Nonideal effects behind reflected shock waves in a high-pressure shock tube

Shock tubes often experience temperature and pressure nonuniformities behin d the reflected shock wave that cannot be neglected in chemical kinetics ex periments. Because of increased viscous effects, smaller tube diameters, an d nonideal shock formation, the reflected-shock nonidealities tend to be gr eater in higher-pressure shock tubes, Since the increase in test temperatur e (DeltaT(5)) is the most significant parameter for chemical kinetics, Expe riments were performed to characterize DeltaT(5) in the Stanford High Press ure Shock Tube using infrared emission from a known amount of CO in argon. From the measured change in vibrationally equilibrated CO emission with tim e, the corresponding dT(5)/dt (or DeltaT(5) for a known time interval) of t he mixture was inferred assuming an isentropic relationship between post-sh ock temperature and pressure changes. For a range of representative conditi ons in argon (23-530 atm, 1275-1900 K), the test temperature 2 cm from the endwall increased 3-8 It after 100 mus and 15-40 K after 500 mus, depending on the initial conditions. Separate pressure measurements using a shielded piezoelectric transducer confirmed the isentropic assumption. An analytica l model of the reflected-shock gas dynamics was also developed, and the cal culated DeltaT(5)'s agree well with those obtained from experiment. The ana lytical model was used to estimate the effects of temperature and pressure nonuniformities on typical chemical kinetics measurements, When the kinetic s are fast (< 300 <mu>s), the temperature increase is typically negligible, although some correction is suggested for kinetics experiments lasting lon ger than 500 mus The temperature increase. however, has a negligible impact on the measured laser absorption profiles of OH (306 nm) and CH3 (216 nm), validating the use of a constant absorption coefficient. Infrared emission experiments are more sensitive to temperature and density changes, so T-5 nonuniformities should be taken into account when interpreting ir-emission data.