EFFECTS OF NONCONDENSABLE GAS ON CAVITATING NOZZLES

This paper focuses on the analysis of low-pressure regions inside fuel injector nozzles, where fuel vapor formation (strictly referred to as cavitation, or vaporous cavitation) and expansion of noncondensable gas (also referred to as pseudo cavitation, or gaseous cavitation) can simultaneously occur. Recently, X-ray radiography experiments of a 500 pin diameter cavitating nozzle showed that the presence of dissolved gas in the fuel can cause significant changes in the apparent distribution of projected void fraction. In this article, the effect of dissolved gas on cavitation measurements is investigated in further detail through experimentation and numerical simulations. Test conditions have been selected to have highly cavitating conditions. Tests with a standard gasoline calibration fluid and equivalent degassed fluid are compared and discussed. Numerical simulations have been conducted under the same conditions as the radiography experiments. The primary goal of the study is a quantification of the separate contributions of gas expansion as opposed to actual cavitation to the measurement of total void fraction. The multiphase flow is represented using a mixture model. Phase change is modeled via the homogeneous relaxation model. Particular attention is paid to quantifying the effective amount of noncondensable gas included in the mixture, in order to predict the response of regular and degassed fuels. The presence of dissolved gas in the multiphase flow is taken into account using a compressible fluid model with three distinct components (liquid, vapor, and gas). Issues surrounding estimation of the effective amount of noncondensable gas are discussed. Numerical simulation results match well with the experiments and indicate that when a sufficient quantity of gas is dissolved in the fuel, a void is evident in the central region of the channel that can be attributed to local expansion of noncondensed gas. Conversely, degassed fuel shows only intense cavitation at the nozzle wall, with very little contribution from noncondensed gas.