Investigation of thermal characteristics and two-phase flows of a star-shape thin heat pipe

The present study develops a thin silicon-glass bonded heat pipe with microchannels fabricated on the silicon substrate. The height of the capillary wick gap was only 10 μm. Two-phase flows inside the pipe can be clearly visualized through the upper transparent glass surface. Under the observation using a high speed microscope camera, the capillary condensation and the boiling phenomena of the sealed ethanol inside the microchannels could be observed. The 3D steady VOF (volume of fluid), CFD (computational fluid dynamics) models have been established to investigate for the dynamic balance of the vapor-liquid interfaces inside the microchannels. The infrared camera measured the temperature distribution on the upper glass cover and the observed vapor-liquid interfaces inside the microchannel were applied to validate the model. The pressure drop in the condenser section could be deduced by the Bernoulli Equation using the measured vapor-liquid velocities. The deduced number (3.2 ± 0.2 Pa) was also matched the simulation result (3 Pa). As the comparison, the longitudinal thermal conductivity of the microchannel chip was about 2.2 times higher than that of the solid bonding silicon-glass plate. The concave microstructure could be useful in the WLP (wafer-level-package) stack heat dissipation module. Therefore, the microfluidic behaviors of various micrometer scale internal heat pipe structures could be investigated utilizing such a visualization platform.