Shock Tube Study of Jet Fuel Pyrolysis and Ignition at Elevated Pressure

Abstract

The development of a compact HyChem reaction mechanisms for jet fuels requires datasets both for pyrolysis products yields to constrain the model and for kinetic targets to evaluate the model. To this end, we have measured selected species time-histories during fuel pyrolysis using laser absorption, and ignition delay times using multiple methods behind reflected shock waves in a heated shock tube. Measurements were performed for three jet fuels over a temperature range of 1000-1400 K and pressures from 12 to 40 atm, for equivalence ratios of 0.5 to 1 and diluted in nitrogen or argon. Fuel loading was measured using an IR He-Ne laser at 3391 nm; ethylene with a CO 2 gas laser at wavelengths of 10532 nm and 10674 nm; and methane with a tunable diode laser at wavelengths of 3175 nm and 3177 nm. Ignition delay times were measured three ways: by monitoring fuel removal with laser absorption, by sidewall pressure, and by OH* emission. Particular care was taken in this study in mixture preparation and transfer of the gaseous fuel mixture to the shock tube. The current HyChem model shows good agreement with these data.

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Sub Topic: Diagnostics
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10th U. S. National Combustion Meeting
Organized by the Eastern States Section of the Combustion Institute
April23-26, 2017
College Park, Maryland
Shock Tube Study of Jet Fuel Pyrolysis and Ignition at Elevated
Pressure
Jiankun Shao1, Yangye Zhu1, Shengkai Wang1, David F. Davidson1,*, Ronald K.
Hanson1
1Department of Mechanical Engineering, Stanford University, Bldg. 520, Room 222
Stanford, CA 94305-3032, USA
*Corresponding Author Email: rkhanson@stanford.edu
Abstract: The development of a compact HyChem reaction mechanisms for jet fuels requires
datasets both for pyrolysis products yields to constrain the model and for kinetic targets to
evaluate the model. To this end, we have measured selected species time-histories during fuel
pyrolysis using laser absorption, and ignition delay times using multiple methods behind reflected
shock waves in a heated shock tube. Measurements were performed for three jet fuels over a
temperature range of 1000-1400 K and pressures from 12 to 40 atm, for equivalence ratios of 0.5
to 1 and diluted in nitrogen or argon. Fuel loading was measured using an IR He-Ne laser at 3391
nm; ethylene with a CO2 gas laser at wavelengths of 10532 nm and 10674 nm; and methane with a
tunable diode laser at wavelengths of 3175 nm and 3177 nm. Ignition delay times were measured
three ways: by monitoring fuel removal with laser absorption, by sidewall pressure, and by OH*
emission. Particular care was taken in this study in mixture preparation and transfer of the
gaseous fuel mixture to the shock tube. The current HyChem model shows good agreement with
these data.
Keywords: Shock Tube, Laser Absorption, Pyrolysis, Ignition Delay Time, HyChem Model
1. Introduction
In support of the development and deployment of alternative engine fuels, many efforts have
been focused on the development of accurate and reliable chemical kinetics models for these
fuels. Traditionally, a surrogate approach has been employed to facilitate the development of
these models, by selecting representative hydrocarbons to mimic the real fuels; for example, iso-
octane and n-heptane were used as surrogates for gasoline [1-6]. However, since many of the
proposed new alternative fuels contain species with large molecular weight and with diverse
compounds such as oxygenates, naphthenes and other types of species, whose physical and
chemical properties and combustion kinetics are poorly known, the surrogate and/or detailed
reaction mechanisms for many alternative fuels are still in their infancy or do not exist. To
exacerbate the situation in the ever-expanding alternative jet fuels industry, the surrogate
approach leads to a slower and inefficient coupling of fuel surrogates and combustion chemistry
mechanism development with mechanisms for individual surrogate fuel components. To produce
a more compact chemical kinetics model for jet fuel that accurately reproduces the pyrolysis and
oxidation behavior of real jet fuel, a new approach, entitled HyChem, has been suggested by
Wang [7]. In this approach, an experimentally-constrained pyrolysis–detailed oxidation model
can provide both a compact reaction mechanism and a direct link to real fuels.

Sub Topic: Diagnostics
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In either approach, the development of kinetic models to describe the combustion behavior of
conventional and alternative fuels requires a set of kinetic targets (such as ignition delay times
and species time-histories) to test, refine and evaluate the reaction mechanism. Fundamental
experimental databases that might be used for comparison and validation purposes for these
models are scarce.
Shock tube/laser absorption methods can furnish reliable, economical and accurate kinetic
targets/tests of ignition, pyrolysis and oxidation that are needed to both validate the reaction
mechanisms and characterize the chemical and physical fit-for-purpose properties for these fuels.
Measurements, behind reflected shock waves, of species concentration time-histories using laser
absorption methods also can provide the necessary data for model generation. Other laboratory
modeling targets include, for example, ignition delay time (IDT), which places global constraints
on model predictions, and species time-histories that place more specific constraints on the
internal sub-mechanisms of these models.
In this paper, experimental methods are first discussed. This is followed by experimental results
and discussion for three conventional jet fuels. Finally, some conclusions are given.
2. Methods / Experimental
2.1.High Pressure Shock Tube
Current pyrolysis and ignition delay time experiments for all fuel mixtures were performed using
the Stanford high-purity, high-pressure shock tube (HPST). Typical uniform test times are of the
order of 2 ms when helium is used as the driver gas. The stainless steel driven section has an
internal diameter of 5 cm and was heated to 110 C to prevent condensation of the test gas
mixture. Diaphragms were made of aluminum of 1.27 mm thickness (with cross-scribing 0.4 mm
deep) to allow measurements over a broad range of pressure (12 - 30 atm). Before introducing
the test gas mixture, ultimate pressures in the driven section of less than 10
-5 Torr and
leak/outgassing rates of less than 10-4 Torr/min were achieved regularly.
The Jet fuels were obtained from the Air Force Research Laboratory. Cat A1 is a JP-8 fuel
sample; Cat A2 is a Jet A fuel sample; and Cat A3 is a JP-5 fuel sample. High-purity Ar and
synthetic air were supplied by Praxair as the bath gas. The liquid mixture was injected into a
heated 12.8-liter stainless-steel mixing tank at 120 C. A test gas mixture of fuel/bath gas was
then prepared manometrically and was stirred using a magnetically-driven vane assembly for at
least 15 minutes prior to the experiments.
2.2. Shock Tube Diagnostics
In the current experiments, narrow-linewidth laser absorption took advantage of the Beer-
Lambert law, i.e. -ln((I/I0)λ) = σλNL, to relate the measured absorbance, -ln((I/I0)λ), to the
unknown species concentrations X ≡ NRT/P, using measured absorption cross sections σλ. When
one product dominated the absorbance at a particular wavelength, and other species have broad,
nearly constant, and featureless absorbance at this wavelength, a two-wavelength differential
method, i.e. on-line minus off-line absorbance, was used to determine the concentration of the
dominant absorber. This is the case for ethylene, C2H4 [8] and methane, CH4 [9] in this work.
The wavelengths and laser types for the species indicated are shown in Table 1.

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Table 1: Wavelengths and laser types for some chemical kinetics target species.
Wavelength(µm) LaserType Usage
3.175 InterbandCascadeLaser Methaneon‐line
3.177 InterbandCascadeLaser Methaneoff‐line
3.391 He‐Ne FuelinRegion1
10.532 CO2 Ethyleneon‐line
10.675 CO2 Ethyleneoff‐line
Fig. 1. Example absorption cross-section data. [8-9]
For each wavelength, absorption cross-section data was collected for each species. Example
cross-section data are shown in Figure 1. The measurement location of all diagnostics was 1.1
cm away from the end wall. Figure 2 shows a schematic of the diagnostics set-up. The left side
shows a typical set-up for ignition delay time measurement, and the right side shows a typical
set-up for pyrolysis laser absorption measurement.
Figure 2. Diagnostics setup.
Representative experimental data traces are shown in Figure 3, including pressure traces and
laser absorbance signals.

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Figure 3. Example pyrolysis and ignition delay time measurements for A2.
Temperatures and pressures in the current shock wave experiments are well-characterized by the
ideal shock relations. Accurate measurements of the incident shock speed directly translate into
accurate determinations of reflected shock temperatures and pressures. The shock arrival times in
the driven section of the shock tube were recorded by five axially-spaced piezoelectric pressure
transducers (PCB 113A) located near the endwall of the driven section. The velocity of the
incident shock at the end wall was then determined by extrapolation, allowing calculation of the
initial reflected shock temperature and pressure, by using one-dimensional shock-jump relations
and assuming vibrational equilibrium and frozen chemistry.
3. Results and Discussion
Figure 4 shows representative time-history data (C2H4 and CH
4) for one reflected shock wave
experiment: pyrolysis of A2 fuel in argon at 1228 K and 12.4 atm. These experiments, to our
knowledge, represent the first of these kind in terms of simultaneous measurements of C2H4 and
CH4 during pyrolysis of heavy distillate fuels. Note that excellent signal-to-noise ratios (SNR)
are achieved for C2H4, and good SNR for CH4 is achieved, considering its weaker product of
absorption cross section with number density. Uncertainty in the species concentrations is
typically ±20% of signal. Evidently, the shape and magnitude of two profiles are dramatically
different. C2H4 appears to rise rapidly in the early 500 µs and then to grow slowly, while CH4
appears to increase at a slow and steady rate. In addition, the measured mole fraction of CH4 is
consistently lower than that of C2H4.
Figure 4. Laser absorption measurements of C2H4 (in blue) and CH4 (in red) during A2 pyrolysis.
Smooth solid lines: simulations using the HyChem approach [10] (version 02).

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A large series of measurements similar to the one shown in Figure 4 were conducted for a range
of temperatures for the three Category-A fuels at pressures near 12 atm. A summary of the
product yields, including both C2H4 and CH4, at three time milestones (i.e. 0.5 ms, 1.0 ms, and
1.5 ms) are shown in Figure 5.
Figure 5. C2H4 and CH4 yields during Category-A fuel pyrolysis at 0.5 ms. Symbols represent
individual experiment. Solid lines: HyChem simulations.
Figure 6. A1, A2 and A3 fuels in 4%O2/Ar and Air IDT data and HyChem simulations
Figure 6 Comparison of measured and simulated IDT in the bath gases of 4%O2/Ar and air of A1,
A2, and A3, respectively. Overall, the HyChem simulations for A, A2 and A3 are in reasonably

Sub Topic: Diagnostics
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good agreement with the measurements and qualitatively capture the corresponding equivalence
ratio and pressure dependences of IDT.
4. Conclusions
A database of pyrolysis product species time-histories was generated for three Category-A jet
fuels. These laser absorption data are characterized by accurately known and uniform test
conditions, fuel loading and species concentrations. Measured ethylene and methane product
yields were found to be similar for all three Category-A fuels over temperatures from 1100 to
1350 K at 12 atm. While ethylene yields appear to plateau or peak at higher end temperatures,
methane yields seem to keep increasing at current temperature range. These ethylene and
methane data provide kinetic targets for HyChem model generation. Ignition delay time data at
different operating conditions are also provided in this work. The HyChem model generated
based on the pyrolysis data predicts the ignition delay times quiet well.
5. Acknowledgements
This research was supported by the Air Force Office of Scientific Research through AFOSR
Grant No. FA9550-14-1-0235, with Dr. Chiping Li as contract monitor and by the US Federal
Aviation Administration (FAA) Office of Environment and Energy as a part of ASCENT Project
35 under FAA Award Number: 13-C-AJFE-SU-016. Any opinions, findings, and conclusions or
recommendations expressed in this material are those of the authors and do not necessarily
reflect the views of the FAA or other ASCENT Sponsors.
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