Continuous-wave laser strategy of probing NO at 220 nm for flow measurements

Abstract

Nitric oxide is a species intrinsic to hypersonic flight, and the capability of nonintrusively monitoring gasdynamic properties of NO could prove useful in experimental studies of wings, inlets or combustors. Accessing both the (0,0) and (1,0) band of NO A ← X with a cw ring dye laser is now possible due to recent wavelength extensions to 210 nm. Furthermore, modifying a ring dye laser for rapid-tuning1 allows scanning the laser frequency 80 GHz repetitively at a rate of 4 kHz. This paper investigates the use of rapid-tuning in single-point fluorescence detection NO A ← X transitions for simultaneous measurements of velocity, temperature, and pressure within an underexpanded free jet. The 80-GHz scan width encompasses two NO rotational lines that, by intentional selection, have differing temperature dependences. Because of the Doppler shift, the laboratory-frame resonance frequency depends on the velocity component of the flow along the beam direction. Imparting a relative Doppler shift between acquired spectral features was accomplished by dividing the UV output into two separate paths that pass through the flow at different angles. Fluorescence signals from these two beams are collected from nearly coincident 1-mm3 probe volumes along the jet centerline. Modeling the acquired spectral lines with Voigt profiles yields the temperature, pressure, and Doppler shift from which the velocity is directly computed.