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Spark Ignition Measurements in Jet A: part II
Spark Ignition Measurements in Jet A: part II
Julian J. Lee and Joseph E. Shepherd
Graduate Aeronautical Laboratories
California Institute of Technology
Pasadena, CA 91125
January 22, 2000
Explosion Dynamics Laboratory Report FM 99-7
Prepared for and supported by the National Transportation Safety Board
Under Order NTSB12-98-CB-0415 Abstract
An improved system for measuring the ignition energy of liquid fuel was built to perform
experiments on aviation kerosene (Jet A). Compared to a previously used system (Shepherd
et al. 1998), the present vessel has a more uniform temperature which can be held constant
for long periods of time. This ensures thermal equilibrium of the liquid fuel and the vapor
inside the vessel. A capacitive spark discharge circuit was used to generate damped sparks
and an arrangement of resistors and measurement probes recorded the voltage and current
histories during the discharge. This permitted measurement of the energy dissipated in the
spark, providing a more reliable, quantitative measure of the ignition spark strength. With this
improved system, the ignition energy of Jet A was measured at temperatures from 35 C to
50 C, pressures from 0.300 bar (ambient pressure at 30 kft) to 0.986 bar (ambient pressure
near sea level), mass-volume ratios down to 3 kg/m
3
, with sparks ranging from 10 mJ to 0.3 J.
Special fuel blends with ash points (T
f p
) from 29 C to 73.5 C were also tested. The statistical
properties of the ignition threshold energy were investigated using techniques developed for
high-explosive testing.
Ignition energy measurements at 0.585 bar with high mass-volume ratios (also referred to as
mass loadings) showed that the trend of the dependence of ignition energy on temperature was
similar for tests using the stored capacitive energy and the measured spark energy. The ignition
energy was generally lower with the measured spark energy than with the stored spark energy.
The present ignition energy system was capable of clearly resolving the difference in ignition
energy between low and high mass-volume ratios. The ignition energy vs. temperature curve
for 3 kg/m
3
was shifted approximately 5 C higher than the curve for high mass-volume ratios
of 35 kg/m
3
or 200 kg/m
3
. The ignition energy was subsequently found to depend primarily on
the fuel-air mass ratio of the mixture, although systematic effects of the vapor composition are
also evident. As expected, the ignition energy increased when the initial pressure was raised
from 0.585 bar to 0.986 bar, and decreased when the pressure was decreased to 0.3 bar. Finally,
tests on special fuels having ash points different from that of commercial Jet A showed that
the minimum ignition temperature at a spark energy of about 0.3 J and a pressure of 0.986 bar
depends linearly on the ash point of the fuel. CONTENTS
i
Contents
1
Introduction
1
2
Description of the heated ignition energy apparatus
2
2.1
Vessel description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2.2
Diagnostic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
2.3
Observation of ignition or failure . . . . . . . . . . . . . . . . . . . . . . . . .
5
3
Development of an improved spark discharge system
8
3.1
Description of spark circuit
. . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3.1.1
Circuit operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
3.1.2
Voltage and current measurement . . . . . . . . . . . . . . . . . . . .
9
3.1.3
Signal recording . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.4
Damping resistors
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2
Spark energy measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.2.1
Signal processing and calibration
. . . . . . . . . . . . . . . . . . . . 13
3.2.2
Spark energy calculation . . . . . . . . . . . . . . . . . . . . . . . . . 14
4
Statistical variation of ignition energy measurements
17
4.1
Bruceton Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.2
One-Shot Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2.1
The ignition energy test series . . . . . . . . . . . . . . . . . . . . . . 19
4.2.2
Estimate of median value of the ignition energy . . . . . . . . . . . . . 19
4.2.3
One-Shot analysis methodology . . . . . . . . . . . . . . . . . . . . . 20
5
Ignition energy tests
22
5.1
Ignition energy dependence on temperature . . . . . . . . . . . . . . . . . . . 22
5.1.1
Uncertainty of the ignition energy measurements . . . . . . . . . . . . 25
5.2
Ignition energy dependence on mass-volume ratio . . . . . . . . . . . . . . . . 28
5.3
Ignition energy dependence on pressure . . . . . . . . . . . . . . . . . . . . . 33
5.4
Ignition energy dependence on ash point . . . . . . . . . . . . . . . . . . . . 38
6
Conclusion
41
6.1
Summary of results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6.2
Relationship to Previous Tests . . . . . . . . . . . . . . . . . . . . . . . . . . 42
6.3
Implications for Airplane Safety . . . . . . . . . . . . . . . . . . . . . . . . . 43
A Standard Operating Procedure
47
B Electrode breakdown voltage
48
C High voltage probe calibration
49
D Table of all tests
51 ii
CONTENTS
E Continuous acquisition program
55
F Offset measurement program
58
G Pressure and temperature acquisition program
61
H Spark energy signal processing program
64
I
Corrections to the One-Shot test series
67 LIST OF FIGURES
iii
List of Figures
1
Schematic diagram of the ignition vessel used for ignition energy measure-
ments of liquid fuels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
2
Schematic diagram of the entire heated ignition energy vessel system. The
elements in the schematic are: 1) and 12) heater-control relays, 2) heating
pads, 3) needle valve, 4) bleed valve, 5) rapid venting valve, 6) MKS pres-
sure gage, 7) electro-pneumatic valve, 8) vacuum valve, 9) vacuum line, 10)
box air thermocouple, 11) insulated box, 13) high-voltage probe terminal, 14)
positive terminal from discharge circuit, 15) and 22) damping resistors, 16) ig-
nition vessel, 17) magnetic mixer, 18) solenoid valve, 19) circulation fan, 20)
circulation duct, 21) current transformer, 23) ground terminal from discharge
circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
3
Schematic diagram of adjustable-gap-width electrodes of the ignition energy
vessel. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
4
Schematic diagram of electrode tip geometry. . . . . . . . . . . . . . . . . . .
4
5
Schematic diagram of the color-Schlieren arrangement used to record the ex-
plosion event in the vessel. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
6
The (a) pressure and (b) temperature histories of the successful ignition of
the fuel-air vapor at 0.585 bar for mass-volume 35 kg/m
3
(80 ml) at 35.5 C
(test# 197). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
7
Video frames showing (a) the failure of ignition (test# 193), and (b) successful
ignition (test# 194). The two columns of three frames progress in time from
top to bottom and time between frames is 2 ms
. . . . . . . . . . . . . . . . .
7
8
Circuit diagram of the discharge circuit used in the present work. . . . . . . . .
9
9
Circuit diagram showing the measurement arrangement of the voltage and cur-
rent used in the spark energy calculation. . . . . . . . . . . . . . . . . . . . . . 10
10
The (a) voltage history and (b) current history of an underdamped spark result-
ing from the discharge of a 0.602F capacitor. . . . . . . . . . . . . . . . . . . 11
11
The (a) voltage history and (b) current history of a damped spark resulting
from the discharge of a 0.602F capacitor with a 14.3 resistor on the positive
electrode and a 7.15 resistor on the negative electrode providing the damping
(test# 197). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
12
The power consumed by the spark and the resistor (R3) together (P
SR3
), and
the power consumed by R3 alone (P
R3
). The power histories shown are for
test# 197, the discharge of a 0.602 F capacitor charged to 5.5 kV with a 14.3
resistor on the positive electrode and a 7.15 resistor on the negative electrode. 15
13
The energy