Pumped Two-Phase Cooling
Pumped two-phase loops utilize the phase change mechanisms (evaporation and condensation) to absorb and reject heat. The working fluid circulation inside the loop is driven by a mechanical pump. When designed properly, pumped two-phase loops offer substantially higher heat flux performance than pumped liquid cooling technologies. Compared to passive two-phase devices such as heat pipes and loop heat pipes, pumped two-phase loops offer improved heat transport capability and reliability, particularly during startups and/or transients.
There have been many studies on pumped two-phase flows in micro or mini channels. Large pressure drops and two-phase instabilities are two main drawbacks of these flows. For example, the two-phase flow pressure gradient in a 100µm diameter channel can be up to 60psi per inch of flow length. The large difference in density between the liquid and vapor phases, combined with the small cross section and high heat flux condition, often causes large pressure fluctuations in these channels.
Evaporative spray cooling is another variation of the pumped two-phase cooling technologies. A nozzle is used to atomize the liquid. The liquid droplets evaporate upon hitting the heat input surface, absorbing the heat with large heat transfer coefficients. Evaporative spray cooling has been used in some high heat flux cooling applications. Its shortcomings include complex fluid control requirements, potential erosion at the nozzle, tight geometric constraints for the development of spray cones and interference between multiple sprays (required for cooling of large area heat sources). It was also found that the spray performance changes substantially with the variation of the body force (e.g. gravity).
ACT has been actively developing the following advanced pumped two-phase cooling technologies for a number of military and space thermal control applications.