Two-Phase Flows in Microchannel/Microimpingement Coolers for Coolant Flow Reduction in High Energy Laser Systems

Estimated water flow requirements for various high power laser systems. In comparison, a typical fire truck has a 1400 GPM capacity.

Increasing waste power and waste power densities generated by semiconductor devices re-quires thermal management systems with low thermal resistance and efficient coolant usage. The advantages of using high aspect ratio micropassages to minimize thermal resistance have been well established since 1980 (Tuckerman & Pease (1980)).

For airborne systems, however, the flow rates used to achieve these low thermal resistances are unacceptably high for large laser and RF systems with high total heat dissipation requirements. DARPA’s Excalibur program is a prime example. Excalibur involves the coherent combination of fiber lasers, each laser supplying up to 3 kW for a total output ranging between 0.1 and 1 MW. Assuming 50% laser diode efficiency, 67% diode-fiber conversion efficiency, and a water coolant, this leads to the coolant flow rates shown in Figure 1. For reference, the 1400 GPM of the 1 MW/4K point on the figure is about equal to the pumping capacity of a full-size firetruck. These flow rates, which are clearly not consistent with a necessity to reduce laser system sizes by 10X, will be typical of any laser diode system operating at these power levels.

Two-Phase Flow. Exploiting the enthalpy of vaporization of a two-phase flow system is an obvious means of reducing flow rates, pump system size, and pumping powers, so long as both: 1) the thermal resistance of the coolers is not compromised at high values of quality (χ, the vapor fraction), and 2) the complexity and size of the 2-phase loop does not excessively penalize the overall system. Among candidate two-phase systems such as porous metals, spray cooling, and enhanced surface boiling, microchannel cooling offers the best combination of practicality, versatility, compact size, and high thermal performance.

The two-phase flow behavior in macroscopic ducts is very sensitive to buoyancy, and so the orientation of the duct is critical. This is a significant issue on airborne platforms where the acceleration forces are large and cover a range of orientations. However, surface tension forces are much larger than gravitational (buoyancy) forces at sub-millimeter scales. This implies that certain combinations of channel dimensions and coolant properties will provide a two-phase system that is immune to large accelerations.

The relative magnitudes of the buoyancy and surface tension forces in a duct are estimated by the Bond (Eotvos) number, Bo = (ρliqvap)gD2/σ (ρliq is the density of the liquid phase, ρvap is the density of the vapor phase, g is the gravitational acceleration for Earth, D is the diameter of the passage, and σ is the surface tension). Bo << 1 implies that surface tension forces dominate over buoyancy forces.

The Problem. However, previous experiments indicate that coolants with high surface tension will perform poorly in two-phase flows. It appears that the surface tension effects which are desirable to reduce sensitivity to high-g accelerations also promote undesirable flow morphologies, which inhibit heat transfer. Finding a solution which satisfies these competing requirements constituted the primary design challenge of this MDA-sponsored R&D program.

Experimental Results. The thermal and flow performance of a prototype microchannel cooler was evaluated in bench testing at ±1g using R245fa. The test results confirmed the predictions of CFD analyses; little difference was seen between the two test orientations. Since R245fa has a Bond number 3X larger than the candidate coolants, it is felt that the candidates will be even less sensitive to acceleration-induced buoyancy effects than R245fa. The numerical model was found to successfully predict R245fa performance over the entire quality range (0 to 1) for a 25 µm x 312.5 µm microchannel geometry.

The experimental results support the notion that coolants with acceptable thermal and flow performance can be used in g-insensitive 2-phase micro-channel heatsinks. These heatsinks offer flow rate savings of 6X to 12X over conventional single-phase water-cooled microchannel heatsinks, representing an enabling technology for high energy lasers on airborne platforms.