Determination of Particle Penetration Factors in a Particle Transfer Line for Aero Gas Turbine Engine Exhaust Particle Measurement
Mohamed A. Altaher
School of Process, Environmental and Materials
Engineering
University of Leeds
Leeds, UK
And now: Petroleum, and Chemical Engineering
Department
Sebha University
Sebha, Libya
Hu Li
School of Process, Environmental and Materials
Engineering
University of Leeds
Leeds, UK
Paul Williams
Center for Atmospheric Science University of Manchester
Manchester, UK
Mark Johnson
Rolls-Royce Plc
Derby, DE24 8BJ,
UK
Simon Blakey
Department of Mechanical
Engineering
University of Sheffield,
Sheffield, UK
ABSTRACT
There is a need to develop a reliable and standard PM (Particulate Matter) measurement method for aircraft engines. Due to safety and practicability of such measurements, a distance is required for the transportation of the exhaust samples from the aircraft engine exhaust exit to particle measurement instruments. The particle line loss during the transportation is therefore a critical issue for the accurate and reliable determination of particle emissions from aircraft engines. The work in this paper investigated the particle penetration/loss along a 25 meters ARP proposed particle sample transfer line by measuring the particle emissions from an aircraft auxiliary power unit (APU) at idle and full power. Two SMPS instruments were used to simultaneously measure exhaust particle size distributions at the entrance and exit of the 25 m transfer line. A catalytic stripper was used to remove volatile particles so that non-volatile particles can be measured. The particle penetration factors for the 25 m transfer line were found to be 0.6~0.7 in general, excluding particles smaller than 10 nm. For the particles smaller than 10nm, particle penetrations were very poor and about 70-100% of parcel losses were observed. The volatile factions were roughly 20~30% of the total concentrations. NOMENCLATURE
AFR: Air Fuel Ratio.
APU: Auxiliary Power Unit.
ARP: Aerospace Recommended Practice.
CAEP: Committee on Aviation and Environmental
protection.
CPC: Condensation Particle Counter.
CS: Catalyst Stripper.
DMA: Differential Mobility Analyser.
EGT: Exhaust Gas Temperature.
EPA: Environmental Protection Agency.
FOA: First Order Approximation.
GMD: Geometric Mean Diameters.
ICAO: International Civil Aviation Organization.
ID: Inside Diameter.
LPM: Liter per Minute.
nvPN: non-volatile Particle Number.
PM: Particulate Matter.
SMPS: Scanning Mobility Particle Sizer.
Proceedings of ASME Turbo Expo 2014: Turbine Technical Conference and Exposition
GT2014
June 16 – 20, 2014, Düsseldorf, Germany
GT2014-25440
SN: Smoke Number.
TPN: Total Particle Number.
INTRODUCTION
Particulate Matter (PM) emitted from aircraft engines is ultrafine particle with peak diameter less than 100nm [1, 2]. These particles can be divided into two categories: primary PM (non-volatile particles) emitted directly from the engine and the second PM which is volatile and semi volatile particles formed through sampling line. Jet engines are one of the main sources of soot particles particularly nano-particles in the atmosphere compared to other combustion sources [3, 4]. Aircraft and other airport activities in the airport have great effects on the air quality of both the airports and the neighboring regions. Emissions from aircrafts are not only injected into planetary boundary layer, but also, into the upper troposphere and lower stratosphere [4-6]. Particulate matter (PM) affects the air quality and has been classified as a potential health hazard according to Environmental Protection Agency (EPA) of the USA. The diameter of turbine engine’s PM is small in size with bimodal peaks in the distribution usually occurring near 30 and 100nm [7]. The measurement of the aircraft engine PM emission provides knowledge on the magnitude of effect this will contribute to the environment and would help both the aircraft manufactures and airport authorities to take actions to reduce emission levels by adopting various methods from manufacturing of low emission aircraft engines, cleaner fuels, effective use of aircraft route of operation to the use of low emission technology infrastructure in the airport. Authors had measured PM size distributions from a series of alternative and renewable aviation fuels/blends using the same APU engine and the same particle size measurement instrument and reported reduced particle emissions compared to Jet A-1 [8].
Currently probe measurement and ICAO methodology are being adopted in the measurement of gaseous and smoke emissions from aircraft engines and its Auxiliary power unit (APU) [6, 9]. The PM emission can be then determined by smoke number (SN) using the FOA method. There are no regulatory and certified methods for direct sampling and measurement of PM emitted from aircraft engines. As the emission regulation is getting more stringent, there is a need to develop a concept sampling system in terms of components, manufacture and operability to standardize PM measurement. However, further validation in terms of practicability and robustness of a sampling system capable of delivering non-volatile PM to both mass and number measuring analyzers is required in order that the SAE E31 can develop a working ARP for the measurement of non-volatile PM mass and number. Thus, the committee on Aviation and Environmental protection (CAEP) within ICAO anticipates the implementation of a non-volatile PM certification requirement by the end of 2016.
Due to safety rules and standards from the aircraft engine exhaust testing, a certain distance from the engine exhaust is required before the extraction of test samples can be taken. In reality there would have been a significant of chemistry and diffusion occurred before the exhaust sample can be extracted for measurement instruments. So it is crucial to determine the particle penetration/loss during transportation in the sample transfer line. The major factors affecting the particle loss in the sampling line include the residence time, coagulation of particles and wall losses due to collisions etc. Particle losses varied depending on particle compositions and sampling system configuration such as dilution method and sample flow rate etc. Timko et al [10] estimated particle losses of aircraft engine exhaust and found it was about 30% for particles ranging from 30nm to 300nm and 80% for particles range 10nm or less.
The work in this paper investigated particle penetration/loss factors along a 25 meters ARP proposed particle sample transfer line using an aircraft APU engine. Both total and non-volatile particle numbers were reported at the entrance and exit of the 25 m transfer line. So penetration factors for total and non-volatile particle numbers at idle and full power can be determined.
EXPERIMENTAL SET UP
Aircraft Auxiliary Power Unit (APU)
A re-commissioned Artouste MK113 APU (Figure 1) gas turbine engine was used. It is a single spool gas turbine engine, in which a centrifugal compressor is driven by two stage turbine through a single rotating shaft [11]. The APU was instrumented to monitor and record key engine operating conditions, such as temperatures, pressures, engine RPM, fuel flow rates etc. Table 1 lists the actual values for Exhaust Gas Temperature (EGT), Air Fuel Ratio (AFR) and fuel flow rate achieved when the APU was burning Jet A-1 at two operating conditions - idle and full power.
Figure 1: Auxiliary Power Unit (APU) Engine
Table 1: APU measured operating conditions
Fuel
Conventional kerosene fuel Jet A-1 was used. The typical physical and chemical properties include ratio of hydrogen
to carbon, sulphur, aromatic content and density are shown in Table 2.
Table 2: Fuel properties . Particulate sampling system
Overview
A direct probe sample extraction measurement method from the exhaust of the auxiliary power unit (APU) was used. Two stainless steel sampling probes with an ID of 7.74mm were used at the exhaust outlet of the APU, with the aid of a stainless steel plate fixed behind the exhaust outlet to hold the sampling probe to sit just half way out of the exhaust diameter allowing for less than 5% space occupied at the exhaust as required by the Aerospace Recommended Practice (ARP1256). One was used to check particulate signature during the test and the other one was used as a sampling system. The temperature of the exhaust was monitored using thermocouple to make sure it was above 160?C before entering the heated line section. Figure 2 shows the schematic view of the sampling system set up. This set up was used for the investigation of a complete non-volatile PM sampling system including the 2nd dilution. In this paper, the focus is concentrated on the particle line penetration/loss across the 25 m transfer line, i.e. from point 2 to 3. Point 1 is the upstream of the first dilutor, followed by a heated dilutor with a temperature of 160 o C and dilution ratio of 10:1 in which nitrogen gas was introduced. The diluted exhaust gas sample was then connected to a 25 meters particle sample transfer line which was heated to 160+/-15 o C and made of grounded carbon-loaded PTFE with a ID of 7.7-8.05 mm. The flow rate was set at 25+/- LPM. Point 2 is the start of the 25 m particle transfer line. At the end of the 25 m heated line is the point 3, followed by a cyclone to remove particles larger than 1 μm. A Vapour Particle Remover (VPR) was installed after the
cyclone to remove volatile fractions of particles by heating and introducing the second dilution by nitrogen. The exhaust gas sample after the VPR was passed through several particle measurement instruments including two SMPS, with one fitted with long DMA and the other one fitted with nano-DMA. These two instruments were connected to different sampling points in turn so the particle concentrations and size distributions at the different locations of the sampling line were determined. A DMS500 particle measurement instrument was employed at the fixed location (point 2) acting as a “policeman” to monitor the exhaust stability. All the measurements were undertaken when the engine was completely stabilized.
Nitrogen gas was used to set a zero calibration of the measurement instrument before the start of the testing thereby giving a much accurate reading when the sample is being passed through the line to the measuring instrument.
Figure 2: Schematic view of the sampling system set up
Catalytic stripper (CS)
A catalytic stripper (CS) was used to oxidize and remove volatile and semi-volatile particulate matter, typically organic carbon, which has been proved to have an removal efficiency of 99.98% [12]. Inorganic compounds such as SO 2 and sulphuric acid are chemically absorbed to the catalyst. This was done by passing the exhaust over the oxidation catalyst which was heated to 300?C. This method differs from other methods such as a thermal denuder that remove gas phase material via physical adsorption.
The catalytic stripper was connected to the sampling line at different sampling points and then connected to two SMPS instruments. So the non-volatile particles can be measured. The results were compared to the measurements without the CS fitted. The volatile particles can thus be determined. A line loss correction factor of 0.3 (30% loss cross the CS) were used and incorporated into the results. The CS flow rate was set to 1.5 L/min.
2nd diluter
1st diluter
Cyclone
1
2 3
4 5
25m
10m APU
Particulate measurement instruments
Two scanning mobility particle sizers (SMPS) coupled with two CPCs (TSI Model 3025&3085) were employed to measure particle size distributions and mean particle diameter according to their mobility through an electric field. The first SMPS with nano-DMA for particle size range measurement of 3-51 nm, while the second one with long-DMA was used for range 16-500 nm. The sheath flow and sample flow are 15 L/m and 1.5 L/m respectively. The scan up time of 100 seconds and retrace time of 15 seconds were selected for SMPS. The high flow rate setting of the CPC at (1.5 L/m) was chosen.
The combination of two SMPS enabled the measurement of particle size distribution of 5-400 nm. Both SMPS instruments were moved from various points to measure particle size and concentrations with and without the CS so total particles, volatile and soot particles can be determined at different locations on the sampling line. Particles at point 1 could not be directly measured due to too high particle concentrations and condensations.
RESULTS AND DISCUSSIONS
Total and non-volatile particle concentrations as a function of particle size and engine power
Figures 3 & 4 present the TPN (Total Particle Number) and nvPN (non-volatile Particle Number) size distributions for point 2 and 3 at idle and full power respectively. TPN here is defined as the number distribution of total particles across the whole particle size range. Similarly, nvPN is the number distribution of non-volatile particles across the whole particle size range. All the results show mono-modal distributions which is different from the size distribution measured by DMS. There are noticeable line losses for TPN and nvPN. The peak particle concentrations for TPN at point 3 are ~60% of that at point 2 at the idle power condition, indicating a ~40% line loss at peak particle size (~30nm), as a result of particle coagulation and particle wall line loss taking place over the 25 meters particle transfer line. At the full power condition, the line losses are reduced with a ~35% at the peak particle size due to higher particle concentrations as shown in Figure 4. The results in Figure 3 and Figure 4 also indicated the increase of solid particle shares at full power compared to that at idle. Figure 3: Total and non-volatile particle size distributions at point 2 and 3 at idle engine power condition
Figure 4: Total and non-volatile particle size distributions measured at point 2 and 3 at full engine power condition Determination of particle penetration factors for total and non-volatile particles as a function of particle size and engine power
From the results in Figure 3 and Figure 4, particle penetration factors along the 25 meter particle transfer line at idle and full power conditions were determined for TPN and nvPN as shown in Figure 5 and Figure 6. Particle line losses can also be determined. The particle penetration factor is defined as:
Particle penetration factor = (dN/dlogDp) at point 3 / (dN/dlogDp) at point 2
Particle line losses = [(dN/dlogDp) at point 2 - (dN/dlogDp) at point 3] / (dN/dlogDp) at point 2)
Here, the unit for dN/dlogDp is Particles/cm3
For the particles smaller than 10nm, particle penetrations were very poor and about 70-100% of the TPN and nvPN were lost in the particle transfer line at both power conditions. The very low penetration factors for these nano-particles are considered due to the diffusion losses in the particle transfer line as the smaller particles have a stronger tendency for diffusion losses.Particle penetration factors increased rapidly as the particle size increased and there are different patterns between the idle and full engine power conditions. At the idle engine power condition, the particle penetration factor for TPN and nvPN was kept at about 60~70% for particles in the size range of 20-200 nm. At the full engine power condition, particle penetration factors continued to increase and finally there were hardly any line losses. The higher penetration factor at full power was likely due to the fact that the exhaust flow rate at full power was much higher compared to the idle
particle size
function of particle size
Ratios of nvPN to TPN at two engine power settings at points 2 and 3
Figures 7 and 8 present the ratio of nvPN to TPN for point 2 at idle and full power respectively. It can be seen that typically 70% of total particles are non-volatile particles or solid particles at idle, except particles smaller than 20nm. The removal of the volatile particles by the CS did not change the shape of the size distribution curve, i.e. there is no shift in the size distribution. This suggests that the volatile particles are distributed across the whole size range. However, this is contradictory to well-known knowledge that pure volatile particles are normally smaller than 20 nm [14]. At the full power condition, the ratio of volatile particles varied depending on particle sizes. There is a large fraction of volatile particles (~40%) in the size range of 60-100 nm, which are agglomeration particles. The relatively higher volatile fractions in this size range may suggest that there was a thin coating layer of liquid or volatile fraction on these particles, which were oxidized by the catalyst stripper.
Figure 10 and 11 show the ratio of nvPN to TPN for point 3 at idle and full power respectively. By comparison with point 2 results, it has shown that the fractions of volatile particles have been reduced to a level of ~25% for the particles in the size range of 20~50 nm at idle and 25~30% for all particles lager than 20 nm at full power along the 25 m particle transfer line. However, the reductions of volatile particles from point 2 to point 3 are not significant, indicating proportional particle losses along the 25 m transfer line between solid and volatile particles.
There were no significant volatile particles detected at nucleation mode, indicating that nucleation mode particles were solid particles.
Figure 7: nvTP to TPN and their ratio at point 2 on idle condition
Figure 8: nvPN, TPN and their ratio at point 2 on full power condition
Figure 9: nvPN, TPN and their ratio at point 3 on idle condition
Figure 10: nvPN, TPN and their ratio at point 3on full power condition
Comparison of total number concentrations, Geometric mean diameter for total and non-volatile particles
Figure 11presents the total number concentrations (summated particle concentrations across the whole size range) for total and non-volatile particles at different sampling points and engine power settings. Figure 12 shows the results normalized to point 2 for idle and full power respectively. These enabled the determination of the overall particle line losses or penetration factors at idle and full power. It can be seen that there are no differences in particle penetration factors between total particles and non-volatile particles along the 25 m sample transfer line at both engine power settings. Also, there are no differences in particle penetration factors between idle and full engine power. A generic penetration factor of 0.6 or 40% particle line loss is thus obtained to represent both total and non-volatile particles at idle and full power conditions.
Figure 13 compared the ratios of non-volatile to total particles in terms of total number concentrations at point 2 and 3 for both engine power settings. The volatile particles account for 20 to 26% of the total particles with the highest one at point 2 in the idle mode. By comparison of the ratios between idle and full power at point 2, it can be seen that the fraction of volatile particles was noticeably reduced at full power, due to less hydrocarbon emissions being generated at full power. There is no difference in the ratios between idle and full power at point 3, indicating some changes in the particle compositions between volatile and non-volatile particles along the 25 m transfer line.
Figure 14 shows the Geometric Mean Diameters (GMD) of total and non-volatile particles for point 2 and 3 at idle and full power respectively. The results showed that there are changes in particle size distribution characteristics from idle to full engine power, indicated by the increase of the GMD. The 25 m particle sample transfer line had shifted the particle size distribution towards slightly larger particles for both total and non-volatile particles. This is a result of more losses for smaller particles as smaller particles have a stronger tendency for diffusion losses along the sample line. The results in Figure 14 also show that the removal of volatile particles was in all size ranges as there is no difference in total particle GMD and non-volatile particle GMD. This is in good agreement with the result in figures 7 to 10.
Figure 11: Total particle number concentrations for total particles and non-volatile particles at point 2 and 3 on idle and full engine power conditions
Figure 12: Total particle number concentrations normalized to point 2 for total particles and non-volatile particles on idle and full engine power conditions
Figure 13: Ratio of non-volatile to total particles at different sampling points and engine power settings
Figure 14: Comparison of Geometric Mean Diameter (GMD) of total and non-volatile particles
CONCLUSIONS
Total and non-volatile particle numbers from the exhaust of a full size aerospace engine - an auxiliary power unit (APU) were measured with an aim to investigate the particle penetration factors or line losses along an ARP recommended particle sample transfer line. Two SMPS instruments (nano-DMA and long-DMA) were used for the measurement of particle size distributions. The following findings can be drawn from the results:
1)For the particles smaller than 10nm, particle
penetrations were very poor and about 70-100% of the
total particles and non-volatile particles in terms of
number were lost along the 25 m particle transfer line
on both idle and full power conditions. There are
almost no differences in particle penetration factors in
terms of total particle numbers and non-volatile
particle numbers. This indicated that these smaller
than 10 nm nano-particles were nucleation mode solid
particles.
2)Particle penetration factors increased rapidly as the
particle size increased from 10 nm onwards. There are
different particle penetration patterns for the idle and
full engine power conditions. At the idle engine power
condition, the particle penetration factors for the total
and non-volatile particle numbers were about 0.6~0.7
for particles in the size range of 20-200 nm. At the full
engine power condition, the particle penetration
factors showed a continuous increasing trend and
finally reached full penetration where there were
hardly any line losses. For total particle numbers, the
full penetration was achieved when the particle size
reached 140nm. As most of the particle sizes was
distributed between 20 to 100 nm, a value of 0.6~0.8
can be used for the total particle number penetration
factor at full power. For non-volatile particle numbers,
the full penetration was achieved at ~80 nm. Different
penetration factors may be needed for different sizes
of non-volatile particles.
3)By comparison of total particle number
concentrations, it is observed that a uniformed
penetration factor of ~0.6 could be applied to total and
non-volatile particles for both engine power
conditions.
4)The volatile particles were mainly detected in the size
range 20 nm and above with a fraction of 20~30%. It
is a surprise that volatile particles were not in
nucleation mode and instead in agglomeration mode.
5)The 25 m particle sample transfer line shifted particle
sizes slightly, reflected by a small increase in particle
GMD from point 2 to 3. Average particle sizes
measured by GMD increase as engine power
increased.
ACKNOWLEDGMENTS
This work was supported by the European Aviation Safety Agency (EASA) project: Studying, sampling and measurement of aircraft particulate emissions III” (SAMPLE III), under the Implementing Framework Contract No: EASA.2010.FC10. Thanks go to the University of Minnesota for providing a catalyst stripper.
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