Replacing many of the traditional mechanical power and control systems with electronic equivalents will enable significant improvements by reducing weight, lowering fuel usage, increasing design flexibility, and enhancing overall functionality. This trend is apparent in “drive-by-wire” and “fly-by-wire” system designs. The past few iterations of Army vehicle programs have focused on increasing the use of electrical systems throughout the vehicle for many of the same reasons. Some of these programs have focused on new fully hybrid vehicles, while others have looked at improvements that can be made to legacy systems.

Because the disparate electrical systems common to these programs need to draw power from a platform’s common electrical bus, they all require improved power conversion electronics to meet operational specifications.

The components typically used to achieve a Two-Phase Cooling Loop. The filter and vapor/liquid separator are optional.
Most vehicle designs have relied on air-cooled heat sinks for low-heat-flux electronics and an antifreeze-based automotive liquid coolant loop to manage the larger waste heat from engine compartment components. However, new power-dense electronic systems are further increasing waste heat and presenting great challenges to the capabilities of conventional air and single-phase liquid cooling systems. Considering strictly air cooling, the effect of

higher heat flux electronics is larger, heavier, costlier heat sinks and fans to compensate for insufficient convective performance. The effect is equally dramatic with single-phase liquid cooling, with higher heat fluxes requiring larger coolant flow rates to sufficiently cool the system devices. These large flow rates and subsequent pumping powers result in increasingly bulky, heavy systems that consume more fuel. Thus, there is a drive to develop improved cooling components that are smaller and lighter, and have increased convective performance relative to conventional liquid cold plates.

With ever-increasing electronic heat fluxes and mounting single-phase thermal management issues, cooling schemes using liquid-vapor phase change (hereafter referred to as “two-phase cooling”) have been examined as practical and cost-conscious steps beyond single-phase cooling.

Most vehicles have a single-liquid coolant loop for the removal of waste heat from the engine and other “underhood” components. This has historically been an antifreeze loop consisting of a 50/50 ethylene glycol and water (EGW) solution, though there has been a recent commercial movement toward the use of lower-toxicity propylene glycol and water (PGW) solutions. This loop is driven by a single-coolant pump, and after acquiring heat from various heat sources will pass through a liquid-air heat exchanger (typically a fan-cooled radiator) to reject heat to the environment.

The single-coolant loop architecture restricts the potential performance of the electronic cooling components. Because the engine is usually the primary cooling concern for this loop, changes to loop parameters, such as fluid type, flow rate, and available pressure, generally cannot be made even if electronics cooling performance could be improved.

Two-phase cooling refers to intentionally using a cooling fluid in a manner where at least a portion of the fluid is transformed into vapor upon heating, thereby resulting in a gas/liquid mixture in a portion of the cooling loop. The boiling event generally occurs when the temperature of the heat acquisition surface exceeds the liquid’s saturation temperature, or boiling point. At this temperature, the vapor pressure of the liquid is equal to the pressure of the environmental surroundings, thus allowing vapor bubbles to form at the solid-liquid interface, grow, and eventually detach from the surface. After this point, multi-phase flow exists in the system until the vapor is either separated from the liquid flow or is cooled to the point of condensing back into a liquid. A liquid may be induced to boil over a range of temperatures, depending on the pressure of the environment, where increasing system pressure will increase the nominal boiling temperature of the fluid. This provides an additional benefit for a system using two-phase cooling, whereby the boiling and condensation temperatures of enclosed coolant loops may be tailored by controlling system pressurization.

Two-phase macrochannel coolers are being implemented as an intermediary step between conventional single-phase methods and future two-phase microchannel designs. At the cost of reduced thermal performance, macrochannel designs have a more extensive research basis and are currently considered less complex and easier to implement than their higher-performing microchannel counterparts.

The difficulties in designing a hybrid electric propulsion system for Army vehicular applications have consistently called for advanced thermal management technologies. However, single-phase forced-convection liquid cooling has reached a mature state, and it is highly unlikely that the order-of-magnitude heat flux improvements desired for future systems will be obtainable by this technology under the current set of platform constraints.

The key limitations can be summarized as:

  1. Minimum allowable orifice size — particles and sediment in the primary engine loop limit cooling scheme geometries and may cause clogging.
  2. Fluid limitations — the primary cooling loop has been designed for an EGW coolant, which is not an ideal fluid for use in electronics cooling.
  3. Pumping inflexibility — fluid delivery requirements cannot be optimized for the electronic cooling components due to engine coolant flow and pressure specifications.
  4. Single-phase fluid limitation — because the system is not designed to accommodate mixed gas/liquid flow, there is currently little opportunity to implement two-phase flow.

This work was done by Darin Sharar, Nicholas R. Jankowski, and Brian Morgan of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at under the Electronics/Computers category. ARL-0116

This Brief includes a Technical Support Package (TSP).
Review of Two-phase Electronics Cooling for Army Vehicle Applications

(reference ARL-0116) is currently available for download from the TSP library.

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This article first appeared in the February, 2011 issue of Defense Tech Briefs Magazine.

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