ABSTRACT
The saturation concentration for dissolved oxygen and nitrogen in water varies considerably with the pressure and the temperature of the water. Therefore, in closed-conduit systems changes in pressure and/or temperature can cause formation of small gas bubbles in the interior of the liquid. Air entrainment into closed-conduit systems is usually undesirable for such reasons as disturbances of pump performance or flow variations due to the detrainment and accumulation of air in the system. Further information on this distinctly different type of air entrainment is contained in the monographs: Swirl-flow problems at intakes and Air-water flow in closed conduits. Free-surface flow configurations leading to local air entrainment are always connected with some form of surface discontinuity. These flows differ from ambient surface aeration in that the local aeration processes entrain air at a rate completely independent of the transport capacity of the flow.
