When companies talk about life or death situations, it’s usually metaphorically. In the case of SureFire, maker of high-end illumination tools used by the U.S. military and law enforcement officers, it is a reality that has made upfront thermal simulation an integral part of the company’s design process.
The latest SureFire product is the X400, the first weapon mounted assembly to combine a flashlight with a red targeting laser in the same lightweight package. At the heart of the X400 is a powerful LED that cranks out more lumens than other light sources, but also generates a great deal more heat that the housing must dissipate. “Upfront thermal analysis with CFdesign enabled us to explore uncharted territories,” says Deepanjan Mitra, SureFire’s thermal analysis engineer. “It gave us the chance to virtually test ideas and optimize designs before physically prototyping the product.”
From concept through the detailed design phase, Surefire engineers made product improvements by probing digital prototypes to pinpoint precise internal and external temperatures driven by natural convection and radiation.
From incandescent to LED
According to Paul Kim, vice president of engineering, SureFire, thermal analysis became necessary when the company’s products converted from incandescent illumination to highpowered LED lighting. “In terms of thermal management, LEDs are like semiconductor devices,” said Kim. “As you put out more light, you generate more heat. The cooler we can run the device, the more photons, or light, we can put out.”
Full product details in the CAD model – such as springs and small washers – were included in final stage of CFdesign analyses, providing a total picture of product performance before physical test validation.
The engineering team created the preliminary design for the X400 in Pro/ENGINEER, then opened it in CFdesign upfront CFD software from Blue Ridge Numerics Inc. CFdesign is integrated with Pro/ENGINEER and other major CAD packages, eliminating the translation step and data loss associated with traditional CFD software.
“Using the CAD package, I can choose the complexity of my models,” said Mitra. “For quick and dirty answers, I can suppress all cosmetic features and perform an analysis on the critical heat paths, which typically answers the more fundamental design questions.”
The quick analysis is followed by analyses on full models with most of the cosmetic features, which can illustrate more complex interactions and also be used as a marketing tool to highlight new features.
Simulating flow-field conditions
Upfront CFD can simulate flow-field conditions such as convection coupled with heat generation. The results from these simulations are used to generate transient simulations that gave the engineers a picture of how fast temperatures rise in the flashlight according to factors such as materials, heat-sink design, LED power, and fin spacing and design.
Thermal analysis was crucial in two major aspects of X400 design: optimizing the design of housings, especially thermal isolation of the laser unit; and determining the best settings for thermal management firmware.
Flashlight housings act primarily as large heat sinks that need to dissipate as much heat into the ambient air as possible, according to Mitra. This is especially important with LEDs – the high-powered LED lamp in the X400 dissipates almost 85% of its input power as heat.
The X400 presents another challenge: isolating the laser housing. The laser diode must be maintained at a temperature well below that of the LED. Since the main lamp body is mechanically coupled to the laser housing for shock isolation, insulation is needed to minimize the amount of heat from the main lamp that reaches the laser housing.
The engineers used CFdesign to perform various “what-if” scenarios, balancing heat dissipation with size and weight of the weapon-mounted light. For housing material and insulation thicknesses, thermal simulations showed how fast heat traveled to the laser housing. From the simulations, the engineers could predict when the laser diode would fail based on a visualized temperature field.
“We simulated several scenarios with different housing materials and insulation thicknesses for the design requirements,” saidMitra. “This is really simple, since it only amounts to changing the material property of the part in CFdesign and rerunning the analysis.”
The other major simulation area for the X400 was thermal management: A micro-processor in the flashlight regulates power to the LED based on feedback from sensors, controlling the overall temperature of the device. Simulations helped determine optimal settings for the thermal management firmware. “Using the results,” said Kim, “we could define a cut-off temperature at which to activate the microprocessor-based thermal management algorithm.”
Time savings and beyond
Using CFdesign software early eliminated three different design prototypes for testing. That saved about 15 days, according to Kim. In other areas, timesavings are harder to quantify, but readily evident. In addition to material evaluation, upfront thermal simulation made it possible to see the interplay between critical elements such as housing fins and convection.
“This would have been difficult to gauge with physical prototype experiments,” said Mitra. “We can now simulate different fin sizes, thicknesses, spacing and shape, then gauge the effects on the thermal performance of the flashlight. In many cases, this could be no improvement, so we have saved time and resources by not making a physical prototype. In other cases, it could mean a tremendous improvement in thermal performance, helping us decide that this is the right design to prototype.
“Cut-section views, x-y plot flow vectors, and 3D temperature isotherms in CFdesign are all important visualization tools that help us make sense of the simulation results and compare them among different designs.”
“We can now do things with upfront thermal simulation that used to take months and several design cycles to achieve by traditionalmethods,” said Kim.
Blue Ridge Numerics Inc.
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