Thermo-fluid dynamics of pulsating heat pipes for LED lightings

Abstract

An effectual thermal management is a very important issue in a light emitting diode (LED) floodlight because its performance and reliability decrease significantly as the junction temperature increases. A cooling device free of moving parts is suitable for lighting with natural convection. A very large heat sink is combined with a relatively small light source in high-power LED floodlights with everlasting miniaturization of microelectronic systems and highly developed high-density packing technology. Pulsating heat pipes (PHPs) lead to a breakthrough solution for passive two-phase flow spreading of LEDs non-radiant heat in case of a high-density mounting device. PHPs have little influence of gravity direction (floodlight attitude), high heat spreading performance, high reliability for long-term use, lightweight, low cost, and ease of production. This paper describes closed loop 8-turn, ethanol-charged PHPs with radial channels. The experimentally investigated PHPs have dimensions of 200 mm × 200 mm × 3 mm with central heating using a diameter of 38 mm heater. A copper plate is covered with polycarbonate plate for visual observations. A time-strip image processing technique has been applied to the flow videos to extract qualitative details of flow regimes concerning the liquid/vapor interface dynamics. Radial channel flow and thermal oscillation characteristics have been discussed using time-strip technique, Fourier power spectrum and autocorrelation function. The U-shaped vapor oscillates in the cooling section. The wall temperature-time histories fluctuate non-periodically.

Keywords

• led floodlight
• heat spreader
• pulsating heat pipes
• oscillating heat pipes
• radial channel
• flow visualization
• video sequences processing