Simulation software accounts for altitude effects

Thermal simulation accurately predicts the performance of cooling system for an ATR-form-factor chassis that dissipates almost 200W and operates at altitudes up to 15,000m.

Hybricon Senior Simulation Engineer Michael Palis was given the challenge by a defence contractor customer of cooling an ATR-form-factor chassis that dissipates almost 200W and operates at altitudes up to 15,000m.

Palis used thermal simulation to evaluate a wide range of possible design configurations, focusing on heatsink 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 defence contractor built the system, which performed almost exactly as predicted by the simulation.

ATR-form-factor systems typically dissipated 50 to 60W several decades ago, but today's systems are more likely to dissipate up to 200W, greatly increasing the thermal management challenges.

The challenge is even greater at high altitudes.

At 15,000m, 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 modelling tools, but the most powerful by far is Flotherm, our thermal simulation tool of choice", said Palis.

"Flotherm provides detailed graphical information on pressures, temperatures and airflows throughout our design, providing detailed insights on how the design can be improved".

Palis used the parametric design capabilities of Flotherm to optimise the heatsink 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 optimised when 21 fins were used.

The simulation results showed that the optimised heatsink design met the temperature requirements at altitudes up to 10,000, but did not meet them at 15,000m.

Palis discussed this information with the customer, who stated that an operating altitude of 10,000m was acceptable for the initial demonstration version but that a solution needed to be developed for operation at 15,000m.

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 15,000m altitude.

"In this application, we were able to meet the customer's demanding thermal requirements by carefully optimising the heatsink 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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