The University of Queensland's X3 facility is a large, free-piston driven expansion tube used for superorbital and high Mach number scramjet aerothermodynamic studies. X3's powerful test flow is initiated by firing its heavy piston into its driver gas -a mixture of argon and helium. Knowledge of the initial and transient temperature behavior of the driver gas is critical to correctly characterize the driver performance, losses and all subsequent expansion tube flow processes. However, the driver gas temperature is not currently measured during routine experimentation and there is limited evidence of its measurement in other international facilities. During recent scramjet flow development in X3, lower than expected experimentally measured shock speeds through helium indicated an effective driver temperature of approximately 2400 K, compared to a temperature of 3743 K at the time of diaphragm rupture for an ideal isentropic driver gas compression. These performance losses motivate this study, which examines initial and peak driver gas temperatures. The study shows that the initial steady-state driver gas temperature, once the filling process ceases, can be approximated by the external driver tube wall temperature. During filling, a net increase in driver gas temperature occurs due to compression heating, as well as, Joule-Thomson effect for helium. Optical emission spectroscopy was then used to resolve the peak driver gas temperature, during piston compression, for a Mach 10 scramjet operating condition. The driver gas emission spectrum exhibits a significant background radiation component, with prominent spectral lines attributed to contamination of the flow. A blackbody approximation of background radiation suggests a peak driver gas temperature of 3200±100 K. Application of the line-ratio method to two argon lines at 763.5 and 772.4 nm suggests a temperature of 3500±750 K. Comparison of these estimates to the ideal isentropic driver gas temperature at diaphragm rupture, 3743 K, suggests losses in the driver gas temperature during the compression process are not extensive for X3, and further that the lower than expected shock speeds are likely primarily due to pressure losses during driver gas expansion through the diaphragm and at the driver-to-driven tube area change. Nomenclature A = Einstein coefficient for spontaneous emission, s -1 a = sound speed, m/s c = speed of light = 3×10 8 m/s E = energy of the upper atomic state, eV g = statistical weight of upper atomic state h = Planck's constant = 4.136×10 -15 eV.s I = spectral intensity, counts K = Boltzmann's constant = 8.617×10 -5 eV/K L = spectral radiance, W/m 2 /sr/nm M = molecular weight, g/mol P i = driver gas initial pressure, Pa P p = driver gas pressure at rupture, Pa R = gas constant, J/mol/K T abs = absolute temperature, K T ex = excitation temperature, K T i = driver gas initial temperature, K T p = driver gas average peak temperature, K γ = ratio of specific heats λ = wavelength, m 1 Undergraduate, School of Mechanical and Mining Engineering, AIA...
