Development of Tunable Diode Laser Absorption Tomography and Application to Scramjet Engines

Abstract

Scramjets are an appealing alternative to traditional rocket engines for many applications such as access to space, long-range strike missile capabilities, and civilian transport. Their advantages over rockets and traditional jet engines include a stationary flow path with no moving parts and higher payload capacity due to their airbreathing operation. A major hindrance to the maturation of scramjet technology is the high-cost and high-risk associated with flight testing. Computational fluid dynamics (CFD) simulations are a promising alternative to flight testing, but much of the complex flow physics present in scramjet engines are not well understood, making the modeling of these high-enthalpy, supersonic, combusting, and three-dimensional flows extremely challenging. Ground testing of scramjet model flow paths allows for experimental measurements of key variables, such as temperature and species number density, in controlled experimental environments. These measurements are guided by preliminary CFD results while also serving to validate the advanced state-of-the-art computational models. This symbiotic relationship between ground test experiments and computational models brings us closer to the realization of operational scramjet technology.Harsh flowfields, such as those found within a scramjet, can present great difficulties for conventional in-stream diagnostics. The high speed, high enthalpy, combusting flowfields make probing the flow very challenging. Optical diagnostics, such as tunable diode laser (TDL) techniques, have an advantage in these harsh flowfields over their mechanical counterparts due to their non-intrusive nature. No hardware comes in contact with the flow itself, and therefore the integrity of the diagnostic is preserved and the flow is undisturbed. Traditional implementations of TDL, such as Tunable Diode Laser Absorption Spectroscopy (TDLAS), are indeed non-intrusive, but are limited by their path-integrated line-of-sight (LOS) nature. The harsh flowfields of a scramjet are highly three-dimensional and thus an optical diagnostic technique capable of producing spatially resolved measurements is required. Tunable Diode Laser Absorption Tomography (TDLAT) is a non-intrusive optical technique which combines TDLAS and computed tomography (CT) to produce a 2-D spatially resolved measurement of two key combustion diagnostic properties: temperature and species number density. Several hundred LOS TDLAS measurements are collected for each experiment and are subsequently reconstructed utilizing a tomographic post-processing algorithm. The TDLAT measurement results in highly resolved 2-D temperature and species number density distributions of the flowfield at the probed measurement plane.The work presented herein describes the development of the TDLAT technique, building upon earlier proof-of-concept demonstrations. Application of the TDLAT diagnostic to two scramjet-relevant ground test facilities is presented and the insight gained from the resulting 2-D distributions is discussed. Of gr...