowledge gap by adopting computational fluid dynamics. The effects of various parameters on pressure and flow patterns (including initial water depth, orifice size, aspect ratio of the surge chamber, and unit closure time) and the thermodynamic response of the air during the compression and expansion phases were analyzed. The results indicate that a smaller orifice size has larger Froude numbers, thus intensifying jet heights and exacerbating wave fluctuations. An increased initial water depth or a reduced aspect ratio of the corridor-shaped chamber decreases the angular velocity of the fluid above the orifice during load increase, thus attenuating the vortex intensity. A method for calculating the heat transfer rate in the chamber was developed by considering the heat exchanges between water, chamber wall, and air. The intense heat transfer at the water-air interface is caused by large wave fluctuations due to velocity gradients. In addition, larger orifice size increases the flow rate and heat transfer rate, leading to an increase in the total heat transfer coefficient of the chamber.