Abstract-We report on the installation and commissioning of two systems for the measurement of cross-sectional distributions of pollutant species in jet exhaust, within the engine ground test facility at INTA, Madrid. These systems use optical tomography techniques to estimate the cross-sectional distributions of CO 2 and soot immediately behind the engine. The systems are designed to accommodate the largest civil aviation engines currently in service, without obstruction of the exhaust or bypass flows and with negligible effect upon the entrained flow behavior. We describe the physical construction and installation status of each system. In the case of the CO 2 system, we examine the challenges of achieving the structural rigidity necessary for adequate suppression of pointing error within 126 laser-based transmittance measurements, each utilizing a 7 m overall path length. We describe methods developed for efficient implementation of co-planarity and 4-degree-of-freedom alignment of individual paths within this beam array. We also present laboratory performance data for three alternative optical designs that differ in their approach to the management of pointing error and turbulence-induced beam wander and spread. The FLITES soot monitoring capability is based on laser induced incandescence (LII) and uses a short-pulse fiber laser and two CCD cameras, in an autoprojection arrangement. We describe the measurement geometry currently being implemented in the test cell and discuss optical design issues, including once again the effect of the plume itself.

The FLITES project (Fiber-Laser Imaging of gas Turbine Exhaust Species) aims to provide a videorate imaging (100 fps) diagnostic tool for application to the exhaust plumes of the largest civil aero-jet engines. This remit, enabled by chemical species tomography (CST) currently targeting Carbon Dioxide (CO2), requires system design that facilitates expansion of multiple parameters. Scalability is needed in order to increase imaging speeds and spatial resolutions and extend the system towards other pertinent gases such as the oxides of Nitrogen and Sulphur and unburnt hydrocarbons. This paper presents a fully-scalable, non-invasive instrument for installation in a commercial engine testing facility, technical challenges having been tackled iteratively through bespoke optical and mechanical design, and it specifically presents the high-speed data acquisition system (DAQ) required. Measurement of gas species concentration is implemented by tunable diode laser absorption with wavelength modulation spectroscopy (TDLAS-WMS) using a custom, high-speed 10-40 MS/s/channel 14-bit DAQ. For CO2 tomography, the system uses 6 angular projections of 21 beams each. However, the presented DAQ has capacity for 192 fully-parallel 10-Hz to 3-MHz differential inputs, achieving a best-case SNR of 56.5 dB prior to filtering. Twelve Ethernet-connected digitization nodes based on field-programmable gate array (FPGA) technology with software control, are distributed around a 7-m diameter mounting "ring". Hence, the high data rates of 8.96-Gb/s per printed circuit board (PCB) and 107.52 Gb/s for the whole system, can be reduced using local digital lock-in amplifiers (DLIAs). We believe this DAQ system is unique in both the TDLAS and CST literature.

We report here the first implementation of chemically specific imaging in the exhaust plume of a gas turbine typical of those used for propulsion in commercial aircraft. The method used is chemical species tomography (CST) and the target species is C O 2 , absorbing in the near-infrared at 1999.4 nm. A total of 126 beams propagate transverse to the plume axis, along 7 m paths in a coplanar geometry, to probe a central region of diameter ≈ 1.5 m . The C O 2 absorption spectrum is measured using tunable diode laser spectroscopy with wavelength modulation, using the second harmonic to first harmonic (2f/1f) ratio method. The engine is operated over the full range of thrust, while data are recorded in a quasi-simultaneous mode at frame rates of 1.25 and 0.3125 Hz. Various data inversion methodologies are considered and presented for image reconstruction. At all thrust levels a persistent ring structure of high C O 2 concentration is observed in the central region of the measurement plane, with a raised region in the middle of the plume assumed to be due to the engine’s boat tail. With its potential to target various exhaust species, the CST method outlined here offers a new approach to turbine combustion research, turbine engine development, and aviation fuel research and development.