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.
Pressure and temperature of the liquid rocket thrust chambers into which propellants are injected have been in an ascending trajectory to gain higher specific impulse. It is quite possible then that the thermodynamic condition into which liquid propellants are injected reaches or surpasses the critical point of one or more of the injected fluids. For example, in cryogenic hydrogen/oxygen liquid rocket engines, such as Space Shuttle Main Engine (SSME) or Vulcain (Ariane 5), the injected liquid oxygen finds itself in a supercritical condition. Very little detailed information was available on the behavior of liquid jets under such a harsh environment nearly two decades ago. The author had the opportunity to be intimately involved in the evolutionary understanding of injection processes at the Air Force Research Laboratory (AFRL), spanning sub- to supercritical conditions during this period. The information included here attempts to present a coherent summary of experimental achievements pertinent to liquid rockets, focusing only on the injection of nonreacting cryogenic liquids into a high-pressure environment surpassing the critical point of at least one of the propellants. Moreover, some implications of the results acquired under such an environment are offered in the context of the liquid rocket combustion instability problem.
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AbstractThe combustion chamber temperature and pressure in many liquid rocket, gas {urbin|, and diesel engines are quite high and can reach above the critical point *¥ the injected fuels and/or oxidizers. A high pressure chamber is used to investigate and understand the nature of the interaction between the injected fluid and the environment under such conditions. Pure N 2 , He, and 0 2 fluids are injected. Several chamber media are selected including, N 2 , He, and mixtures of CO+N 2 . The effects of chamber pressure ranging from a subcritical (i.e.*elative pressure, P r = P/Pinjeoantcridca!
An experimental investigation was conducted on a coaxial jet, similar to those used in cryogenic liquid rockets, under sub-, near-, and supercritical pressures, with the intent of gaining a better understanding of an aspect of combustion instability pertaining to interactions of an externally imposed acoustic field with the jet. Past research on this subject has shown both the relevance and importance of geometrical changes in an injector's exit-area and its nearby physical and fluid mechanical processes. Special attention is paid in collecting spatially resolved time averaged temperatures and documenting the aforementioned interactions at the exit of this injector. Short-duration and high-speed framing digital images provided information on the behavior of this jet under various conditions. Mean and root mean square values of the "dark-core" length fluctuations were measured from the acquired images via a computer-automated method, and results are discussed. There appears to be a good correlation between this length and the outer-to-inner-jet momentum ratio, but the form of this dependence was found to be different at subcritical pressures than the rest of the conditions. The root mean square values of the dark-core length fluctuations suggested a possible explanation for the observed improvement in instability limit at increasingly higher outer-toinner-jet velocity ratios. Nomenclature= diameter with subscripts L = length of dark core, length of injector tube M = momentum flux ratio of outer-jet to inner-jet m = mass flow rate n = exponent of M P = pressure R = radius with subscripts R h = hydraulic radius of outer-jet, equal to the gap width T = temperature U = velocity VR = velocity ratio of outer-jet to inner-jet = viscosity = density = surface tension Subscripts 1, 2, 3, 4 = four diameters or radii of the coaxial injector from the smallest to largest values i = subscript denoting inner-jet o = subscript denoting outer-jet
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