Fractal geometry and growth rate changes of cryogenic jets near the critical point

Abstract

A high pressure chamber with a variety of ambient fluids is used into which pure I$. , He, and O2 fluids are injected. The effects of chamber pressure (density) ranging from the thermodynamic subcritical to supercritical values at a supercritical chamber temperature are observed by acquisition of shadow images from the injector exit region using a CCD camera illuminated by short-duration light pulses. At sufficiently low subcritical chamber pressures, the jet interface disturbances amplify and eventually break up downstream into irregularly-shaped small entities. Increasing the chamber pressure causes the formation of many small ligaments and droplets on the surface of the jet only within a narrow regime below the thermodynamic critical pressure of the injected fluid, resembling a second wind-induced liquid jet breakup. At even higher chamber pressures, near but below the critical pressure of the injectant, the expected transition into a full atomization regime to produce a liquid spray is inhibited due to sufficient reduction of both the surface tension and the heat of vaporization. The jet appearance changes abruptly at this pressure and resembles a turbulent gas jet injection for all higher chamber pressures. The jet initial growth rate is plotted together with available data on liquid fuel injection in diesel engine environments, and turbulent incompressible, supersonic, and variable-density jets and mixing layers. The resulting plot is unique on its own right. At nearand super-critical pressures, the present measurements agree well with theoretical growth rate equations proposed by Brown [l]/Papamoschou and Roshko (21 and Dimotakis [3] for incompressible but variable-density turbulent mixing layers. This constitutes the first quantitative evidence in support of qualitative observations that the jet appears to evolve into a gas-like behavior. The geometry of the jet interface has also been examined for the first time by fiactal analysis. The results clearly indicate a transition Ii-om a Euclidean to a fractal interface, with a tiactal dimension close to values measured for gaseous turbulent jets. This provides additional quantitative evidence for the hypothesis that the jet evolves into a gas-like behavior. Finally, an “intuitive/smart” equation is proposed that agrees well with the experimental growth rate data, based on a proposed physical mechanism and characteristic gasification times and interfacial bulge formation/separation times.