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

Abstract

The current study used large eddy simulations to investigate the sonic and mixing characteristics of turbulent under-expanded hydrogen and methane jets with various nozzle pressure ratios issued into various ambient pressures including elevated conditions relevant to applications in direct injection gaseous-fuelled internal combustion engines. Due to the relatively low density of most gaseous fuels such as hydrogen and methane, DI requires high injection pressures to achieve suitable mass flow rates for fast in-cylinder fuel delivery and rapid fuel-air mixing. Such pressures typically form an under-expanded fuel jet past the nozzle exit. Test cases of hydrogen injection with nozzle pressure ratio (NPR) of 10 issued into quiescent air with pressure P ∞ ≈1, 5 and 10 bar were simulated. Direct comparison between hydrogen and methane jets with NPR=8.5 and P ∞ ≈1 was also made. The effect of ambient pressure on features of transient development of the nearnozzle shock structure and tip vortices (vortex ring) was investigated. It was observed that at constant NPR, higher ambient pressure resulted in slightly faster formation of the Mach reflection and shorter Mach disk settlement time. Different mechanisms were observed between hydrogen and methane with regards to transient formation of their initial tip vortex rings. It was found that the initial transient tip vortices of hydrogen jets may also contribute to the flow instabilities at the boundary of the intercepting shock and, unlike for methane, promote fuel-air mixing before the Mach reflection. It was also shown that the nearnozzle shock structure was only affected by NPR regardless of the ambient pressure. Furthermore, no flow recirculation zone was found just downstream of the Mach disk, a finding comparable to all previous experimental investigations. Also, it was observed that a locally richer mixture was created for jets with higher NPR or with higher ambient pressure at constant NPR. Based on the results of the current study, correlations were proposed for the shock cell spacing and jet tip penetration of highly under-expanded jets issued from millimetre-size circular nozzles.

Characteristics of trans-critical propane spray discharged from multi-hole GDI injector

“…Figure 8). This same tendency was also observed in the cases of the under-expanded ideal gas [50]. Those factors dominate the process of spray and shorten the maximum penetration.…”

Real-Gas Effects and Phase Separation in Underexpanded Jets at Engine-Relevant Conditions

“…14 on the right side. A blocking cone is formed in front of the jet consisting of a higher pressure and therefore higher density region which redirects the main flow away from the centerline to the sides, an effect which was also noticed for high pressure gas injections by other researchers, see, e.g., Hamzehloo et al [61]. After the cone vanishes the jet reattaches to the centerline and the injection profile is in a very good agreement with the correlation of Hill and Ouellette [70] with the fitting parameter of Γ = 2.968 obtained in Banholzer et al [10], see dashed lines in Fig.…”

Flow characteristic of highly underexpanded jets from various nozzle geometries

“…The gird around the nozzle exit, the jet potential core, and the jet shear is specially refined to obtain high resolution. With these careful arrangements, the grid resolution in the main region of interest in this study is similar as that used in the previous LES of supersonic jets [31][32][33][34][42][43][44], which are summarized in Table 2. On the other hand, coarse cell sizes with a resolution of 1.0 mm in the far field and 0.5 mm at outflow boundaries are used to avoid wave reflections by introducing additional dissipation.…”

“…For example, Vuorinen et al [31] performed LES modeling of underexpanded jets at different NPR, and further compared the flow differences between the underexpanded methane and nitrogen jets [32]. Hamzehloo and Aleiferis [33] focused on the flow characteristics of the underexpanded hydrogen jets, and then compared the mixing characteristics of hydrogen jets with the methane jets at various NPR [34]. Li et al [35] investigated the differences of flow structures and screech characteristics between the underexpanded hydrogen and nitrogen jets using LES, and also explored the turbulence transition mechanism and three-dimensional instability of underexpanded jets at different NPR [36].…”