Model of air entrainment in central jet vertical dropshafts in ordinary working conditions

One of the most important hydraulic devices affected by the phenomenon of air entrainment is vertical dropshaft. The evavuation of the rate flow of entrained air is essential for verification of the working conditions and for the correct design of the structure. In scientific research and in technical practice, a coefficient called "beta" is used for the assessment of the phenomenon: it is the ratio between flow rate of entrained air and water flow rate. The present paper reports the results of a study carried out on a physical model of large dimensions reproducing a central jet vertical dropshaft supplied by an orifice at the base of a tank. The authors used an original experimental technique, called volumetric method, and a particular elaboration method, which provided reliable values of coefficient "beta". On the basis of a wide experimental data set, a new analytical model for the determination of coefficient "beta" has been proposed. The results, though achieved in a particular case, contribute to the choice and the specification of parameters by which the phenomenon is characterized.