Visualization and comparison of methane and hydrogen jet dynamics using schlieren imaging

In the present work, methane (CH4) and hydrogen (H2) jet dynamics from a hollow-cone piezoelectric injector are experimentally investigated. The jet characteristics are studied under the effect of (1) pressure ratio, which is the ratio of the injection pressure (Pi) to the chamber pressure (Pch), (2) needle lift, and (3) nozzle geometry (hollow-cone outwardly opening nozzle without any caps versus the same nozzle with a single-hole cap). Z-type schlieren imaging is applied in a constant volume chamber equipped with optical access. The main characteristics of CH4 and H2 jets, i.e., penetration and cross-sectional area, are compared via MATLAB-based image post-processing. The novelty originates from the comparison of H2 and CH4 jet characteristics in similar conditions, from two different nozzle geometries, and investigating the effect of pressure ratio by changing the injection pressure and chamber pressure, separately. The results show that a 20% increase in the pressure ratio leads to at least a 13% increase in the H2 jet penetration and a 20% increase in its cross-sectional area. For CH4, the same rise in the pressure ratio enhances the jet penetration by at least 17% and its cross-sectional area by at least 21%. Change of Pi or Pch at the same pressure ratio leads to at least 3% and 10% difference in H2 and CH4 jet penetration, respectively, and at least 4% difference in their cross-sectional area. Increasing the needle lift leads to a greater jet penetration and cross-sectional area because of a larger injected mass and thereby a larger jet momentum, as well. Moreover, placing a single-hole cap on the hollow-cone outwardly opening nozzle causes approximately 50% pressure loss. However, the single-hole jet can still provide deeper penetration and enough cross-sectional area for efficient mixing. Overall, the comparison of H2 and CH4 jet characteristics shows that despite the large differences between their physical properties, jet penetrations are almost similar whereas the H2 jet possesses at least a 3.5% larger cross-sectional area than CH4.