In spite of improvements in technology and operational procedures, total aviation emissions have increased, because increased demand for air transport has outpaced emission reduction effects. Next (subsonic) and future (supersonic) generation aircraft must be equipped with low-emission engines. The development of appropriate combustor and engine technology, as well as their certification, maintenance, quality assessment and control requires advanced emission measurement methods and equipment. Among the different possible measurement techniques, mid-infrared laser diagnostics is particularly promising because of its inherent high sensitivity, remote sensing capability, the possibility of spectroscopic identification of minority effluents, its high spatial resolution, and its high potential of compactness allowing potentially faster, easier and low cost testing of engine effluents.
The MENELAS project intends to give a contribution to this topic, and responds to the Thematic Programme "Competitive and Sustainable Growth", Key Action 4. "New Perspectives in Aeronautics", and specifically addresses Objective 4.1 "Reducing aircraft development cost and time to market" and Objective 4.3 "Improving Environmental Friendliness of Aircraft". The exploitation of results is expected in a long term time period.
The overall objective of the project is the development of mid-infrared laser sources for spectroscopic determination of minority pollutant and demonstration of their ability to measure the exhaust gas traces for the most important aircraft engine exhaust gases (i.e. NO, NO2, CO, CO2, H2O, according to ICAO Annex 16 and beyond). These measurements will permit online inspection (no sampling required), long range tests (from few meters to hundred of meters) and verification. Moreover, the apparatus will be compact enough (in the litre range) to easily allow on-site or on-board measurements. In comparison to the EC projects AEROJET and AEROPROFILE the concept developed by MENELAS introduces very high spatial and spectral resolution and significantly expand the list of species to be possibly investigated. This is achieved by the innovative adaptation of different laser spectrometric techniques.
More specifically, the scientific research objective of the project is to determine advantages and disadvantages of different laser and related non-intrusive instrumentation and associated analysis methodology; while the technical research objective is the development and application of the necessary instrumentation and software (especially various laser systems) for the demonstration of the technology. This includes joint measurement campaigns on ground and on-site.
Specific objectives are:
- To develop two innovative Mid-Infrared (MID IR) spectrometers for measurements of trace species like CO, CO2, H2O, NO and others which have an impact on engine efficiency or pollution emissions. The two instruments are: the MIDROPO, which is based on tuneable optical parametric oscillator (OPO) for wide spectral tunability address several species simultaneously; the MIRPL which is based on difference frequency generation and uses a picosecond LIDAR for high spatial resolution.
- To calibrate and optimise the use of these instruments with two other benchmarking instruments properly adapted beforehand: The DLAS/SDLA technique (mid-infrared spectroscopy by tuneable diode laser absorption); the Hot cell calibration.
- To demonstrate the feasibility of the new instruments through two field experiments (after adequate preparation characterisation with classical intrusive techniques and computational evaluations of flow conditions): The DLR atmospheric primary zone combustor (APZ) in Cologne; A
The scientific work was divided into the following groups of activities.
- Requirements and definitions for the development and validation of the instruments. The technical work starts with a comprehensive study of infrared spectral characteristics of the gases to be investigated in the engine and combustor exhaust, on the basis of the experience from previous projects on hot gas spectroscopy. Single lines and small groups of lines within the tuning range of the prospective laser systems are to be considered applying existing data bases (like HITRAN). Goals are the identification and selection of spectral lines (microwindows) with respect to the dependence of their strength and width on temperature and pressure, as well as their detectability (superposition of neighbouring lines of other species) and required accuracy of the measured spectra. In parallel investigations are to be performed and decisions made with regard to the question which gases are to be detected by which of the participating laser systems. This is useful, because at this basic research status it is not necessary that all laser systems cover the whole spectral range required for all gases. Finally, a study is performed about which detection techniques can be the most promising in order to perform exhaust gas velocimetry and to provide high spatial resolution gas concentration measurement. This study is to be a first step towards non-intrusive measurement of the exhaust gas emission index, i.e. grams of COX and NOX per kg burned fuel.
- Laboratory development or adaptation. This group of activities deals with the development or adaptation of instrumentation based on the requirements established in the previous phase. The laser systems are to be designed with respect to their spectral and tuning range, their power, their sensitivity in order to assess the gas spectra with the required accuracy. The subsequent construction, assembly and prototyping of the instrumentation is to be specifically guided by provisions to ruggedize the subsystems for operation in aircraft engine test-bed environment as well as by fitting them to a unified setup with optical integration elements to allow a reproducible, combined, and common observation geometry.
- Laboratory calibration. After completion of hard- and software tools, the project will deal with the calibration and validation of the subsystems in laboratories with cross-verifications with other mid-infr
The main results of the project are as follows.
- Prototypes of two novel optical instruments. Two novel optical instruments, namely the MidDropo and the Pico-second Lidar as originally planned, have been developed for probing in the infrared spectrum some minority species like CO, NO and CO2 in the effluents of combustors for aeronautics. Unfortunately and although that there has been much progress in the laboratory developments, it appears that these two sophisticated instruments are not yet mature to perform measurements at exit of a combustor or of an aircraft engine on an airfield as planned in the project.
- Field experiments to demonstrate the capability of spectroscopic measurements based on SDLA/DLAS. Field experiments to demonstrate the capability of spectroscopic measurements at such locations have been performed with classical infrared absorption spectrometers using diode lasers initially envisioned in the project for benchmarking the novel instruments. These experiments were done using the ONERA SDLA/DLAS instrument to probe an APZ combustor at DLR Koln and the exhaust gases of NLR’s Cessna Citation II research aircraft at a test site at Amsterdam Airport Schiphol. For the latter, there were also measurements by TUC who developed an absorption spectrometer using a Quantum Cascade Laser.
- Calibration and infrared beam propagation studies. The calibration hot cell at RISOE has been successfully upgraded for high temperature purposes but all the potential of this facility has not been exploited : there were only a few calibration experiments by TUC for high temperature CO2 and H2O. When the spectroscopic instruments will be mature, calibration in such a calibration facility will be essential before any field experiments.
There have been also preliminary study of the use of infrared fiber optics to ease propagation of infrared laser beams to measurement locations in hostile environment.
The field experiments carried with the infrared absorption spectrometers using diode lasers (SDLA) on an APZ combustor at DLR Koln and the exhaust gases of NLR’s Cessna Citation II research aircraft at a test site at Amsterdam Airport Schiphol showed the potential of optical probing in terms high repetition rate, multi-species non-intrusive measurements of minority effluents CO, CO2, H20,…in aircraft engine exhaust gases. This method is thought to replace advantageously intrusive sampling techniques for quick online inspection or more basic diagnostics during combustor development. The further developments of the MidDropo laser source and the Pico-second Lidar will bring higher spatial resolution as well as long distance range measurements and can also be envisaged for inflight measurements.