Visualization research on hydrogen jet characteristics of an outward-opening injector for direct injection hydrogen engines

•Hydrogen jet structure in near-field is conical, while in far-field is spherical vortex.•The spread angle is not sensitive to pressure ratios of injection to ambient (PR).•The jet penetration, projected area and volume are positively related to PR.•The jet has good self-similarity under all PRs, and the penetration constant is 13.18.•Proposed a scaling correlation of axial penetration for this kind of hydrogen jets. The hydrogen jet characteristics have effect on the performance of direction injector (DI) hydrogen engines because they can influence the process of mixture formation and heat release. In this study, the hydrogen jet characteristics of an outward-opening injector were studied with high-speed schlieren imaging in a constant volume chamber at different injection and ambient pressure ratios (PRs), which ranged from 10 to 140. Results show that the hydrogen jet in the near-field is a conical structure, while the jet structure develops into a spherical vortex in the far-field. The jet axial penetration, radial penetration, and volume increase with the increasing of the PR. The jet spread angle is not sensitive to the PR except for the low PRs. The entrainment rate decreases with the increasing of the PR. The normalization analysis of jet penetration shows that the hydrogen jets have a good self-similarity under all PRs. A non-dimensional scaling correlation of axial penetration is proposed for this kind of hydrogen jets. In the scaling correlation, the exponents of the non-dimensional penetration and time term are 0.18 and 0.83, while the penetration constant is 13.18. The discovery of the above jet characteristics can predict the free jet shape under any PR in the range of 10 to140. These results can also promote the application of DI hydrogen engines.