Use of degenerate four-wave mixing for one-dimensional thermometry in high-enthalpy test facilities

The use of non-intrusive optical techniques to make measurements of temperature and species in chemically reacting environments has received much attention over the last three decades. More recently, non-linear techniques have been employed to make measurements in demanding environments, characterized by one or more of the following: limited optical access, strong attenuation of the beams, or multi-phase flow. This project investigated the use of one such technique, Degenerate Four-Wave Mixing, to make temperature measurements in a strongly attenuating environment. Previously, it was shown that when employing this technique with laser intensities below the line center saturation intensity, temperatures derived from a Boltzmann population distribution analysis yielded temperatures an order of magnitude too high. In this follow-on project, a Nd: YAG pumped dye laser was used to provide laser intensities that were much above the saturation intensity. A pair of OH transitions were identified which offered very good temperature sensitivity over the range of interest and were both spectrally resolvable and close enough together that they could be reached without tuning the BBO doubling crystal. By measuring the OH population in each of these rotational energy states with sufficient intensity, five short line segments were imaged in a near adiabatic, lifted diffusion flamelet burner to yield temperature. These measured temperatures were in very good agreement with those calculated using a one dimensional chemical kinetics code. Although the region imaged was small, the results from this project show that with sufficiently high laser intensity, longer one-dimensional temperature profiles can be measured with sufficient accuracy. This technique can there for provide valuable insights into the flowfield and turbulence-chemistry interaction in high enthalpy test facilities.