Release of dissolved nitrogen from water during depressurization

Experiments were run to study depressurization of water containing various concentrations of dissolved nitrogen gas, the primary case being room-temperature water saturated with nitrogen at 4 MPa. In a static depressurization experiment, water with very high nitrogen content was depressurized at rates from 0.09 to 0.50 MPa per second and photographed with high-speed movies. The pictures showed that the bubble population at a given pressure increased strongly with decreasing depressurization rate. Bubbles rarely appeared before the pressure reached half the initial pool pressure. Flow experiments were performed in an axisymmetric converging-diverging nozzle and in a two-dimensional converging nozzle with glass sidewalls. Depressurization gradients were roughly 500 to 1200 MPa per second. Both nozzles exhibited choked flow behavior even at nitrogen concentration levels as low as 4 percent of saturated. The flow rates were independent of concentration level and could be computed as incompressible water flow based on the difference between stagnation and throat pressures; however, the throat pressures were significantly different between the two nozzles