Cool LEDs, Thanks to CFD

Jan. 8, 2009
A European manufacturer built lamps intended for retirement and nursing homes that used light emitting diodes (LEDs)

A European manufacturer built lamps intended for retirement and nursing homes that used light emitting diodes (LEDs) to cut energy consumption by more than 90%. Unfortunately, the first prototype overheated and using fans to cool the lamps wasn’t an option because they would be too noisy. So Voxdale, a Belgium-based engineering consulting firm, did a thermal simulation with FloEFD.Pro, CFD software that works inside Pro/Engineer and comes from Mentor Graphics Mechanical Analysis Div. (formerly Flomerics) in Marlborough, Mass.

Mentor Graphics Mechanical Analysis Div.

Each lamp contains six LEDs mounted on a 6-cmdiameter PCB cylinder. Each LED dissipates 2 W, and their maximum operating temperature is 105°C. A power supply in the enclosure has a maximum operating temperature of 70°C. The relatively small size of the housing (11 × 8 × 8 cm) and its close proximity to the ceiling complicated the thermal-management problem.

Voxdale President Ken Beyers modeled the light in Pro/Engineer. Program inputs for CFD included materials for the housing, heat sink, and PCB. The CFD analysis also included values for properties of the material connecting the LEDs to their aluminum heat sink, as well as the direction of gravity, important because the lamp is cooled by natural convection. Other parameters entered included the heat generated by the LED and power supply. Defining all the information needed to convert CAD geometry to a CFD model took about 30 min.

The next step was getting the CFD software to mesh the fluid path around and through the housing. First came generating a coarse mesh to perform a fast simulation for a quick check. Results appeared realistic, so a finer mesh was generated for more accurate results. With the finer mesh, results took about an hour. They highlighted the basic design flaw: The housing needed more outlets to get air flowing freely through it.

Beyers increased the number of the vents while being mindful that the manufacturer wanted to minimize them for aesthetic reasons. “The change boosted airflow through the housing,” he says. “In fact, air sped through it at an amazing 6 to 7 m/ sec. This actually created another problem. The air did not stay in the housing long enough for much heat to transfer. So we reduced the size of the vents a bit. Interestingly, we found vents that are too big actually make the LEDs run hotter, which seems counterintuitive.”

Physical testing of the new prototype showed temperatures of the LEDs and power supply matched the simulation prediction.

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