N2O Molecular Tagging
A new seeded velocity technique, Nitrous Oxide (N2O) Molecular Tagging Velocimetry (MTV), has been developed to measure velocity in air wind tunnels by photochemically creating a Nitric Oxide (NO) tag line. Although the Hydroxyl Tagging Velocimetry (HTV) method works well at high temperatures, at room temperature the OH lifetime is too short limiting low temperature application to moderate and high-speed flows. And below freezing, water vapor concentration is insufficient to implement the HTV method. N2O MTV does not suffer from this issue, as Nitrous has a low boiling point and can be added to low temperature, high-speed flows without condensation.
While both OH and NO tags are stable and proven MTV tags, HTV relies on common water as the source of tracer in time of flight measurements. Historically, using NO tracer required the addition of toxic gas [e.g., NO, NO2, (CH3)3CONO] to the flow, limiting the application due to safety concerns. N2O, otherwise known as “laughing gas,” is a non-toxic gas that is commonly inhaled as a mild anesthetic in dentistry and surgery or used as an aerosol agent in foods.
Using an ArF excimer laser, NO is photochemically created from N2O; the initial and displaced NO lines are imaged with laser-induced fluorescence. The very short NO formation time (< 10 ns) makes the N2O MTV method amenable to low- and high-speed air flow measurements.
Adapted from:
A. M. ElBaz and R. W. Pitz, “N2O Molecular Tagging Velocimetry in Experimental Air Flows,” Applied Physics B, 106, pp. 961-969, 2012.