Numerical simulation of fuel dribbling and nozzle wall wetting

Abstract

The present work describes a numerical methodology and its experimental validation of the flow development inside and outside of the orifices during a pilot injection, dwelt time and the subsequent start of injection cycle. The compressible Navier-Stokes equations are numerically solved in a six-hole injector imposing realistic conditions of the needle valve movement and considering in addition a time-dependent eccentric motion. The valve motion is simulated using the immersed boundary method; this allows for simulations to be performed at zero lift during the dwelt time between successive injections, where the needle remains closed. Moreover, the numerical model utilises a fully compressible two-phase (liquid, vapour) two-component (fuel, air) barotropic model. The air’s motion is simulated with an additional transport equation coupled with the VOF interface capturing method able to resolve the near-nozzle atomisation and the resulting impact of the injected liquid on the oleophilic nozzle wall surfaces. The eccentric needle motion is found to be responsible for the formation of strong swirling flows inside the orifices, which not only contributes to the breakup of the injected liquid jet into ligaments but also to their backwards motion towards the external wall surface of the injector. Model predictions suggest that such nozzle wall wetting phenomena are more pronounced during the closing period of the valve and the re-opening of the nozzle, due to the residual gases trapped inside the nozzle, and which contribute to the poor atomisation of the injected fluid upon re-opening of the needle valve in subsequent injection events.

A direct forcing immersed boundary method for cavitating flows

“…An accurate simulation of the needle movement, including full-closed position, is essential to capture the pressure-wave dynamics and the residual fuel remaining in the injector's sac volume between successive injection events, a phenomenon that plays a major role in nozzle wall wetting and emissions, as demonstrated by a recent work of the author's group. 38 The geometry employed is a 6-hole valve covered orifice (VCO). The computational domain, as well as a general and a detailed view of the mesh are visible in Figure 34.…”

“…For this simulation, the density-based solver is employed, which uses 10 3 times smaller steps compared to the previously simulated cases, 38 where a pressure-based solver was used, and thus, better captures the dynamic effects linked with the residual fuel inside the nozzle's sac volume and injection hole during the dwelt time. Moreover, although recent studies [83][84][85] consider temperature effects, for the cases examined here, that refers to pilot injection at a rail pressure of 1600, temperature effects can be ignored.…”

“…[86][87][88][89][90] The diesel properties employed in the simulations are the ones considered in the aforementioned study. 38 The air properties were computed using the NIST (REFPROP) library. 91 Regarding the turbulence, instead of RANS, a LES approach is employed and the turbulent viscosity is modeled using the wall adaptive large Eddy (WALE) method.…”

“…The observations and obtained results agree with that of the previously published study. 38 The evolution of the cycle and the two distinct cavitation formation mechanism are illustrated in Figure 35 for the start of injection transients (SOI) and Figure 36 for the end of injection (EOI). At the start of the first injection, a small amount of air from the combustion chamber is suctioned into the orifices and at 32 μm lift, geometric-induced cavitation is initiated at the inlet edge due to the high flow speed generated between the needle and nozzle walls, as it is illustrated in the time-instances of Figure 35.…”

“…The recent works of Huang et al 34 and Park et al 35 on cavitation induced by the motion of underwater projectiles, and Xu et al 36,37 on cloud cavitation around blunt bodies followed a cut-cell approach as well. On the other hand, a direct forcing method has been chosen by the authors' group to study cavitation induction in diesel injectors, 38 in gear pumps 39 and during the closure of the claw of the pistol shrimp. 40 The authors also presented in a previous article 41 a study of shock-wave generation and the induced cavitation by the high-velocity impact (Mach = 0.7) of a solid projectile on a water jet; in this work, the projectile was modeled via a similar formulation of the direct forcing immersed boundary approach proposed herein.…”