Laser absorption and infrared emission measurements in a high-pressure shock tube
© 1997 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. Issues related to optical diagnostics development and their application to a high-pressure shock tube environment are presented. In laser absorption experiments behind high-pressure reflected shock waves, the transmitted laser intensity can experience fluctuations as high as 60% of the incident intensity at 300 atm. A major source of these fluctuations is the turbulent wall boundary layer that interferes with the laser beam through refractive index perturbations. Recent results indicate a modified window design may minimize the problem, but further development is required in this area. Additional optical perturbations are the result of stress-induced birefringence of the windows. Birefringence alters the polarization and intensity of the transmitted laser light. Experiments designed to diagnose and remedy the polarization problems are being conducted in a high-pressure static cell. Elevated pressures also lead to increased spectral line broadening, shift, and asymmetry of molecular line absorption profiles. These details must be included when measuring and calculating the lineshapes and strengths of the absorbing molecule, since accurate absorption coefficients at elevated pressures and temperatures are needed to infer species mole fractions. Additionally, the thick turbulent boundary layer produces a nonuniform temperature profile in line-ofsight absorption measurements; the adverse effects of the colder boundary layer can be reduced, however, through proper spectral line selection. High pressures affect infrared emission diagnostics via self absorption of the emitting species and increased thermal radiation from the shock tube walls, both of which can be minimized and accounted for.
author list (cited authors)
Petersen, E. L., Bates, R. W., Davidson, D. F., & Hanson, R. K.