N ₂O molecular tagging velocimetry

A new seeded velocity measurement technique, N ₂O molecular tagging velocimetry (MTV), is developed to measure velocity in wind tunnels by photochemically creating an NO tag line. Nitrous oxide "laughing gas" is seeded into the air flow. A 193 nm ArF excimer laser dissociates the N ₂O to O( ¹D) that subsequently reacts with N ₂O to form NO. O ₂ fluorescence induced by the ArF laser "writes" the original position of the NO line. After a time delay, the shifted NO line is "read" by a 226-nm laser sheet and the velocity is determined by time-of-flight. At standard atmospheric conditions with 4% N ₂O in air, ∼1000 ppm of NO is photochemically created in an air jet based on experiment and simulation. Chemical kinetic simulations predict 800-1200 ppm of NO for 190-750 K at 1 atm and 850-1000 ppm of NO for 0.25-1 atm at 190 K. Decreasing the gas pressure (or increasing the temperature) increases the NO ppm level. The presence of humid air has no significant effect on NO formation. The very short NO formation time (<10 ns) makes the N ₂O MTV method amenable to low- and highspeed air flow measurements. The N ₂O MTV technique is demonstrated in air jet to measure its velocity profile. The N ₂O MTV method should work in other gas flows as well (e.g., helium) since the NO tag line is created by chemical reaction of N ₂O with O( ¹D) from N ₂O photodissociation and thus does not depend on the bulk gas composition.