Inception of cavitation

Abstract

Cavitation due to flow separation may occur at pressures higher than the vapor pressure (e.g., where PwP_wPw? is the measured pressure in the cavity and PvP_vPv? is the vapor pressure at the corresponding temperature).

The point of inception of cavitation is a unique point and can be accurately determined by graphical methods based on pressure measurements in a closed circuit. It can be defined as the point at which a sudden loss in the total head of the system occurs.

In these investigations, the point of inception of cavitation was determined by noting the changes in pressure that occur while the flow changes from a cavitating condition to a non-cavitating condition. From Figs. 33 to 37, it is clear that at this point of inception, the minimum pressure-constant as long as the flow is cavitating-suddenly changes when cavitation stops. This shows the analogy of cavitation to boiling.

Since all the points of inception of cavitation corresponding to different total pressures lie on a straight line, it proves that there is no head-scale effect on cavitation inception in a closed circuit.

The critical cavitation coefficient as suggested in this work:

?=Ps?PtPs\sigma = \frac{P_s - P_t}{P_s}?=Ps?Ps??Pt?? where PsP_sPs? = Wet positive suction head, PtP_tPt? = Total pressure, and PtP_tPt? = The intercept on the P-axis made by producing the straight-line curve of the plot of PsP_sPs? vs PswP_{sw}Psw?, can be adopted for correlating cavitation inception in model tests.

Surface roughness of the model, as explained in the thesis, affects cavitation inception.

Pressure recovery, as defined by the slope of the "S" and "O" curves in the cavitating zone, increases with increasing total pressure.

The effect of various admixtures on cavitation inception is difficult to generalize. An admixture affects several properties of the liquid-such as surface tension, viscosity, and vapor pressure-simultaneously, making it hard to predict which effect will dominate in determining the inception point.

An alcohol-water mixture lowers the surface tension of water while increasing its viscosity. A decrease in surface tension enhances cavitation tendency, while an increase in viscosity resists it. In these experiments, viscosity was found to be the predominant factor (Figs. 62 and 63). Addition of salts to water increases its surface tension and boiling point. Both effects resist cavitation, but salts also favor gas release, which enhances cavitation. Hence, generalization is difficult. A decrease in cavitation tendency was noticed with small percentages of various salts. With charcoal, a definite increase in cavitation tendency was observed (Fig. 64), supporting the statement that a substance providing structural discontinuity in the liquid enhances cavitation.

There is a transition stage from fully cavitating to non-cavitating flow. During this stage, the cavity over the model oscillates, enlarges to envelop the model, and then contracts rapidly toward the upstream end. During this period, the reference pressure fluctuates, as indicated by the mercury column. This shows that in this type of cavitation, there is no definite line of demarcation between fully cavitating and non-cavitating stages.

When the model was enveloped by the cavity, small vapor bubbles were seen traveling very fast from the downstream end of the cavity toward the upstream end. These bubbles were quite different from ordinary air bubbles, which could be seen in large numbers in the diffuser.