Large eddy simulation of highly turbulent under-expanded hydrogen and methane jets for gaseous-fuelled internal combustion engines

Abstract

Burning hydrogen in conventional internal combustion (IC) engines is associated with zero carbon-based tailpipe exhaust emissions. In order to obtain high volumetric efficiency and eliminate abnormal combustion modes such as preignition and backfire, in-cylinder direct injection (DI) of hydrogen is considered preferable for a future generation of hydrogen IC engines. However, hydrogen's low density requires high injection pressures for fast hydrogen penetration and sufficient in-cylinder mixing. Such pressures lead to chocked flow conditions during the injection process which result in the formation of turbulent under-expanded hydrogen jets. In this context, fundamental understanding of the under-expansion process and turbulent mixing just after the nozzle exit is necessary for the successful design of an efficient hydrogen injection system and associated injection strategies. The current study used large-eddy simulation (LES) to investigate the characteristics of hydrogen under-expanded jets with different nozzle pressure ratios (NPR), namely 8.5, 10, 30 and 70. A test case of methane injection with NPR=8.5 was also simulated for direct comparison with the hydrogen jetting under the same NPR. The near-nozzle shock structure, the geometry of the Mach disk and reflected shock angle, as well as the turbulent shear layer were all captured in very good agreement with data available in the literature. Direct comparison between hydrogen and methane fuelling showed that the ratio of the specific heats had a noticeable effect on the near-nozzle shock structure and dimensions of the Mach disk. It was observed that with methane, mixing did not occur before the Mach disk, whereas with hydrogen high levels of momentum exchange and mixing appeared at the boundary of the jet. This was believed to be the effect of the high turbulence fluctuations at the nozzle exit of the hydrogen jet which triggered Gortler vortices. Generally, the primary mixing was observed to occur after the location of the Mach disk and particularly close to the jet boundaries where large-scale turbulence played a dominant role. It was also found that NPR had significant effect on the mixture's local fuel richness. Finally, it was noted that applying higher injection pressure did not essentially increase the penetration length of the hydrogen jets and that there could be an optimum NPR that would introduce more enhanced mixing whilst delivering sufficient fuel in less time. Such an optimum NPR could be in the region of 100 based on the geometry and observations of the current study.3

Gas dynamics and flow characteristics of highly turbulent under-expanded hydrogen and methane jets under various nozzle pressure ratios and ambient pressures

“…For instance, Ashkenas and Sherman [11], Velikorodny and Kudriakov [12] and Vuorinen et al [13] suggested C H values of 0.67, 0.63 and 0.62, respectively. The authors of the current work found that for hydrogen jets with values of NPR up to 10, C H had a value of ~0.65 [14]. An average value of C H ≈0.71 has been measured for hydrogen jets with NPR in the range 8.5-70 [14].…”

“…The authors of the current work found that for hydrogen jets with values of NPR up to 10, C H had a value of ~0.65 [14]. An average value of C H ≈0.71 has been measured for hydrogen jets with NPR in the range 8.5-70 [14]. An empirical correlation comparable to Equation (1) can also be used to estimate the Mach disk width with a respective constant C W .…”

“…However, very limited experimental and computational studies are available in the literature on under-expanded hydrogen and methane jets and specifically with respect to applications in DI gaseous engines [36][37][38][39][40][41]. Recently, the authors of the present paper used LES to investigate the near-nozzle shock structure and mixing characteristics of highly under-expanded hydrogen and methane jets with NPR=8.5-70 issuing from a circular nozzle into a low ambient pressure (P ∞ ≈1 bar) [14,42]. The present computational work aims to extend the previous studies on under-expanded hydrogen and methane jets by investigating effects of ambient pressure on the key characteristics of this type of flow.…”

Aspects of Numerical Modelling of Flash-Boiling Fuel Sprays

“…see [40] for a discussion on high-speed gaseous fuel jets), causing recirculation of vapor and a widening of the vapor footprint. This effect may also have an influence on spray collapse.…”

The Influence of Mixedness on Ignition for Hydrogen Direct Injection in a Constant Volume Combustion Chamber

“…A parametric study has been carried out on high speed hydrogen injection, which shows the effect of grid resolution, Adaptive Mesh Refinement (AMR), and injection timing on mixture formation inside the cylinder [8]. The characteristics of hydrogen under-expanded jets has been comprehensively investigated using Large Eddy Simulation (LES) with different nozzle pressure ratios [9].…”