A Model for the Prediction of I2 Fluorescence in the Presence of Pulsed Laser Radiation utilizing Computational Fluid Dynamic Simulation Datasets

Abstract

LIF as a flow imaging technique originates in the work of Daily Imaging of fluid dynamic flowfields using non-intrusive diagnostics is a matter of practical interest to both experimentalists and theoreticians. All flow diagnostics such as Pitot tubes, hot-wire probes, pressure taps, particle image velocimetry, etc., offer the ability to obtain valuable information about the flow characteristics but perturb the local flow state to some degree. Evaluations must be made comparing the diagnostic need, the type of information provided, and the cost of the diagnostic among other considerations to determine the best choice. Non-intrusive diagnostics, particularly those using coherent radiation to generate a secondary radiative response from the particles in the gas, are attractive because of the inherent ability provide local measurements with minimal perturbation of the flow. In the case of chemical oxygen-iodine laser (COIL) flowfields, laser induced fluorescence (LIF) techniques are attractive for a variety of reasons, the non-intrusive nature of LIF, the ability to leverage existing COIL development infrastructure, the ability to get detailed localized information about flow structure, and the natural combination of LIF with the presence of I2 in COIL flowfields, termed laser induced iodine fluorescence (LIIF). A drawback to the use of LIIF in COIL flowfields is the compressible nature of the flows, with large changes in pressure, temperature, and local species densities occurring within the field that must be accounted for in making quantitative interpretations of LIF generated imaging. The development of a model for the I2 fluorescence that accounts for the thermodynamic variations while describing the spatial variation of photon production is useful from this standpoint as it allows for quantification of the compressibility effects from an interpretive standpoint while at the same time providing a tool for validation and verification of computational fluid dynamic (CFD) models. 1 Wrobel, and Levy who applied the non-intrusive nature of the technique as a convenient mechanism for imaging the development of flow structure for a variety of disparate flows including mixing layers, reacting flows, and expanding jets. Original work by Rapagnani and Davis applied the technique as a flow diagnostic for the interrogation of the mixing systems in chemical lasers as a steady state flow diagnostic utilizing a cw laser source exciting the X B transitions of I2, although the issues associated with thermodynamic and quenching rate constant variation within the flow due to compressibility were not addressed. McDaniel addressed the issue of use of LIIF as a quantitative, steady state diagnostic for compressible flows. His work found particular application in the mixing systems of high speed engines where the ability measure local quantities over small volumes was particularly useful for diagnosing mixing rates in these systems and creating multidimensional datasets for validation and verification of CFD models. The issue of LIF intensity variation resulting from compressibility effects was addressed by Fletcher and Hartfield, again for cw laser sources and steady state diagnosis. Given the analysis performed by McDaniel in developing the LIF for compressible flow application, the use of this model on CFD datasets was an attractive extension of the theoretical development. Hartfield performed such work, applying the model as a computational fluid imaging technique to steady state, 3-D CFD results and comparing directly to LIF imaging of high speed mixing system experiments. As mentioned earlier, LIF is an obvious candidate for