Thomas Benoy

Progress towards non-intrusive optical measurement of gas turbine exhaust species distributions

We report on the development of three systems intended to provide fast, non-intrusive measurement of cross-sectional distributions of pollutant species within gas turbine exhaust flows, during ground-based testing. This research is motivated by the need for measurement systems to support the introduction of technologies for reducing the environmental impact of civil aviation. Tomographic techniques will allow estimation of the distributions of CO2, unburnt hydrocarbons (UHC), and soot, without obstruction of the exhaust, bypass or entrained flows, from measurements made in a plane immediately aft of the engine.

Recovery of Absorption Line Shapes With Correction for the Wavelength Modulation Characteristics of DFB Lasers

Tunable diode laser spectroscopy combined with wavelength modulation spectroscopy (WMS) is an important technique for noninvasive measurements of gas parameters such as pressure, concentration, and temperature in high-noise harsh environments. A variety of laser types are used for these applications, and the modulation characteristics can have significant effects on line shape recovery. Here, we identify important characteristics of distributed feedback (DFB) lasers that need to be taken into account in the context of WMS and illustrate the effects with a 2-μm wavelength multiquantum-well DFB laser used for CO2 detection. The modulation response of the laser is measured, and we demonstrate how the phasor decomposition method (PDM) may be used to obtain accurate line shapes from the first harmonic WMS signals by correcting for phase variation across the lasers low-frequency current sweep. We also demonstrate how the PDM approach can be improved by removing the need to preset the orientation of the lock-in axis to isolate the residual amplitude modulation component, making it more suitable for field applications.

Constrained models for optical absorption tomography

We consider the inverse problem of concentration imaging in optical absorption tomography with limited data sets. The measurement setup involves simultaneous acquisition of near-infrared wavelength-modulated spectroscopic measurements from a small number of pencil beams equally distributed among six projection angles surrounding the plume. We develop an approach for image reconstruction that involves constraining the value of the image to the conventional concentration bounds and a projection into low-dimensional subspaces to reduce the degrees of freedom in the inverse problem. Effectively, by reparameterizing the forward model, we impose, simultaneously, spatial smoothness and a choice among three types of inequality constraints, namely, positivity, boundedness, and logarithmic boundedness in a simple way that yields an unconstrained optimization problem in a new set of surrogate parameters. Testing this numerical scheme with simulated and experimental phantom data indicates that the combination of affine inequality constraints and subspace projection leads to images that are qualitatively and quantitatively superior to unconstrained regularized reconstructions. This improvement is more profound in targeting concentration profiles of small spatial variation. We present images and convergence graphs from solving these inverse problems using Gauss-Newtons algorithm to demonstrate the performance and convergence of our method.

Measurement of CO 2 Concentration and Temperature in an Aero Engine Exhaust Plume Using Wavelength Modulation Spectroscopy

In this paper, the 2f/1f tunable diode laser wavelength modulation spectroscopy is used for the simultaneous measurement of concentration and temperature of CO2 in the exhaust plume of an aero engine. The suitability of the R48 spectral feature of CO2 at 1997.2 nm is discussed for this project and for further application of CO2 tomographic imaging on large-scale aero-engines. To ensure accurate recovery of gas parameters at the high exhaust temperatures, a full spectral characterization of the absorption feature is presented using a direct spectroscopy. The 2f/1f method is validated in the laboratory for controlled gas mixtures and temperatures to recover concentration and temperature. Good agreement with the actual temperature and concentration values is demonstrated. Finally, single path measurements are presented for an aero engine exhaust showing good correlation with the measured engine conditions.

Fibre laser imaging of gas turbine exhaust species – a review of CO2 aero engine imaging

Here we present the latest developments in tomographic imaging of gas turbine exhaust species, particularly CO2. The optical source, detection methodology and mechanical optical distribution design are discussed in detail.

Fibre Laser Imaging of Gas Turbine Exhaust Species - A Review of CO 2 Aero Engine Imaging

Here we present the latest developments in tomographic imaging of gas turbine exhaust species, particularly CO2. The optical source, detection methodology and mechanical optical distribution design are discussed in detail.