Internal Nozzle Flow Simulations of the ECN Spray C Injector under Realistic Operating Conditions

Abstract

<div class="section abstract"><div class="htmlview paragraph">In this study, three-dimensional large eddy simulations were performed to study the internal nozzle flow of the ECN Spray C diesel injector. Realistic nozzle geometry, full needle motion, and internal flow imaging data obtained from X-ray measurements were employed to initialize and validate the CFD model. The influence of injection pressure and fuel properties were investigated, and the effect of mesh size was discussed. The results agreed well with the experimental data of mass flow rate and correctly captured the flow structures inside the orifice. Simulations showed that the pressure drop near the sharp orifice inlet triggered flow separation, resulting in the ingestion of ambient gas into the orifice via a phenomenon known as hydraulic flip. At higher injection pressure, the pressure drop was more significant as the liquid momentum increased and the stream inertia was less prone to change its direction. Two fuels were tested in both experiments and simulations, namely iso-octane and n-dodecane. With the former, the gas species in the low-pressure region consisted of both fuel vapor and non-condensable gas. With n-dodecane, due to its low saturation pressure, fuel vapor was practically absent. Furthermore, it was found that fuel cavitation might not be the only phenomenon able to trigger flow separation, as “pseudo-cavitation” caused by non-condensable gas expansion played a similar role in promoting the conditions that led the flow to detach from the wall. Finally, it was found that a minimum mesh size of 10 μm within the orifice was sufficient to ensure the main flow features were captured. However, it was shown that finer meshes allowed for better resolution of the near-wall gas layer, resulting in more flow features to be resolved.</div></div>

Three-component multi-fluid modeling of pseudo-cavitation phenomenon in diesel injector nozzles

“…Recent diagnostics have shown the relevance of void fraction contributions from the de-gassing of non-condensable gases which occurs at a pressure higher than the saturation pressure of the fuel. 8 The discrepancies observed in the size and location of the cavitation zone between the experiments and numerical model predictions are attributed to ''pseudo-cavitation,'' which is the formation of bubbles attributed to non-condensable gases such as oxygen and nitrogen dissolved in the fuel also known as de-gassing of non-condensable gases. 11213 While the level of dissolved gas in the fuel is not typically recorded or measured, it is noted that the selection of fuel properties and dissolved gas concentration have a pronounced effect on the cavitation development and structure.…”

“…Vortex-based cavitation on the other hand, occurs when there is a local pressure drop due to the turbulence generated in the flow recirculation zones. 1,8 The possible locations of this type of cavitation are in the sac and the nozzle region. 1,9 The cavitation in the sac region is termed as needle-originated cavitation 10 while needle-string cavitation 2 occurs at high injection pressures.…”

“…14 Pseudo-cavitation typically occurs near the central axis of the nozzle downstream of the cavitation zone. 14 Numerical studies of Guo et al 8 showed that iso-octane demonstrated a hydraulic flip (separation of fuel from the nozzle wall) as the local pressure dropped below the saturation pressure. However, the same hydraulic flip was observed even for n-dodecane which has a significantly lower saturation pressure as compared to iso-octane indicating an absence of classical cavitation.…”

Investigation on the effect of the flow passage geometry of diesel injector nozzle on injection process parameters and engine performances

“…In this scenario, the nitrogen gas degasses and leads to void formation throughout the nozzle orifice. The flow characteristics with non-condensable gas in diesel injector nozzles have also been simulated with the homogeneous relaxation cavitation model, 55,56 in which a specified mass fraction of non-condensable gas (nitrogen gas) is included in the fuel liquid phase. Cavitating channel flow simulations performed by Magnotti et al 55 uncovered that the content of nitrogen gas has a negligible effect on the flow field predictions due to the dilute concentration and low density of nitrogen gas but the non-condensable gas content is found to affect the cavitation predictions.…”

“…At low non-condensable gas contents, the expansion of nitrogen gas is considerably reduced. A simulation of internal flow in a divergent injector nozzle (prone to cavitate) under super cavitation conditions was conducted by Guo et al 56 It is found that the expansion of nitrogen gas inside the orifice triggers flow detachment from the orifice wall, which extends up to the orifice outlet and results in hydraulic flip. Two fuels with different saturation pressures were investigated.…”