Thermal Simulation Helps Cool Aircraft Electronic Equipment at 50,000 Feet
Hybricon Inc. Senior Simulation Engineer Michael Palis was given the challenge by a defense contractor customer of cooling an ATR form-factor chassis that dissipates almost 200 watts and operates at altitudes up to 50,000 feet. Palis used thermal simulation to evaluate a wide range of possible design configurations, focusing on heat sink design and fan performance at high altitudes. The simulation helped identify several alternative designs that would meet the customer's demanding requirements. Based on Palis' recommendations, the defense contractor built the system, which performed almost exactly as predicted by the simulation.
ATR form-factor systems typically dissipated 50 to 60 watts several decades ago, but today's systems are more likely to dissipate up to 200 watts, greatly increasing the thermal management challenges. The challenge is even greater at high altitudes. At 50,000 feet, air has only one-eighth of its density at sea level which means that the volumetric flow of the air must be increased by a factor of eight in order to maintain the same level of cooling.
"We use a variety of tools to address these challenges, including hand calculations and flow network modeling tools, but the most powerful by far is FloTHERM, our thermal simulation tool of choice," said Michael Palis, Senior Simulation Engineer for Hybricon Inc. "FloTHERM provides detailed graphical information on pressures, temperatures and airflows throughout our design, providing detailed insights on how the design can be improved."
Palis utilized the parametric design capabilities of FloTHERM to optimize the heat sink design. He set up the software to vary the fin count and thickness over a range. FloTHERM software then automatically set up each design iteration and simulated flow velocity and temperatures throughout the enclosure. The results showed that the design was optimized when 21 fins were used.
The simulation results showed that the optimized heat sink design met the temperature requirements at altitudes up to 35,000 feet, but did not meet them at 50,000 feet. Palis discussed this information with the customer who stated that an operating altitude of 35,000 was acceptable for the initial demonstration version but that a solution needed to be developed for operation at 50,000 feet. Palis then ran further simulations to evaluate the impact of using a higher capacity air-moving device. The simulation showed that this high-performance fan would meet the customer's original specification for operation at 50,000 feet altitude.
"In this application, we were able to meet the customer's demanding thermal requirements by carefully optimizing the heat sink design to squeeze the last bit of cooling out of the limited mass of air that could be drawn through the chassis at high altitude," Palis concluded. "Then we were able to answer the what-if questions that made it possible for them to develop their thermal management strategy for the both the current demo version and future production version of the system."
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