Toluene Laser-Induced Fluorescence (LIF) Imaging of Supersonic Flow within a Diverging Duct

Abstract

Non-intrusive optical diagnostic tools are becoming increasingly popular in various fields. The potential of LIF imaging as a tool to measure various flow field parameters, which can aid in observing mixing and reaction chemistry, has been realised by several researchers particularly in the combustion community [1-3]. The strong dependence of toluene-LIF on parameters such as temperature and oxygen partial pressure, and the available photophysical studies on toluene have made this tracer attractive for studying different gaseous flows. Examples include the temperature and oxygen measurements shown by Koban et. al. [4, 5]. Such work has mainly focused on engine related conditions involving high temperatures. Yoo et. al. use toluene-LIF imaging for flow visualization and quantitative thermometry behind the incident and reflected shocks within a shock tube [6]. The lowest temperature measured in their study is 296 K. However, in this study toluene-LIF imaging is applied at low temperatures that are applicable to supersonic flows. The geometry of the duct utilised in this work is designed as a model system for the synthesis of nanoparticles from the gas phase, using gasdynamic shocks within a supersonic reactor with diverging walls. An injector mixes a gaseous metal-organic precursor into the subsonic region of the flow and the shock system downstream of the reactor provides high temperatures in which the precursor decomposition and particle synthesis is initiated. Further details of this reactor may be found in [8]. The aim is to utilize toluene-LIF imaging in order to study the mixture formation and flow conditions for various injectors and to provide validation data for computational fluid dynamic (CFD) simulations of the reactor. In this paper the first set of data comprising of instantaneous LIF images obtained for four different injector flows that are seeded with a toluene-N2 gasmixture are presented, revealing qualitative differences in flow structure. The flow within the duct, with one injector (essentially a symmetric aerofoil body) in place but no flow through it, is visualized using schlieren. Initial CFD simulations comprise only the duct flow and have been performed using the ANSYS CFX commercial code. These are compared to the expermiental observations.