Note on Observations of Cavitation in Different Fluids

Many observations [1, 2, 3] have shown that the \nperformance of a centrifugal pump with different fluids or with the same fluid at different temperatures is not the same at the same cavitation number when the latter is based upon the vapor pressure of the bulk fluid. Various similarity rules have been put forward in these works to account for the observed effect; namely, that lower net positive suction heads are achievable in \nmost cases compared to those observed in cold tap water. This difference is ascribed to the thermal effect associated with evaporating a certain fraction of the bulk fluid and the attendant decrease of vapor pressure. Scaling rules of the vapor-pressure decrease are made by assuming the process static and that all of the fluid in the inlet of the pump is at the same pressure. The measurements \nof Salemann [4] show that such a simple concept is \ninadequate, and he offers further speculations about the nature of the cavitation process as do Acosta and Hollander [5]. The purpose of this note is to describe an experiment intended to show the types of cavitation that occur and, where possible, to measure directly the reduction of vapor pressure or net positive suction head observed in pump experiments.