Cryogenic shear layers: experiments and phenomenological modeling of the initial growth rate under subcritical and supercritical conditions

A jet of a cryogenic fluid, typically liquid N2, is injected into a chamber whose ambient pressure is varied to values exceeding the critical pressure of the injectant. The structure of the jet and the shear layer between the jet and the ambient have been examined. Results from visualization, jet initial growth rate, fractal analysis, and Raman scattering measurements indicate that the behavior of the injected fluid changes from liquid spray-like to gaseous jet-like behavior as pressure increased. This is 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 indicating the growth rate of the jet is measured at the jet exit. These values are then compared with those measured from a large number of other mixing layer flows, including atomized liquid sprays, turbulent incompressible gaseous jets, supersonic jets, and incompressible but variable density jets covering 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 injected fluid, the jet initial growth rate measurements agrees well with the theory and measurements of incompressible, 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 mere qualitative physical appearance. The initial growth rate using Raman scattering is also in reasonably good agreement with our measurements using shadowgraphy if twice the FWHM of the normalized intensity radial profiles are used. Finally, an equation based on a proposed physical mechanism combined with the characteristic gasification time (τg) and interfacial bulge formation/separation time (τb) is proposed, θ=0.27[τb/(τb+τg)+(ρg/ρl)0.5], that shows good agreement with the measured initial growth rate data. It is found that the transition point from sub- (liquid-jet like) to supercritical (gas-jet like) behavior occurs when the time scale ratio (τb/(τb+τg)) is approximately equal to 0.5.