The ultimate ambition of the LITECS research programme is to reduce the environmental impact of aviation and industrial gas turbine engines, through development and deployment of new measurement technologies to enhance understanding and modelling of combustion and emissions generation processes and the role of alternative fuels.
The new technologies will revolutionise the measurement of gaseous and particulate combustion species and provide spatially and temporally resolved maps of strategic high priority species such as soot, CO2, CO, water and NO. The instrumentation systems developed by Strathclyde, Edinburgh and Manchester Universities will be installed on combustion research rigs at NCCAT (Loughborough) and LCCC (Sheffield) for experimental GTE research in this programme and far into the future. The instrumentation development programme focuses on tuneable diode laser spectroscopy (TDLS) as the basic measurement technique for gaseous species, laser induced incandescence (LII) for soot, multiple beam TDLS for tomographic species imaging and beam scanning for soot imaging. Southampton University is developing the new near-infra red (NIR) and mid-infrared (MIR) technology required for light amplification and delivery. An introduction to the various technologies targeted is provided below.
2. Tuneable Diode Laser Spectroscopy
2.1 Spectroscopy - basics
The vibrations of the molecular bonds can combine with molecular rotation at varying speeds / energies leading to many absorption lines associated with particular vibration-rotation combinations. The study of such absorption line is known as spectroscopy [1-3]. Figure 3 shows the rotation-vibration absorption spectra of CO2 and water at ambient temperature and 600°C at light wavelengths just less than 2000nm. Note that this can result in overlaps between line sets at specific frequencies and that we need to choose the lines we target for measurement to ensure no cross-species interference (Note that the CO2 line selected for measurement in Fig. 4 does not overlap with any water line).
If we can measure two lines with different temperature dependence, we can determine temperature from the ratio of the signal amplitudes (absorption strengths) – see Figure 6.
2.2 Tunable Diode Laser Spectroscopy (TDLS)
In TDLS , the laser is tuned repetitively over the line by applying a saw-tooth (repeating ramp) current to the diode laser (Fig. 7) and recording multiple measurements of the line-shape which can be averaged over many scans to clean up the signal to noise ratio (SNR). The laser output power increases with the increasing current in each ramp phase and the final line-shape is recovered by normalising, point by point, the recorded signal to the background intensity signal. The measurement process using line-shape models is as described above (Section 2.1). This is referred to as direct TDLS as the line-shape is recorded directly at the output of the photodiode [5-8]. However, this approach has significant disadvantages in terms of improving signal to noise ratio. Since the signal is recovered at low frequencies, at or near DC, it suffers from 1/f noise, laser intensity and phase noise and receiver noise (probably thermal). More significantly, for gas turbine engine measurements, any noise arising from beam propagation issues in a turbulent environment such as dynamic beam wander off and on the receiver, beam break-up or mechanical vibration are particularly destructive.
To beat the noise issues an advance on direct TDLS was developed referred to as wavelength modulation spectroscopy.
2.3 Tunable Diode Laser Spectroscopy (TDLS) with wavelength modulation spectroscopy (WMS)
In WMS the centre wavelength of the laser is repetitively scanned over the line using a saw-tooth current function, as before, but with a high frequency sinusoidal modulation (dither) current superimposed on it (Fig. 8, 9). As the laser centre wavelength traverses the line, the sinusoidal current dithers the laser wavelength (frequency). The interaction of the beam subjected to wavelength modulation (WM) with the absorption line-shape generates an intensity modulation (Fig. 9) at multiple harmonics of the fundamental modulation frequency [4, 9-16].
Transmitted signals at the modulation frequency are strongest and purest when the laser centre frequency is at a linear point of the line-shape and the higher harmonics are strongly generated when the centre frequency is passing over the highly non-linear regions. The intensity modulation harmonic signals arising from the wavelength dither are isolated in frequency and their amplitude measured by a lock-in amplifier (LIA – Fig. 8). The output of the LIA then traces of the amplitudes of the WM induced intensity modulated (WM-IM) signals. For the LIA set to record at the fundamental modulation frequency the output trace (1f signal) is a close approximation of the first derivative of the line-shape and, if set to record the second harmonic (the 2f signal), the output (2f signal) is a close approximation of the second derivative of the line-shape (Fig. 9) and so on.
In the WMS approach we can choose the modulation frequency and hence the frequency detected by the LIA to minimise the noise and maximise SNR. Figure 10 shows the measured noise spectrum on a recovered TDLS signal during a measurement on the exhaust of a gas turbine engine. If we make the measurement using a WMS approach to capture 1f or 2f signals above 400kHz we can, to a large extent, suppress the influence of all the noise sources. The information we want is recovered from a high frequency signal and thus we avoid the worst excesses of lower frequency 1/f noise, laser intensity noise and receiver noise, and suppress the noise arising from mechanical sources (turbulence in the beam path, vibrations etc.). Figure 11 shows a comparison of recovered signals from different approaches the black signal is from direct TDLS, the red is a 1f signal and the green signal is a 2f WMS signal from the LIA. This illustrates clearly the power of 2f WMS to recover signals with high SNR from extremely noisy photo-diode outputs and why it became the preferred choice for many applications.
2.4 Calibration free TDLS-WMS
Notwithstanding the above, 1f and 2f TDLS-WMS presents some difficulties. There is no intensity referencing and the signal amplitudes are strongly dependent on system parameters in a very complex way. As a result, it is mostly used with calibration of recovered signals to those from known gas mixtures. However, as system and component parameters tended to drift with time the integrity of stand-alone instruments was always questionable. In its simplest form, based on measurement of signal amplitude, they also suffer inaccuracies arising from varying pressure and temperature.
Since 2007 there have been many significant advances in TDLS-WMS to yield calibration free techniques that are very robust and ideally suited to stand-alone instruments in the field [14-16]. As well as the WM-IM signals carried by the beam onto the receiver, a direct IM signal is present from the intensity modulation of the laser by the high frequency sinusoidal drive current (known more widely as the residual amplitude modulation or RAM signal). The result is phase related distortion of the recovered IM and WM-IM signals at 1f or 2f with accompanying in accuracies. Due to the nature of the laser modulation, the WM-IM signals are phase delayed relative to the direct modulation IM signals. Recognising this led to techniques to isolate the direct IM and WM-IM signals by phase selection or a phasor decomposition approach [14, 16]. These led to calibration free approaches based on the phase isolated direct IM signals, which enabled direct recovery of the absorption line-shape [14, 16]. Extraction of gas concentration and pressure (T known) then followed the same measurement procedure as for direct TDLS. The techniques required accurate, in-situ, characterisation of the system parameters  and refinements of the models particularly for high temperature use .
A particularly useful calibration free approach  that provides intensity referencing normalises the WM-IM 2f signal to the 1f signal (takes the point to point ratio of the 2f to 1f signals). The two signals contain the same common-mode optical noise components, including from the laser and the beam path generated noise from turbulence and vibration. Hence, the normalisation process largely eliminates such noise from the processed signals vastly improving the SNR. The resulting signals, though, are strongly dependent on system parameters particularly the laser modulation characteristics, such as relative IM to WM-IM phase and the laser wavelength tuning coefficient. This approach is made more accurate by the new understanding of phase related IM distortion and laser tuning, how to characterise them in-situ on stand-alone instruments and take account of them in the signal analysis [14, 16-22]. Even with such complex and careful signal generation, recovery and processing techniques, the recovered signals are uniquely related to C, P and T. Hence, with accurate models of the 2f/1f signals, accounting for system parameters and the variations of key spectral parameters, we can again simply fit the models to the measured signals by varying C and P / T in the model (with T or P known). This approach was used in a study of CO2 in the exhaust plume of a GTE . Measured 2f/1f signals for CO2 in that study for two different thrust levels are shown in Figures 12a and 12b along with the theoretical simulations of these signals for those concentrations
The excellent SNR confirms the efficacy of the normalisation in eliminating the common-mode noise. The technique was used to investigate CO2 concentration in the exhaust plume of a GTE as the thrust was cycled. The results are presented in Figure 13 along with the engine temperature measured by a thermocouple. The strong correlation in trends is evident as the engine thrust is cycled, again indicating the efficacy of the approach.
A further calibration free technique with intensity referencing and noise cancellation  was developed by the Strathclyde team in 2018. This technique normalised the 1f WM-IM signal to the 1f isolated pure IM (RAM) signal, recorded in quadrature, giving excellent results for SNR and CO2 measurement integrity  in measurements made on a GTE combustor at Cardiff University. A sample measured signal compared with a simulated signal fitted to it is shown in Figure 14a. For comparison the 2f/1f signal derived from the same measurement is shown in Figure 14b. This RAM normalised 1f-WMS approach  and the 2f/1f normalisation approach  will be used in LITECS.
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3. Chemical Species Tomograpy (CST)
By exploiting the absorption of light at carefully chosen wavelengths, Chemical Species Tomography (CST) yields images of the spatial distribution of a target species of molecules within engineering processes. Images can be obtained at thousands of frames per second (fps), and in favourable cases it’s also possible to obtain images of the spatial distribution of temperature. Since CST uses collimated light beams, it operates in a similar way to medical Computed Tomography scanning using X-rays, except that it doesn’t normally involve any moving parts. In the literature, CST is often referred to as “TDLAS tomography” (see below), or “hyperspectral imaging”, among other names.
CST for engineering applications became feasible following the advent of diode lasers and optical fibre technologies in the communications industry in the 1980s. Practical implementation of high-speed CST exploits the technique of Tunable Diode Laser Absorption Spectroscopy (TDLAS), which has been developed for many gas sensing applications in non-imaging mode. Typically, spectral absorption is measured along several dozen separate optical beams, which demands bespoke electronics. In common with X-ray CT, the reconstruction of images in CST uses mathematical methods from the field of Inverse Problems, and it also borrows heavily from data inversion methods used in other modalities of Industrial Process Tomography.
An overview of the development and status of CST is given in , and its challenges are unique:
- The target molecular type is usually in a mixture of gases, often at high temperature and/or pressure, demanding exquisite spectroscopic knowledge;
- Sophisticated optical access engineering is required in order to launch many light beams through the measurement subject, along with complex multi-channel Data Acquisition (DAQ) technology;
- With many fewer beams than X-ray CT, and by measuring an absorption spectrum rather than just attenuation at a single photon energy, the image reconstruction process is particularly challenging;
- Reconstruction of meaningful images requires that the spectral absorption measurements have very low noise content, i.e. high Signal-to-Noise Ratio, SNR, despite the difficult environment.
After the first demonstration of high-speed CST two decades ago , many research groups around the world have used it to image species and/or temperature distributions in a wide variety of engineering processes.
In LITECS, we are focusing on gas turbines and working to establish CST systems to provide:
- Independent images of CO2 concentration and temperature (using absorption spectroscopy of water), in the exhaust plume;
- Independent images of temperature and the concentrations of both CO and CO2 in the combustion zone;
- Exemplar applications of the above under a variety of turbine combustion conditions, with various fuels.
The motivation for this research is to facilitate the development of improved environmental performance of gas turbines, which are deployed in a variety of industrial settings, notably in aviation and in power generation.
3.2 Chemical Species Tomography in action
Automotive Compression Ignition Engine:
Swirl Flame Dynamics:
Towards CO2 Imaging in Civil Aero Engine Exhaust:
Preparatory Research for LITECS:
- McCann H, Wright P, and Daun KJ, Chemical Species Tomography, Ch.5 in Industrial Tomography: Systems and Applications, (Ed. Mi Wang), Woodhead Publishing, 2015, DOI: 10.1016/B978-1-78242-118-4.00005-8
- Hindle FP , Carey SJ, Ozanyan KB, Winterbone DE, Clough E, and McCann H, Measurement of gaseous hydrocarbon distribution by a Near Infra-Red absorption tomography system, J. Electronic Imaging 10 (2001) 593-600, DOI: 10.1117/1.1377306
- Tsekenis SA, Ramaswamy KG, Tait N, Hardalupas Y, Taylor A and McCann H, Chemical species tomographic imaging of the vapour fuel distribution in a compression-ignition engine, Int. J. Engine Research 19 (2018) 718-731 DOI: 10.1177/1468087417730214
- Chang Liu, Zhang Cao, Yuzhen Lin, Lijun Xu, Hugh McCann, On-line cross-sectional monitoring of a swirling flame using TDLAS tomography, IEEE Trans. Inst. and Meas. 67 (2018) 1338-1348 DOI: 10.1109/TIM.2018.2799098
- Fisher E, Tsekenis S, Yang Y, Chighine A, Liu C, Polydorides N, Wright P, Kliment J, Ozanyan K, Benoy T, Humphries G, Lengden M, Johnstone W, and McCann H, A Custom, High-Channel-Count Data Acquisition System for Chemical Species Tomography of Aero-Jet Engine Exhaust Plumes, IEEE Trans. Inst. and Meas. 69 (2020) 549-558 DOI: 10.1109/TIM.2019.2895932
- Lengden M, Stewart G, Johnstone W, Upadhyay A, Wilson D, Polydorides N, McCann H, Liu C, Enemali G, Tsekenis A, Wright P, Kliment J, Archilla V, Velasco J, Sánchez-Valdepeñas J, Beltran M, Polo V, Armstrong I, and Mauchline I, Recent Progress in the Development of a Chemical Species Tomographic Imaging System to Measure Carbon Dioxide Emissions from Large-Scale Commercial Aero-Engines, OSA Optical Sensors and Sensing Congress 2020, paper JM2F.3, DOI: 10.1364/AIS.2020.JM2F.3
- Tsekenis S-A, Polydorides N, Fisher E, Chighine A, Wilson D, Humphries G, Lengden M, Benoy T, Johnstone W, Kliment J, Wright P, Feng Y, Nilsson LJ, Victor DA Prat, Jia J, and McCann H, Chemical Species Tomography of Carbon Dioxide, Proc. 8th World Congr. Industrial Process Tomography, ISBN 978-0-853-16349-7, Paper B17, Iguassu Falls, Brazil, September 2016
- Polydorides N, Tsekenis SA, Fisher E, Chighine A, McCann H, Dimiccoli L, Wright P, Lengden M, Benoy T, Wilson D, Humphries G, and Johnstone W, Constrained models for optical absorption tomography, Appl. Opt. 57 (2018) B1-B9, DOI: 10.1364/AO.57.0000B1
- Godwin Enemali, Rui Zhang, Hugh McCann and Chang Liu, Cost-Effective Quasi-Parallel Sensing Instrumentation for Industrial Chemical Species Tomography, IEEE Trans. Ind. Electronics (accepted), DOI: 10.1109/TIE.2021.3063963
- Yong Bao, Rui Zhang, Godwin Enemali, Zhang Cao, Bin Zhou, Hugh McCann and Chang Liu, Relative Entropy Regularised TDLAS Tomography for Robust Temperature Imaging, IEEE Trans. Inst. and Meas. 70 (2021) 1-9 DOI: 10.1109/TIM.2020.3037950
4. Laser Induced Incandescence (LII)
[Section to be supplied]
EPSRC Programme: Laser Imaging of Turbine Engine Combustion Species
Contact: Professor Walter Johnstone
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