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Employing effective thermal management techniques such as spray cooling is becoming a necessity in new small form-factor and power dense electronic architectures. Spray cooling can solve an array of problems ranging from: electronics cooling, power density, airborne/structure vibration, motor/generator and electric contact life/maintenance, EMI and magnetic field contributions. Its near term expansion looks promising and may enable new key components into areas presently limited by traditional thermal management technologies. Successful spray cooled tests performed on electric motors are directly applicable to COTS-based electronic systems.
Spray Cooling Benefits
Spray cooling can help create uniform temperatures and reduce magnetic field fluctuations and electrical noise. Spray cooling technology can be used to spray dielectric fluids, such as per-flouro compounds (PFCs), e.g. Fluorinert, directly onto energized components such as motor coils, controls and brushes (in the case of brushed motors), and brushed excitation rings in generators. A closed loop system provides for a hermetic seal against airborne contaminants such as moisture, dirt or other particulates. The process of spray cooling removes latent heat through vaporization.
The liquid PFC change of state provides a higher BTU reduction without depending on substantially higher temperature surface areas required by convection methodology (liquid or air). The vaporization and condensation of the PFC, or other dielectric, usually occurs inside the motor case and its drive. With the design of an intercooled source you can: a) achieve even higher power densities, and b) remove heat to a desirable location.
This is often the case where multiple high-powered, and high-density units are located in command and control centers (Fire Control, Command Information Centers and Communication spaces). Using existing heat exchange sources for cold plate technology can be expanded by integrating more efficient spray cooling techniques without increasing the existing exchange infrastructure. This is an important factor when considering total operating costs (TOC) and complexity.
The PFC fluids that are presently utilized are capable of comparable dielectric properties in vapor form as well as a liquid state. This provides for reduced EMI (electromagnetic interference) and EMC (electromagnetic compatibility). The high dielectric strength provides for significantly reduced packaging requirements due to reduced interference in compact electronic architectures. PFCs and some other dielectric liquids capable of phase change can minimize the damaging effects of spaceborne radiation as well.
Testing Results on Electric Motors
U.S. Navy Testing
In the quest to modernize, integrate, automate and reduce fixed costs, the Department of the Navy has initiated several programs: DD21, Integrated Power Systems (IPS), Advanced Electric Power Systems (AEPS) and others. Ships of the future will be used in hostile littoral (shallow waters) that increase the detection of shipborne radiated acoustic noises. They are also in the process of electrifying the propulsion systems in new ships, which eliminate the need for reduction gears and makes for acoustically quieter ships.
There are numerous objectives that are fundamental to future designs, which include the following:
• reduced Total Operating Costs (TOC)
• reduced manpower
• reduced maintenance
• increased reliability
• increased power density
• improved thermal management
After several years of systems development, thermal management has proven to be a significant obstacle for achieving many of these objectives. The proliferation of new high-powered equipment, communications, electronic warfare systems and directors for missiles, satellites and guns has taken thermal management to its limits. The expansion of compartments and cooling systems is required to cool these new systems and is contrary to cost reduction.
The Naval Systems Warfare Command, Carderock Division (NSWCCD), has been monitoring the development of “Spray Cooled” rotating electric equipment and their controls. This technology was demonstrated at the NAVSEA-operated Fox Island Acoustic Laboratory (FIAL), located on Fox Island in Washington State (Figure 1).
The demonstration was on Efficient Propulsion Inc.’s spray cooled 28 kW DC electric motor and funded as Project 96K. The test was to determine the effect of the spray cooling system on radiated acoustic noise (airborne noise and machine vibration) as well as the other objectives listed above.
High dielectric per-flouro compounds (PFCs) were sprayed directly onto the armature, and structureborne vibration and airborne noise data were taken. The tests were performed under normal air-cooled conditions versus spray cooled with the motor in a low load condition. The spray cooled test results are shown in Table 1. The airborne noise levels with no spray cooling increased with RPM. The motor with spray cooling reduced higher frequency vibration levels, especially in the axial and radial motor free-end bearing positions.
The other non-acoustic benefits included lower operating temperatures, lower voltage and current levels for the same RPM, resulting in higher operating efficiencies (Table 2). Visual observation of the brushes showed significant reduction in arcing, which reduces heat and wear.
When conductors heat up, resistance increases, creating more heat and energy loss. In motors and generators, in either the armature or field windings, the temperature around the coils is rarely uniform, creating hot spots where resistance is higher or lower in different areas. This higher or lower resistance area can be caused by non-uniform cooling, resulting in hot spots. With this non-uniform continuity, there will be areas where the electrons will move faster or slower creating minute surges and sags, which will cause the magnetic field to fluctuate around the conductors in the device and in associated conductors. This contributes to the magnetic field fluctuations and induced currents in the structure. Uniform temperatures reduce the higher and lower resistance areas caused by heat; uniform temperatures will therefore reduce magnetic field fluctuations.
Electromagnetic interference (EMI) emissions and radiation are also a concern. As electronics power density increases, EMI emissions also increase. Electromagnetic compatibility (EMC) standards create more pressure on finding solutions to reducing these phenomenon. Irregular magnetic fields create unbalanced loads yielding decreased efficiency, reduced life, increased maintenance and electrical and mechanical disturbances. In generators and electronics these micro disturbances affect equipment downline as electrical noise. These disturbances are particularly detrimental to sensitive communication systems and multi-spectral sensors.
Southern California Edison Test
Testing by Southern California Edison on a GE 85HP traction motor was significant and consistent with NAVSEA’s FIAL efficiency data. The GE 85HP traction motor was fully loaded and tested with air-cooling and spray cooling (Figure 2).
In the mid- to 3/4-loads, a 25% to 30% improvement in efficiency was reported with spray cooling. Also, a 10% improvement was reported at 85 HP load and a 15% improvement in sustained overload at 100 HP (total power, in direct contact with the liquid, exceeded 73,000 watts). The test rig and shaft were not designed to operate at higher loads; however, the designer indicated that sustained overloads of at least 200% of rated load were possible. The overload results show that the use of less costly, lighter and more efficient generators and motors in ship, air and spaceborne applications is possible with spray cooling.
Second Annual U.S. Navy Sponsored Spray Cooling Symposium
At the recent second-annual U.S. Navy-sponsored symposium in Scottsdale, AZ, all major service branches were represented and discussed the immediate need for evaporative cooling technologies. Numerous projects have implemented spray-cooling technologies due to the lack of other high-density cooling techniques. These programs include upgrades to the Navy’s EA-6B Prowler, the Marine’s Amphibious Assault Vehicle, AAAV, the Air Force’s “Rivet Joint” RC-135 surveillance aircraft, Lockheed Martin’s upgraded U-2S, Northrop Grumman’s Global Hawk UAV, Army laser projects and an undisclosed project built for NSA. Project “Rivet Joint” expansion has been halted due to having reached the maximum thermal capacity of its cooling system. Recent spray cooling developments have provided an increase in power density for this program.
Spray cooling’s superior power density characteristics make it 50 times more effective than typical air-cooling. As a result, on-board avionics systems in many platforms could expand by an order of magnitude in power output—providing heretofore unheard of electronic capabilities. The condensation side of the spray cooled system can be tied to the existing AC infrastructure. This provides for a power density improvement since evaporative cooling needs less condensation-related inter-cooling and is far more efficient than conductive “cold plate” technologies. Evaporative cooling provides for more efficient use of existing cooling systems without increasing their size.
Other prospective projects include the Joint Strike Fighter, F-22 JSF and updates to the B2 Stealth Bomber, STARWARS, Airborne Warning and Control System, AWACS and Joint Surveillance Target Attack Radar System, Joint-STARS E-8C. The symposium was also well represented by VITA who is working toward the standardization of sealed enclosures for spray cooling applications. This will be an important step for reducing costs and increasing the supply of VME-oriented cases, particularly for the use of COTS boards.
Future of Spray Cooling
The evolution of evaporative electronics cooling is beginning to closely mimic that of mammals. This natural and highly efficient biological means of evaporative cooling has evolved through natural selection and has taken millions of years to perfect. The biological functions hold answers for perfecting viable and cost-effective thermal management techniques. New gas and liquid permeable materials will provide for the advancement of evaporative cooling techniques.
These materials may enhance superconductivity, approaching ambient conditions. Many answers likely exist within permeable biological metabolic processes, which include electrochemical, intracellular, atomic and sub atomic infrastructure heat transfer. This type of heat exchange is dependent upon relatively simple conductive and evaporative cooling processes.
The subsequent combination of electronics and these highly evolved “biological” processes may be referred to as “bioelectronics” when the working liquid is introduced through the core of a component (Figure 3). This method more closely mimics the biological cooling mechanism, which secretes substances from within its core. Electronic core uses include that of a rotating coil, electronic component or the inner sides of an electronics enclosure. This process provides both conductive and evaporative cooling, which is vital as mass, power density and high cycle and clock rates increase. Higher cycle and clock rates are necessary for enhanced electric weapons, signals intelligence systems and other communication system designs.
This secretion permeable method provides for significant thermal performance improvements and a more consistent component temperature. This provides for both stability and longevity of multi-layered high-performance components such as semiconductors. This method has demonstrated a 1600% increase in the amount of power applied to a coil using this “Internal Injection Method” when compared to air-cooling (Figure 4).
Ultimately, liquid and gas permeable membranes will enable individual semiconductor junctions to benefit by rapidly shunting thermal byproduct during each sequence. This more closely mimics a biological synapse, which benefits from conductive and gas heat transfer via cellular membranes. High-power coils, such as those found in linear motors and weapons systems, benefit from increased power density and increased cycle rates as do other high-power electronic components. For high-power devices such as silicon rectifiers, integrated bipolar transistors, radar klystrons, laser diodes and sensors, this method of cooling may compliment COTS for components in the near future.
This method further emulates that of a biological function by moving the heat directly away from the heat source. It benefits from the natural migration of heat to the cooler region such as toward select areas of an enclosure or to a heat exchanger. This increases the efficiency by reducing vapor, liquid vortices and boundary layers found in more traditional “spray cooling” methods (Figure 5).
As an alternative, introducing the liquid through the sides of an enclosure via integral atomizers would provide for a simplified and inexpensive way to manufacture standardized enclosures for COTS board applications. This method utilizes the enclosure’s sides as an integrated reservoir. A side panel can also be used as an integrated port for vapor uptake or condensation to maintain a more natural cycle of: a) spray b) phase change to vapor and c) back to liquid while keeping vortices to a minimum. This standard would work well with COTS in mind; there might even be interchangeable enclosure sides for a specific COTS board application and environment.
These enclosures would include the option for VME backplane quick disconnects (QDs) directing fluid delivery directly from the case to the board or enclosure components. [Editor’s note: refer to the article “Liquid Cooling Looks for Connector Mechanical Standards” by APW Electronic Solutions, page 81, August 2003, COTS Journal.] This basic QD methodology, for board attachment to a VME case, has been successfully utilized and has been evolving over time in applications that have utilized Polyalpha Olefin (PAO) oil for conductive cooling. A liquid, capable of phase change, could also be directed to an evaporative heat sink via QD for a specific high-temperature COTS component located on a board.
An evaporative heat sink is a compartmentalized and internally injected heat sink capable of cooling a specific component on a COTS board or other high-temperature component. This is a way to incorporate evaporative cooling into an existing location that does not have a sealed case. Since the heat sink has its own enclosure, the condensed liquid can be re-circulated without a sealed case. This method is most useful where there may be only one or two high-temperature components.
The VMEbus International Trade Association (VITA) who assists industry in developing standards for VME enclosures, has already developed a method for strengthening the enclosures backplane. The backplane is where the integrated circuit boards meet the power bus in the enclosure. This upgrade will accommodate high cycle swap rates, reliability, and precise QD alignment for long-term durability. This board locking method includes a simple mechanism with a “quick release” cam-operated lever to exchange COTS boards in the field.