Effect of direct liquid cooling on light emitting diode local hot spots: Natural convection immersion cooling

Abstract

The increased popularity of solid state systems with the technological developments have led them to be a favorable choice for many lighting applications besides electronics. However, the development of denser high lumen packages has been accompanied by increasing heat fluxes at the LED chip and package levels. Especially, the chips driven at high currents may experience local hot spots, which may cause thermal degradation or even catastrophic failures. As the air cooling has been widely used over the years and significant advances have been made to manage increased heat fluxes. It has been recognized as very difficult to rely solely on it to have an efficient cooling in higher heat fluxes. Moreover, active cooling methods may provide necessary thermal performance but at the expense of high cost and energy consumption. Hence, an efficient cooling capability in high heat fluxes (100 W/cm2) can be accommodated through the use of immersion liquid cooling. Immersion cooling has been studied for electronics circuits since last several decades where the thermal capability of such cooling systems have proved several orders of magnitude higher heat fluxes capability due to phase change heat transfer. Thus, direct liquid cooling with the usage of fluorocarbon liquids, generally considered as the most suitable liquids, has been applied in the current study. The thermal and optical performances of a multi chip LED light engine has been investigated with a series of computational fluid dynamics models and experimental validation studies. Heat transfer mode has been kept at the single phase in dielectric fluids. Effect on the local temperatures, peak and dominant wavelength shifts with respect to temperatures, and impact on total lumen extraction has been presented. Finally, a close form first order correlation has been developed for total lumen extraction depending on driving current and chip temperature.