Flow characteristics of under-expanded jets Instantaneous snapshots of fuel mole fraction (H2 or CHal, temperature, velocity and Mach number are presented 0.2 ms after the start of injection in Fig, 11 for various values of NPR. A semi opaque mask has been applied on these snapshots to interest in the core of the jets. Hydrogen-air mixing prior to the location of the Mach disk is evident. It is also clear that for the same NPR of 8.5, the hydrogen jet was more voluminous than the methane jet. This can be explained by the lower density and higher diffusivity of hydrogen which speed up the mixing process and increase the radial penetration rate of hydrogen in com- parison to methane. Furthermore, it is also clear that the fuel core with X 1 penetrated further in the axial direction with hydrogen than with methane. Very similar spatial variation of mole fraction was noticed between hydrogen jets with NPR 8.5 and NPR 10. For NPR 30 the hydrogen mole fraction snapshot of Fig, 11 displays clearly a wider jet in which the majority of the highlighted area had X20.7 and the core with X 1 penetrated beyond the area shown in the snapshots.