Investigation of the effects of fluid properties representation and boundary condition selection in numerical simulations of micro scale flows with phase change

Abstract

Cavitation is a phenomenon affected considerably by the underlying pressure waves that occur on similar time and length scales as the bubble dynamics. Thus appropriate representation of wave dynamics within numerical frameworks is of paramount importance for the prediction of the phase change process in the nozzle as well as the subsequent spray formation. In this paper we focus on investigating the sensitivity of the wave dynamics within a compressible Large Eddy Simulation framework with regards to downstream geometry and boundary representation. Diesel was used as working fluid and was injected at various pressures through a micro-channel. Results in terms of vapour fraction, velocity and pressure are compared with the experimental data of Winklhofer [30,31]. The downstream domain length and reflectivity properties are shown to exert a significant effect on in-nozzle processes.

Cavitation Bubble Cloud Break-Off Mechanisms at Micro-Channels

“…The solution procedure is described in Appendix A and Table 3 shows the thermodynamic properties that have been used. The selection of these properties was based on the review carried out in our previous work [14]. The choice of the individual fuel properties compared to the real physical properties of the diesel fuel have to be considered.…”

“…The dimensionless wall distance y + was calculated to be 2.4 so turbulence was handled implicitly rather than requiring explicit wall functions. [14]. Figure 4 shows the numerical grid used for sensitivity analysis and, as can be seen, while the domain length was varied, the mesh quality in the upstream or nozzle domains was kept the same.…”

“…The study presented 'line of sight' distributions of the vapour void fraction, produced by shadowgraph imaging, in a plane micro-channel between an upstream and a downstream plenum chamber, with the flow 'sandwiched' between two windows. Measurements exist for several different pressure differences between the chambers and this case is commonly used for numerical validation due to the simple orthogonal geometry and realistic Diesel injection pressure conditions [12][13][14][15]. The limitations of the shadowgraph images though should be noted; the visual data provided are averaged over 20 different time-steps and are the 'line of sight' average over the depth of the cross-section.…”

“…This is particularly the case when the full domain is not modelled to reduce computational cost. To the best of the authors' knowledge no previous studies have been performed on the role of the boundary conditions in the prediction of cavitation dynamics, especially in terms of cloud break-off, apart from the preliminary work of Pearce et al [14]. In this work we adopted a non-reflective boundary condition developed by Poinsot and Lele [24] within a compressible multiphase solver.…”

“…After having established the extent of the validity, as well as the limitations, of our framework for both cases of cavitation inception and critical cavitation conditions, in the second part of our paper we turn our attention to the mechanisms that govern the cloud break-off dynamics. Using the Winklhofer geometry we have the opportunity to examine the cloud dynamics in conditions close to "choking" that have not been investigated before by numerical studies performed in the same geometry [12,14,15,27]. Previous studies predicted the shape of the free surface of the cloud to be largely flat and generally parallel to the wall for large pressure differences, due to low-order discretisation schemes used, which is inconsistent with experiments.…”