Over the last five years, the aviation sector has experienced a growth of around a 6% in air traffic due to several factors that favour its access on a large scale not only for tourism but for transportation. And the forecast is to continue the increase after recovering pre-COVID levels, expected as late as 2025.
However, air transport have associated adverse consequences such as pollutant emissions which have a direct effect on human health and the environment. This is why stringent regulations have been published all over the world to reduce aircraft pollutant emissions. An example of this is the Committee on Aviation Environmental Protection CAEP, which assists the International Civil Aviation Organization ICAO in formulating policies and adopting Standards and Recommended Practices related to aircraft noise and emissions.
In the eighth meeting of this Committee: the CAEP/8, Aircraft engines were committed to reduce in an average 30% nitrogen oxides from the standards established in CAEP/2 in 1992. Besides, from the tenth meeting, the First edition of Aeroplane CO2 Standard was published. And, between 2016 and 2019, in CAEP 10 and 11, standards for non-volatile particulate matter emissions were published.
AVIATOR project adopts a multi-level measurement, modelling and assessment approach in order to develop an improved description and quantification of the most relevant aircraft engine emissions, as well as their impact on air quality under different climatic conditions. In this case, special attention will be paid on non-volatile PM and volatile PM, and their gaseous precursors. In the work presented here, different results related to particulate matter and gases emissions obtained in the framework of this project are introduced.
Regarding the Facilities and Methodology section, here we have a scheme of INTA’s facilities. As it can be observed the multi-orifice probe, is installed 50 meters away from the engine exit plane. The probe is fitted in the exhaust stack and it has 6 sampling points, as can be observed in the image.
Gases emitted by the engine, and the exhaust flow pass through the probe and reach the mixing chamber. At the inlet of this chamber a nozzle is installed. in order to favour the mixture and homogenisation of these exhaust gases. As they pass through the mixing chamber, they reach a laminar regime and they are sucked out by the emission equipment.
To characterise the particle number concentration, a CPC from TSI and a Partector-2 from Naneos have been used. Besides, in order to include non-volatile particle number concentration, a catalytic stripper, which works at 350 ºC, has been installed upstream of these devices.
Non-volatile particle mass concentration has been assessed with a laser-induced incandescence device from Artium. A NanoDMA from TSI and an ELPI+ from Dekati were employed for the evaluation of particle size distributions. CO2 concentration has been studied with both devices the X-Stream from Emerson and LI-850 from Li-Cor.
Results: particle number and mass concentrations are presented. The total particle number concentration is depicted in green colour, the non-volatile particle number concentration appears in yellow colour and the non-volatile particle mass concentration in pink. At low engine power levels, we find a high particle number concentration, which can be explained by the low exhaust temperatures, insufficient to oxidize the particles emitted. Besides, the low exhaust speed, which implies a higher resident time of the particles play an important role because the adsorption process of volatile compounds on the surface of the particle is favoured.
In the way that the engine power level increases, the temperature increases and particle oxidation is favoured as it can be observed in the graph. At high engine power levels, it is observed a slight increase in the total particle number concentration. This can be explained through the rapid cooling process that particles suffer as a consequence of the high velocities of the exhaust plume. In this cooling process, volatile compounds condense on the particle surfaces.
The non-volatile particle mass concentration can be explained taking into consideration the particle size distribution. As it can be observed, as the engine power level increases, the particle size distribution change from a single-mode distribution around 8 – 9 nm to a bimodal distribution with an accumulation mode around 30 – 40 nm. Bigger particles have more importance in the particle mass concentration than smaller ones and for this reason the particle mass concentration increases as the engine power level increases.
The intercomparison between CPC and Partector 2 is shown. In the graph is presented the engine power level in blue colour and the results obtained with the CPC and the Partector 2 in green and red colour respectively. Furthermore, the average concentration values for each power level are included with dots. Here we can see how the low-cost sensor follows the same trend that high fidelity sensor. Another important conclusion is that the dispersion obtained with the Partector-2 is higher than those from the CPC. Besides, in general, the concentration obtained with the Partector 2 is higher.
Finally, there’s a comparison of the average particle number concentrations obtained with Partector and CPC as the engine power level evolves. It can be observed that the particle concentration follows the same trend in the previous slide.
Regarding CO2, in the graph presented it is shown the engine power level in blue colour and the results obtained with X-Stream and LI-850 in orange and in brown colour respectively. The average CO2 concentration values for each power level is presented with dots as well. As it can be observed X-Stream and LI-850 follow the same trend. In addition, values obtained with LI-850 are around 4% higher than those from X-Stream. Finally, in the graph in which is depicted the average CO2 concentration against the engine power level, both parameters have a direct relationship, as a consequence of a more efficient combustion process.
Summarizing the presentation, it has been observed that higher total particle number concentrations are related to low engine power levels, but an important proportion is due to volatile compounds. At high engine power levels, it has been observed that the total particle number concentration undergoes a slight growth, which is explained through the rapid cooling process that suffers the particles as a consequence of the high velocity of the exhaust plume. In this cooling process, the volatile compounds condense over the particle surface.
Another important conclusion is that as the engine power level increases, the non-volatile particle mass concentration increases do it so. This can be explained taking into account the particle size distribution which changes from a single-mode distribution around 8 – 9 nm to a bimodal distribution with an accumulation mode around 30 – 40 nm.
And finally, it has been seen that the results obtained with the low-cost sensors are in agreement with those obtained with the high fidelity sensors.