Characterization of hydroxyl tagging velocimetry for low-speed flows

Abstract

Hydroxyl tagging velocimetry (HTV) is characterized for the first time at extended pressures (1 and 3 atm) and temperatures (295 K to 673 K) in an attempt to improve measurement precision for low speed flows. While previous investigations have focused on ambient and flame temperatures (1400 K), the present study investigates between these two extreme conditions, wherein both the local chemistry and thermodynamic state of the gas may hinder or aid the functionality of the technique. Effects of temperature and pressure on the hydroxyl (OH) excitation spectrum are assessed and compared to simulations to determine the optimal laser frequency, and OH species lifetime and tracer line diffusion are examined to determine the relative efficiency of the photo-dissociation process and the quality of the resulting signal. A two-fold increase in the photo-dissociation efficiency is observed at elevated temperatures. Tag line spread was found to be dominated by shear rather than molecular diffusion. Velocity measurement precision was characterized for time delays ranging from 5 μs to 3.2 ms and was found to be inversely proportional to the time delay selected, supporting the need for the extended tracer lifetimes observed at higher temperatures when used for low-velocity applications. Velocity profiles measured in heated jets of nitrogen and air indicate measurement uncertainties as low as 0.1 m (at confidence level), while comparison with particle image velocimetry (PIV) measurements showed peak deviations in the observed velocity profiles to be less than . The results suggest the high utility of HTV at making measurements in low-velocity flows at moderate temperatures.