Below are the questions asked during the event, along with their respective answers.

Q: How can we make these concepts and products to use in the small size power electronics?
A: In theory everything we discussed is scalable. From a manufacturing perspective, it depends on how small you’re trying to go. Depending on your application, a gravity-driven two-phase loop (Loop thermosyphon) may be a good alternative. These systems are also quite scalable to smaller sizes and are less expensive to implement. They do however require a gravity aided orientation in order to return the liquid to the heat-generating section

Q: Does a pumped 2-phase work like a refrigerator?
A: No. Pumped two-phase technology transfers heat very efficiently, but it does not provide refrigeration. The devices being cooled with P2P systems will always run hotter than the ultimate cooling fluid, whereas, a refrigeration system can cool below the sink temperature.

Q: How far does “long-distance” refer to in the pumped two-phase system?
A: In theory there is no hard distance limit for the pump, it just needs to overcome the system-level pressure drops. ACT has built both pumped and passive two-phase systems with 10m (30ft) transport lengths for a variety of powers, up to 30kW.

Q: What are the challenges to modeling two-pressure drops in two-phase systems?
A: Accurately modeling two-phase flow, particularly vaporizing flow, is numerically challenging. The most efficient way to model these systems with reasonable accuracy is often to use empirical correlations, which are derived from experiments that most closely mimic the application. There are a number of them in the literature to choose from.

Q: What is the recommended simulation software for designing with these products?
A: Modeling the physics of true two-phase heat transfer is complex and computer-intensive. Most designers rely on correlations for heat transfer and pressure calculations, and then input the appropriate information into commercially available CFD codes for higher-level system solutions.

Q: Do you have any interest in collaborating with Universities to explore new advanced concepts; like femtosecond functionalization of metal surfaces for mini-channel flow?
A: ACT has a large R&D group, and we do collaborate with universities on relevant opportunities. Please contact us directly for more details.

Q: Does a pumped two-phase system use more refrigerant than a compressor based two-phase system?
A: Yes, I would say for a comparable system, a P2P version would use more refrigerant than a vapor compression system.

Q: Different material (pumps, pipes, valves) have to be used with these fluids than with water, right?
A: As with any fluid system, your materials need to be compatible with the fluid. Refrigerants are fairly inert and can be used with a much broader range of materials than water systems.

Q: Any comment for selecting pump for two-phase loop?
A: Yes, the pump needs to be hermetic and provide long life at the designed operating conditions.

Q: What are the maximum heat flux levels in the evaporators of passive and pumped systems?
A: Typical critical heat flux limitations for refrigerants are significantly lower than for water. However, flow boiling increases those values, and flow boiling in channels increases it significantly. ACT has demonstrated heat fluxes well over250 W/cm2 for pumped refrigerant systems. This link will take you to a page on our website that contains more detailed information.

Q: Your seminar addressed the magnitude of heat rejection (w=energy/time) and not heat fluxes (energy/time*area) which brings in the discussion the size of the heat source and of the device.
A: See above answer with link

Q: What are the subcooling effects on two-phase thermosyphon loop performance?
A: The loops are not designed to operate with any significant sub-cooling, the only available sub-cooling would be that which is possible due to the system pressure drops. The evaporator temperature will almost always follow the sink temperatures at a constant power.

Q: Can you go into the costs?
A: Our cabinet cooler pricing is listed on our web site, www/ Please contact ACT directly for pricing for other systems specific to your application.

Q: Are the pumps magnetically coupled?
A: The pumps need to be hermetic, but they can be either mag driven or sealed shaft-driven pumps.

Q: Max size of pumps?
A: There is no theoretical limit to the pump size and transferred power. It is a simple mater of scaling the diameter of the liquid and vapor lines to accommodate the flow for very high-power systems.

Q: Is there a minimum heat flux for P2P?
A: No. Since the fluid entering the cold plate is already at saturated pressure, the addition of even a low heat flux load can cause the fluid to vaporize. If the system is designed for higher powers and fluxes and runs at a low flux, the device will be cooled by single-phase flow until the fluid reaches saturation.

Q: Is there a maximum temperature difference, or more specifically a minimum ambient temperature required to allow the system to work? Could it be so cold outside of the enclosure that all the refrigerant “hangs up” outside? (on a passive system)
A: There is a thermal gradient required for both the Passive Loop-Thermosiphon and Pumped-Two-Phase systems to operate. First, they need to generate sufficient vapor pressure to allow the vapor to flow to the condenser. Secondly, they need to be able to dissipate the heat to the ambient air or ultimate heat sink. As far as going too cold with refrigerants as the working fluid, that should not be a concern.

Q: Can a passive 2-phase fit into a typical desktop chassis?
A: Yes. A passive loop thermosyphon is an excellent option for high power desktop cooling. We have also developed pumped-two-phase systems to fit in a 3-U rack.

Q: What are the major design considerations for cold plates for two-phase cooling systems?
A: Just like single-phase cold plates: flow rate, pressure drop, flow distribution, and heat flux into the fluid are key design considerations. Some of these are more critical for a Pumped-Two-Phase (P2P) system, but once properly designed, P2P evaporators are very robust in response to flow, power variations.

Q: Are any of these products available off the shelf from distributors? I am only aware of heat pipes available from DigiKey?
A: Our Enclosure cooling products are standard and available to order through our web site. All of our other products are based on custom designs resulting from consultations with our customers.