Examination of NO Tag Formation for Unseeded Molecular Tagging Velocimetry

摘要

In order to fully understand a fluid flow (and the resulting implications, e.g. lift, drag, vortex shedding, turbulence, etc) accurate measurements under experimental conditions are absolutely necessary. A common way to measure these flow parameters is to use laser diagnostics, such as Particle Image Velocimetry (PIV) or Molecular Tagging Velocimetry (MTV) which utilize Laser Induced Fluorescence (LIF) in order to calculate a velocity vectors in the flow. The investigated method here aims to simulate and experimentally validate the use of NO as a molecular tag formed from quiescent air and highly focused 355 nm light from a tripled Nd:YAG laser. Simulations were performed with the sensitivity analysis suite of CHEMKIN (SENKIN). The findings are that there is a large discrepancy between simulated concentrations of NO and experimentally observed NO. This discrepancy lies in the assumptions of the simulation, the amount of N_2 ionized and therefore NO produced. It is theorized that despite other methods being able to achieve 1% ionization of N_2, which the 355 nm method is not efficient enough to meet this target, and the actual ionization is well below this value. Only under experimental conditions of 5 atm of pressure and higher was NO detectable in trace amounts. The detected level was only at a SNR of 1.25, which is too low for MTV work. Through equipment/procedural optimization, it is believed that the SNR may be able to be increased to 4, a critical threshold. However this may only be achievable at high pressures which further limits the applicability of a 355 nm method of NO MTV. Further experiments are underway in order to explore this proposed method further.