Experimental Study on Mach Disk Characteristics of an Extremely Under-Expanded Hydrogen Jet with High Pressure Direct Injection

Abstract

The high-pressure direct injection hydrogen internal combustion engines represent a significant technical path towards achieving zero-carbon power systems. However, current research lacks systematic experimental data and empirical models for the near-field Mach disk structure of high-pressure hydrogen jets. This study experimentally investigates the near-field Mach disk structure of the high-pressure hydrogen jets based on high-speed schlieren/shadow imaging technology. Firstly, the transient evolution laws of the Mach disk height and diameter are revealed. It is found that the Mach disk first appears approximately at 0.075 ms after injection and enters a quasi-steady state at around 0.25 ms. Secondly, based on the quasi-steady state data, empirical relationships between the dimensionless Mach disk height and diameter and the pressure ratio are established. Using the least squares fitting, empirical coefficients for the dimensionless Mach disk height and diameter are obtained as 0.4384 and 0.1442, respectively, with determination coefficients of 0.9977 and 0.989. By comparing with the empirical coefficients of classical air jets, the reasons for the lower empirical coefficients are attributed to the high-pressure real gas effect, the squeezing suppression by the high ambient pressure, and the flow loss at the outlet caused by the guide slot. The research results can provide a direct theoretical basis for CFD simulation of high-pressure direct injection hydrogen internal combustion engines.