47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition
5 - 8 January 2009, Orlando, Florida
AIAA 2009-501
Fuel Composition and Coking Analysis of Endothermically
Heated Hydrocarbon Fuels for Use in a Pulsed Detonation
Engine
Chris A. Stevens * and Paul I. King †
Air Force Institute of Technology, Wright Patterson AFB, OH, 45433
Eric A. Nag ley ‡
Helicopter Anti-Submarine Squadron, Naval Air Station Jacksonville, FL 32212
and
Fred R. Schauer §
Air Force Research Laboratory, Propulsion Directorate
Wright-Patterson AFB, OH 45433
Three key characteristics of JP-8 and JP-7 that have been heated endothermically are compared.
Samples of both fuels were collected after being heated by the waste process heat of a Pulsed Detonation
Engine (PDE). It was initially expected that JP-7 woul d have better heat absorption and carbon deposit
formati on properties, and that JP-8 woul d have lower ignition ti mes due to i ts lower thermal stability.
Samples of reacted fuel were collected from the endothermic cooling system of a PDE, and each of the three
metrics was calculated. As expected JP-7 proved to have better heat abs orption, and lower presence of the
precursors to carbon deposit; however, it was also discovered that JP-7 produced more of the gaseous
products that reduce ignition ti mes in the engine. With this result, JP-7 outperforms JP-8 in all three metrics.
Nomenclature
a, b, c, d, e
Cp
h
!"q
T
#
$
= specific heat polynomial fit coefficients
= specific heat (J/kg-K)
= enthalpy (J/kg)
= net heat addition
= temperature at (K)
= detonation cell size
= mass fraction
Subscripts
in
out
= measured at heat exchanger entrance
= measured at heat exchanger exit
I.
Introduction
With the advent of pulsed detonation engines (PDE’s) and Supersonic Co mbusting ramjets (SCRAMjets) it
has become important to understand the endothermic cooling properties of hydrocarbon fuels used to power these
types of engines1 . JP-8 is the standard jet fuel used in all United States Air Force aircraft2 . Work has already been
*
Graduate Student Department of Aeronautics and Astronautics, 2950 Hobson Way, AIAA Student Member
Professor, Depart ment of Aeronautics and Astronautics, 2950 Hobson Way, Senior AIAA Member
‡
LT, USN, Graduate Student, Depart ment of Aeronautics and Astronautics, 2950 Hobson Way, AIAA Member.
§
Senior Eng ineer, Head PDRF, AFRL/ RZTC, 1950 5th Street, Sen ior AIAA Member.
†
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This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.
completed showing that JP-8 can be effect ively used as an endothermic cooling fuel for PDE’s 3 . This paper presents
a comparison between the performance of JP-8 and JP-7. The metrics used for comparison are ignition time, heat
addition, and quantity of poly-aromatic co mpounds in the fuel.
II.
Background
A PDE operates on the principle of fixed volu me detonation of fuel air mixtu re. Figure 1 outlines the PDE
operating cycle for the research engine.
33 ms
33 ms
33 ms
Fill
Fire
Purge
4 ms
15 ms
2.5 ms 1.5 ms
Ignition
Ignition Delay
10.3 ms
Blow Down
Transition
Detonation
Figure 1 – Research PDE operating cycle at 10 Hz pulse rate
In the fill phase, a mixture of fuel and air is forced into an open ended tube. Then during the firing phase, the
mixtu re is detonated. The resulting shockwave and hot, high pressure gas exits the open end of the tube creating
thrust. In the purge phase, after the thrust tube blows down to ambient pressure, the spent mixture is purged with a
blast of pure air before being refilled with another charge of fuel/air mixture. Because the detonation travels through
a stagnant fuel mixture, much of the heat generated during detonation is transferred into the walls of the thrust tube.
The purge air helps to carry away some of the heat, but it must be supplied by the engine and increases the span
between thrust pulses. An alternative to purging the thrust tubes is to cool the walls of the thrust tubes using fuel.
The process of detonation in a PDE has three stages4 . The times shown are representative of the experimental
setup. The first stage is ignition of the mixture. The second stage is transition from deflagrat ion to detonation. The
third is progression of the detonation. Ignition is long on the order of 15 ms. Transition is shorter at about 1.5 ms,
and progression is very short, fro m one to two milliseconds. The obvious target for reducing the length of the fire
phase is the ignition time.
The length of the fire phase of the PDE depends on the ignition time of the fuel, the length of time and tube
required to transition to detonation, and the blow down time5 . The ignit ion time and transition length are related to
the chemical co mposition of the fuel used, and shorter times result in higher cycle frequencies for the engine and
higher thrust. Elimination of the purge phase and reduction of the length of the fire phase both lead to increases in
the performance of the PDE6 .
Because the detonation process produces large amounts of heat, a regenerative cooling system is useful for
two purposes. The first is cooling of the thrust tubes. For this purpose it is best to use a fuel with high heat capacity,
resulting in smaller heat exchangers. The second is the addition of energy to the fuel prior to detonation7 . Liquid
hydrocarbon fuels are formulated to have high energy densities; however, their comp lex mo lecular structures make
them difficult to detonate8 . Figure 2 shows some representative fuels and their detonation initiation energies.
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Figure 2. Initiation energies of various fuels1
Liquid hydrocarbon fuels have initiation energies close to that of propane(cite), and fro m Fig. 2 that init iation energy
is ~2x105 J. Hydrogen and acetylene have much lower in itiation energies, 5x103 J and 200 J respectively. A lo w
initiat ion energy is desirable because the length of the spirals used to transition to detonation is reduced10 . However,
both of these fuels are impract ical for use in aircraft fo r many reasons including fire safety, fuel tank weight, lo w
energy density, and production costs 2 . When used as an endothermic coolant, a liquid hydrocarbon fuel such as JP-8
can be decomposed into products including hydrogen and acetylene7 . The presence of these species in the fuel
mixtu re should reduce the initiation energy allowing a reduction in spiral length. The downside of endothermic
cooling is the production of solid carbon particles in the fuel10 . These particles adhere to passage walls, and clog
both filters and nozzles. In order to design an effective endothermic cooing system, the format ion of these particles
needs to be kept to a minimu m.
The characteristics of JP-7 and JP-8 make these two fuels ideal for the study of endothermic cooling. JP-8 is
currently the standard jet fuel for the USAF2 . It has a high energy density and is distilled direct ly fro m crude oil. It is
available in large quantities, and is the standard to which new fuels are co mpared. Developing the capability to
operate a PDE on JP-8 precludes the need to operate the engine on more expensive, less available fuels. JP-7,
however, was designed for applications where high thermal stability and low aromatic concentrations are important.
Thermal stability is the resistance to chemical change brought on by heating. Originally developed for the SR-71
Blackbird 2 , it is blended from special stocks to ensure that it has a very low concentration of aromatics <3% and
high thermal stability 2 . The low aro mat ic properties of JP-7 make it attractive, as aro matic co mpounds are
precursors to coke format ion in endothermic reactions, and the high thermal stability makes it mo re effective as an
endothermic coolant than JP-8. Knowing the behavior of these two fuels is important to deciding how specialized a
fuel needs to be in order to be viable as an endothermic coolant for PDE’s.
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III.
Experimental Setup
In each experiment, fuel was heated above its known endothermic temperature and a samp le of the reacted
fuel was collected before it passed on to the engine for detonation. The test rig used is identical to that used by
Nagley 11 . A schematic of the test rig is shown in Fig. 3.
Figure 3 - S chematic of test rig
Neat fuel flowed fro m the pressurized reservoir to the shell passage of the thrust tube heat exchangers (Fig. 4)
where it was heated and the endothermic react ion took place. After heating, the fuel was split into two streams. The
proportion of the flow through each stream was governed by the flow nu mber of nozzles in the lines. Most of the
fuel was then injected into the airstream feed ing the PDE. The smaller portion of the fuel was then cooled to room
temperature in a cold water bath (Fig. 5). Cooling the fuel both halted the endothermic reaction, and protected the
hardware down stream fro m damage. A remotely operated valve then directed the cool fuel either to the intake
man ifold or to the liquid collection flask (Fig. 6). Any gas in the cooled fuel then passed through the gas sample
cylinder (Fig. 7) and into the gas fuel collect ion bag (Fig. 8). The bag is tubular and unrolled fro m a spool increasing
in volu me as more gas accumulated. The volu me o f gas collected was calcu lated by measuring the length that the
bag unrolled.
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Figure 4 - Thrust tube heat exchangers
Figure 5 - Cold water bath
Figure 6 - Liquid sample flask
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Figure 7 - Gas sample cylinder
Figure 8 - Gas collection bag
At the beginning of a test the engine was operated with the sample collect ion valve in the bypass position until
the fuel flow reached the desired temperature. Then the valve was activated drawing off a sample for co llect ion.
After a few minutes of operation, the valve was returned to bypass and the engine was shut down. The gas sample
cylinder was isolated to contain a portion of the gas sample for chemical analysis and then was replaced.
Measurements of the liquid sample volu me and the length of the unrolled gas bag were taken. Then up to 120 mL of
the liquid was bottled, and stored with the gas collection cylinder for chemical analysis.
The endothermic cooling capacity of a fuel is best measured by the heat absorbed by the fuel as it is heated.
This net heat addition is a bulk quantity that applies regardless of the level of reaction the fuel undergoes. Another
applicable figure of merit is the liquid-to-gas conversion ratio, the ratio of mass that was converted to gaseous
species by the endothermic reaction to the mass of neat fuel heated. The liquid-to-gas conversion ration was
determined fro m measurements of the volume of gas collected, volume of liquid collected, the gas composition, the
liquid density, and the flow rate of fuel into the heat exchangers. Nagley explains the process in full in h is thesis3 .
The net heat addition (!q) was calculated by modeling JP-7 and JP-8 as a collection of 117 chemical species
believed to be present in either the raw or reacted fuel. Part icular selections included long chain paraffins and olefins
for raw fuel, light hydrocarbons for gaseous products, hydrogen, and poly-aromatic species involved in solid carbon
formation. The heat addition was calculated using a control volume approach taking the pair of thrust tube heat
exchangers as the control volu me and calculat ing the enthalpy of the fuel at the entrance and exit (Eq. 1).
(1)
The enthalpy at each station was calculated by assuming negligib le changes in potential and kinetic energy leaving
only the effects of temperature fo r each chemical species (Eq. 2).
(2)
The mass fraction of each species ($) was measured by gas chromatograph (GC), gas chro matograph mass
spectrometer (GCMS), or high precision liquid chro matography (HPLC) depending on the phase, and molecu lar
weight. Gas samples were analyzed using GC, poly-aro mat ic co mpounds using HPLC and all others using GCM S.
The mass fractions were then normalized and mult iplied by specific heat (Cp ) and temperature (T) resulting in the
modeled enthalpy of the fluid. Specific heat functions for each species were calcu lated by fitting fourth order
polynomials (Eq 3) to National Institute for Standards and Technology (NIST) and Thermodynamic Research
Center (TRC) data tables. The fits were calculated using at least nine points between 273 K and 1000 K for a least
squares fit of the data.
(3)
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The resulting calculated heat addition is Joules per kg of fuel and is not dependant on fuel flow rate, heat exchanger
geometry, or the metering of fuel between the intake and sampling rig.
For both JP-8, and JP-7 there was some difficulty in startup of the engine as ignition time of the fuel was so long
that it did not transition to detonation before the end of the thrust tubes. While the fuel did not detonate it did burn
and began to heat the fuel in the thrust tube heat exchangers. The problem corrected itself as the temperature of the
fuel rose into the endothermic range. Ignition times were calculated by measuring the time between the spark
discharge at the head of the tube, and the beginning of pressure rise within the tube. Figure 4 shows a representative
set of spark and pressure traces. The ignition time is the time between the drop of the spark trace when the energy
discharges, and the time when the slope of the pressure increases beyond a preset threshold.
Figure 9 - Representative spark and pressure traces
Formation of deposits on the walls of fuel system is a major drawback of endothermic cooling systems. In an
endothermic reaction such as the one that occurs in fuel, solid carbon forms via the assembly of poly aro matic
hydrocarbons into graphite sheets. It is believed that by limit ing the presence of aromatic co mpounds in the raw fuel,
solid carbon formation will be limited to that formed by pyrolysis alone. The concentrations of several polyaromat ic co mpounds were measured by High Performance Liqu id Chro matography (HPLC). The poly-aro matics are
intermediate species in the solid carbon format ion process. Direct comparison of the concentrations in JP-8 and JP-7
samples with similar heat additions will indicate whether or not the low concentration of aromatic compounds in the
JP-7 reduced the amount of solid carbon formed.
Together, heat addition, ignition time, and poly-aro mat ic co mpound concentration provide a set of measurements
that outline the benefits and drawbacks of JP-8 and JP-7 as working fluids in an endothermic cooling system for
PDE’s. Heat addition demonstrates the fuels ability to cool much like heat capacity for a non-reacting fluid. Ignition
time supplies a measure of the improvement in the fuel’s detonation properties, and poly-aromat ic concentration
reveals the relative tendency of the fuel to form solid carbon particles. Fro m this knowledge, fuels can be selected
for endothermic cooling systems that provide high heat absorption, short ignition times, and low tendency to form
solid carbon particles during endothermic heating.
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IV.
Results and Discussion
Due to its high thermal stability, JP-7 is a better heat sink than JP-8. Figure 3 co mpares the heat addition for JP-7
and JP-8 as a function of the temperature of the fuel leaving the thrust tube heat exchangers. JP-7 consistently
absorbed more heat than JP-8 at the same exit temperatures. Because of the limited volume of the pressurized fuel
reservoir, the maximu m attainable temperature was limited to about 845 K for JP-7 and 940 K for JP-8. The
resulting data set is more co mpact than desired, but a linear least squares fit of the data still shows higher heat
additions for JP-7.
3000
Heat Addition (J/g)
2500
2000
JP-7
1500
JP-8
%&'()*"+,--7)
1000
%&'()*"+,--8)
500
0
800
820
840
860
880
900
920
940
960
Heat Exchanger Exit Teperature (K)
Figure 10 - Heat sink capability
While the heat exchanger exit temperature is easily measured, it fails to capture the effects of residence time.
The mass flow rate of fuel through the heat exchanger is not constant due to the changing restrictions caused by
coking of nozzles and filters. This becomes apparent when liquid-to-gas conversion is plotted as a function of heat
exchanger exit temperature, and separately as a function of heat addition. Figures 6 and 7 show that the heat addition
is a better independent variable for co mparisons of endothermic heating properties. Heat addition does not assume a
constant fuel flow rate like analyses based on temperature alone.
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0.35
Liquid to Gas Conversion
(kg gas/ kg fuel)
0.3
0.25
0.2
0.15
0.1
0.05
0
810
815
820
825
830
835
840
845
850
Heat Exchanger Exit Temp. (K)
Figure 11 - Temperature as the independent variable (JP-7)
0.35
Liquid to Gas Conversion
(kg gas/kg fuel)
0.3
0.25
0.2
0.15
0.1
0.05
0
800
1000
1200
1400
1600
Heat Addition (J/g)
Figure 12 - Heat addition as the independent variable (JP-7)
It is now appropriate to examine the liquid-to-gas conversion used by Nagley3 . By plotting his JP-8 conversion
fractions against heat addition and superimposing the results from JP-7. It is clear that JP-7 produces more gaseous
products than JP-8 (Fig 8). In fact, the more energy added to the fuel the greater the increase. Th is should lead to
improved ignition times for JP-7 also due to the larger quantities of hydrogen, ethylene and ethane. The greater gas
production was unexpected since JP-7 is formu lated for high thermal stability. A high thermal stability should mean
a reduced level of reaction and low gas concentrations. This result strengthens the case for high thermal stability
fuels.
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0.5
L to G conversion (kg gas/kg fuel)
0.45
0.4
0.35
0.3
0.25
JP-7
0.2
JP-8
0.15
0.1
0.05
0
0
500
1000
1500
2000
2500
3000
Heat Addition (J/g)
Figure 13 - Gas production
The larger quantities of gas produced by the JP-7 should reduce the ignition time of the fuel. Figure 9 shows that
to be the case. Again high thermal stability is superior with lower ignition times by two to three milliseconds. Two
milliseconds is an imp rovement of appro ximately 25% in the ignit ion times. At this point, two of the three metrics
point toward high thermal stability as the better choice for fuels. Stable fuels have both better heat sink capacity and
lower ignit ion times.
9
Ignition Time (ms)
8
7
6
JP-7
JP-8
5
4
3
800
820
840
860
880
900
920
Heat Exchanger Exit Temperature (K)
Figure 14 - Ign ition time
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940
960
It was expected that the low levels of aromatics in JP-7 would reduce the amount of carbon formed by
heating. Figure 10 shows that the total concentrations of poly-aro matics were consistently higher for JP-8. The
reduced quantities of these precursor compounds resulted in fewer carbon particles forming in JP-7. The
reduction was visually evident by the amount of carbon that collected in the fuel filter after each run. Careful
formulat ion of fuel to reduce the presence of aromatic co mpounds in the raw fuel resulted in reduced format ion
of solid carbon deposits.
0.06
0.05
Mass Fraction
0.04
0.03
JP-7
JP-8
0.02
0.01
0
0
500
1000
1500
2000
2500
3000
Heat Addition (J/g)
Figure 15 - Poly-aromatic total mass fractions
V.
Conclusion
At the beginning of testing, it was believed that there were qualit ies of both JP-7 and JP-8 that would give
each fuel an advantage in some aspects of the endothermic heating. Due to high heat capacity, and thermal stability
JP-7 would be a better heat sink, and that the low aro mat ic content would reduce carbon formation. However, the
reduced levels of reaction would result in higher ignit ion times than JP-8. It became clear during the data analysis
that JP-7 also produced more gaseous products than JP-8 including those that reduced ignition times. This
unexpected result shows that JP-7 is an all around better fuel for an endothermic cooling system. In general, it means
better cooling and reduced clogging. As applied to PDE’s it also means higher pulse frequency because of reduced
ignition times. It is reco mmended that fuels selected for endothermic cooling systems in general, and especially
those used with PDE’s, be supplied with fuels that have the high thermal stability, and low aro mat ic content.
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Acknowledgme nts
Appreciation is expressed to the tech team at the PDE Research Facility who made this research possible especially
to Dr. John Hoke and Curt Rice. In addition, Dr. Matt DeWitt and Linda Schafer (AFRL/ RZTG) were helpful in
developing the fuel systems and conducting fuel analysis for experiments performed. Appreciation is also expressed
to Dr. Robert Hancock (AFRL/RZTC) for his technical leadership. Funding was provided by the Air Force Research
Laboratory, Propulsion Directorate and AFOSR.
References
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Wornat, M.J., “Fuel Composition Influence on Deposition in Endothermic Fuels,” AIAA
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