Electronics continue to increase in power while shrinking in size, so future thermal control systems will have to accommodate higher heat fluxes and powers than advanced liquid cooling, advanced heat pipes or even commercial pumped single and two-phase solutions can handle. ACT’s advanced R&D in Pumped Single-Phase and Pumped Two-Phase (P2P) cooling is discussed below.
ACT’s Advancements in Pumped Two Phase Cooling Technology
ACT has experience using advanced pumped two-phase approaches to solve the most unique and challenging thermal requirements, including tight spatial and temporal thermal control (high isothermality) requirements for Directed Energy Weapons, high heat flux and power removal for power electronics, and challenging acceleration requirements for missiles, space and aircraft applications.
Some of ACT’s previous projects accomplished the following:
- Remove heat fluxes greater than 500 W/cm2 over several cm2.
- Remove heat fluxes greater than 1200 W/cm2 with a hybrid P2P loop.
- Develop a spray cooling system for a bare chip, with a fireball (>1,000 W/cm2). The system allowed the instrument to move and probe the entire chip.
- Cool dual 2 ft2 (0.2 m2) cards with multiple laser diodes, maintaining ±3°C isothermality while switching between full and half power. No active flow control was needed.
- Develop sub-components and systems capable of operating in low, high, and variable acceleration environments.
Figure 1. High-Heat-Flux Pumped Two-Phase cooling in a mini-channel cold plate.
ACT has developed a pumped two-phase cooling system for high heat flux electronic components and laser diodes that efficiently handles fluxes up to ~500W/cm2from several parallel heat sources. The technology uses sintered wick materials to provide ample and uniform nucleation sites for boiling. These nucleation sites prevent flow maldistribution during transients and help provide lower evaporator thermal resistances.
The Hybrid Two-Phase Loop (HTPL) technology combines the robust operation of mechanically pumped loops with the passive flow control of capillary-driven loops. The mechanically pumped liquid loop supplies liquid to the evaporator and returns any excess liquid to the reservoir. The capillary-driven two-phase loop acquires and transports large, high heat fluxes (>1200 W/cm2) at the low thermal resistances associated with evaporation off a wick structure.
Momentum-driven vortex phase separators are typically used in systems where gravity cannot be relied on to separate vapor in liquid. Pumped Two Phase (P2P) applications include systems in microgravity as well as on aircraft, where the acceleration vector varies as the aircraft maneuvers.
Pumped Single-Phase Cooling is a standard method for cooling electronics, typically high power systems with low to moderate heat fluxes. ACT has also shown that single-phase systems can be used for heat spreading, and to remove very high heat fluxes over small areas. (Researchers now at ACT have removed heat fluxes of over 10,000 W/cm2 over several square centimeters.
ACT developed a spray impingement system for an electronic device with high overall heat flux (70 W/cm2) as well as a fireball in one location (>1000W/cm2) over a small area (<0.25mm2).
Oscillating Liquid Cooling
ACT’s oscillating liquid cooling technology incorporates a mechanical actuator to generate an oscillatory motion of the liquid. The oscillating liquid absorbs heat with very high efficiency, thanks to the disrupted liquid-wall boundary layers. The fluid oscillation effectively “relays” the heat from the heat input area to the heat dissipation area. The heat transfer performance can be controlled by adjusting the amplitude and frequency of the actuator output.
The figure below shows the test results of oscillating water in a 1/8″ OD copper tube. The heat source and sink were attached to the two ends of this 12″ long tube. The oscillation was generated by a mechanical assembly capable of producing fluid oscillations at frequencies of 0 to 20Hz and amplitudes of 0 to 12″. As shown, the oscillating flow heat transfer, with a proper combination of oscillating frequency and stroke, can remove heat fluxes in excess of 1,300W/cm2 at an equivalent thermal conductivity close to 250,000W/m-K. For comparison, a 1/8″ OD and 12″ long copper/water heat pipe can only handle heat fluxes up to 40W/cm2. The thermal conductivities of copper and diamond materials are on the order of 380 and 1,200W/m-K, respectively.
The following conclusions can be drawn from the test results:
- Maximum deviations between test results and model predictions are within a few percent.
- Hot spot temperature is reduced by 6%.
- Temperature non-uniformity is reduced by 75%.