This document discusses passive thermal management techniques for cooling avionics equipment in aircraft. It describes how avionics generate heat that must be rejected to prevent temperature increases that reduce equipment life. Two passive approaches are discussed: heat pipe assemblies that reduce internal temperature gradients by 25 degrees Celsius, and loop heat pipes that transport thermal energy to alternative heat sinks like aircraft fuel or external surfaces. The loop heat pipe was tested using methanol as the working fluid and able to exchange heat over a flight envelope by using two condensers connected to liquid nitrogen sinks.

1. *Passive Thermal Management for
Avionics in High Temperature
Environment

2. • Avionics are the electronic systems used on
aircraft, artificial satellites, and spacecraft.
• Avionic systems include communications, navigation,
the display and management of multiple systems.
• The cockpit of an aircraft is a typical location for
avionic equipment
*

3. *
*Under a program funded by the Air Force
Research Laboratory(AFRL), Advanced Cooling
Technologies(ACT) has developed a series of
Passive Thermal Management techniques for
cooling avionics.

4. *
*The rejection of the waste heat generated by the
electronics in the avionics to the surrounding
environment, results in a significant ambient heat
gain.
*As a result this high temperature reduces the life of
the electronics.
*Heat must be transported to a sink.
*The only viable sink is aircraft fuel, some times the
fuel temperature can become too high to effectively
cool avionics.

5. *
*Two passive thermal approaches for avionics
packages.
1. Heat Pipe Assemblies(To reduce the internal
temperature gradient)
2. Loop Heat Pipe(to transport thermal energy to
alternative sinks)

6. *Heat pipe assemblies were capable of reducing the
internal temperature gradient by approximately 25
degree Celsius.
*This reduction translates directly to an increase in the
allowable sink temperature that will still provide
sufficient cooling for the electronic components.

7. *
*LHP was designed to transport thermal energy from
the fuel to two heat rejection section.
*Two heat rejection sections were necessary as
aircraft sink condition can vary considerably
throughout the flight.
*Since these sinks can approach temperature much
higher than the intended operating temperature of the
LHP.

9. *
*Two sinks that together were capable of providing
heat rejection over the entire flight envelope.
*Methanol was selected as the working fluid.
*Stainless Steel was used for the evaporator body,
compensation chamber and transport lines.

10. *

11. *Both condenser exchanged waste heat with Liquid
Nitrogen(LN)
*The flow of LN was controlled to maintain the sink
temperature at the desired value.
*Each condenser also had a surface exposed to an
electric heater.
*This allowed the sink temperature to be raised above
the LHP vapor temperature.

12. *Heat pipes transport Heat by two phase flow of a
working fluid
*Liquid Phase
*Vapor Phase