Draining of a water tank under inadequate ventilation conditions

Draining processes through a bottom orifice of a water tank under different ventilation conditions were studied experimentally. The water pressure fluctuations at the tank bottom and the air pressure in the headspace were measured simultaneously, based on which the air–water flow features were analyzed. From the experiments, three drainage modes were identified, including a periodic draining mode with periodic bubble generation at the orifice, a full-flow draining mode without bubble formation in the tank, and a combined draining mode with bubbles generated when the water level fell to a certain water depth. The drainage efficiencies varied among the three modes, resulting from the different conditions of bubble generation at the bottom orifice and the negative pressure in the headspace due to limited ventilation. The mechanism for bubble generation at the orifice was analyzed theoretically, and the effect of the negative pressure on drainage efficiency was discussed. The transition criteria among the modes were developed by a mathematical model. This study provides new knowledge on the drainage from a water tank under different degrees of ventilation.