Flat heat pipes (heat spreaders) are similar to cylindrical heat pipes. But they have received significant attention recently because of their advantages over conventional cylindrical heat pipes with regard to their large surface area, isothermal heat delivery, geometry fit. For example, Thermal Ground Planes (TGPs) are flat, thin (less than 3 mm thick) heat pipes which utilize phase change cooling. The goal is to use TGP’s as universal heat spreaders in microelectronic cooling applications. These TGPs will act as a new generation of high-performance, integrated systems to work at a high power density without difficulties from temperature gradients, increased weight, or extra complexity. In addition to being able to dissipate high thermal powers, they have very high effective thermal conductivities and can operate in high adverse gravitational fields due to nano-porous wicks.

This work shows the effect of vapor pressure, wall shear stress and the interfacial shear stress in the liquid pressure of the flat heat pipes and a comparison with CFD results. Also, this paper offers a design for flat heat pipe charts that avoids the effects of vapor pressure, wall shear stress and the wick-vapor interface to the liquid pressure for most well-known working fluids.

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