Pumped two-phase cooling provides a compact, low pumping power option for thermal management in high heat flux applications (300-500W/cm2). Compared to single-phase convection, significantly higher heat transfer coefficients can be achieved at substantially lower flowrates. Two-phase cooling also provides a high degree of isothermality, which is important in many applications such as lasers whose emission wavelengths are temperature-dependent.

Figure 1. Schematic of a pumped two-phase cooling system showing key components.
The phase change (latent heat) of the coolant enables two-phase systems to handle high heat fluxes with low pumping power compared to single-phase systems for a given heat load. Two-phase cooling systems are prone to flow/ thermal instabilities, yet engineering solutions are available to address such issues. Specifically, two-phase systems are not new and techniques to effectively manage instabilities have been studied and are available, including the application of engineered microporous coatings on the heated surface(s), evaluated in this study. The porous coatings considered here enhance the boiling performance by increasing the nucleation site density and provide a capillary-driven mechanism for resupplying the liquid/ coolant to the heat transfer surface that postpones dry-out.

Figure 2. a) Experimental pumped two-phase cooling system; b) test section containing a minichannel heat sink/evaporator, housing and heater block used to simulate the heat load.
A schematic of a typical pumped two-phase cooling system is shown in Figure 1. Key components include a pump, preheater, surge tank, evaporator/ heat sink, condenser and accumulator. The surge tank and the preheater differentiate this system from a conventional liquid cooling loop. 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 to a temperature close to the saturation temperature before it enters the evaporator/ heat sink. This is important as boiling heat transfer is most efficient at saturation (minimal subcooling).

For the sake of illustration, representative experimental results on the cooling performance of a pumped two-phase cooling loop are presented here, along with some practical considerations concerning the design and operation of two-phase cooling systems.

Experimental Hardware

The laboratory test setup shown in Figure 2a is outfitted with a copper minichannel heat sink having overall dimensions 20.4mm × 12.3mm × 6.0mm with 8 rectangular channels each having a hydraulic diameter 1.8 mm and a channel aspect ratio (H/W) of 2.8 (housed within the Test Section Assembly). The heat load applied to the heat sink was simulated using a copper heater block containing cartridge heater inserts. As shown in Figure 2b, the heater block narrows down to a pedestal through which heat is transferred to the heat sink. The heat flux applied to the minichannel heat sink is approximated by the product of the thermal gradient (ΔT/Δx) measured with thermocouples positioned along the pedestal length using the 1-D heat conduction formula:

where kc is the thermal conductivity of the copper heater block.

Boiling Enhancement Coatings

Figure 3. Minichannel heat sinks (a) without coating, (b) with porous sintered powder coating, and (c) with epoxy-based boiling enhancement coatings prepared compliments of Dr. You at University of Texas Dallas.
Micro- and nano-textured porous surface coatings can help stabilize boiling and suppress undesirable flow oscillations 1, 2. In addition, the use of coatings in pool boiling experiments have been shown to lower wall superheat ΔT (difference between the wall temperature and the saturation temperature of the coolant) for a given input heat flux as well as increase the Critical Heat Flux (CHF)3. In the current study, the performance of porous coatings was evaluated in flow boiling conditions where both nucleate boiling and forced convection play important roles. Images of select uncoated and coated heat sinks/ evaporators considered in this study are shown in Figure 3.

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