Reliability of Fluid Cavitation Analysis by Means of an Equivalent Fluid Characteristics Modelling

The modelling of two-phase flows, and in particular the modelling of cavitating flow s, represents a remarkable field of interest for fluid power machines and components analysis. Even if the cavitation is a phenomenon physically well known and experimentally well detectable, it is very difficult to introduce its development and effects into numerical models built up for machines and components behaviour analysis. The aim of this paper is to analyse the alternate approach to the two-phase flow description, based on the definition of an "equivalent fluid", having physical-chemical characteristics defined by a proper combination of a given mixture of different fluids (liquid-gas included). In deeper detail, the correctness of an "equivalent fluid" approach, which is made with the aim to reduce the number of differential equations to be solved in presence of twophase flows, has to be thoroughly investigated. This is achieved by the simulation of an adiabatic cylinder fi11ed with a water-air mixture, which expand isentropica11y from atmospheric pressure towards the vapour tension. This simple test case is investigated applying both the "equivalent fluid" model and a homogeneous pressure zero-dimensional two-zone model, which is solved by direct integration. The final scope of the paper is to determine to which extent the "equivalent fluid" approach is able to reproduce the actual behaviour of a cavitating flow, with particular attention paid to the correct representation of heat transfer phenomena. Some of the examples are referred to water, but no major limitation exists to the application of the approach to mineral oil.