Transcript (WA6) Oral Presentation

Parametric Study of the Ignition of Metal Powders
by Electric Spark
Bhavita Patel
July 6th 2008
Graduate Mentor: Ervin Beloni
Faculty Mentor: Prof. Edward Dreizin
Background: Metals as Fuel Additives
• Aluminum, other metals used as
fuel additives
– Example: Propellants, explosives,
pyrotechnics
• Advantages of Metals: high energy
density
• Shortcomings of Metals:
– relatively low reaction rates
New Approaches
• New approaches to increase reaction rate:
• Synthesis can be done by:
•Mechanically alloyed powders
•Reactive nanocomposites
• By using new approaches the Reactivity increased
• But, Sensitivity increases as well
Sensitivity needs to be decreased
Sensitivity needs to be understood
Electro-Static Discharge (ESD) for Sensitivity Testing
• The ESD testing are based on US Bureau of Mines Report from 1940s.
• Part of current MIL-STD-1751A/NATO-AOP-7, this is the standard followed
by many countries for such ESD testing.
•
ESD offers a qualitative ranking of ESD sensitivity between different
powders by comparing Minimum Ignition Energy (MIE)
Magnesium
(Mg)
Powder
Ignited
Pin
Electrode
Sample Cup
Skinner, D., Olson, D., Block-Bolton, A. “Electrostatic Discharge Ignition of Energetic Materials” Propellants, Explosives,
Pyrotechnics 23, pp. 34-42 (1997)
ESD Ignition: Current Issues
• A most common ignition sensitivity test
• Many new reactive materials fail, not clear why
– Production of new materials are
not scaled up
• Test results can be affected by
–
–
–
–
–
Equipment model
Powder amount
Testing location
Testing personnel
Weather…such as Humidity
• Main problem: mechanism of ESD ignition is poorly understood for
powders
• Possible processes causing ignition:
– Thermal ignition as a result of direct heating of powder in the discharge
– Thermal ignition as a result of Joule heating
Technical Approach
•
Perform an experimental parametric study
•
Determine how ignition is affected by the process
parameters
•
Establish a model that can adequately describe
experiments
–
•
Different ignition mechanisms are expected to result in
different effect of discharge parameters on ignition
Challenge: design a parametric study to produce meaningful results
Basic Setup for ESD Testing
Switch 1
Switch 2
DC
High Voltage
Pin Electrode
Capacitor
Distance
(Gap)
Voltage
Powder bed
•
•
•
•
Free powder placed in an electrode cup
Capacitor charged to a specific voltage
Capacitor discharges through pin electrode to powder bed
Pulse parameters
– Voltage, duration, current, overall energy
Setting of a Parametric Study
• How is ignition affected by?
– Material
• Output: optical trace
(emission from ignited powder)
• Magnesium & Aluminum
– Particle size
• Spherical Mg 10.3 µm
• Spherical Al 3.0 - 4.5 µm
• Spherical Al 4.5 - 7.0 µm
– Applied energy (capacitance & voltage)
• 2000 pF, 5000pF, 10000pF, etc.
– Applied voltage
• 8kV, 10 kV, 12kV, 16kV, etc.
– Pulse duration (Capacitance & Resistance)
– Spark Configuration (gap)
– Measure ignition delay time
( the Delay time measured from the
Spark to the increasing slope)
– Other optical measurements to be
considered in the future (spectral,
intensity, etc.)
Experimental Setup with Diagnostics
Oscilloscope
Voltage
Inductance
Coil
Current
Inductance
Coil
Firing Test System
Switch
Resistor/
Capacitor
Selector
Pin
Electrode
PMT 1
DC High Voltage
Power Supply
Fiber
Optics
Interference
Filters
Spectrometer
Sample
Cup
PMT 2
Chromel
Wire
Cup diameter: 6 mm
Voltage Inductance Coil: 1 V = 1 A
Cup depth: 0.45 mm
Current Inductance Coil: 1 V = 10 A
Outline of Experiments Conducted
• Powders
– Spherical Mg 10.3 µm
– Spherical Al 3.0 - 4.5 µm
– Spherical Al 4.5 - 7.0 µm
• Repeat same experiments for Al
powders and Mg powder with smaller
weight.
• Test Al powders at 8 kV
– Capacitances
• 2000pF (Does not Ignite)
• 5000 pF
• 10000 pF
– Gap
• From each set of runs determine
– Ignition delay
– Spark energy
• 0.2 mm
• 1.5 mm
– Resistance
• 0Ω
• Vary voltage at a given
capacitance
Aluminum Powder
Size Distribution for Mg and Al
Volume %
Al (3.0-4.5) µm
Al (4.5-7.0) µm
Mg 10.3 µm
0.1
1
10
Partical size (µm)
100
Processing of Current and Voltage
Current, A; Voltage, V
600
Current-Voltage-traces
Experimental current
Fit current
Experimental voltage
400
200
0
E   IV t
-200
-400
 1

R2
 R 
I t   
 sin 
 2 t     exp   t 
 LC 4 L

 2L 
4 LC  R 2C 2


2CVA
0
1
2
3
Time, µs
4
5
6
Short Signal (V)
Emission Traces of AL
6
5
4
3
2
1
0
-1
5
4
3
2
1
0
-1
0.00
Spark
Shorter Ignition Delay
Spark
Longer Ignition Delay
0.01
0.02
0.03
0.04
Time (s)
0.05
0.06
0.07
Derivative, V/s
Emission signal, V
Processing of Delay Pulse
6
5
4
3
Raw Derivative
Slope at the peak of
the derivative
Ignition delay
Spark
2
Zero-level signal
1
0
1500
First peak
of the signal derivative
1000
500
0
9.0
9.5
10.0
10.5
Time, ms
11.0
11.5
12.0
Ignition Delay v Energy for Mg powder
Ignition Delay (ms)
3.0
Recovered2
2000 pF - 0.2 mm - (6 - 16 kV, in 2 kV steps)
5000 pF - 0.2 mm - 8 kV
10000 pF - 0.2 mm - 8 kV
2000 pF - 1.5 mm - 8 kV
5000 pF - 1.5 mm - 8 kV
10000 pF - 1.5 mm - 8 kV
2.5
2.0
1.5
1.0
0.5
0.0
0
20
40
60
80
100
Measured Spark Energy (mJ)
120
Ignition Delay v Energy for Al
Al (3.0-4.5)
Al (3.0-4.5)
Al (3.0-4.5)
Al (3.0-4.5)
Al (4.5-7.0)
Al (4.5-7.0)
Al (4.5-7.0)
Al (4.5-7.0)
Ignition Delay (ms)
10
8
m 10000pF-0.2mm-8kV
m 5000pF-0.2mm-8kV
m 10000pF-1.5mm-8kV
m 5000pF-1.5mm-8kV
m 10000pF-0.2mm-8kV
m 5000pF-0.2mm-8kV
m 10000pF-1.5mm-8kV
m 5000pF-1.5mm-8kV
6
4
2
0
0
20
40
60
80
100
120
Measured Spark Energy (mJ)
140
Summary / Future Work
SUMMARY:
• For Mg powder: ignition delay is a function of energy
– Shorter delays at higher spark energies
– Ignition delays do not decrease below about 0.5 ms
• For Al powders: ignition delay is a function of particle size
– Shorter delay for finer particles
– No detectable effect of energy
– Larger error bars compared to Mg results: explained by a more
difficult ignition
FUTURE WORK:
• Reprocess data to attempt reducing the error bars.
– Using another criterion to analyze previous data: by choosing some
threshold value that is above the base signal noise.
• Additional experiments with new materials, varied settings