Visual characteristics and initial growth rates of round cryogenic jets at subcritical and supercritical pressures

Cryogenic liquids initially at a subcritical temperature were injected through a round tube into an environment at a supercritical temperature and at various pressures ranging from subcritical to supercritical values. Pure N2 and O2 were injected into environments composed of N2, He, Ar, and various mixtures of CO+N2. The results were photographically observed and documented near the exit region using a CCD camera illuminated by a short duration backlit strobe light. At low subcritical chamber pressures, the jets showed surface irregularities that amplified downstream, exhibiting intact, shiny, but wavy (sinuous) surface features that eventually broke up into irregularly shaped small entities. A further increase of chamber pressure at constant jet initial and ambient temperatures caused the formation of many small droplets to be ejected from the surface of the jet similar to what is observed in the second wind-induced jet breakup regime. As the chamber pressure was further increased, the transition to a full atomization regime was inhibited near but slightly below the critical pressure. The jet structure at this point changed and began to resemble a turbulent gas jet with no detectable droplets. The reason was attributed to the reduction of the surface tension and enthalpy of vaporization as the critical pressure of the injectant is approached. The initial divergence angle of the jet was measured at the jet exit and compared with the divergence angle of a large number of other mixing layer flows, including atomized liquid sprays, turbulent incompressible gaseous jets, supersonic jets, and incompressible but variable density jets. The divergence angle for all these cases was plotted over four orders of magnitude in the gas-to-liquid density ratio, the first time such a plot has been reported over this large a range of density ratios. At and above the critical pressure of the injectant, the jet growth rate measurements agreed quantitatively with the theory for incompressible but variable density gaseous mixing layers. This is the first time a quantitative parameter has been used to demonstrate that the similarity between the two flows extends beyond a mere qualitative physical appearance. Finally, as the pressure is reduced to progressively more subcritical values, the spreading rate approaches that measured by others for liquid sprays.