EXPERIMENTAL AND SYNTHETIC LASER ABSORPTION SPECTROSCOPY MEASUREMENTS IN HIGH-TEMPERATURE REACTING FLOWS

This work describes the development and implementation of multiple LAS diagnostics to acquire measurements in either post-detonation fireballs or ablative hypersonic flows. The first diagnostic presented uses wavelength-modulation spectroscopy (WMS) to measure temperature and water mole fraction at 500 kHz in post-detonation fireballs. This diagnostic was utilized to acquire measurements in large scale (25 g) hemispherical charges of pentaerythritol tetranitrate (PETN) or N5 which consists primarily of high melting explosive (HMX). This required the use of a hardened optical probe in order to provide localized measurements within the blast chamber. The second diagnostic uses scanned-wavelength direct-absorption to measure temperature, pressure, and CO at 1 MHz while also obtaining CO₂ measurements at 500 kHz in post-detonation fireballs. Using this diagnostic, measurements were acquired in post-detonation fireballs produced by a small scale (<1 g) hemispherical PETN charges. Lastly, the development and implementation of a laser absorption imaging (LAI) diagnostic for CO absorption and emission measurements in ablating hypersonic flows is presented. The utility of all three diagnostics was significantly enhanced by comparing experimental measurements to synthetic (i.e., CFD-based) measurements. This was particularly important for the post-detonation fireballs measurements due to the presence of pronounced line-of-sight (LOS) non-uniformities.

Experimental and synthetic measurements were used to evaluate the accuracy of CFD models for the post-detonation fireballs and ablating hypersonic flows studied here. In general, the CFD predictions of all measured quantities were reasonably accurate, however, the LAS measurements reveal several shortcomings in CFD models specifically for post-detonations fireballs. (1) The model does not take into account energy loss specifically the effect of non-idealities including venting of detonation products and the deformation of solid boundaries to name a few. (2) Underpredicting mixing for both H₂O and carbon-containing species (i.e., CO and CO₂). (3) Need to account for experimental details such as the exploding-bridgewire detonator (EBW) used to initiate the charge of PETN and air gap between this EBW and PETN charge. (4) Overprediction of CO₂ in post-detonation combustion gases which could indicate a need to model soot formation. Lastly, these synthetic measurements contain inherent uncertainty which this work will call the confidence interval of synthetic measurements. The effects of turbulence, broadening model, beamsteering, beam size, and spectroscopic model uncertainty on synthetic measurements are all quantified here. It was found that the turbulent 3D structure of these fireballs contributes the most to the confidence interval of synthetic measurements.