The underexpanded jet Mach disk and its associated shear layer

Edgington-Mitchell, Daniel; Honnery, Damon; Soria, Julio

doi:10.1063/1.4894741

Cited by 82 publications

(84 citation statements)

References 32 publications

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“…The normalized values of these transverse velocities (u/U 1 ) were higher for methane than for hydrogen. Therefore, it may be concluded that the concavity of the Mach disk is due to gradient of the transverse velocity component (see the lines of Z/D=1.8 for the transverse profiles of Figure 10), not directly by the initial expansion fans at the nozzle lip as suggested in [10]. In fact, by moving away from the nozzle centreline, the direction of the velocity vector produced by the transverse and axial velocity components would be more inclined with respect to the nozzle centreline.…”

Section: Near-nozzle Shock Characteristicsmentioning

confidence: 99%

“…Similarly to [10,59], the transverse velocity graphs of Figure 10 show that just after the Mach disk location (Z/D≈1.9 and 1.85 for methane and hydrogen, respectively), at 2.0D downstream of the nozzle exit, the flow has a transverse velocity component towards the jet boundary (positive value of u/U 1 ). This was attributed to the relative concavity of the Mach disk (see Figure 9) due to the initial expansion fans at the nozzle lip [10]. However, after a radial distance of ~0.55D, the flow was turned inwards by passing through an oblique shock which was formed by the reflection of the shock at the triple point.…”

Section: Near-nozzle Shock Characteristicsmentioning

confidence: 99%

“…This behaviour has been explained in [10] through a set of schlieren and PIV images and was attributed to the direction of the transverse component of the velocity field. Similarly to [10,59], the transverse velocity graphs of Figure 10 show that just after the Mach disk location (Z/D≈1.9 and 1.85 for methane and hydrogen, respectively), at 2.0D downstream of the nozzle exit, the flow has a transverse velocity component towards the jet boundary (positive value of u/U 1 ). This was attributed to the relative concavity of the Mach disk (see Figure 9) due to the initial expansion fans at the nozzle lip [10].…”

Section: Near-nozzle Shock Characteristicsmentioning

confidence: 99%

“…Density, velocity and entropy are discontinuous across the slip line but pressure and streamline deflection must be continuous [8]. This may trigger a turbulent mixing process within the jet core [10]. For relatively high levels of under-expansion, e.g.…”

Section: Near-nozzle Shock Structurementioning

confidence: 99%

“…The slip line shows the existence of an annular shear layer within the jet volume. Highly under-expanded jets have two annular shear layers, the inner and outer layer [9][10]. The inner shear layer lies between the high-velocity gas near the jet boundary and the low-velocity jet core, while the outer shear layer (or the mixing layer) lies within the jet boundary and the surrounding medium.…”

Section: Near-nozzle Shock Structurementioning

confidence: 99%

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

Hamzehloo

Aleiferis

2016

International Journal of Hydrogen Energy

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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.

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Section: Near-nozzle Shock Characteristicsmentioning

confidence: 99%