Compact high speed MWIR spectrometer applied to monitor CO2 exhaust dynamics from a turbojet engine

R. Linares Herrero*a, G. Vergara a, R. Gutiérrez Álvarez a, C. Fernández Montojo a, L.J. Gómez a, V. Villamayor a, A. Baldasano Ramírez a, M.T. Montojoa, V. Archillab, A. Jiménezb, D. Mercaderb,

A. Gonzálezc and A. Enteroc aNew Infrared Technologies, S.L., Calle Vidrieros 30, Boadilla del Monte, 28660 Madrid, Spain;

bInstituto Nacional de Técnicas Aeroespaciales (INTA), Ctra. Ajalvir s/n, Torrejón de Ardoz, 28850 Madrid, Spain;

cISDEFE, Calle Beatriz de Bobadilla, 3, 28040 Madrid, Spain

ABSTRACT

Due to international environmental regulations, aircraft turbojet manufacturers are required to analyze the gases exhausted during engine operation (CO, CO2, NOx, particles, unburned hydrocarbons (aka UHC), among others). Standard procedures, which involve sampling the gases from the exhaust plume and the analysis of the emissions, are usually complex and expensive, making a real need for techniques that allow a more frequent and reliable emissions measurements, and a desire to move from the traditional gas sampling-based methods to real time & non-intrusive gas exhaust analysis, usually spectroscopic. It is expected that the development of more precise and faster optical methods will provide better solutions in terms of performance/cost ratio. In this work the analysis of high-speed infrared emission spectroscopy measurements of plume exhaust are presented. The data was collected during the test trials of commercial engines carried out at Turbojet Testing Center-INTA. The results demonstrate the reliability of the technique for studying and monitoring the dynamics of the exhausted CO2 by the observation of the infrared emission of hot gases. A compact (no moving parts), high-speed, uncooled MWIR spectrometer was used for the data collection. This device is capable to register more than 5000 spectra per second in the infrared band ranging between 3.0 and 4.6 microns. Each spectrum is comprised by 128 spectral subbands with a bandwidth of 60 nm. The spectrometer operated in a passive stand-off mode and the results from the measurements provided information of both the dynamics and the concentration of the CO2 during engine operation. Keywords: infrared, uncooled, MWIR, LVF, high-speed, spectroscopy, VPD PbSe

1. INTRODUCTION An increasing concern in the developed countries for the environment welfare/preservation implies legislative changes oriented to improve the efficiency of aircraft engines [1,3]. Their emissions and their potential influence on local and global environmental have become the subject of intensive study by scientists. Environmental problems to be considered in relation to these emissions include:

• Climate change such as global warming. • Increase of ultra-violet (U.V.) radiation resulting from stratospheric ozone depletion. • Local and regional air pollution due to volatile organic compounds (VOC) such as UHC, and particle’s

emissions. One of the phenomena to which aircraft engine emission contribute are the effects of aerosol and emissions of particulate matter, which impact covers a variety of different effect categories in the local areas, such as:

* rlinaresh@niteurope.com; phone: +34916324363; www.niteurope.com

Thermosense: Thermal Infrared Applications XXXV, edited by Gregory R. Stockton, Fred P. Colbert, Proc. of SPIE Vol. 8705, 87050E · © 2013 SPIE · CCC code: 0277-786X/13/$18 · doi: 10.1117/12.2015894

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• Public health. • Agriculture production and ecosystems. • Buildings and infrastructure.

It is well known that airport’s environment and the local air quality in its surroundings represent a strategic key in European research programs, and new aircraft engine designs are requested to incorporate new and significant technologies to improve the combustion efficiency and fuel consumption reduction. The global air transportation industry has grown more than 4% annually since 1990. The forecast for the future estimates similar growth rates during the next decades. This increment results in a equivalent net increase in fuel burn. For every kilogram of jet fuel burnt more than 3 kg of CO2 are emitted to the atmosphere. As a consequence, the International Civil Aviation Organization (ICAO) foresees that global CO2 emissions from aviation would increase by 2036 an additional 150% above 2006 levels. At this rate, aircraft CO2 emissions would quadruple by 2050. In order to minimize the environmental impact associated to the growth of the demand, the aviation industry is being pushed to develop technologies focused to increase the fuel efficiency of the aircraft engines for reducing their greenhouse gas emissions. The fuel consumption of an aircraft is a function of its weight, engine efficiency (i.e. specific fuel consumption) and aerodynamic efficiency (i.e. lift-to-drag ratio) for a specified range and speed. The exhaust from jet turbine engines contains many chemical species, including carbon monoxide (CO) and dioxide (CO2), nitric oxide and dioxide (NO and NO2), unburnt hydrocarbons (UHCs) and others [4]. The detection of the gases from jet engines is needed to certify the engine performance and monitor environmental pollutants. Traditionally, in-situ techniques for the analysis have been used, including chemiluminescence, flame ionization or proton transfer reaction mass spectrometry, which analyze the emissions from aircraft on the ground demanding expensive systems and skilled engineers. Sampling hardware capable of withstanding plume conditions so close to an engine is expensive. In most cases, the samples have to be transferred by heated lines to a variety of measuring instruments, creating many measurement uncertainties and the engine has to be run for a considerable time to be stabilized on the required number of measurement conditions. At the present time there is a real need for more feasible aircraft engine emissions measurements [5]. The future technologies are moving from the traditional gas sampling-based methods to non-intrusive (stand-off) analysis technologies. Optical methods fulfill the most important requirements demanded for the next generation of turbojet gas analyzing techniques:

• Not interference in the flow pattern • Lower operational and logistics costs • Real-time, in-situ, standoff measurements • Compatible and complementary with other techniques and probes • Wide area coverage • Spatial information

In this work the first results on CO2 dynamics of a standard large turbofan engine (76000 lbf (pounds of force) thrust) are presented. The data was collected using a high-speed MWIR infrared spectrometer during scheduled trials at the Turbojet Testing Centre (CET) at INTA. The work has been divided into four main sections, including this introduction. Section 2 will provide a detailed description of the spectrometer developed and the experimental set- up; in section 3 some experimental results will be presented, obtained during tests in different configurations. The conclusions will be presented in the last section.

2. INSTRUMENT DESCRIPTION AND EXPERIMENTAL SET UP The experimental results shown in this work were obtained during independent, scheduled engine test trials carried out during September 2012 at Turbojet Testing Centre (CET) at INTA. The next paragraphs describe the experimental set-up and a detailed description of the high-speed infrared spectrometer used for the data collection.

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2.1 Ground facilities for testing aircraft engines - Turbojet Testing Centre (CET) at INTA

The Test Cell for turbojet engines at INTA is the result of a European technological effort that provides service to clients of international prestige: Rolls Royce PLC, General Electric and Eurojet Turbo GmbH. In the modern testing facilities new techniques are becoming available to investigate the environmental impact of these systems, in order to improve their design. In the test cell, tests are carried out to certify models, investigate new technologies and assess the environmental impact of modern and future engines. Having in mind the principles of safety, reliability, economy and ecology, in an atmosphere of international cooperation with the clients, the INTA Test Cell is equipped with modern facilities and highly qualified personnel that increases its value with the experience of ever even more challenging tests. INTA performs both developments and analysis of complete jet engine tests as well as assembly of their components and accessories. As described in figure 1 the facility has two chimneys (entry and exit), both with height of 22 m (buried 4.5 m). The total length of the building is 114 m, where the length of the testing cell is 60m. The section of the testing cell is 14.5 m x 14.5 m. The length of tube (in which all the emissions are directed) is 48 m long with an inner diameter of 5.8 m. The total width of the building is 16 m.

Figure 1. Sea level test cell at INTA facilities.

2.2 High speed infrared spectrometer

The MidIR spectrometer used in this work has been developed by New Infrared Technologies (NIT S.L.) and can be considered a breakthrough in the field of low cost and compact (no moving parts) infrared spectrometers [6]. The spectrometer is able to provide more than 5000 MWIR spectra per second in real uncooled, stand-off operation and has two main components:

- Infrared spectrometer Core: The spectrometer is based on a VPD PbSe linear array with 128-pixel, which is capable to be operated at room temperature in real uncooled performance. Due to the short time constant of the sensing material, the spectrometer is able to work in a high-speed mode operation providing more than 5000 spectra/sec. The readout electronics (Analog-to-Digital Conversion stage) that the system uses have been specifically designed for VPD PbSe photoconductive detectors. The ADC stage is controlled by a VIRTEX 4 FPGA from Xilinx, which has a PowerPC IP embedded and a LinuxOS running in real-time. The data is transferred to a host computer using TCP/IP communications. All these electronic components form a set called CORE module, which communicates with a PC through an Ethernet port (100 Mbps). Software provided by the manufacturer allows the control of the acquisition parameters (integration time) as well as other processing and storing functions, such as the 1-pt and 2-pt Non Uniformity Corrections. In addition, optics with F#1.1 has been used during the data acquisition. The main characteristics of the FPA and the CORE electronics are summarized in the next table. An image showing the module is shown on the right side.

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Table 1. Summary of the FPA and CORE characteristics

Parameter Value Units FPA material VPD PbSe, uncooled operation Number of pixels 128 pixels Pixel size 1000 x 100 microns Detection band 1.0 – 5.0 microns Peak detectivity 3.7 microns D*(λpk) 3·109 Jones Time constant < 3 us Readout method MUX (blocks of 16 pixels) Readout electronics CORE module Integration time Variable, from 7.68 to 40 us Frame rate Variable, 5000 lps (standard) Communications Ethernet, using TCP/IP Data Raw format, 16 bits/px Optics F#1.1

- The Linear Variable Filter (LVF): The device’s spectral capabilities are provided by an infrared Linear Variable Filter (LVF) coupled to the linear array. The LVF lets the IR radiation comprised between 2.995 um and 4.6 um pass, blocking the rest of wavelengths. The characteristics of the LVF can be seen in the next table, and an image of the filter set on top of the FPA is shown on the right side:

Table 2. Summary of the LVF characteristics

Parameter Value Units LVF range 2.995 – 4.600 um Out of Band Blocking ≤ 0.1 % HPBW < 1.5 % In Band Transmission Peak T > 50 % Filter Dispersion <130 nm/mm Dimensions 16.0 x 4.0 mm Thickness < 1 mm Clear Aperture Length 12.8 mm

Theoretically, the filter would provide hyperspectral information in the horizontal dimension (i.e., very small bands tending to single wavelengths). However, due to the pixel size of the FPA different to zero, it behaves as a set of bandpass filters (128) with the center wavelengths placed on top of the center of the pixels. A model of the linear variable filter, based on the data provided by the manufacturer, has been calculated and it’s shown in the left side of Figure 2. If the response of the filter is calculated for one individual pixel (same figure, right side, calculated for pixel #63), the result is that each pixel is covered by a bandpass filter of approximately 60 nm bandwidth (-3dB cut-on and cut-off wavelengths), centered in the pixel center axis.

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During the tests, the infrared spectrometer was set for working with an integration time of 30.4 µsec leading to a frame rate of ~2k spectra/sec. According with previous measurements using a high-speed infrared camera (see Figure 5) we would expect to see a temporal modulation in the CO2 flow, which was observed. Figure 7 shows the temporal evolution of infrared spectra (y-axis shows the wavelength and x-axis shows the number of frame (temporal information)).

Figure 7. Infrared spectra vs time obtained during test. Green line is the signal corresponding to CO2 and red area corresponds to the infrared signal coming from the black body type radiation coming from the engine cone.

The signal measured between 3.0 and 3.8 um corresponds to the blackbody radiation coming from the engine metal cone. In this case, we do not see any fluctuation due to the thermal inertia of the piece. On the other hand, the infrared radiation between 4.1 and 4.4 um, coming from hot CO2, present a clear modulation of the IR intensity, matching the results

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obtained in previous high-speed infrared measurements. Figure 8 shows a detail of the temporal modulation of gas flow. The reconstructed image has both, temporal (gas dynamics) and spectral information. The green line corresponds to the temporal evolution of infrared signal of 4.45 um (bandwidth 60 nm) obtained during trials. As far as we know it is the first time that these type of curves have been measured on a aircraft engine exhaust.

Figure 8. Modulation of CO2 measured at exhaust during engine operation. Green line (right) corresponds to the temporal evolution of 4.45 microns IR signal.

The high-speed spectrometer also provides effective qualitative information about CO2 concentration of exhausted gases flow. Depending on the AFR (Air Fuel Ratio) there are three modes of combustion: lean (air rich mixture, AFR > 16:1), rich (fuel rich mixture, AFR < 15:1 ) and stoichiometry (AFR = 15-16:1). When the AFR is rich, then a complete combustion does not take place and there is a loss of fuel in the form of unburnt hydrocarbons (UHC), resulting in less efficiency to the engine. When the AFR is lean, then the combustion is complete but cooling down of the engine occurs, therefore causing loss of efficiency. The stoichiometric ratio is the chemically correct AFR to ensure complete combustion of fuel but it produces the maximum amount of CO2 (see figure 9).

Figure 9. Normalized CO2 emission dependence on Air Fuel ratio (AFR).

Instantaneous AFR has a strong impact on CO2 emissions. During the trials we have observed this effect in fast transients events such as slam acceleration/deceleration. Figure 10 shows synchronized exhaust gases temperature (red), fuel consumption (green) and CO2 red shift IR signal (blue) vs time normalized curves during a slam. It is possible to observe that the IR signal from CO2 emission, which has a strong dependence on the gas concentration, is very dependent on the instantaneous air/fuel ratio (AFR).

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4. CONCLUSIONS A compact high-speed MWIR infrared spectrometer has been used for monitoring gas exhaust of a turbojet engine. The device is based on a linear array (1x128 pixels) on top of which a LVF has been assembled. It is a compact spectrometer capable to provide spectral (from 3.0 to 4.6 um) and dynamic information of the gases exhaust flow in a passive stand-off observation, and uncooled operation (affordable). The spectrometer captures more than 5000 infrared spectra/sec providing information about both the flow dynamics and relative concentrations of gases and compounds such as CO2 and UHCs. The results of trials carried out at Turbojet Testing Center (CET) at INTA with a commercial turbojet engine show a strong dependence of CO2 concentration emitted on instantaneous changes of AFR during engine operation. Non equilibrium conditions during combustion and fast transients of fuel feeding would produce big amounts of CO2. Test demonstrated the importance of monitoring gas exhaust during transient operation. Uncooled high-speed MWIR spectrometry has demonstrated to be an excellent tool in terms of affordable cost and technical and logistic simplicity.

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