AMT - Measurement of black carbon emissions from multiple engine and source types using laser-induced incandescence: sensitivity

AMT - Measurement of black carbon emissions from multiple engine and source types using laser-induced incandescence: sensitivity to laser fluence
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© Author(s) 2022. This work is distributed under
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© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.
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Research article
19 Jan 2022
Research article |
19 Jan 2022
Measurement of black carbon emissions from multiple engine and source types using laser-induced incandescence: sensitivity to laser fluence
Measurement of black carbon emissions from multiple engine and source types using laser-induced incandescence: sensitivity to laser fluence
Measurement of black carbon emissions from multiple engine and source types using laser-induced...
Ruoyang Yuan et al.
Ruoyang Yuan
Prem Lobo
Greg J. Smallwood
Mark P. Johnson
Matthew C. Parker
Daniel Butcher
and
Adrian Spencer
Ruoyang Yuan
CORRESPONDING AUTHOR
ruoyang.yuan@sheffield.ac.uk
Department of Mechanical Engineering, University of Sheffield,
Sheffield, S1 3JD, United Kingdom
Prem Lobo
Metrology Research Centre, National Research Council Canada, Ottawa,
Ontario, K1A 0R6, Canada
Greg J. Smallwood
Metrology Research Centre, National Research Council Canada, Ottawa,
Ontario, K1A 0R6, Canada
Mark P. Johnson
Rolls-Royce plc, Derby, DE24 8BJ, United Kingdom
Matthew C. Parker
Rolls-Royce plc, Derby, DE24 8BJ, United Kingdom
Daniel Butcher
Department of Aeronautical and Automotive Engineering, Loughborough
University, Loughborough, LE11 3TU, United Kingdom
Adrian Spencer
Department of Aeronautical and Automotive Engineering, Loughborough
University, Loughborough, LE11 3TU, United Kingdom
Abstract
A new regulatory standard for non-volatile particulate
matter (nvPM) mass-based emissions from aircraft engines has been
adopted by the International Civil Aviation Organisation. One of the
instruments used for the regulatory nvPM mass emissions measurements in
aircraft engine certification tests is the Artium Technologies LII 300,
which is based on laser-induced incandescence. The LII 300 response has been
shown in some cases to vary with the type of black carbon particle measured.
Hence it is important to identify a suitable black carbon emission source
for instrument calibration. In this study, the relationship between the nvPM
emissions produced by different engine sources and the response of the LII 300 instrument utilising the auto-compensating laser-induced incandescence
(AC-LII) method was investigated. Six different sources were used, including
a turboshaft helicopter engine, a diesel generator, an intermediate pressure
test rig of a single-sector combustor, an auxiliary power unit gas turbine
engine, a medium-sized diesel engine, and a downsized turbocharged direct-injection gasoline engine. Optimum LII 300 laser fluence levels were
determined for each source and operating condition evaluated. It was found
that an optimised laser fluence can be valid for real-time measurements from
a variety of sources, where the mass concentration was independent of laser
fluence levels covering the typical operating ranges for the various
sources. However, it is important to perform laser fluence sweeps to
determine the optimum fluence range as differences were observed in the
laser fluence required between sources and fuels. We discuss the
measurement merits, variability, and best practices in the real-time
quantification of nvPM mass concentration using the LII 300 instrument and
compare that with other diagnostic techniques, namely absorption-based
methods such as photoacoustic spectroscopy (using a photoacoustic
extinctiometer, PAX, and a micro soot sensor, MSS) and thermal-optical
analysis (TOA). Particle size distributions were also measured using a
scanning mobility particle sizer (SMPS). Overall, the LII 300 provides
robust and consistent results when compared with the other diagnostic
techniques across multiple engine sources and fuels. The results from this
study will inform the development of updated calibration protocols to ensure
repeatable and reproducible measurements of nvPM mass emissions from
aircraft engines using the LII 300.
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Yuan, R., Lobo, P., Smallwood, G. J., Johnson, M. P., Parker, M. C., Butcher, D., and Spencer, A.: Measurement of black carbon emissions from multiple engine and source types using laser-induced incandescence: sensitivity to laser fluence, Atmos. Meas. Tech., 15, 241–259, https://doi.org/10.5194/amt-15-241-2022, 2022.
Received: 16 Jul 2021
Discussion started: 16 Aug 2021
Revised: 20 Nov 2021
Accepted: 22 Nov 2021
Published: 19 Jan 2022
Copyright statement
The works published in this journal are distributed under the Creative Commons Attribution 4.0 License. This license does not affect the Crown copyright work, which is re-usable under the Open Government Licence (OGL). The Creative Commons Attribution 4.0 License and the OGL are interoperable and do not conflict with, reduce, or limit each other. The co-authors Prem Lobo and Greg J. Smallwood are employees
of the Canadian Government and therefore claim Crown copyright for the respective contributions.
© Crown copyright 2022.
Introduction
Short- and long-term exposure to particulate matter (PM) can lead to serious
health problems such as lung or heart disease (AQEQ,
2005). There is increasing emphasis on reducing PM emissions from energy
conversion systems, especially for the transport sector. The World Health
Organisation (WHO) reported that up to 50 % of PM emissions in OECD
countries were caused by road transport, in which diesel traffic was the
majority (United Nations Environment Programme World Health
Organisation, 2009). In addition to road traffic, aviation and shipping are
also significant sources of PM emissions in the transport sector. Therefore,
it is strategically important to limit the exposure to PM from various
sources, and accurate measurement of PM is a key enabler to achieving this
goal and developing low-PM-emission transport solutions.
The International Civil Aviation Organisation (ICAO) Committee on Aviation
Environmental Protection has adopted new standards and recommended practices
limiting aircraft engine non-volatile particulate matter (nvPM) (also
referred to as soot or black carbon) number and mass emissions to mitigate
the impact of aircraft engine emissions on local air quality
(ICAO, 2017). The nvPM refers to the particles that exist at the
aircraft engine exhaust nozzle exit plane that do not volatilise at
temperatures greater than 350
C (ICAO, 2017). A standardised
sampling and measurement methodology for aircraft engine nvPM emissions has
been developed by the Society of Automotive Engineers (SAE), which has been
detailed in Aerospace Information Report (AIR) 6241 (SAE,
2013) and Aerospace Recommended Practice (ARP) 6320 (SAE,
2018) and adopted by ICAO (ICAO, 2017). The SAE and ICAO documents also
define the performance criteria and calibration protocols for the
instruments to be used in the standardised measurements. Currently, the two
instruments that satisfy the performance criteria for nvPM mass measurements
are the Artium Technologies LII 300, based on laser-induced incandescence
(LII), and the AVL micro soot sensor (MSS), based on photoacoustic
spectroscopy. The standardised systems for aircraft engine nvPM emissions
measurements using these instruments have been previously evaluated and
inter-compared
(Lobo
et al., 2015a, 2020).
The instruments used to measure nvPM mass concentration in the exhaust of
aircraft engines are calibrated by reference to elemental carbon (EC)
content, as determined by thermal-optical analysis (TOA) of a diffusion
flame combustion aerosol source
(Durdina et al., 2016). TOA is
an industry-accepted method for measuring the mass of both organic carbon
(OC) and elemental carbon (EC) aerosols sampled on a quartz filter
(Baumgardner et al., 2012). The only
criteria for the source used for calibrating nvPM mass instruments is that
it be capable of producing an average EC content of the collected mass on
the filter of
≥80
% (SAE, 2018).
The LII techniques have been shown to be sensitive to different black carbon
types, for both continuous-wave laser LII (Baumgardner
et al., 2012; Gysel et al., 2012; Laborde et al., 2012) and pulsed laser
LII, as used in the LII 300
(Durdina et al., 2016). The
relationship between the nvPM emissions produced by these different sources
and the response of the LII 300 instruments has not been fully investigated.
It is important to identify a suitable nvPM emissions source that meets the
requirements to provide reliable and robust calibration of the LII 300
instrument used in the emissions certification of aircraft engines.
The LII technique has been widely applied in combustion studies to obtain
in situ black carbon characteristics, such as soot volume fraction
(Boiarciuc et al., 2006; Choi and Jensen, 1998; Liu et al., 2011; Melton,
1984; Quay et al., 1994; Zhang et al., 2019) and primary particle size
measurements (Axelsson et al., 2000; Boiarciuc et al., 2006; Boies et al.,
2015; Mewes and Seitzman, 1997) from both laboratory flames (Axelsson et
al., 2000; Mewes and Seitzman, 1997; Tian et al., 2017; Zhang et al., 2019)
and aircraft engines (soot concentration: Schäfer et al., 2000; Delhay
et al., 2009; Black and Johnson, 2010; Petzold et al., 2011; Lobo et al.,
2015a; soot particle sizes: Boies et al., 2015). Previous reviews on LII have provided insight on current challenges, questions associated with this
technique (Schulz et al., 2006), and current model developments (Michelsen
et al., 2015). Several models of the LII technique have been reviewed to
quantify the soot concentration, primary particle size, and peak temperature
(Eckbreth, 1977; Melton, 1984; Roth and Filippov, 1996; Smallwood et al.,
2001; Schittkowski et al., 2002; Lehre et al., 2003; Snelling et al., 2004;
Kock et al., 2006; Michelsen et al., 2007; Sipkens and Daun, 2017). These
were required because the complex mechanisms incorporated in the LII  model strongly influenced the predicted signal magnitudes and time evolution. The
signal dependence on the laser fluence and in turn the quantification of
soot concentration were highlighted in each model. As the laser fluence
increases, soot absorbs increasing amounts of light energy, resulting in
greater particle peak temperature and radiation magnitude. The subsequent
decay rate of this radiation is related to the particle temperature, which
in turn is related to the primary particle size. Sublimation can also occur
at excessively high laser fluence levels, characterised by the
particle-volume reduction. According to the model by
Michelsen et al. (2003), the maximum LII signal
may level off or be reduced due to this mass loss. Although there are
differences in the temporal profiles, the various models provide similar
results for the relative signal magnitudes as a function of laser fluence
(Schulz et al., 2006; Sipkens and
Daun, 2017). In conventional LII techniques, the determination of the LII fluence level directly affects the
accuracy of the quantitative measure of the soot concentration.
Additionally, further calibration (via laser extinction for example) is
required to obtain absolute rather than relative measurements. The
requirement to accurately represent the target nvPM absorption and
sublimation characteristic during calibration is an additional complexity
associated with the LII technique. An improved approach, described in the
literature
(Snelling
et al., 2005; Thomson et al., 2006), utilises an auto-compensating
laser-induced incandescence (AC-LII) method. By measuring LII signals at two
separate wavelengths and temporally resolved decay rates, AC-LII has the
potential to resolve the soot temperature, volume fraction, and particle
size without the need for additional calibration sources. This method was
incorporated into the commercial LII 300 instrument by Artium Technologies
Inc.
The goal of this study is to assess the suitability of the nvPM emissions
from a variety of different engine sources and fuels to be used as a
calibration source for the LII 300 instrument for application to the
emissions certification of aircraft engines. It was important to identify a
source that met all the requirements, produced a similar response from the
LII 300 to that obtained from nvPM emissions from an aircraft gas turbine
engine, and was economical and practical to operate. In this study, we evaluated a range of different combustion sources that could potentially be used as calibration sources, including a turboshaft helicopter engine (already verified as an applicable aircraft engine calibration source for LII 300), an intermediate-pressure single-injector combustor test rig, an auxiliary power unit gas turbine engine, a diesel generator, a medium-sized diesel engine, and a downsized turbocharged direct-injection gasoline engine. The turboshaft helicopter engine, intermediate-pressure single-injector combustor test rig, and auxiliary power unit gas turbine engine were fuelled with kerosene, while the diesel generator and medium-sized diesel engine burned diesel, and the downsized turbocharged direct-injection gasoline engine utilised gasoline.
The relationship between the nvPM emissions produced by the different engine
sources and the response of the LII 300 instrument was investigated. Laser
fluence sweeps were performed at different operating conditions, and the
fluence dependence of and variability in the technique for field measurements
of different nvPM sources were studied. The measurements using the LII 300
were compared with other diagnostic techniques, including thermal-optical
analysis and photoacoustic spectroscopy, for comparison. The relationship
between laser fluence and nvPM mass concentration measurement is discussed.
The results from this study will help identify nvPM sources that can be used
to calibrate the LII 300 instrument and inform the development of updated
calibration protocols to ensure repeatable and reproducible measurements of
nvPM mass emissions from aircraft engines using the LII 300, including a
procedure to optimise the laser fluence.
Experimental method
2.1
Diagnostics methods
2.1.1
Laser-induced incandescence (LII)
Laser-induced incandescence (LII) measures the thermal emission from soot
particles heated by a laser to temperatures in the 2500–4500 K range
(Bachalo et al., 2002). Assuming that all the
volatile and semi-volatile OC that condenses on the BC particles will be
evaporated promptly at these temperatures, the LII signals are directly
related to the non-volatile particles, primarily refractory black carbon
(rBC), a form of carbon directly related to EC
(Baumgardner et al., 2012). The LII 300
instrument calculates a mass concentration from the particle emission signal
and the temperature determined from the signal at two wavelengths (446
and 720 nm) using two-colour pyrometry. As the optical properties of the
particle may vary from source to source, a correlation factor may need to be
applied to this optical determination of the mass concentration to relate
the result to that of EC determined from TOA. A multimode pulsed Nd:YAG
laser, operating at a fundamental wavelength of 1064 nm, with a pulse duration
of 7 ns full width half maximum (FWHM) and a repetition rate of 20 Hz was
used as the excitation source. The laser beam passed through a set of optics
and formed into a square cross-section light beam uniformly incident
throughout the probe volume. The measurement volume is approximate 14.7 mm
, and the range of fluence levels is typically 0.6–3.2 mJ mm
−2
At low laser fluence levels, the peak LII signal intensity rises with the
laser fluence as peak particle temperatures increase. As laser fluence is
increased, a threshold is reached where the measured mass concentration
becomes independent of laser fluence. This plateau region extends for a
range of increasing laser fluence until a level is reached where soot
sublimation begins to dominate. At these very high laser fluences, the mass
loss associated with soot sublimation has an influence, and a reduction in
the mass concentration may be observed (Michelsen et al., 2015). Various soot
fragments evaporate from the particle surface when sublimation occurs
(Schulz et al., 2006). Models usually assume these to
range from C1 to C7 (molecules having one to seven carbon atoms), dependent on the
particle temperature (Leider et al., 1973).
Additional research is required to understand the sublimation loss, which is
complicated by uncertainties in physical parameters of the carbon species,
limiting the ability of the models to fully predict phenomena in this regime
(Michelsen et al., 2015).
In the AC-LII technique (Snelling et al., 2005), the soot temperatures are
determined by the two-colour pyrometry method (in the LII 300, narrow
bandpass filters centred at
∼446
nm and
∼720
nm are used with bandwidths of 40 and 20 nm, respectively). The peak soot
temperature and absolute intensity of the LII signal were used to calculate
soot volume fraction (
). With soot particle material density (
) from the literature, the mass concentrations of the non-volatile
particles, or EC, are obtained from
. The
primary particle sizes are determined from the temperature decay rates with
the assumption that conduction dominates the particle cooling rates for
conditions below the sublimation limit
(Smallwood, 2008).
Theoretically, the mass concentration obtained from the two-colour method is
valid regardless of the laser fluence applied, and a lower laser fluence
level might be preferred to avoid the reduction in particle volume
(sublimation) at high laser fluence. Practically, care should be taken with
low laser fluence levels for particles from unknown sources with unknown
combination of size distributions and morphologies as the assumption of the
uniform temperature in the sampling volume could be invalid (Liu et al.,
2016). The particles with various sizes could reach different peak
temperatures at separate times, which induce uncertainties with the
determination of the effective peak particle temperatures and in turn
uncertainties with the derived mass concentration values. For a typical
effective soot temperature of 4000 K, a 100 K error (2.5 % of the soot
temperature) can lead to 15 % error in the estimated soot volume fraction
value (Boiarciuc et al., 2006). It is therefore important
to operate the LII 300 with optimum laser fluence, in a region of laser
fluence where the reported mass concentration is relatively independent of
fluence, to minimise uncertainties. In the current work, laser fluence sweep
experiments (varying laser fluence from low levels to high fluence
conditions with all other parameters held constant) were performed for each
source at different operating conditions. This was performed to examine the
relationship between the measured mass concentrations and LII fluence levels
for multiple sources and to subsequently determine the optimum laser
fluence level for both calibration and application to real-time nvPM mass
measurements from aircraft engines.
2.1.2
Thermal-optical analysis (TOA)
TOA measures the mass of both organic carbon (OC) and elemental carbon (EC)
of the particulate matter sampled on a quartz filter mounted in a stainless
steel filter holder. This is not a gravimetric measurement where the mass of
the elemental carbon was measured, but rather a chemical analysis of the
black carbon on the filter. The NIOSH 5040
(NIOSH, 2003) standard method was used for the
TOA, with a modified temperature ramp as specified by ARP 6320 (SAE, 2018),
to obtain the EC mass concentrations. The uncertainty associated with this
method for EC mass concentration is stated to be 16.7 % (NIOSH, 2003) due
to additional uncertainty in assigning the split between OC and EC. The
uncertainty associated with the total carbon (TC) mass concentration is
8.4 % (Conrad and Johnson, 2019). A basic overview of the process is presented here.
The sample flow through the quartz filters was set to 22.7 slpm for
all measurement conditions, which was controlled by a calibrated mass flow
controller. Mass flow controllers were calibrated using a Sensidyne
Gillibrator-2 bubble flow meter and corrected to the standard temperature and pressure reference conditions of
C and 1 atm. The total volume of sample on the quartz filters
varied from 360 to 1820 L, depending on the loading for a particular
condition to achieve a target mass density of
10
mg  cm
−2
The particulate matter (mostly black carbon) in the sample flow was captured
by the filter. After collection, a 1 cm
1 cm punch from the filter
was cut out and mounted in an analyser (Sunset Laboratory OC-EC Aerosol
Analyser) to obtain the EC and OC content. The filter punch was heated in
the analyser oven in stages. From 0 to 425 s, the sample was surrounded by
an inert helium atmosphere, and the heating process drove off any embedded
volatile organic compounds (VOCs) from the sample. The desorbed VOCs were
further drawn into a series of catalyst beds, where they were converted to
CO
and then to methane, whose concentration was measured by a flame
ionisation detector (FID) to give a measure of the organic carbon fraction
on the filter punch. As the VOCs are heated, it is possible that some were
pyrolysed and converted to char, which stayed on the filter punch, instead
of being driven off the surface. The quantity of the char was measured via
laser extinction propagated through the sampled filter punch. From 425 to
800 s, the sample was surrounded by a reactive oxygen–helium atmosphere. As
the oven heated the sample, the elemental carbon reacted with the oxygen to
form CO
, which was again converted to methane and measured by the FID.
The elemental carbon fraction of black carbon on the filter punch was then
measured by subtracting the char (which was originally organic carbon)
content from the total. With the two mass values quantified, i.e. the OC
mass fraction and the EC mass fraction, the total carbon (TC) on the filter
punch can be obtained. By multiplying by the stain area of the filter, the
total mass of EC, OC, and TC on the entire filter was quantified. Having
also measured the total volume of sampled gas, the mass concentration of the
EC, OC, and TC was obtained. One uncertainty in the OC quantification from
the TOA measurements was the artefact effect caused by the gas-phase
semi-volatile organic compounds absorbed on the filter
(Durdina et al., 2016). To
correct this artefact, the OC determined from a secondary filter that
contained exclusively absorbed gas-phase OC was subtracted from the OC mass
determined from the primary filter.
2.1.3
Photoacoustic spectroscopy
In this study, a photoacoustic extinctiometer (PAX; Droplet Measurement
Technologies, Inc.) and a micro soot sensor (MSS; AVL, model 483) were used
to provide additional measurements for nvPM mass concentrations. Both the
PAX and MSS are based on the photoacoustic method
(Adams et al., 1989),
measuring the acoustic wave generated by the heated gases surrounding the
light-absorbing particles due to their increased temperature
after
interaction with a laser. Unlike filter-based absorption methods, where
uncertainties related to organic aerosol coating and light-absorbing
properties are issues
(Baumgardner
et al., 2012; Corbin et al., 2019), photoacoustic measurements can
potentially be affected by the humidity via latent heat and mass transfer
from volatile droplets (Arnott, 2003) and
absorption by molecular species present in the gaseous medium. The PAX and
the MSS measurements rely on the light-absorption properties of the PM
sources, for which a mass absorption cross-section (MAC) is usually
assumed. In this work, a MAC of 7.5
1.2 m
−1
at a
wavelength of 550 nm was applied to the PAX following the recommendation by
Bond and Bergstrom (2006), noting that the MAC
will vary with the black carbon source and operating condition. The MAC for
the PAX (with a measured absorption at a wavelength
of 870 nm)
was converted using MAC(
MAC(550 nm)
(550 nm
), assuming the refractive index was the same at those
wavelengths, equivalent to stating that the aerosol Ångström
exponent (AAE) is 1. The PAX operates at a wavelength of 870 nm and the MSS
at 808 nm. The MSS was previously calibrated with a soot source (CAST) of
known concentration (determined by comparison with EC from TOA), while the
PAX was calibrated separately with ammonium sulfate and Aquadag solutions.
2.1.4
Scanning mobility particle sizing
Although the focus in this work was on the quantification of nvPM mass
concentration, the particle size distributions from different engine sources
were measured using a scanning mobility particle sizer (SMPS; TSI, USA). The
SMPS consisted of an electrostatic classifier (model 3082), a differential
mobility analyser (model 3081), a soft X-ray neutraliser (model 3088), and
an ultrafine condensation particle counter (model 3776). The particle size
distribution data of the nvPM emissions from the various sources provide
additional information on the physical properties of the particles. This
will also provide information for the LII model development in terms of
particle size distribution functions. The typical size range for a SMPS scan
was 7–206 nm.
2.2
Test rigs and fuels
Multiple sources (test rigs) were used to study the LII 300 instrument's
response to the nvPM exhaust emissions at steady-state conditions. These
rigs included an aircraft gas turbine turboshaft engine (Rolls-Royce Gnome
helicopter engine, Rig A), an intermediate-pressure gas turbine combustor
rig (Rolls-Royce IP rig, Rig B), a gas turbine auxiliary power unit (Rover
2S/150 APU, Rig C), a diesel generator (Stephill Generators SE6000D4, Rig D), a naturally aspirated medium-sized (4.4 L) diesel engine (Rig E),
and a downsized (1.0 L) turbocharged gasoline direct-injection (GDI)
engine (Rig F). A schematic of the typical experimental set-up for the
different sources is shown in Fig. 1. The source exhaust was diluted with
HEPA-filtered air heated to 160
C. A cyclone with a 1
cut size at a flow rate of 8 L min
−1
was installed immediately downstream of the
diluted engine exhaust. The output flow from the cyclone was transferred
using a 3 m long heated (100
C) carbon-loaded PTFE sample line
and split to a pair of ejector diluters (Dekati, Model DI-1000), operating
in parallel to further dilute the sample. The flow from the ejector diluters
was combined and transported through a mixing section to the inlet of a
custom-built sampling tunnel with 12 ports, which was used to distribute
the diluted exhaust sample while ensuring there was excess flow. Carbon-impregnated silicone tubing (nominal
”) was used to transfer the sample to
the real-time instruments and the filter holders (for TOA, URG Corp. model URG-2000-30FVT). Quartz filter cassettes (model URG-2000-30FL) mounted in
the filter holder were used to hold the quartz filters.
Figure 1
Schematic diagram of the experimental set-up, illustrated for Rig A.
In this study, the focus was not on the performance of the test rig or the
operating conditions but rather the response of the diagnostic instruments
for a range of different combustion emission sources and associated fuel
types – kerosene (Jet A-1) for the gas turbine engines and IP rig, diesel
(EN590) for the diesel engines, and gasoline (EN228) for the GDI engine. The
IP combustor rig design was developed as a tool for gas turbine fuel spray
nozzle hardware ranking for soot production, similar to Makida et al. (2006). The soot formation–oxidation process and maturity are different to
those of typical rich-quench-lean gas turbine combustion. The real-time
measurements of the nvPM mass emissions from these rigs using the LII 300
and the other diagnostic instruments were recorded and compared. Table 1
summarises the operating conditions of the various test rigs. Fuel samples
were acquired from each of the test rigs and were subsequently analysed
(Table 2). The Jet A-1 fuels used in test rigs A, B, and C had similar
properties, as did the EN590 fuels for test rigs D and E.
Table 1
Test rigs and operating conditions.
Download Print Version
Download XLSX
Table 2
(a)
Physical and chemical properties for the Jet A-1 fuels (kerosene)
used in Rigs A, B, and C.
(b)
Physical and chemical properties for the EN590
fuels (diesel) used in Rigs D and E.
(c)
Physical and chemical properties for the EN228
fuel (gasoline) used in Rig F.
Download Print Version
Download XLSX
To ensure stabilised conditions were reached, the sources (test rigs) were
operated at the set point for a short period prior to nvPM data collection.
The exhaust temperature, measured with a thermocouple fitted to the exhaust
of each rig, along with other operating condition data was monitored and
available as an indication of combustion stability. Once these operational
parameters were determined to be stable, the data collection for that
particular set point was initiated. There were no exhaust aftertreatment
devices on any of the rigs in the current work.
Figure 2
Laser fluence optimisation procedure for mass concentration showing
(a)
all raw measurements;
(b)
initially normalised measurements using the TN
method; and
(c)
laser fluence sweep, superimposed with the best-fit curve
(black) of the normalised mass concentration data using the Loess method
with a second-order local polynomial regression.
Results and discussion
3.1
LII 300 fluence optimisation
For real-time nvPM mass quantification with the LII 300, a single laser
fluence level is typically used for the emission sources of interest. The
laser fluence level is important for the calibration of the instrument and
its application. For the experiments reported here, the laser fluence levels
were deliberately tuned over a wide range, from low to sublimation levels of
fluence, by adjusting the
-switch (
sw
) delay settings. Laser fluence
sweep measurements (adjusting from low to high fluence with a number of
discrete steps) were performed at steady-state engine operating conditions
to determine the optimum fluence for a particular nvPM source and operating
condition. For the laser fluence sweep tests at steady-state engine
operation, measurements are unlikely to be influenced by substantial variation
in the source emissions. A time-weighted normalisation (TN) method was used
to account for fluctuations in the measured concentration caused by any
modest variations in the concentration of the source emissions. This
involved many repeats at a reference
-switch delay during the fluence sweep
and using the mean value at this reference
-switch delay for initial
normalisation of all data. Figure 2 shows an example of the mass
concentration time series during one laser fluence sweep while measuring
the nvPM emissions from the IP gas turbine (Rig B). The red line denotes the
reference points at which the
-switch setting was fixed corresponding to
the maximum laser energy output. The colour map of data points shown in Fig. 2a correlates to the
-switch settings applied. Figure 2b is a time series
of the normalised mass concentration results from the time-weighted method.
A best fit was calculated on the TN-normalised mass concentrations using the
local polynomial regression method (Loess method). The final normalisation
was achieved by normalising the TN values to the peak value of the best-fit
curve, as shown in Fig. 2c. Fluence levels shown in Fig. 2c and in this
work are estimated from an internal energy meter and the laser beam
dimensions (measured by a beam profile camera) in the probe volume.
To further examine the role of source emissions variability in the LII 300
fluence sweep measurements, the reference points of the 135
-switch
delay from the LII 300 were plotted together with the data simultaneously
acquired by the PAX in Fig. 3. Results from the two sources (nvPM emissions
from the IP rig (Rig B) and from the diesel engine (Rig E)) are presented.
Despite subtle differences in the variability in the two sources, the trends
of the two signals between the PAX and the LII 300 are similar for both
cases, suggesting that the uncertainties observed in the LII 300 fluence
sweep measurements at the reference fluence points (red line in Fig. 2a)
were likely influenced by the variability in the source. The trend also
suggests engine thermal equilibrium was achieved after 600 s, and there was
∼2
% variability in mass concentration following this. In
the subsequent analysis, the TN method was used for the normalisation to
minimise the impact of source variability.
Figure 3
Mass concentration (normalised) measurements by the PAX and the LII 300 at its reference laser fluence points during fluence sweep tests using
two different sources: nvPM emissions from the IP combustor (Rig B) and from
the diesel engine (Rig E).
3.2
Laser fluence dependence – Rig A
3.2.1
Profiles of the measured mass concentrations
Figure 4 shows the LII 300 laser fluence sweep vs. normalised mass
concentration (solid symbols) from the Gnome engine exhaust (Rig A) fuelled
with kerosene at two different steady-state engine operating conditions –
(1) idle and (2) high power output (HPO). It is anticipated that the idle
and HPO conditions are two extremes in terms of the properties of the nvPM
being emitted from Rig A, with those at idle being less mature and with more
volatile organic compounds and the opposite for the HPO condition.
Performing fluence sweep tests at these two conditions aided in determining
an optimum laser fluence level to cover the full range of conditions from
this source for real-time nvPM mass measurements. At laser fluence levels
below 1.7 mJ mm
−2
(Fig. 4), the measurements showed an almost monotonic
increase with fluence between the measured nvPM mass concentrations and the
laser fluence. The measurement of nvPM mass concentration was independent of
the laser fluence levels in the range of 1.9 to 2.5 mJ mm
−2
for the HPO
condition and 2.2 to 3.2 mJ mm
−2
for the idle condition. The shaded
area in Fig. 4 corresponds to the data range that is within 2 % of the
peak value of the Loess best-fit curve
from the HPO condition. To determine an optimum laser fluence level
for real-time nvPM mass concentration measurements, a fluence level at which
the normalised mass concentration (NMC) data fell within the optimum range
(within 2 % error) at both conditions would be ideal as these two engine
conditions are anticipated to represent the extremes in terms of the
properties of the nvPM emissions. For the Gnome engine, a fluence level at
2.4 mJ mm
−2
(Fig. 4 arrow) was selected as optimum for this source
across all operating conditions and used in subsequent comparisons with
results from the other diagnostic techniques (as discussed in Sect. 3.5).
While not the focus of this study, it is interesting to note the impact of
laser fluence and source operating condition on the effective primary
particle diameters (ePPDs) resulting from the LII 300 measurements, as shown
in Fig.4 (via the decay rate of the LII signal) (Schulz et al., 2006). The
relationship between the effective primary particle diameter and the laser
fluence is most significant at low fluence levels (
1.9
mJ mm
−2
). As the reported sizes are larger than those reported for
similar sources (Saffaripour et al., 2017) and as the ePPD is dependent on
the rate of cooling of the particles, it is likely that there is an
additional mechanism inhibiting the conduction of energy from the particles
at low fluences. At high fluence levels (
2.5
mJ mm
−2
sublimation occurs, and there may be a modest mass loss and potential
reduction in particle size. In the region of optimum fluence the effective
primary particle diameter is relatively insensitive to laser fluence. At all
fluence levels, the effective primary particle diameters measured at idle
were larger than those measured at the HPO condition. The obtained ePPDs at
optimum fluence levels are within a similar range (6–19 nm by LII)
reported for other gas turbine engines (Boies et al., 2015; Saffaripour et
al., 2020). Further investigation into the ePPD measured by the LII 300 and
corresponding size comparisons with other techniques such as transmission electron microscopy (TEM) images will
be addressed in future work and is not discussed further as the focus of
this study is on nvPM mass concentration measurements.
Figure 4
Normalised mass concentration (NMC) mean and the derived effective
primary particle diameter (ePPD) mean across the different laser fluence
levels using the nvPM source from the kerosene-fuelled Rig A at high power
output (HPO) and at idle conditions (idle). The shaded area indicates the
±2
% data range of the HPO case. The arrow indicates an optimum laser fluence
level for real-time nvPM mass concentration measurements for all power
conditions.
3.2.2
Sublimation
It was observed that at the HPO condition for Rig A, the maximum nvPM mass
concentration was reached at a laser fluence of
∼2.25
mJ mm
−2
, beyond which the nvPM mass concentration decreased moderately
with increasing laser fluence, attributable to sublimation of the particles.
However, at the idle condition, no obvious sublimation was observed over the
range investigated for this laser fluence sweep experiment. The peak
particle temperatures reported by the LII 300, shown in Fig. 5a, permit
further investigation into the difference in response to the nvPM produced
at the idle and HPO conditions. The particles reach a lower peak
temperature
for the idle condition compared with that for HPO
across the range of laser fluence levels applied. The combustion is fairly
inefficient at the idle condition for Rig A, and
2000
ppm
of unburnt hydrocarbons has been previously observed at this condition. It is
suspected that due to the combustion inefficiency at the idle condition, the
soot particles measured at the engine exit plane were coated with VOCs
(Olfert et al., 2017). To overcome this, additional laser energy was
required to vaporise the VOCs, energy that was no longer available to heat
the nvPM. To illustrate the potential for this phenomenon, the data obtained
at the idle condition was aligned (via a shift by an arbitrary laser
fluence) with the HPO condition and replotted (Fig. 5b), with the mass
concentrations normalised as described previously. It was observed that the
relationship between the normalised mass concentration and laser fluence
exhibit very similar behaviour at both the HPO and idle conditions. At both
conditions, measured nvPM mass concentrations were found to be independent
of laser fluence levels in the range 1.8–2.7 mJ mm
−2
, noting that the
scale for the idle condition has been shifted.
Figure 5
(a)
Peak particle temperature mean from the LII 300 for the range of
laser fluence levels at the two operating conditions.
(b)
Shifting of the
data in Fig. 4 with the idle condition's data aligned with that for HPO. NMC (normalised mass concentration) using the nvPM source from the kerosene-fuelled Rig A.
3.2.3
Repeatability
The impact of the laser fluence on the mass concentration measurements shown
in Fig. 4 were also evaluated across multiple LII 300 instruments (LII 1
to LII 4) using the same nvPM source from Rig A to better understand
instrument-to-instrument differences and exclude potential single-instrument anomalies in the observed trends with laser fluence levels. The
results obtained from the HPO condition are plotted in Fig. 6a. The trends
of mass concentration measurements during the laser fluence sweep analysis
across the four different LII 300 instruments were similar, with minor
differences observed between the individual LII 300 instruments likely due
to modest variations in the beam profiles from the lasers, the
specifications of the optics, and the resulting shape and size of the laser
sheet in the probe volume location. All the instruments demonstrated the
existence of a plateau region, identified as the range of fluence levels
where the mass concentration was independent of the fluence level applied.
The fluence levels from each instrument were estimated from the energy meter
in the instruments and laser beam cross-section area in the probe volume,
obtained from a beam profile camera, as mentioned previously. It should be
noted that while the internal meter in the LII 300 used for determining
fluence is not a precision instrument, it does provide representative and
consistent fluence values. However, without calibrated energy meters, the
fluences reported for each instrument may not be directly compared to the
other instruments. To aid in interpretation of the results, fluence shifts
were applied for different operating conditions (HPO, Idle in Fig. 5b) and different instruments (LIIs 1 to 4 in Fig. 6b) and can be used for
different rigs (Rigs A, C–F in Fig. 11b) to align the fluence sweep data.
The data from LII 3 and LII 4 were shifted with an offset laser fluence
value to fit with the data from LII 1 in Fig. 6b. The data from LII 2 did
not require shifting as the fit with the data from LII 1 was good. The
normalised and shifted mass concentration profiles from the four different
LII 300 instruments were found to repeatedly demonstrate similar behaviour.
While interpretation of the behaviour in the low fluence region (fluence
1.8
mJ mm
−2
for Rig A) remains a topic to be explored, it is
recommended that measurements be acquired in the plateau region (
1.8
fluence
2.7
mJ mm
−2
for Rig A; Fig. 6b) and that
extra care be taken in interpreting the results from the low fluence and
sublimation regions.
Figure 6
Normalised mass concentration (NMC) mean across the different laser
fluence measured from the kerosene-fuelled Rig A by multiple LII instruments
(LIIs 1 to 4):
(a)
original output from the multiple LII instruments and
(b)
the laser fluence axis of instruments LII 3 and LII 4 was shifted to fit the
data from LII 1.
3.2.4
Laser fluence –
-switch delay mapping
During the laser fluence sweep tests, the laser fluence levels were adjusted
by changing the
-switch delay settings. The correlations in between the
-switch delay settings and the laser fluence levels from the multiple LII instruments are different due to modest variations in the characteristics of
each laser used in the LII 300 instruments. The correlation maps obtained
during the laser fluence sweep tests at the HPO condition of Rig A are shown
in Fig. 7. Each point indicates an average fluence over a period of 15 s duration. Raw data are shown here with no normalisation or
adjustments included. A fifth-order polynomial function was fit to the data
from each instrument and shown as a solid line in Fig. 7. The fit was
essentially linear except at the lowest
-switch delay settings, where there
was evidence of saturation observed for the four different LII 300
instruments. LII 1 was also used for measurements from Rigs C to F. For the
test on Rig B (IP rig) the LII 1 was not available, and LII 2 was used
instead. LII 1 and LII 2 have a very similar relationship between laser
fluence and normalised mass concentration, as shown in Fig. 6.
Figure 7
Correlation between the
-switch delay settings and the laser
fluence levels from the LII 300 instruments (LIIs 1–4).
3.2.5
Particle size distribution
Figure 8a shows the particle size distributions in electrical mobility
diameter (
) measured using the SMPS in this study from multiple
sources. For spherical particles,
is equivalent to the volume
equivalent diameter
ve
, whereas for aggregate particles,
is
larger than
ve
(Decarlo et
al., 2004). In Fig. 8b and c, example images of the typical soot morphology
collected in a previous study (Saffaripour et al., 2017)
from a turboshaft engine exhaust are also shown for reference. The TEM
images from two conditions are shown here: condition E1-1 (speed of 13 000 rpm,
load 70 shaft horse power (shp), and the estimated global equivalence ratio
of 0.25) and condition E1-2 (speed of 21 000 rpm, load 630 shp,
and
of 0.18). The image shows the primary particles and the
fractal-shaped soot aggregate analysed via transmission electron microscope
(TEM). In general, the primary particle diameters were small (17.2 nm
(condition E1-1) and 20.8 nm (condition E1-2)), as were the aggregates, and
they were non-volatile, with no evidence of significant semi-volatile
coatings (clear boundaries) (Saffaripour et al., 2017).
The aggregate sizes that were determined by projected equivalent-area
diameter from TEM images were 33.5 nm (condition E1-1) and 45.7 nm
(condition E1-2) (Saffaripour et al., 2017). The
geometric mean diameter determined from the particle size distributions from
Rig A is in close
agreement with the prior results of aggregate diameter
from TEM image analysis by Saffaripour et al. (2017).
Figure 8
(a)
Typical particle size distributions measured using the SMPS for
the multiple sources and operating conditions: Rig A (kerosene-fuelled, HPO
condition), Rig B (kerosene-fuelled, four operating conditions), and Rig D
(diesel-fuelled, two load conditions);
(b, c)
typical TEM images of soot
produced by a turboshaft engine (speed: 13 000
(b)
and 21 000 rpm
(c)
; shaft
horse power: 70
(b)
and 630 shp
(c)
; and the estimated global equivalence
ratio of 0.25
(b)
and 0.18
(c)
, respectively) at a magnification of
45 000×
(Saffaripour et al., 2017).
3.3
LII 300 fluence sweeps from multiple kerosene-fuelled rigs – Rigs  B and C
Figures 9 and 10 compare the mass concentration profiles as a function of
laser fluence levels from the different test rigs, Rig B (IP rig) and Rig C (APU), respectively, both operating with kerosene fuel. The laser fluence
sweep tests for the four operation conditions of Rig B (Fig. 9) resulted in
similar trends with fluence for the NMC. In general, the mass concentration
results (Fig. 9a, original output) were less dependent on the laser fluence
levels (plateau) over the fluence range of 1.8 to 2.5 mJ mm
−2
, where
peak mass concentration was observed at
∼2.1
mJ mm
−2
for the
two low-pressure cases and
∼2.2
mJ mm
−2
for the two
high-pressure cases. For the NMC at the low-pressure (LP) condition, both
the low-temperature (LT) and high-temperature (HT) cases were coincident; however, the two high-pressure (HP) cases were modestly shifted to higher
fluence, with the high-pressure, low-temperature (HPLT) results having the
larger shift, only 0.12 mJ mm
−2
. Sublimation was observed above a
fluence of 2.5 mJ mm
−2
for all four test conditions, with the HPLT
condition requiring slightly more fluence than the other conditions. The TOA
measurements for the four cases indicate similar organic carbon content,
with the OC
TC ratio of 0.43 (LPLT), 0.41 (LPHT), 0.43 (HPHT), and 0.47
(HPLT). The relatively high OC
TC ratio suggests that additional laser
energy was used to evaporate volatile organic material coating the nvPM. The
higher OC
TC for the HPLT condition compared to the rest of the conditions
is consistent with requiring additional fluence across the entire range,
from low fluence
to sublimation. Shifts to the laser fluence axis were
applied to the HP cases to account for evaporation of organics, shown in
Fig. 9b, demonstrating the self-similarity of the fluence data observed for
all four conditions with Rig B.
Figure 9
Laser fluence sweep performance using Rig B as the nvPM source, with
four operating conditions. NMC – normalised mass concentration.
Superimposed are the best-fit curves from the Loess method.
(a)
Original
output from the LII instrument and
(b)
the laser fluence axis of HPHT and
HPLT was shifted (left shift of 0.05 and 0.12, respectively) to fit the LP
curves.
Figure 10
Laser fluence sweep results using sources from Rig C. NMC –
normalised mass concentration. The best-fit curve from the Loess method is
shown with a black line.
The laser fluence sweep measurements using the nvPM source from Rig C are
shown in Fig. 10. Peak mass concentration was obtained at
∼2.25
mJ mm
−2
laser fluence of Rig C, similar to that from Rig A
∼2.25
mJ mm
−2
) and Rig B (2.2–2.5 mJ mm
−2
),
operating with the same fuel (kerosene). Mass concentration measurements
were less dependent on the laser fluences in the range of 2 to 3 mJ mm
−2
for Rig C.
The similar performance of the LII 300 measurements from multiple rigs
demonstrated the robustness of this technique and revealed that an optimised
laser fluence can be valid for real-time measurements from a variety of
sources. On the other hand, it is shown that laser fluence shifts may be
required to align the fluence sweeps for different operating conditions,
even from the same sources (such as shown in Figs. 4–5 from Rig A). This
suggests that possible differences in the composition (such as volatile
organic coatings), morphological characteristics (internal structure of the
primary particles or aggregation), or optical absorption properties of the
particles at different operating conditions may be important. This
observation suggests that care must be taken in selecting the optimum laser
fluence when using a single or different source (such as a laboratory flame)
for calibrating LII 300 instruments such that it is valid (in the plateau
regime) for both the calibration source and the intended application. In
most cases it is possible to select one laser fluence that is in the plateau
regime for all operating conditions for a single source.
3.4
LII 300 fluence dependence – different sources and fuels
The laser fluence dependence was further investigated for Rigs D–F, which
were reciprocating engine sources with intermittent combustion and burned
diesel or gasoline fuels, unlike Rigs A–C, which were steady-state combustion
sources with kerosene fuel. The results are shown in Fig. 11a and
demonstrate that, similar to the results for Rigs A–C, there is a wide range
of laser fluence levels (1.7–2.7 mJ mm
−2
) where the measurements of
mass concentrations were insensitive to the laser fluence levels (plateau
regime). This allows a moderate laser fluence to be applied in practical
applications to avoid the mass loss due to sublimation and at the same time
reduce the reliance on a critical laser fluence value as the same result is
obtained across this range of laser fluences. Mass concentrations obtained
from Rigs D–F were normalised by the maximum of each rig individually and
plotted together with those from Rig A and Rig C in Fig. 11b, with all the
data acquired using the same instrument (LII 1). As with Rigs A–C, it should
be noted that at low fluence levels (
1.7
mJ mm
−2
for Rigs D–F), the reported mass concentrations were lower than those measured when
the laser fluence level is in the plateau regime.
Figure 11
(a)
Laser fluence dependence of mass concentration measurements
from LII 300 using multiple nvPM sources from diesel- (Rigs D and E, Load 2)
and gasoline-fuelled (Rig F) engine exhausts at typical operation
conditions. Superimposed are the best-fit curves from the Loess method.
(Arrows point at the corresponding
axis.)
(b)
Normalised mass
concentration vs. laser fluences from Rigs A and C to F. Measurements were
carried out by the same instrument LII 1 for the kerosene cases (Rigs A, C)
as for the diesel and gasoline cases (Rigs D to F). Rig E, Load 1: speed
1200 rpm, load 165 Nm; Load 2: speed 2200 rpm, load 300 Nm.
(c)
Combined
data from all rigs with the laser fluence axis of Rig C and Rig E, Load 1, shifted to the left by 0.27 and
−0.05
mJ mm
−2
, respectively.
In general, similar behaviour is observed in the normalised data from all
the rigs and fuels in the plateau ranges. Figure 11b illustrates that the
optimum fluence from Rig A overlaps well with that from other rigs, except
for Rig C. The results from Rig C require a shift on the fluence axis (of
around 0.3 mJ mm
−2
) in order to align with those from Rig A (HPO
condition), as shown in Fig. 9c, where the fluence data from 6 rigs and a
total of 11 conditions converge after the fluence shifts were applied. This
shift was due to the combustion-formed soot particles emitted from Rig C,
which had different physical and chemical characteristics compared to the
soot formed from Rig A. Further analysis would require much greater
understanding of the soot morphology, structure, and composition
characteristics from the various operation conditions, which is beyond the
scope of this paper.
The laser fluence sweeps showed similar results in terms of achieving an
optimum fluence range with a plateau regime for the NMC measurements across
multiple rigs and fuels, which may suggest the option of utilising
cost-effective rigs as nvPM sources for calibration prior to measurements on
aircraft gas turbine engines. The data suggest a near universality of the
fluence sweeps, with the need to shift some by a fixed amount of fluence to
compensate for differences in the particle properties, but all with the same
shape and exhibiting a plateau regime over which the response is uniform for
a range of laser fluence values. Utilising an optimum fluence level of 2.2 mJ mm
−2
, which corresponds to the peak of the best fit from Rigs D to F,
will fall within the 2 % error band, which defines the plateau regime for
the HPO condition of Rig A. It should be noted that the shift in the
fluence levels measured on Rig A (shown in Fig. 4) required to align the
fluence sweeps at the idle and HPO conditions (shown in Fig. 5b) would
lead to a 4 % bias error for the idle condition of Rig A if the
fluence is set at 2.2 mJ mm
−2
. At engine idle or at conditions where
less mature soot (Migliorini et al., 2011) or fewer volatile coatings are
encountered, the laser fluence range for the plateau regime (with a 2 %
error band) will need to be shifted to a higher fluence level, covering the
range from 2.4 to 3.2 mJ mm
−2
. The optimum fluence to cover the full
range of operating conditions for Rig A is the region where these two
plateau regime ranges overlap in the range of 2.4 to 2.5 mJ mm
−2
3.5
Comparison of mass concentration results from different diagnostic techniques
The nvPM mass concentration was simultaneously measured using LII 300, PAX,
and MSS for a range of conditions for
Rigs A–F. Here the real-time nvPM mass
concentration measurements from Rig A are discussed (Fig. 12) as an example
for interpreting the results from the laser-induced incandescence and
photoacoustic real-time diagnostic techniques.
Figure 12
(a)
Mass concentration profiles for LII 300, PAX, and MSS and
(b)
correlations between LII 300 and MSS nvPM mass concentrations to those from
PAX (Rig A).
Figure 12 shows that a high degree of correlation was found between the LII 300, PAX, and MSS for nvPM mass concentration measurements. From the linear
curve fits over nearly 2 orders of magnitude (
– LII vs. PAX – and
– MSS vs. PAX – with an intercept value 0), the mass concentrations
from the LII 300 and PAX are within 2.9 % of each other, and the mass
concentrations from the LII 300 and MSS are within 1.9 % of each other,
much less than the measurement uncertainties associated with each of the
instruments.
The LII 300 measurements for nvPM mass concentrations were further compared
with the EC results obtained from TOA. The results from the multiple rigs
are shown in Fig. 13, normalised to the EC results. The measurements from the
PAX and MSS instruments are also included for comparison. The nvPM mass
concentrations measured by the real-time instruments, i.e. LII 300, PAX,
and MSS, were averaged for the same duration as the filter collection by the
corresponding TOA measurements. The error bars of real-time instruments were
computed by calculating the relative standard deviation, RSD (standard
deviation of the sampled mass concentration data normalised to their average
value), indicating the variability in the nvPM mass measurement. The LII 300, PAX, and MSS results generally are not significantly different than
those for EC from TOA, with error bars overlapping within the uncertainty
(16.7 %) of the TOA EC determination. The real-time instruments' results
were higher (1 %–20 %) than the EC results from TOA on nvPM
emissions from most rigs, except the PAX result from Rig D (3 kW). The PAX
result from Rig D (3 kW) was 4 % lower than that for the EC result; however, it was still within the uncertainty in the TOA EC measurement.
Unlike other cases investigated where the nvPM mass concentrations were
observed to be in the range of 100–800
g m
−3
, the nvPM mass
emissions of Rig F were lower,
∼40
g m
−3
. The TOA
EC uncertainty from the NIOSH method is greatest at the lowest mass
concentrations (NIOSH, 2003) and is reduced at higher mass concentrations.
In comparison of the real-time instruments' performance, for the cases
investigated, PAX results were on average 6 % lower than those for the LII 300 from the kerosene-fuelled rigs, 10 % lower than those for the
LII 300 from the diesel-fuelled
rigs, and 6 % lower than those for the LII 300 from the
gasoline-fuelled rig. MSS results were on average 7 % higher than those for
the PAX from both kerosene-fuelled and diesel-fuelled rigs. This agrees in
general with the finding of previous studies (Smallwood et al., 2010; Durdina
et al., 2016; Lobo et al., 2020; Corbin et al., 2020), and the discrepancy
between the various instruments from source to source and amongst the
different fuels is likely caused by difference in the properties of the
particles (morphology, structure, and optical absorption), the different
content of non-refractory components of the particles (i.e. quantities of
bound H and O, volatiles, nitrates, ash, sulfates, etc.)
(Smallwood et al., 2010), and uncertainties
associated with varying relative humidity content in the heated sampling
cell (for photoacoustic instruments)
(Arnott, 2003) as well as the choice of the
split point in determining EC from TOA
(Baumgardner
et al., 2012). In terms of identifying a substitute for the aircraft gas
turbine helicopter engine (Rig A) for calibrating the LII 300, Rig C (APU)
appears to be the closest in terms of LII 300 response, 1 % higher
than EC from TOA on the same source, well within the uncertainty in the
methods. In terms of the variability in nvPM mass concentrations results,
the LII 300 exhibited a lower variation of 5 % on average among all the
rigs and operating conditions investigated compared to that for the other
instruments, which was 6 % for PAX, 7 % for MSS, and 7 % for EC from
TOA. A higher variability was shown for cases of low nvPM mass emissions
(such as from Rig F or Rig D at 3 kW; refer to nvPM mass concentrations'
ranges detail in Fig. 13 caption) for both PAX and MSS than the variability
exhibited for cases of high nvPM mass emissions (such as in Rigs B and E).
This trend of a low variability in nvPM mass at high concentrations is
consistent with results from emissions measurements of the miniCAST soot
generator (Lobo et al., 2020) and other aircraft engines (Lobo et al.,
2015b, 2016).
Figure 13
Comparison of the mass concentration results from the multiple rigs
and diagnostic techniques. The shaded area marks the 16.7 % uncertainty in
the TOA from the NIOSH method (NIOSH, 2003). The nvPM mass concentrations are
in the range of 90–470
g m
−3
from Rig A, 500–600
g m
−3
from Rig B, 670–780
g m
−3
from Rig C, 300–550
g m
−3
from Rig D 5 kW, 200–270
g m
−3
from Rig D 3 kW, 230–590
g m
−3
from Rig E, and 20–70
g m
−3
from Rig F.
Summary
New standards and recommended practices adopted by the ICAO require the nvPM
mass and number emissions from aircraft engines to be quantified during
emissions certification tests. The LII 300, the only commercial LII instrument utilising the AC-LII method, has been used to measure nvPM mass
emissions from aircraft engines. Previous studies reported the sensitivity
of the LII technique to the type of black carbon sources. In this study the
response of the LII 300 instrument to different nvPM from a range of
different sources and fuels was investigated to understand the relationship
between the laser fluence values and the resulting nvPM mass concentrations
and to evaluate the suitability
of different sources and fuels to be used as an
nvPM calibration source for the LII 300. For all the tests using multiple
rigs as sources of nvPM emissions, LII 300 measurements demonstrated a
plateau regime with a range of laser fluence values where the resulting nvPM
mass concentration measurements were insensitive to the laser fluence levels
applied. The shape of the fluence sweep curves was nearly universal for all
sources, operating conditions, and fuels investigated. Optimising the laser
fluence for the plateau regime over the range of source operating conditions
was shown to reduce potential uncertainties for the LII 300 associated with
the corresponding range of nvPM properties.
Data demonstrated that the LII 300, PAX, and MSS had similar response and
performance in the real-time measurements of nvPM emissions from multiple
rigs studied. Compared to other diagnostic instruments, the real-time
measurement output of LII 300 exhibited no significant differences and high
correlation (
97
%) with the photoacoustic instruments. To
assess suitability of replacing an aircraft gas turbine engine as a
calibration source, further work is required to establish the repeatability
and reproducibility of particle sources as well as investigating
additional laboratory sources, including the miniCAST, MISG (mini-inverted
soot generator), and nebulised carbon black particles. In addition, future
work should include investigating the morphology characteristics,
composition, and optical absorption of the various particulate matter
sources from multiple operating conditions to further understand the
relationship between soot particle characteristics and the response of
real-time instruments used for the measurement of nvPM mass concentration.
Data availability
The datasets analysed during the current study are available from the
corresponding author on request.
Author contributions
GJS, MPJ, PL, MCP, and AS conceived and planned the study and
performed the experiments. RY processed the experimental data and
performed the analysis. RY drafted the manuscript with input from PL and
GJS and designed the figures. PL and GJS assisted in interpreting
the results. DB contributed to the discussion on the time-weighted
normalisation method. All authors discussed the results and contributed to
the final paper.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
The authors would like to acknowledge the funding from Transport Canada for
this project and support from Rolls-Royce. The authors also thank Dan Clavel and Brett Smith, who assisted with the data collection. Ruoyang Yuan
would like to acknowledge the funding from the EPSRC and the David Clarke
Fellowship to support her work.
Financial support
This research has been supported by Transport Canada and the Engineering and Physical Sciences Research Council (grant no. EP/S017259/2).
Review statement
This paper was edited by Pierre Herckes and reviewed by four anonymous referees.
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The region near the peak of
the Loess best fit within the 2 % error band is defined and referred to as a
plateau regime (relatively uniform response to the fluence level) in the
following sections. The optimum fluence discussed later is in the plateau
regime.
The temperature is determined by two-colour pyrometry
and therefore influenced by the nvPM optical properties, i.e. the relative
value of
between the two detection wavelengths. In this study, it is
assumed that the value of the absorption function,
, is the same at both
wavelengths (Snelling et al., 2005). The different particle properties at
HPO and idle may invalidate this assumption, but the potential effect would
only account for
100
K (Snelling et al., 2004) of the observed difference
in the peak temperatures.
Articles
Abstract
Copyright statement
Introduction
Experimental method
Results and discussion
Summary
Data availability
Author contributions
Competing interests
Disclaimer
Acknowledgements
Financial support
Review statement
References
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Short summary
The relationship between the non-volatile particulate matter (nvPM) mass emissions produced by different engine sources and the response of the LII 300 instrument, used for regulatory measurements of nvPM mass emissions in aircraft engine certification tests, was investigated for different sources and operating conditions. Laser fluence optimisation was required for real-time nvPM mass concentration measurements. These results will inform the development of updated calibration protocols.
The relationship between the non-volatile particulate matter (nvPM) mass emissions produced by...
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Sections
Abstract
Copyright statement
Introduction
Experimental method
Results and discussion
Summary
Data availability
Author contributions
Competing interests
Disclaimer
Acknowledgements
Financial support
Review statement
References