Characterization of fuel drop impact on wall films using SPH simulation

Abstract

The ability to accurately predict the outcome of the drop/wall interaction is essential to engine spray combustion modeling. In this paper, the process of fuel drop impact on a wet wall was simulated using a numerical method based on smoothed particle hydrodynamics (SPH). The present numerical method was first validated using experimental data on the crown height and crown diameter resulting from water drop impact on a liquid film. Then, the impact process of iso-octane drops on wet walls under engine relevant conditions were studied. The presence of a wall film will affect not only the splash threshold but also the crown evolution and the secondary droplets ejected from the rim of the crown. Numerical results show that the splash threshold increases with the film thickness; the splashed mass ratio increases as the kinetic energy of the incident drop increases. The effect of film thickness on the splashed mass ratio is determined by two competing mechanisms. On the one hand, as the film thickness increases, more incident energy will be absorbed and transferred into the crown, thus producing more secondary droplets. On the other hand, more impinging energy will be dissipated during the spreading as the film thickness increases, thus generating fewer secondary droplets. The properties of the secondary droplets are very different as the film thickness increases. Instead of moving outward, the secondary droplets will move upward and even congregate to the center when the film becomes thicker. The impact angle will affect not only the distributions of the secondary droplets but also the splashed mass. The locations and velocities of the secondary droplets were analyzed. These outcomes were incorporated into formulas that can be further developed into a model for simulating engine spray/wall interactions.

Numerical study on the Leidenfrost behavior of a droplet stream impinging on a heated wall

“…The present SPH method has been validated for the impacts of a single drop on the wall [10,26,27,29,30]. The present method has also been validated for crown rim propagation [20] against the experimental studies for the droplet train impact of ethanol [17] and Novec HFE-7100 [31] at different impingement frequencies.…”

Modelling liquid film in modern GDI engines and the impact on particulate matter emissions – Part 1

“…Several numerical approaches have been proposed in the literature to model spray-wall interaction (the outcome of each droplet-wall impact) in the context of an engine cylinder, with some still under development to this day. [50][51][52][53][54][55][56][57] The results presented in this paper are obtained using the Bai-Onera spray-wall interaction model. Based on the work by Bai and Gossman, 50,58 the Bai-Onera model was developed using numerous experimental data attained by the ONERA laboratory.…”

Particle-based approach to model fuel droplet impact and thermal spray coating processes

“…The governing equations to describe the dynamics of drop-wall interactions in fuel spray dynamics include the unsteady Navier-Stokes equations, the energy equation, and the continuity equation of the vapor species in the gas phase. Basic governing fluid flow equations for both the liquid and gas phases [13,[74][75][76][77] are modified to simulate the vaporization process by incorporating proper equations of state and necessary source terms. The convective/Lagrangian form of unsteady, laminar Navier-Stokes equations and continuity equation of vapor species in the gas phase governing the drop-wall interaction process can be written as…”

“…The overall non-dimensional film thickness in the experiment was typically 1/6 [19]. The effect of surface roughness may not be significant for a thick liquid film on a surface, but for thin film ( 0.2   ) created by the preceding droplets, the succeeding droplet will push the liquid film away, and its exposure to the wall surface can cause the post-impingement outcomes to be affected by the surface roughness underneath the film [25,74,87,88].…”

“…Finally, the polished specimen of stainless steel used in the experiment and the preference of the present SPH method for a rough surface could have contributed to the differences, as explained before. Further validation and applications of the present SPH method for a single droplet impact can be found in [13,74,77,89].…”