Aerospace & Defense Technology - October 2024 - 9

Weapons & Armament
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and around the grain voids, enveloping some grains head-on with
a stagnation point and boundary layer flow and then brushing by
other grains generating a more convective flow. The individual
grains are not homogeneous but have various layers of graphite or
inhibitors that influence heat transfer. Grains close to the primer
vent(s) may be exposed to hot solid particle impacts and grains
one or two layers deep are only exposed to hot gases. The goal is to
transfer heat energy until all propellant grains reach ignition and
start their own exothermic reaction. The dynamics then become
notably more complex in that some grains fully ignite; some partially
ignite and others may not burn. The ignited grains are quickly
adding heat and hot gases to the chamber that then expose and
ignite the remaining grains at an exponential rate.
The energy conversion processes throughout this ignition train
are not well understood and are plagued with assumptions. Current
gun propellant ignition models are empirical and based on an
aggregate initial state of the system. To make matters worse, temperature
data is often highly fluctuating and based on measurements
of the condensed phase that may not be a good indication
of gas phase heating. To improve the safety, reliability and performance
of our gun systems, scientists need to understand the heat
transfer and energy conversion processes controlling the ignition
event on a granular level or even smaller, on a molecular level.
The study at Texas Tech will explore the step prior to the main
combustion event to focus on the fluid dynamics and heat transfer
events that ignite the propellant. Understanding how and
when the propellant ignites will give modelers more detailed
information to optimize current gun systems and design better
future gun systems. Researchers at Texas Tech and Element U.S.
Space and Defense will use a new diagnostic tool known as highspeed
thermography to analyze the coupled flow and thermal
events of the igniter exhaust as it interacts with propellant grains.
This tool uses specialized lenses and software to convert the RGB
color data of a modern digital high-speed camera to high-resolution
and high-speed thermal mapping. This technique is similar
to two-color pyrometry but rather than a single point, temperature
is measurable at all pixels within the high-speed image.
One goal of the research is to measure the heat transfer time
scales of individual grains in actual and simulated primer
exhaust flow fields. Engineers will isolate and vary flow conditions
such as gas temperature, gas velocity, solid particulate size
and solid particulate concentration within the gaseous flow
field. A second goal is to measure thermal conditions at various
radial and axial positions within the gun chamber. The measurements
will provide important information about the uniformity
of heating and root causes for ignition delays and pressure
waves throughout the porous propellant bed.
Experiments have not started yet, but the tests will provide
the potential to reveal fundamental mechanisms leading up
to the ignition event, such as the temperature gradient of
the igniter exhaust as it flows through the choked channels
of the porous propellant bed. It is hypothesized that grains
more proximal to the igniter vent are exposed to higher temperatures
and higher convective heat transfer therefore
reaching ignition thresholds early. In contrast, grains that
are further from the igniter vent are exposed to reduced heat
flux and ignite later. This non-uniform ignition is on the
Aerospace & Defense Technology, October 2024
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Aerospace & Defense Technology - October 2024

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