DTIC ADA454350: Heat Transfer ad Thermal Management in a Pulsed Detonation Engine

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by  Defense Technical Information Center

AIAA 2003-0852
HEAT TRANSFER AND THERMAL
MANAGEMENT IN A PULSED DETONATION
ENGINE
* *
John Hoke Royce Bradley , and Frederick Schauer
Air Force Research Laboratory, Propulsion Directorate
Wright-Patterson AFB, OH 45433
*
Innovative Scientific Solutions, Inc.
Dayton, OH 45440

th
41 AIAA Aerospace Sciences
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Heat Transfer ad Thermal Management in a Pulsed Detonation Engine
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AIAA 2003-6486
HEAT TRANSFER AND THERMAL MANAGEMENT IN A PULSED DETONATION ENGINE

John Hoke and Royce Bradley
Innovative Scientific Solutions Inc
2766 Indian Ripple Rd
Dayton, OH 45440

Frederick Schauer
Air Force Research Labs
Wright-Patterson Air Force Base
Dayton, OH 45433

Abstract

The unsteady nature of the Pulsed Detonation Engine (PDE) cycle creates a thermal environment
fundamentally different from steady flow cycles. Gas velocities in a detonation tube range from O(-1) to
O(1000) within a single cycle. This broad range of velocities and flow reversal make it difficult to
determine analytically the contribution to the heat load from the purging, filling, detonating, and blow
down portions of the cycle. In this paper, the overall heat load on a detonation tube is measured
calorimetrically in an aluminum water-cooled detonation tube. The effects of operating parameters such as
fill fraction, purge fraction, ignition delay, equivalence ratio, and cycle frequency are examined.
Equivalence ratio and cycle frequency are found to have the largest effect on detonator tube heat load.

Introduction Research Lab’s Pulsed Detonation Research
Facility. This facility was described in detail by
Questions have arisen as to the thermal Schauer et al.(2001), and only the details
load of a pulsed detonation engine (PDE). pertinent to this study are given here. The
Lower heat loads than experienced in experiments were conducted with a spark-ignited
conventional turbines are expected since the hydrogen-air PDE. The cycle of the research
detonating portion of the cycle is relatively short; Pulsed Detonation Engine (PDE) was divided
however, the temperatures and velocities of the into three temporally equal portions: i)
flow during this period are higher than in detonation window, a third of the cycle is
conventional devices and potentially lead to allotted for detonation initiation and blow-down.
higher heat loads. In this paper, heat-loads on Depending on the cycle frequency, a portion of
the detonator tube of a PDE under several the time in this window was unused. ii) purge
different operation conditions are experimentally process, a third of the cycle was used to pump air
measured and presented. Previously, Eidelman et into the detonation tube to separate the exhaust
al. (2001) have performed numerical simulations products from the pre-mixed fuel-air charge and
to determine the transient temperature profile, to cool the internal geometry. iii) fill process,
the rise time and steady state temperature along during the last third of the cycle, a pre-mixed
the detonation tube. Ajmani and Breisacher charge of fuel and air was pumped into the
(2002) have modeled the heat flux to a detonator detonation tube.
tube and present several measurements using The fuel and airflow into the detonation
heat flux gauges. Here, the overall heat load on tube were measured using a choked nozzle and
the detonator tube is measured calorimetrically, choked orifices respectively. The flow rates
while tube wall temperatures are measured in were actively controlled by measuring the flow
separate experiments using thermocouples spot- rate and adjusting the pressure upstream of the
welded to the detonator tube. measuring devise to achieve the desired flow
rate. The desired flow rates were calculated
Experimental Apparatus and Procedure from the detonation tube volume, operating
frequency and atmospheric pressure. With the
Experiments were performed on a 36” flow control system, the equivalence ratio (f),
tube fill fraction (FF), and purge fraction (PF)
(0.91m) detonation tube at the Air Force
1
could be adjusted with the engine running. The frequency increases the number of detonations
FF and PF are defined as the fraction of the
25
detonation tube volume filled during the filling
and purging process respectively.
y = 21.428x R= 0.99786
A water-cooled detonation tube was 20
constructed by inserting the 2"schedule 40
W]
aluminum detonation tube into a 2.5" schedule
k
40 aluminum tube of similar length. The tubes d [15
a
were oriented to be concentric and the space o
L
between the tubes at either end was welded at 10
closed. Water entered the cavity between the He 20 Hz
two concentric aluminum tubes at the head end FF=1.0
5
of the detonation tube and exited at the tail of the PF=0.5
detonation tube. Type T thermocouples inserted Ign Delay~5 ms
in the water flow were used to measure the inlet 0
and exit temperatures of the cooling water. The 0 0.2 0.4 0.6 0.8 1 1.2
water flow rate was measured by a rotometer. Equavalence Ratio
The heat load on the detonation tube could then

be calculated. The absolute error in the heat load
Figure 1 Heat load verses equivalence ratio, 36"
calculated was approximately 15% due to
(.91m) detonation tube
measurement error, conduction losses and
convection losses from the outer water jacket. 35
The relative error between experiments was
much less being dominated by the measurement 30
errors and was less than 7%.
Thermocouples were also used on un- W] 25
cooled detonation tubes to determine thermal rise d [k 20
time and equilibrium temperatures. The a
o
thermocouples were spot-welded at intervals L
along the detonation tube. Accuracy of the high at 15
temperature thermocouple measurements was He 10 FF=1.0
PF=0.5
estimated to be ± 50 °F.
Ign Delay~5 ms
5
f=1.0
Experimental Results and Analysis
0

0 10 20 30 40 50
The effect of frequency, fill fraction, ignition
Frequency [Hz]
delay, equivalence ration and purge fraction on
the overall heat load was investigated. The
equivalence ratio was found to have the largest Figure 2. Heat load verses frequency, 36"
impact on detonator tube heat load, see Fig. 1. (.91m) detonation tube
The heat load at 20 Hz increased almost linearly
from 0 to 21.8 kW as the equivalence ratio was
increased from 0 to 1. Below an equivalence increased but the time for each portion of the
ratio of 0.5 it became increasingly difficult to cycle was fixed, and the time that the exhaust
achieve a detonation in the 36" (.91 m) products (after blow-down) remained in the
detonation tube using a deflagration to detonator tube decreased with increasing
detonation transition and the heat load measured frequency. Additionally, the average velocity of
was lower than that predicted using the the fill and purge increased with increasing
measurements recorded at higher equivalence frequency. For fully developed flow in a smooth
ratios. Experiments were not conducted above pipe, the heat transfer coefficient increases by
an equivalence ratio of one. velocity to the 0.8 power according to the Dittus-
The influence that frequency has on Bolter correlation (White 1988). Therefore, at
heat load is shown in Fig. 2. Note that unlike the higher frequencies, the purge and fill process
thrust, doubling the frequency from 20 to 40 Hz would be more effective at removing heat from
does not double the detonator tube heat load but the detonation tube. Because of the developing
only increased the heat load by 58%. As the boundary layer, the heat transfer coefficient was
2
expected to be significantly higher than that
predicted by Dittus-Bolter. The heat flux during 34
the fill and purge process was not measured in 35 Hz
32
these experiments.
Decreasing the fill fraction of the
detonation tube decreases the heat load; W] 30
k
however, since the exhaust products must exit d [ 28 FF=1.0
through the unfilled portion of the detonation oa f=1.0
tube, the decrease in heat load was not as at L 26 Ign Delay 5 ms
pronounced as that found for equivalence ratio. e
H 24
If Fig. 3, the heat load on the detonation tube is
shown to decrease by only 19% between a fill 20 Hz
22
fraction of 1 and 0.5. At a fill fraction of 0.5, the
PDE was consuming half of the fuel that it 20
would be consuming at a fill fraction of 1, and 0.2 0.4 0.6 0.8 1 1.2 1.4
14% less fuel than it would consume at an Purge Fraction
equivalence ratio of 0.5 and a fill faction of 1.

Even though the PDE was using 14% less fuel at
Figure 4. Heat load verses purge fraction, 36"
a fill fraction of 0.5 than it was at an equivalence
(.91m) detonation tube
ratio of 0.5 and a full fill fraction, the fuel

distribution in the detonation tube created a 62%
difference in heat load. The lower fuel
23 10
consumption condition of FF=0.5, f =1 had the
higher heat load. The majority of this difference 22
was attributed to the theoretically lower exhaust 8
21
temperatures of the 0.5 equivalence ratio W]
cond2i5tion. eat Load [k 112890 Heat Load Thrust 46 Thrust [lbf]
H 20 Hz
20 17
FF=1.0 2
W] 16 PF=0.5
k f=1.0
d [15 15 0
a
o 0 2 4 6 8 10 12 14
L
at 10 Ignition Delay [ms]
He 20 Hz
PF=0.5 Figure 5. Heat load verses ignition delay, 36"
5
Ign Delay~5 ms (.91m) detonation tube
f=1.0
0 Lastly, the tube heat load was measured
0 0.2 0.4 0.6 0.8 1 1.2 as a function of ignition delay. This parameter
Fill Fraction
affects the time that the exhaust products remain
in the detonation tube as well as the initial
Figure 3. Heat load verses fill fraction, 36" pressure at which the mixture was detonated, see
(.91m) detonation tube Hoke et al. (2002). The heat load on the
detonation tube varied approximately 1.5 kW
The purge fraction had a limited affect with ignition delay, see Fig. 5. The maximum
on tube heat load and in Fig. 4, the effect of thrust occurred at an ignition delay of
purge faction on tube heat load is shown for two approximately 5 ms and the maximum heat load
frequencies, 20 and 35 Hz. At 20 Hz, increasing occurred earlier, between 2 and 4 ms. There are
only small differences in the combustion
the purge air from .25 to 1.25reduced the tube
heat load by a kilo-watt. At 35 Hz, this effect temperatures with initial pressure, however, there
was almost doubled and attributed to the higher can be a significant difference in thrust pressure.
purge velocities and greater purge mass flow The higher thrust pressure leads to higher heat
required for the higher frequency. transfer however, the longer residence time of
3
the early ignition will also contribute to higher load to the significant parameters. From this
heat loads. Therefore, the peak of heat load study, it is evident that PDE’s should be run at
occurs before the peak in thrust. lower frequencies for lower heat loads; however,
The heat load on an un-cooled the heat load per unit thrust is lower at higher
detonation tube should be lower than that frequencies. Additionally, throttling of the PDE
measured for the cooled detonation tube because resulted in lower tube heat loads if the throttling
the temperature difference between the exhaust is done by lowering the equivalence ratio rather
gases and tube walls will be smaller. Shown in than decreasing the fill fraction. The ignition
Fig.6 are outside wall temperatures along the delay or cycle of the PDE should be designed to
length of a 72" detonation tube that was run to minimize the length of time the exhaust products
thermal equilibrium. The temperature along the remain in the detonation tube after blow-down.
detonation tube varied over 300 °F and thermal Additionally, there are aspiration issues that
equilibrium was reached in approximately 2 encourage minimization of the time the exhaust
gasses stay in the detonation tube after blow-
1500 FF= 1.0, 0.5, 0.25, 0.35, 1.0 down. Experiments with longer and larger
Av gas diameter detonation tubes are planned as well as
20 Hz
a numerical model.
PF=0.5
IG = 10
F]
g 1000 f appox 1.0 Acknowledgements
e
d
e [ Gratitude is expressed to the technicians
ur Head Tube
at 21" who worked on this project: Curtis Rice, Walt
er 35"
mp 500 49.5" Balster and Dwight Fox (ISSI). The authors
Te 66.5" would also like to thank Jeff Stutrud
(AFRL/PRTS) for his computer programs used
to collect and analyze the data. The technical
0
5000 5500 6000 6500 7000 7500 leadership of Dr. Mel Roquemore and Dr. Robert
Hancock (AFRL/PRTS) continues to be
Run Time (sec)
invaluable. Funding was provided by the
Figure 6. Detonation tube temperatures verses propulsion directorate and Dr. Julian Tishkoff at
run time for several fill fractions, 72" AFOSR.
(1.83m) detonation tube

minutes. The hottest region on the detonation References
tube occured in the region where transition from
deflagration to detonation occurred. In this Ajmani, K. and K. J. Breisacher (2002).
transition region, an overdriven detonation, with Qualitative Study of Cooling Methods
higher wave velocities, and pressures, increased for a Pulse-Detonation Engine. 51st
the heat transfer to the tube wall. The fill JANNAF Propulsion Meeting, Lake
fraction of the detonation was varied during this Buena Vista, Florida, Chemical
experiment and the heat load for a 72" detonation Propulsion Information Agency.
tube appeared to be higher at a fill fraction of 0.5
than at a fill fraction of 1. This was contrary to Eidelman, S., D. Sharov and D. Book (2001).
the results found for the shorter 36" detonation The Thermal Balance of PDE.
tube. At lower fill fractions, the peak Computational Methods and
temperatures decreased and a detonation wave Experimental Measures: 711-720.
was not produces since transition from
deflagration to detonation occurred around 48" Hoke, J. L., R. P. Bradley and F. R. Schauer
from the closed end of the detonation tube. (2002). The Effect of the Dynamic
Filling Process on PDE Perfomance and
Summary and Conclusions Nozzle Selection. 51st JANNAF
Propulsion Meeting, Lake Buena Vista,
Although cooled-wall heat load Florida, Chemical Propulsion
measurements will not produce identical results Information Agency.
to hot wall tests, the results found in this study
give some indication as to the sensitivity of heat
4
Schauer, F., J. Stutrud and R. Bradley (2001).
Detonation Initiation Studies and White, F. M. (1988). Heat and Mass Transfer.
Performance results for PDE New York, Addison-Wesley Publishing
Applications. 39th AIAA Aerospace Co.
Sciences Meeting and Exhibit, Reno,
AIAA.

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