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.

本工作描述了一种数值方法及其在先导注入，停留时间和随后的注入周期开始期间孔内外流动发展的数值方法及其实验验证。可压缩的Navier-Stokes方程在六孔喷油器中进行数值求解，从而施加了针阀运动的实际条件，并另外考虑了随时间变化的偏心运动。阀的运动是用浸入边界法模拟的。这样就可以在连续进样之间的停顿之间的零停留时间内以零升程进行模拟，此时针头保持关闭状态。此外，该数值模型利用了完全可压缩的两相（液体，蒸气）两组分（燃料，空气）正压模型。空气的运动是通过附加的输运方程式与VOF界面捕获方法相结合来模拟的，该方法能够解决近喷嘴雾化以及所注入液体对亲油性喷嘴壁表面的影响。发现偏心针运动是造成孔内强烈旋流的原因，这不仅会导致注入的液体射流破裂成韧带，而且会导致其向着注射器外壁的向后运动。模型预测表明，由于残留在喷嘴内的残留气体，在阀门关闭和喷嘴重新打开期间，这种喷嘴壁润湿现象更加明显。