Rayleigh Scattering Density Measurements, Cluster Theory, and Nucleation Calculations at Mach 10

Abstract

In an exploratory investigation, quantitative unclustered laser Rayleigh scattering measurements of density were performed in the air in the NASA Langley Research Center's 31 in. Mach 10 wind tunnel. A review of 20 previous years of data in supersonic and Mach 6 hypersonic flows is presented where clustered signals typically overwhelmed molecular signals. A review of nucleation theory and accompanying nucleation calculations are also provided to interpret the current observed lack of clustering. Data were acquired at a fixed stagnation temperature near 990 K at five stagnation pressures spanning 2.41 to 10.0 MPa (350 to 1454 psi) using a pulsed argon fluoride excimer laser and double-intensified charge-coupled device camera. Data averaged over 371 images and 210 pixels along a 36.7 mm line measured freestream densities that agree with computed isentropic-expansion densities to less than 2% and less than 6% at the highest and lowest densities, respectively. Cluster-free Mach 10 results are compared with previous clustered Mach 6 and condensation-free Mach 14 results. Evidence is presented indicating vibrationally excited oxygen and nitrogen molecules are absorbed as the clusters form, release their excess energy, and inhibit or possibly reverse the clustering process. Implications for delaying clustering and condensation onset in hypersonic and hypervelocity facilities are discussed.

Overview: Homogeneous nucleation from the vapor phase—The experimental science

“…Detection of condensation has used light scattering, pitot pressure, and total temperature measurements. [180][181][182] Although these techniques have not routinely reported nucleation rates directly, the flows have been modeled extensively to determine the conditions that could lead to condensation.…”

Krypton tagging velocimetry of an underexpanded jet

“…We focus our efforts on tagging velocimetry with photosynthesized metastable krypton as a chemically inert tracer, as proposed by Mills et al [45] and Balla and Everhart [46]. Mills et al proposed that krypton tagging velocimetry could be performed as follows:…”

Krypton Tagging Velocimetry for Use in High-Speed Ground-Test Facilities

“…59 Mills et al 58 proposed that tagging velocimetry could be performed as follows: Given that the krypton may be seeded into the flow in dilute concentrations, the thermo-physical properties of the flow are nominally unchanged. Estimates of the effect of dilute krypton concentrations on the transport properties can be done using Cantera 61 with the appropriate thermodynamic data.…”