Moira Hilton

Nonintrusive optical measurements of aircraft engine exhaust emissions and comparison with standard intrusive techniques

Nonintrusive systems for the measurement on test rigs of aeroengine exhaust emissions required for engine certification (CO, NO(x), total unburned hydrocarbon, and smoke), together with CO(2) and temperature have been developed. These results have been compared with current certified intrusive measurements on an engine test. A spectroscopic database and data-analysis software has been developed to enable Fourier-transform Infrared measurement of concentrations of molecular species. CO(2), CO, and NO data showed agreement with intrusive techniques of approximately ?30%. A narrow-band spectroscopic device was used to measure CO(2) (with deviations of less than ?10% from the intrusive measurement), whereas laser-induced incandescence was used to measure particles. Future improvements to allow for the commercial use of the nonintrusive systems have been identified and the methods are applicable to any measurement of combustion emissions.

Quantitative analysis of remote gas temperatures and concentrations from their infrared emission spectra

Techniques for obtaining quantitative values of the temperatures and concentrations of remote hot gaseous effluents from their measured passive emission spectra have been examined in laboratory experiments and on field trials. These emission spectra were obtained using an adapted FTIR spectrometer with 0.25 cm-1 spectral resolution. The CO2 and H2O vapour content in the plume from a 55 m smoke stack and the temperature of these gases were obtained by comparing the measured emission spectra with those modelled using the HITRAN atmospheric transmission database. The spatial distributions of CO2, CO and unburnt CH4 in a laboratory methane flame were reconstructed tomographically using a matrix inversion technique.

Development of techniques to characterize particulates emitted from gas turbine exhausts

Particles emitted from aircraft play a role in the formation of contrails and it is essential to characterize them to understand the physical and chemical processes that are happening. Current methods for measuring aircraft particulate emissions study the reflectance of samples collected in filter papers.A series of experiments to more fully characterize particulates has been performed on a small-scale gas turbine engine. An intrusive sampling system conforming to current ICAO regulations for aircraft emissions was used with a scanning mobility particle sizer (SMPS). Non-intrusive measurements were made using laser induced incandescence (LII) and samples were taken from the exhaust to analyse using a transmission electron microscope. Results obtained from different techniques showed good agreement with each other. As engine power conditions increased, both the SMPS and LII indicated that the mass of soot had decreased. Differences were observed between measurements of diluted and undiluted samples. The mean particle size decreased with dilution but the size distribution became bi-modal.The study has shown how significant the sampling environment is for measuring particulates and careful techniques need to be used to ensure that accurate, consistent results can be obtained.

Gas Turbine Exhaust Emissions Monitoring Using Nonintrusive Infrared Spectroscopy

Infrared (IR) spectra of the exhaust emissions from a static gas turbine engine have been studied using Fourier Transform (FT) spectroscopic techniques. Passive detection of the infrared emission from remote (range {approximately}3 m) hot exhaust gases was obtained nonintrusively using a high spectral resolution (0.25 cm{sup {minus}1}) FTIR spectrometer. Remote gas temperatures were determined from their emission spectra using the total radiant flux method or by analysis of rotational line structure. The HITRAN database of atmospheric species was used to model the emission from gas mixtures at the relevant temperatures. The spatial distribution of molecular species across a section transverse to the exhaust plume {approximately}10 cm downstream of the jet pipe nozzle was studied using a tomographic reconstruction procedure. Spectra of the infrared emission from the plume were taken along a number of transverse lines of sight from the centerline of the engine outwards. A mathematical matrix inversion technique was applied to reconstruct the molecular concentrations of CO and CO{sub 2} in concentric regions about the centerline. Quantitative measurement of the molecular species concentrations determined nonintrusively were compared with results from conventional extractive sampling techniques.

Passive remote detection of atmospheric pollutants using Fourier transform infrared (FTIR) spectroscopy

Feasibility studies on the use of passive FTIR spectroscopy for the remote detection of atmospheric pollutants have shown that gases may be identified remotely with an optically modified commercial FTIR spectrometer when only a small 7 degree(s)C) temperature difference exists between the gas and a background IR emitter. A correlation technique was used to extract information from noisy (SNR on the order of magnitude 1) emission spectra from which low temperature differential or low concentration gases or mixtures with interferant species were analyzed. Quantitative measurements of gas concentrations were made by first determining the temperature of the remote gas from rotational line intensity distributions. The product of concentration and pathlength was obtained by comparing the remotely detected emission spectra with simulated spectra obtained from computer models at the measured temperature. Laboratory experiments measured the temperature of CO from its emission spectrum to +/- 5% and the concentration also to +/- 5%. Tomographic reconstructions of molecular distributions within a methane flame have recently been obtained.

Measurement of Smoke Particle Size and Distribution Within a Gas Turbine Combustor

The objective of the work described in this paper was to identify a method of making measurements of the smoke particle size distribution within the sector of a gas turbine combustor, using a Scanning mobility particle sizing (SMPS) analyzer. As well as gaining a better understanding of the combustion process, the principal reasons for gathering these data was so that they could be used as validation for computational fluid dynamic and chemical kinetic models. Smoke mass and gaseous emission measurements were also made simultaneously. A water cooled, gas sampling probe was utilized to perform the measurements at realistic operating conditions within a generic gas turbine combustor sector. Such measurements had not been previously performed and consequently initial work was undertaken to gain confidence in the experimental configuration. During this investigation, a limited amount of data were acquired from three axial planes within the combustor. The total number of test points measured were 45. Plots of the data are presented in two-dimensional contour format at specific axial locations in addition to axial plots to show trends from the primary zone to the exit of the combustor. Contour plots of smoke particle size show that regions of high smoke number concentration once formed in zones close to the fuel injector persist in a similar spatial location further downstream. Axial trends indicate that the average smoke particle size and number concentration diminishes as a function of distance from the fuel injector. From a technical perspective, the analytical techniques used proved to be robust. As expected, making measurements close to the fuel injector proved to be difficult. This was because the quantity of smoke in the region was greater than 1000 mg/m 3 . It was found necessary to dilute the sample prior to the determination of the particle number concentration using SMPS. The issues associated with SMPS dilution are discussed.

Detection of soot particles in gas turbine engine combustion gases using nonintrusive FTIR spectroscopy

Fourier transform IR (FTIR) spectroscopy for making non- intrusive measurements of gas turbine exhaust gases and laser induced incandescence for measuring soot content are being evaluated in EU Brite EuRam project AEROJET. Soot concentrations in modern aero-engine exhausts are very low with mean particle sizes < 100 nm. The standard extractive filter paper soot measurement gives results expressed in terms of SAE smoke number, typically < 10 SAE for modern engines.

Vehicle Exhaust Gas Monitoring by Remote FT-IR Spectroscopy

An optically modified Unicam 0.25 cm-l resolution FT-IR spectrometer has been used for remote monitoring of absorption and emission spectra of vehicle exhaust gases. In the active mode, a beam of modulated infrared radiation was emitted by the spectrometer and the absorption by the exhaust gas was measured along the beam path to a retroreflector. The return signal was monitored by a liquid-N2 cooled mercury cadmium telluride (MCT) detector external to the instrument. The concentrations of CO and other gaseous molecular species in the exhaust were compared, with good agreement, with prepared laboratory samples and models based on the AFGL atmospheric transmission data-base HITRAN. In the passive mode, the emission spectra of the warm exhaust gases have been recorded. The use of a liquid-N2 background gave the spectrometer greater sensitivity to CO present in the exhaust gas. Laboratory studies at moderate (0.25 cm-1) spectral resolution have been conducted to determine the spectrometer sensitivity to trace concentrations of nitrogen and sulphur dioxide.

Mass and size distribution measurement of particulates from a gas turbine combustor using modern mobility analyzer and particle sizer

SMPS and DMS500 analysers were used to measure particulate size distributions in the exhaust of a fully annular aero gas turbine engine at two operating conditions to compare and analyse sources of discrepancy. A number of different dilution ratio values were utilised for the comparative analysis, and a Dekati hot diluter operating at a temperature of 623°K was also utilised to remove volatile PM prior to measurements being made. Additional work focused on observing the effect of varying the sample line temperatures to ascertain the impact. Explanations are offered for most of the trends observed, although a new, repeatable event identified in the range from 417°K to 423°K – where there was a three order of magnitude increase in the nucleation mode of the sample – requires further study.

Measurement of Fourier transform infrared spectrometer instrument lineshape

This work describes the use of a He:Ne laser based test rig, developed at NPL, and a low pressure gas cell containing CO to measure the instrument line shape (ILS) function of a Unicam Research Series FTIR spectrometer, which has been extensively used for the detection and measurement of fugitive gas emissions. During the ILS function measurements minor optical misalignments were introduced into the optical system, and their effect on the measured ILS function observed. The ILS functions obtained using the laser system and low pressure gas cell containing CO were in good agreement, both having a full width at half maximum of 0.3 cm-1. The minor optical misalignments had a significant effect on the ILS function. The initially symmetric function became more asymmetric as the degree of misalignment was increased. Gas concentrations have been retrieved using a symmetric and an asymmetric ILS function, and the differences presented.