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.

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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.

Public reporting burden lor this eolation of information is estimated to average 1 hour per respo-se n-udrg ~e . ~e ■-• revewng instructions, searching existing datasources gathering and maintaining the data needed, and completing and reviewing this collection of information Send ccmme-fs recarcna tr s ouroen estimate or any other aspect of this collection of information including suggestions for reducing this burden to Department of Defense. Washington Headquarters Se vices Directorate tor Info-mation 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! 1) value|"at a supercritical chamber temperature (i.e.^ relative temperature T r = TAr injectam critical >1) are photographically observed and documented near the injector hole exit region using a CCD camera illuminated by a shortduration back-lit strobe light. At low subcritical chamber pressures, the jets exhibit surface irregularities that amplify downstream, lookjggintact, shiny, but wavy (sinuous) on the surface mimS evenwglly break up into irregularly-shaped small entities./^urther increase of chamber pressure causes formation of many small droplets on the surface of the jet ejecting away only within a narrow region below the critical pressure of the injected fluid similar to a second wind-induced liquid jet breakup regime. Raising the chamber pressure, transition into a full atomization regime is inhibited by rejghing near, but slightly lower than, ^"critical pressure «fthe injectant where both surface tension and heat of vaporization are sufficiently reduced. The jet appearance changes abruptly at this point and remains the same «ter resemb}£a?turbulent gas jet injection. In this region, and within the imaging resolution limitation, no droplets are seen forming and/or departing from the jet. The jet initial total divergence angle, indicating initial growth or spreading rate, is extracted from a large set of images and plotted along with ^"available data on liquid fuel injection in £ diesel engine environment, turbulent incompressible, supersonic, and variable-density jets and for incompressible but variable-density turbulent mixing layers, thus quantitatively strengthening the gas-jet like appearance. Considering this agreement, the inhibition of Tfc*-transition to atomization regime and its confirmation through the examined atomization criteria, and the visual lack of any drop; the relevancy of current injection models and some drop vaporization/combustion results under conditions where...

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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Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. REPORT DATE (DD-MM-YYYY) SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)Air Force Research Laboratory (AFMC) AFRL/PRS SPONSOR/MONITOR'S Pollux Drive NUMBER(S)Edwards AFB CA 93524-7048 AFRL-PR-ED-TP-2007-164 14. ABSTRACT An experimental study on the effects of an externally-imposed transverse acoustic field in sub-, near-, and supercritical N2 coaxial jets is presented. Such fields and their interaction with the jets (i.e., breakup, mixing, etc.) is believed to play a critical role during combustion instabilities in liquid rocket engines. The shear coaxial injector used here is similar to those used in cryogenic liquid rockets. By using N2 as the working fluid, the chemistry effects on combsution instability are separated from the effects of a transverse acoustic field on coaxial jets. Furthermore, through this choice, ambiguities associated with compostion dependence on mixtures critical properties are eliminated. The acoustic oscillations are generated by a piezo-siren and have a frequency of ~3kHz. The pressures in the chamber range from 215-716 psia to span sub-, near-, and supercritical conditions. The outer to inner jet velocity ratio varies from ~1.2 to 23 and the momentum flux ratio varies from ~0.2 to 23. These ratios are mainly varied by changing the temperature and flow rates of the outer jet. At least 2000 backlit images were taken at 41kHz for each run. The main metric investigated is the length of the dark, or inner jet, core length. Both the axial length of the jet and the total, or curved, length are studied. A functional relation of the form A/MR n describes the behavior of the axial length with the exponent being 0.2 (A:20-25) for subcritical conditions and 0.5 (A:5-12) for near and supercritical conditions. These results agree with historical data. The standard deviation of the axial length, which due to the large number of data points is within 0.03% of the RMS of the jet lengths fluctuations, also decreases with velocity ratio, for sub-, near-, and supercritical conditions. For momentum flux ratios ~O(1) the axial and total length differences between ac...

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number.