Two-phase cooling will benefit large lasers

July 1, 2005
The laser of choice for military directed-energy-weapons (DEW) research has traditionally been the chemical ­laser.

The laser of choice for military directed-energy-weapons (DEW) research has traditionally been the chemical ­laser. The hydrogen fluoride/­deuterium fluoride (HF/DF) laser, emitting at 2.7 μm for HF or 3.8 μm for DF, has produced short-term bursts of continuous-wave power to 2 MW in the form of the U.S. Air Force’s Mid-Infrared Advanced Chemical Laser. The Tactical High Energy Laser, built by Northrop Grumman Space Technology (Redondo Beach, CA) for the U.S. Army, is a 100-kW-class ground-based DF laser that has shot down mortar rounds and small rockets in flight. The chemical oxygen-iodine laser (COIL) emits at 1.3 μm and is the used in the megawatt-class Airborne Laser being developed by the U.S. Air Force, to be housed in a modified Boeing 747.

But these chemical lasers are bulky and heavy and require large quantities of chemical reactants. Although a mobile form of the THEL is being developed, called the MTHEL, it still must be housed in a large trailer (and its fuel in a second trailer). Military ground vehicles and small aircraft would benefit from carrying an electrically powered laser emitting in the 100-kW range, if it were small enough (say, the size of a refrigerator for the laser head itself). As a result, DEW researchers are developing high-power diode-pumped solid-state (DPSS) lasers, hoping to push them to the 100-kW range necessary to destroy targets.

But many problems must be overcome, one of which is heat removal-not just from the solid-state-laser medium, but from the laser-diode-array (LDA) pumps, which are the source of much of the heat. Improving diode efficiency will help reduce the heat load, but still there will be hundreds of kilowatts of heat to remove. Single-phase liquid cooling, used in commercial high-power DPSS lasers, is limited by the amount of heat per unit mass that the liquid (usually water) can carry away.

Engineers at Raytheon Missile Systems (Tucson, AZ) and elsewhere are taking another approach-that of two-phase cooling, in which a fluid is vaporized. Because a phase change from fluid to gas absorbs far more heat than does a simple rise in fluid temperature, two-phase cooling can handle very large heat loads.

Three approaches

A presentation prepared by Chad Boyack, a senior mechanical engineer at Raytheon Missile Systems, and Kevin Hopkins, a principal mechanical engineer, also at Raytheon, and given by Boyack at ­PennWell’s Military Technologies Conference (March 15-16; Boston, MA), outlined progress made on three different approaches to two-phase cooling: bonded-laminate jet impingement, vapor-injected spray, and slot-jet.

In a slot jet with adjacent exit slots, a fluid jet impinges on a heated surface to cool it. The temperature gradients across the cooled surface are controlled by the slot spacing. Either single- or two-phase operation is possible with this design.
Click here to enlarge image

Bonded-laminate jet impingement, which is being developed at Hamilton Sundstrand (Windsor Locks, CT) in the form of its evaporative compact high-intensity cooler (ECHIC), uses thin copper foils that have been etched to produce an intricate pattern. The foils are stacked together and diffusion bonded to create a single continuous heat-dissipating device. The intricate design features etched in each of the individual foils produce a complex coolant flow in a compact device that maximizes the heat dissipating capability of the device. The CHIC technology can be used in either single-phase or two-phase heat-transfer devices, and has been demonstrated to heat-flux levels in excess of 500 W/cm2, noted Boyack.

The vapor-injected-spray cooling device, developed by Rini Technologies (Orlando, FL), works on the principle of atomizing a stream of coolant liquid as it passes through a series of channels and orifices that break up the liquid into smaller and smaller liquid droplets, explained Boyack. The atomization process is enhanced by injecting coolant vapor into the liquid stream to assist in the breakup of liquid into very small diameter droplets. The resulting fine spray vaporizes as it impinges upon the target surface, maximizing the heat dissipating capability of the device. ­Vapor-­injected-spray cooling devices have been demonstrated to heat flux levels of 500 W/cm2 and higher.

The slot-jet cooler with adjacent exit slots, developed at Raytheon and patent pending, combines liquid-jet-impingement cooling technology with a novel packaging concept to create a very high-heat-flux cooling device (see figure). Liquid coolant enters the slot-jet cooler, impinges on the surface to be cooled, makes a 180° turn, and leaves through exit ports. The slot-jet cooler can be designed to operate as a single-phase or two-phase heat-transfer device, said Boyack.

“Each of these three cooling technologies could potentially be used to dissipate the waste heat generated in LDA packages,” said Boyack. “In some applications, the technologies can be packaged such that the new cooling technology simply replaces existing microchannel cooling devices with minimal impact on LDA packaging design. In other applications, thermal transport from the LDAs can be enhanced with additional consideration of the particular details of a given LDA concept. The primary benefit of using any of these two-phase cooling technologies is the significant reduction in coolant mass-flow rate required for a given thermal load.”

Vapor-injected-spray cooling is probably the most mature in terms of demonstrated performance in the lab, noted Boyack. “The other two are close behind and in some respects could be easier to implement in the end because of the system-level considerations of ­handling/separating the two-phase fluids required by vapor-injection-spray cooling technology,” he said. “There are also potential issues with spray cooling in high-g environments if the application requires operation in that environment. The CHIC concept has been around for more than 30 years, but has seen very limited application. And the CHIC concept has only recently been extended into the two-phase regime. Neither the CHIC nor slot-jet technologies are hindered by the unique challenges facing vapor-injection sprays. But they are not quite as far along in terms of demonstrated performance in the lab.”

Vapor-injected-spray cooling of LDAs is in the early demonstration phase. Small-scale (a few square centimeters of surface area) LDAs have been cooled at low dissipated power-flux levels (less than 100 W/cm2) using a common refrigerant as the coolant. Researchers are interested in pursuing vapor-injected-spray cooling of LDAs at dissipated power flux levels approaching 500 W/cm2 using a higher-capacity coolant fluid.

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