Nonideal Effects Behind Reflected Shock Waves in a High-Pressure Shock Tube

Abstract

044REPORT Public reporting burden for this collection ot information is estimated to average 1 hour per response, including the time for review ng instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection information. Send comment regarding this burden estimates or any aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for information Operations and reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302, and Shock tubes often experience temperature and pressure nonuniformities behind the reflected shock wave that cannot be neglected in chemical kinetics experiments. Because of increased viscous effects, smaller tube diameters, and nonideal shock formation, the reflected-shock nonidealities tend to be greater in higher-pressure shock tubes. Since the increase in test temperature (AT 5 ) is the most significant parameter for chemical kinetics, experiments were performed in the Stanford High Pressure Shock Tube using infrared emission from a known amount of CO in Argon. From the measured change in vibrationally equilibrated CO emission with time, the corresponding dT 5 /dt (or AT5 for a known time interval) of the mixture was inferred assuming an isentropic relationship between post-shock temperature and pressure changes. For a range of representative conditions in Argon (20-530 Atm, 1240-1900, the test temperature 2 cm from the endwall increased 3-8 K after 100 )J,s and 15-40 K after 500 u,s, depending on the initial conditions. Separate pressure measurements using a shielded piezoelectric transducer confirmed the isentropic assumption. An analytical model of the reflected-shock gas dynamics was also developed. The measured incident-shock axial velocity profile and a model of the boundary layer growth provide the upstream boundary conditions needed to define the properties behind the moving reflected shock. The calculated AT 5 's agree well with those obtained from experiment. The analytical model was used to estimate the effects of temperature and pressure nonuniformities on typical chemical kinetics measurements. When the kinetics are fast and occur in less than 300 |is, the temperature increase is typically negligible, although some correction is suggested for kinetics experiments lasting longer than 500 (xs. The temperature increase, however, has a negligible impact on the measured absorption profiles of OH and CH 3 when using laser absorption diagnostics at 306 and 216 nm, respectively, validating the use of a constant absorption coefficient. Infrared emission experiments are more sensitive to temperature and pressure changes, so T 5 nonuniformities should be taken into account when interpreting IR-emission data. NONIDEAL EFFECTS BEHIND REFLECTED SHOCK WAVES IN A HIGH-PRESSURE SHOCK TUBEEric L. Petersen f and Ronald K. Hanson AbstractShock tubes often experience temperature and pressure nonuniformities behind the reflecte...