Pumped single or two phase cooling is generally used to remove and dissipate heat from high-power heat sources such as electronics and lasers, or when the thermal energy must be transferred a significant distance between the heat source and the heat sink. Pumped single phase cooling is commonly used today in automotive systems and power electronics equipment, where the heat fluxes are relatively low and/or temperature uniformity is not required. In a pumped single phase loop, the liquid coolant is pumped through a cold plate which is attached to the heat source being cooled. The temperature of the liquid coolant increases as it passes through the cold plate, absorbing and storing the heat in its sensible heat capacity.
What Are Pumped Two Phase Cooling Systems?
In pumped two phase cooling systems, heat is transferred by the evaporation and condensation of a portion or all of the working fluid. Typically, a liquid near saturation is pumped into the cold plate, where it starts to boil, cooling the electronics and storing the energy in the latent heat of the fluid. The two phase (liquid and vapor) fluid then flows to the condenser, where the heat is removed, condensing the vapor, so that a single phase (liquid) exits the condenser, and the cycle repeats.
When properly designed, two phase pumped loops can do the following:
- Transfer heat over long distances
- Cool high heat flux electronics
- Accommodate & cool multiple cold plates in parallel
- Use quick disconnects to swap out electronics
- Heat can be applied and removed from any combination of cold plates, with passive flow control to each cold plate
- Operate in any orientation
- Cooling over large areas (ACT has demonstrated two phase cooling with multiple 1.8 ft2 (1700 cm2) cold plates)
Pumped two phase systems require additional design, since flow instabilities must be suppressed, and the system must accommodate both liquid and vapor flows. However, pumped two phase has the following benefits when compared with single phase cooling:
- Reduced Size, Weight and Power (SWaP), which is an important concern on aircraft and military vehicles
- Lower flow rates and pumping power
- Mini-channel heat exchangers instead of micro-channel heat exchangers, reducing pumping power and clogging issues
- Isothermal temperatures over large cold plates (±0.5°C has been demonstrated at ACT)
- Thermal management of multiple electronics cards that need to operate at the same temperature (±3°C has been demonstrated at ACT)
- When properly designed, turning some of the electronics off will not affect the temperature of the remaining cold plates
Two Phase Cooling System Layout
The basic layout of a pumped two phase cooling system is similar to a pumped single phase system, except that a two phase reservoir is used to accommodate changes in fluid volume, rather than the accumulator that is used in a single phase system. An example pumped two phase system is shown in Figure 1, where quick connects allow cold plates (and associated electronics) to be swapped out without the need to drain and recharge the system. Flexible lines allow the cold plates to be tested in any orientation, and at different elevations.
The cold plates (heat sinks) are the two evaporators in the front of Figure 1, with a transparent top plate. In the left evaporator, single phase flow enters from the top, a fraction of the liquid boils to remove the heat, and the two phase mixture exits at the bottom (note the location of the bubbles). For the right evaporator, the single phase flow enters from the bottom, with a two phase mixture exiting the top of the cold plate. ACT has demonstrated the ability of these systems to remove heat as the orientation of each evaporator is changed independently from the others.
Some electronics cooling applications have large numbers of parallel electronics boards, where it is desirable to apply electrical power and cool an arbitrary number of boards, without having to adjust the flow to each board. This is easily accommodated with a pumped two phase cooling system, where large numbers of cold plates can be cooled in parallel (series flow is not generally used when temperature uniformity is important, so that each cold plate has the same entrance conditions).
Figure 2 (A) shows the test set-up for four cold plates, each of which can be heated independently. The individual cold plates are marked with blue, orange, yellow and red stickers.
Note that the valves are used in this setup to provide a fixed pressure drop, adjusting them as flow conditions change is not necessary. Figure 2 (B) shows an individual cold plate in more detail.
Figure 3 (A) is a video demonstrating that power can be applied and removed to any of the cold plates during operation. When heater power is supplied to a given cold plate, a color coded dot appears next to the cold plate, and bubble formation can be seen. Figure 3 (B) shows the temperatures and flow rates for all four cold plates. When the electrical power to an individual cold plate is turned off, the temperature of that cold plate drops. However, as expected, the temperature of the powered cold plates is basically unaffected as the power changes.
ACT’s Pumped Two Phase (P2P) products are ideal for cooling of high-power electronics where heat loads have increased to a level beyond what traditional air- and water-cooling systems can effectively manage. Our standard Pumped Two-Phase Cooling Systems leverage common components and have been designed for 8kW, 30kW and 50kW capacities. Fully customized P2P solutions are also available.
ACT developed a P2P cold plate for a high-performance computing application with a total heat load of 4 kW and over 50,000 individual nodes in a 1U Blade configuration.
ACT’s P2P FAQ page answers many common questions about pumped two phase, including “What is Pumped Two Phase”, “How Does It Work”, “When is It Used”, and “What are the Benefits?”.
Advancements in Pumped Two Phase Cooling Technology
ACT has developed a P2P system for laser diodes and high-heat-flux electronics systems. It efficiently handles fluxes up to ~500W/cm2 from several parallel heat sources. Temperature uniformity over large surfaces has been demonstrated.
Hybrid TwoPhase Loop (HTPL) technology combines the robust operation of mechanically pumped loops with the passive flow control of capillary driven loops to transport high power (2 kW), high heat fluxes (>1200 W/cm2) at the low thermal resistances associated with evaporation off a wick structure.
Most pumped two phase systems rely on gravity to separate vapor from liquid. Momentum driven vortex phase separators are typically used in systems where gravity cannot be relied on to separate vapor from liquid, e.g., in micro-gravity, as well as on aircraft, where the acceleration vector varies as the aircraft maneuvers.