Density scaling and calibration of FLEET temperature measurements

Bulk gas temperature is a critical parameter in characterizing the dynamics, species concentration, quantum state and chemical reactivity of high speed flow. A myriad of techniques have been developed recently to non-intrusively and remotely measure temperature in different conditions and environments. Often these experiments require external flow seeding, are only applicable in a relatively limited range of temperature and rely on a complicated apparatus for thermometry alone. Femtosecond Laser Electronic Excitation Tagging (FLEET) avoids many of the common issues encountered with laser-based techniques and has been shown to be an effective tool for simultaneous measurements of flow velocity and temperature^1,2. A spectrum of the second positive and first negative emission of molecular nitrogen is captured from a region of interest and the intensity of rotational features can be correlated to the translational gas temperature within the natural lifetime of the fluorescence. Either the experimental spectrum is fit directly to a simulation or certain portions of the spectrum are analyzed to build a database of area ratios from which temperature can be extracted. It is observed that the intensity, shape and appearance of certain prominent rovibrational features change with pressure. The present study investigates the nature of FLEET fluorescence at various temperatures and pressures and establishes a direct scaling relationship to properly account for changes in gas density to the area ratio methodology.