An Assessment of Homogeneous Mixture Method Cavitation Models in Predicting Cavitation in Nozzle Flow

The homogeneous mixture method (HMM) is a popular class of models used in the computational prediction of cavitation. Several cavitation models have been developed for use with HMM to govern the development and destruction of vapor in a fluid system. Two models credited to Kunz and Schnerr–Sauer are studied in this paper. The goal of this work is to provide an assessment of the two cavitation submodels in their ability to predict cavitation in nozzle flow. Validation data were obtained via experiments which employ both passive cavitation detection, (PCD) via acoustic sensing and optical cavitation detection (OCD) via camera imaging. The experiments provide quantitative information on cavitation inception and qualitative information on the vapor in the nozzle. The results show that initial vapor formation is not predicted precisely but within reason. A sensitivity analysis of the models to input parameters shows that the Schnerr–Sauer method does not depend upon the estimation of nuclei size and number density. Small changes in the vapor formation rate but not the total vapor volume can be seen when weighting parameters are modified. In contrast, changes to the input parameters for the Kunz model greatly change the final total vapor volume prediction. The assessment also highlights the influence of vapor convection within the method. Finally, the analysis shows that if the fluid and nozzle walls do not support nuclei larger than 40 μm, the methods would still predict cavitation when indeed there would be none in practice.

Experimental Investigation of Turbulent Cavitating Flows in a Small Venturi Nozzle

The cavitating flows created in a small Venturi tube with throat cross section 4 × 15.34 mm2 are investigated based on ultra-fast x-ray imaging. The instantaneous velocities of the liquid and vapor are measured simultaneously by tracking seeding particles and vapor structures respectively while the vapor volume fraction is derived from the different x-ray attenuation. Wavelet decomposition with appropriate thresholds is used to separate seeding particles from vapor structures, so that image cross-correlations could be applied on the two phases separately. This study presents data on mean velocity and void ratio field, statistical turbulent quantities in three different cavitation levels with the same reference velocity. A type of cavitation associated with a weak but persistent re-entrant jet is described. The comparison between the cavitation and the noncavitating flow shows that the averaged flow field is significantly altered by the presence of cavitation and the vapor formation near the throat area is observed to suppress velocity fluctuations.

Experimental determination of the induction period for the onset of intensive boiling of a subcooled water flow under conditions of unsteady heat release

This paper studies prediction of the boiling crisis onset under conditions of the rapid temperature growth of the heat releasing surface washed by a water flow subcooled to the saturation temperature. We obtained experimental data on time delay for rapid vapor formation and showed that the existing technique can be extended to the case with a forced flow. It is shown that the characteristic layer thickness is smaller than the thickness of superheated layer.

Study of the Sloshing in a Fuel Tank Using CFD and EFD Approaches

This paper is focused on the study of the sloshing in the fuel tank of vehicles. As well known, fluid dynamic in an automotive fuel tank have to be studied and optimized to allow the correct fuel suction in all driving conditions, prevent undesired slosh noise and limit its influence on fuel vapor formation and management. Experimentation to predict the sloshing with a good accuracy depends on the ability to replace real working parameters and conditions like accelerations, decelerations, slope variations and rotations. This paper shows results obtained studying the sloshing inside a reference tank with computational fluid-dynamic and experimental approaches. The test bench for automotive fuel tank, employed in this analysis, has been designed by Moog Inc. on specification from Fiat Chrysler Automobiles and it is aimed at covering the wider possible range of dynamic conditions. It basically consists of a hexapod, which uses six independent actuators arranged in three triangles and connecting a base and a top platform, thus allowing all six DOFs. Above the top platform is mounted a tilt table with two additional actuators, to extend pitch and roll envelope, thus the name of “8-DOF bench”. A dedicated CFD model has been built up using a CFD commercial code. The model has been integrated with the multiphase tool in order to correctly reply the real free surface. Results, numerical and experimental, have been post-processed with Matlab® comparing percentage gaps of the free surfaces each other. The comparison has shown a good agreement. This research is the result of a scientific collaboration between the Industrial Engineering Department of University of Naples Federico II and FCA Fiat Chrysler Automobiles.

Entrapped Gas and Vapor Formation Influence on Surge Pressure in Liquid Pipeline Systems

Predicting the effects of entrapped gas or vapor formation on surge is very important in design and operation of liquid pipelines. This paper identified the scenarios in which entrapped air and vapor formation need to be considered in pipeline operation and design. Useful modeling methods utilizing common liquid pipeline transient hydraulics software are provided. Validation of the presented methods was completed using experimental data from published literature. Examples are presented in showing the implementation of the provided modeling methods on real pipeline design scenarios. Finally, advantages and limitations of the presented methods was discussed. The methods presented in this paper enable pipeline operators and design engineers to properly estimate the complicated surge issues such as the influence of air bubble venting and column separation and collapse using commonly available single phase hydraulics tools. The operators and engineers will benefit from the provided methods in finding and validating reliable surge mitigation solutions and creating pipeline design with higher integrity level. The paper also presents the limitation of the methods and continuous improvements that can be achieved in the future.