Comparison of Mixture and Multifluid Models for In-Nozzle Cavitation Prediction

Abstract

Fuel injectors often feature cavitation because of large pressure gradients, which in some regions lead to extremely low pressures. The main objective of this work is to compare the prediction capabilities of two multiphase flow approaches for modeling cavitation in small nozzles, like those used in high-pressure diesel or gasoline fuel injectors. Numerical results are assessed against quantitative high resolution experimental data collected at Argonne National Laboratory using synchrotron X-ray radiography of a model nozzle. One numerical approach uses a homogeneous mixture model with the volume of fluid (VOF) method, in which phase change is modeled via the homogeneous rela.xation model (HRM). The second approach is based on the multifluid nonhomogeneous model and uses the Rayleigh bubble-dynamics model to account for cavitation. Both models include three components, i.e., liquid, vapor, and air, and the flow is compressible. Quantitatively, the amount of void predicted by the multifluid model is in good agreement with measurements, while the mixture model overpredicts the values. Qualitatively, void regions look similar and compare well with the experimental measurements. Grid converged results have been achieved for the prediction of mass flow rate while gridconvergence for void fraction is still an open point. Simulation results indicate that most of the vapor is produced at the nozzle entrance. In addition, downstream along the centerline, void due to expansion of noncondensable gases has been identifled. The paper also includes a discussion about the effect of turbulent pressure fluctuations on cavitation inception.

Large-eddy simulation of cavitating nozzle flow and primary jet break-up

“…Wang et al 47 propose an eight-equation model based on the work of Saurel et al, 48,49 to assess both internal and external 2-D nozzle flows simultaneously including cavitation effects by a stiffened gas equation of state, whereas gas and vapor are represented as one single miscible phase. In their recent work, Battistoni et al 50 compare a non-equilibrium thermal mixture model with a Eulerian multi-fluid description with Rayleigh bubble dynamics for phase change. In their simulations of cavitating nozzle flows, a free gas content was considered, but effects on the primary jet break-up were not assessed.…”

Large-Eddy Simulation (LES) of Spray Transients: Start and End of Injection Phenomena

“…Sandia penetration data, based on diffuse backillumination imaging, are also considered, but the available time information is only relative to the apparent SOI, which we assumed to be the same as in the X-ray dataset. Numerical penetration is calculated based on a radiographic projection of density distribution (Battistoni et al, 2014a), in analogy with the experimental methodology. The numerical predictions and the experimental data match quite well in terms of both start timing and slope of the liquid penetration curve.…”

“…In the current work, the value is set to Y 3 = 2910 -5 by mass, according to previous works (Battistoni et al, 2014a, and the assumption is based on an analysis of the characteristic time-scale of air mass diffusion in the liquid solution.…”

“…First principles can never be applied to resolve locally the dynamics of single bubbles, therefore simplified models based on Rayleigh-Plesset equation or on the homogeneous assumption (either equilibrium or non-equilibrium) are generally employed (Wang and Greif, 2006;Habchi, 2014). Significant improvements have been demonstrated if noncondensable gases are accounted for in the multi-phase model (Battistoni et al, 2014a).…”

Topology and Distinct Features of Flashing Flow in an Injector Nozzle

“…The two-fluid Eulerian model has also been for the simulation of two-phase flows (Battistoni et al, 2014;Habchi 2015), since it offers generality of the solution, as the pressure and velocity fields are resolved for each phase, however it is associated with additional computational cost and increased risk of numerical instability compared to the mixture model, since a set of governing equations must be solved for each of the two phases. However, the use of the mixture model has been proven adequate for predicting cavitating/flashing flows Schmidt et al, 2010), even high-velocity compressible flows within complex geometrical layouts (Koukouvinis et al, 2016).…”