Flow and heat transfer limitations in micro heat pipes

This paper examines the basic physics governing the operation of micro heat pipes. It also explores the operating limits that will determine the maximum heat transfer capability of these devices. These devices, which utilize latent energy to transport thermal energy at very uniform temperatures, will be extremely useful for dissipating the large heat fluxes expected in the next generation of computers. Micro heat pipes, which have sharp-edged cross-sections, utilize capillary action instead of a wick to return condensate to the evaporator. The small size of these devices is necessary for both capillary wicking and to allow them to be used on the chip level. A simple analytic model is shown to predict the operating limit for 1 mm hydraulic diameter devices. This model is then used to show the expected effectiveness for 100 μm hydraulic diameter devices currently being built. Not only is the expected heat flux large, 10-15 W/cm², but also the temperature drop should be very small, of order 0.01°C.