With the increase of both heat dissipation and heat flux of electronic devices, the cooling problem becomes more and more challenging. For example, high power light-emitting diodes (LEDs), have shown promising energy efficient features as a new generation of lighting source. However, thermal management has also become a crucial issue in the further development of high-power LED lighting due to the high operating current densities causing hot spots and heat dissipation. While air cooling in conjunction with an integrated heat sink is typically limited to dissipating less than 100 W/cm², phase change based cooling techniques, such as pool boiling, spray cooling and flow boiling, have the potential to remove heat flux in the order of 100~1000 W/cm² utilizing the latent heat of vapo...[ Read more ]

With the increase of both heat dissipation and heat flux of electronic devices, the cooling problem becomes more and more challenging. For example, high power light-emitting diodes (LEDs), have shown promising energy efficient features as a new generation of lighting source. However, thermal management has also become a crucial issue in the further development of high-power LED lighting due to the high operating current densities causing hot spots and heat dissipation. While air cooling in conjunction with an integrated heat sink is typically limited to dissipating less than 100 W/cm², phase change based cooling techniques, such as pool boiling, spray cooling and flow boiling, have the potential to remove heat flux in the order of 100~1000 W/cm² utilizing the latent heat of vaporization. Therefore, two-phase cooling is considered as one of the most promising methods to address those thermal management needs. Phase change heat transfer devices are widely used for high heat flux removal applications for their low thermal resistance and temperature uniformity over solid conduction heat spreaders.

A novel asymmetric vapor chamber was proposed in this research. The effects of surface modification and asymmetric arrangement on phase change heat transfer, droplet dynamics and evaporation were investigated. In this vapor chamber, a nanostructure was patterned on the inner top surface of the condensing wall. And this condensing wall was made to be super-hydrophobic to replace the conventional porous wick. It was found that the proposed asymmetric structure not only improves the heat transfer in drop-wise condensation which has a much higher heat transfer coefficient compared with filmwise condensation, but also provides a shortcut for the condensed water to drop back directly to the center wick. Thus, smaller liquid flow resistance and high anti-dryout capability can be achieved. The optimum working pressure was determined by testing the performance of the vapor chamber under different initial pressures. Heater temperature, horizontal thermal resistance and vertical thermal resistance were defined as key parameters to evaluate the performance of the heat spreader. It was found that the heater temperature increased with an increase in the heat flux but the vertical resistance showed the opposite tendency. The performance of the asymmetric vapor chamber was compared with that of a commercial vapor chamber and a copper plate of the same size. The newly developed vapor chamber could greatly reduce the heater temperature. The critical heat flux (CHF) could reach over 220 W/cm² and the effective thermal conductivity K_eff could reach 27489 W/(mK), better temperature uniformity and lower vertical resistance were found for the newly developed vapor chamber which is promising for the thermal management of high power electronics, such as LEDs.

Furthermore, a multiscale micro/nano structured wick was introduced into the novel asymmetric vapor chamber. The performance of the vapor chamber with this kind of multiscale wick structure was experimentally studied. In order to study the effects of different working fluids, an amount of ethanol was mixed in the deionized water as the working fluid. The ratio of the ethanol to the mixture of DI water and ethanol varied from 0% to 80%. The effects of the acoustic excitation on the asymmetric vapor chamber were also studied by attaching a Piezoelctric actuator on the bottom surface of the evaporator. It was found that the thermal resistance of the vapor chamber with a multiscale micro/nano structured evaporator was much lower than that of a vapor chamber with a bare sintered wick. The existence of the ethanol would deteriorate the performance of the asymmetric vapor chamber. The mechanisms of the multiscale and asymmetric micro/nanostructured surfaces in phase change heat transfer enhancement are analyzed in detail. The performance of the asymmetrical vapor chamber would be enhanced by acoustic excitation, because the acoustic excitation will accelerate the departure of the bubbles from the porous wick.