Custom Pumped Two Phase Test Rigs
Pumped Two Phase cold plates (P2P) are suitable for cooling high heat fluxes, and for maintaining an isothermal surface over large areas. Custom P2P test rigs for cold plate evaluation that ACT has designed and fabricated include:
- Pumped Two-Phase Cooling for Thermal Management of High Heat Flux Devices: This test rig is used to test different high heat flux cold plates, at heat fluxes in excess of 300 W/cm2.
- Pumped Two-Phase Isothermal Cooling Test Rig: A second two phase test rig used to test two large 60 x 60 cm (2 ft. x 2 ft.) cold plates, maintaining a 25°C ± 2°C temperature across each plate.
- High Heat Flux Spray Impingement Cooling Test Apparatus: A spray cooling test rig capable of removing 1000W/cm2 from an exposed CPU, taking single jet impingement data, with three different fluids (HFE-7000, methanol, and water).
Pumped Two-Phase Cooling for Thermal Management of High Heat Flux Devices
As the power density of electronic devices and lasers continue to increase, new cooling strategies such as pumped two-phase (P2P) cooling are needed to maintain their temperatures below maximum operating limits and provide a necessary high degree of isothermality to optimize performance and lifetime. ACT has developed bench-scale and standalone two-phase cooling systems that can handle several hundred Watts per square centimeter of thermal power and provide excellent temperature control and uniformity.
Figure 1 shows a stand-alone two-phase cooling system developed at Advanced Cooling Technologies, Inc. (ACT) that is capable of handling heat loads upwards of 300W/cm2 while maintaining tight temperature control and isothermality over the heat transfer area. In addition, the unit can handle multiple, separate, non-uniform and transient heat loads on the different evaporators. As noted, the unit is self-contained and simply has to be plugged in, setpoint temperatures input and thermal loads mounted to the two-phase evaporators. This apparatus has been used to test advanced two-phase evaporators, some of which are quite large and some of which include the use of microporous coatings to improve boiling performance. Read more on two-phase evaporators and the use of advanced boiling enhancement coatings.
In the design of the unit, particular attention has also been given to address flow maldistribution between multiple evaporators and flow and thermal instabilities internal to the evaporator and at the system level. Regarding flow maldistribution, it is important since adequate flow to and within each evaporator is essential to avoid local dry-out and overheating. In addition, internal controls automatically adjust the saturation conditions of the refrigerant prior to the evaporator inlet(s) such that the refrigerant boils as it enters the evaporators providing optimal cooling. A two-phase mixture of liquid and vapor exits each evaporator, which is eventually condensed onboard the unit.
For those unfamiliar with pumped two-phase cooling systems, it should be noted that the key components in the system include a pump, preheater, surge tank, condenser, accumulator and obviously the evaporator(s). The surge tank consists of vapor and liquid at saturation; by controlling the pressure in the tank, the saturation (boiling) temperature of the working fluid can be controlled. The preheater heats the subcooled liquid exiting the condenser prior to entry into the evaporator(s) to again adjust saturation conditions and achieve optimal cooling performance.
Pumped Two-Phase Isothermal Cooling Test Rig
ACT was approached by a customer to develop custom two-phase cold plates for a pumped two-phase (P2P) thermal management system to be used in a high power directed energy weapons application. Under this program, ACT designed, built, and tested two large cold plates to cool and isothermalize a large array of high-power density electronics. To qualify the cold plate design, ACT also designed, built, and delivered a custom test cart with a full P2P loop that interfaced with an external chiller.
The two-phase isothermal cooling test rig contained a full P2P loop consisting of the custom cold plates, a condenser heat exchanger for heat rejection to an external chiller loop, a reservoir, and a pump. A control system was developed for the P2P loop for maintaining the cold plate inlet temperature and system pressure. This control system included a temperature-actuated flow control valve for inlet temperature control, and heating and cooling of the two-phase reservoir to maintain saturation pressure. The heat load on the cold plates were mimicked by heaters epoxied to the surface in the electronics footprint. The two-phase isothermal cooling test rig was fully instrumented to monitor fluid temperature and pressure at multiple locations, cold plate temperature, and coolant flow rates. The final delivered test rig, shown below in Figure 2, was equipped with a control panel and data acquisition system and was contained on a movable cart.
The pumped two-phase isothermal cooling test rig was used to evaluate the performance of the cold plates, which had strict temperature uniformity (isothermality) requirements. The customer required that the temperature of the cold plates be maintained within ±2°C of a target temperature, and vary by less than 3°C over the entire cold plate. Figure 3 shows the cold plate temperatures along the length of a channel on each plate, which satisfactorily met the customer requirements.
Spray Impingement Cooling Test Apparatus for High Heat Flux Cooling
One of ACT’s customers was interested in using spray and jet impingement cooling to remove high heat fluxes (1000W/cm2) from the surface of a CPU. A test section was designed capable of taking single jet impingement heat transfer data using Intel’s supplied 34980A thermal test platform. Figure 4 shows an overall view of the test section with the Sil-clamp bolted to the base plate. The Thermal Test Vehicle is also shown in place. The 8 bolts hold the Sil-clamp in a precise location above the base plate and also provide even pressure around the O-rings to the acrylic cylinder. Figure 4 shows the flow exiting the nozzles at the projected angle as well as the projected impact of the spray with the die. In the design process the 3-D CAD software was used to verify no flow interactions. The test apparatus was designed to test at various inlet temperatures from -10°C to 125°C and work with three different fluids (HFE-7000, Methanol, and Water). The apparatus successfully measured heat transfer coefficients at various locations along the die, providing the customer with the precise measurements required for their application; see Figure 5.
If you are interested in learning more, please contact ACT today.