Tuesday, November 16, 2010

Electronics thermal management for defense systems

By Courtney E. Howard

Few issues plague military and aerospace systems designers, developers, and integrators as persistently as electronics thermal management. Size, weight, and power (SWaP) constraints are challenging, but they also exacerbate the thermal management problem. At the same time, mil-aero electronics are employed in a vast array of environments, each with their own unique challenges -- be they sand, snow, humidity, or electronics temperature extremes. Today’s mil-aero electronics must withstand the extremes of deserts and space, and virtually everything in-between.

“Thermal management is absolutely critical in military and aerospace environments,” says Curtis Reichenfeld, chief technical officer at Curtiss-Wright Controls Electronic Systems in Santa Clarita, Calif. “Functionality, reliability, and safety require maintaining electronics within qualified temperature limits.”

“We all know that heat can adversely affect the performance of electronics,” says Dr. Erich Buergel, general manager of the Mentor Graphics Mechanical Analysis Division (formerly Flomerics) in Frankfurt, Germany. “Minimizing weight and optimizing space are always key design goals, but an effective cooling solution is essential to reliability. Since field failure is not an option, thermal management is of utmost importance for mil/aero applications.”

Hot topic

Several trends are influencing thermal management in military and aerospace environments. “As we automate more functions to minimize the need for human involvement, we are increasing our reliance on electronic devices,” admits Buergel. “This, in turn, creates a need to design smaller electronic packages, as well as components that fit into tighter spaces.

“At the same time, chips and IC packages are becoming more powerful. Lastly, most devices used for mil-aero applications now must be self-contained/air-tight to ensure contaminants do not enter the device and adversely affect performance,” Buergel adds. “Therefore, the increased density that results from fitting more powerful components into smaller air-tight spaces creates major heat dissipation problems.”

On the business side, those serving the mil-aero community are “urgently pursuing cost-saving measures to deal with today’s economic and market realities,” Buergel says. “Due to the complexity of electronic products for mil-aero applications, plus the safety and regulatory issues that uniquely affect them, saving costs is not easy.”

CFD and avionics

Component designers and systems architects are increasingly turning to electronic design automation (EDA) to reap cost saving and timesaving benefits. EDA software tools enable the design, simulation, analysis, and testing of electronic components and systems in the digital realm, rather than investing considerable time and money in crafting physical prototypes. “Simulation in its many forms is proving to be the salvation of manufacturers working under pressure to deliver end products of proven quality at lower cost, and in less time,” Buergel says.

Computational fluid dynamics (CFD) software, in particular, saves a tremendous number of engineering man hours through the employ of algorithms to perform the millions of calculations required to analyze and solve problems that involve fluid flows -- such as the movement of air throughout an electronics system or enclosure.

“CFD simulation is one of the most reliable methods of understanding and managing thermal issues,” Buergel notes. “Not surprisingly, CFD has become a cornerstone of aerospace mechanical design. With CFD simulation, manufacturers can greatly reduce the cost of getting a product out to market.” For example, instead of testing multiple physical prototypes, design engineers can simulate and test multiple variations of the design in a fraction of the time and cost.

Mentor Graphics in Wilsonville, Ore., has a strong pedigree in electronics cooling, Buergel says, given that the company’s software simulation tools have been employed in thermal simulation applications for more than 22 years. Of late, mil-aero systems designers have increasingly adopted the Mentor Graphics FloEFD software with Concurrent CFD technology.

The tool embeds within mechanical computer-aided design (MCAD) platforms, bringing thermal analysis inside the digital design environment. FloEFD automatically prepares the design model in a host MCAD system for CFD analysis, and therefore requires no special user training. “This process saves engineers a tremendous amount of time because they no longer need outsource their designs to CFD specialists for evaluation. Mechanical designers can now perform accurate thermal analysis routinely without leaving their accustomed MCAD environment,” Buergel explains.

All in all, thermal management is a major challenge from the very first design phase and it can be a significant design bottleneck, Buergel admits.

Cooler chassis

Tecnobit, an industrial and defense electronics company in Madrid, Spain, designed a special chassis to house cockpit avionics in an enclosure whose maximum dimension was approximately 10 centimeters, or 4 inches. The system was designed to be completely sealed without any ventilation slots, requiring heat transfer through the outside surface by conduction, radiation, and natural convection. Tecnobit’s preliminary design did not meet the design requirements; it was unacceptable from a thermal standpoint and conflicted with the trend toward higher power and heat dissipation in avionics systems.

Tecnobit’s design team employed FloTherm to evaluate avionics chassis design options in virtual form, with no need for expensive and time-consuming hardware prototypes. The simulations enabled the Tecnobit engineers to optimize the thermal design rapidly, while observing the effects of their design changes.

The company’s engineers modified the internal chassis structure to increase heat conduction from the components to the chassis walls. At the same time, the Tecnobit design team added special heat-dissipating fins to the enclosure’s outer surface to transport heat away from the box. Sand-blasting treatment and electrostatic painting further enhanced convection and radiation exchange with the external ambient air. Tecnobit engineers used FloTherm 3D thermal simulation to perform steady-state and transient thermo-fluid simulations, and predict system thermal behavior as various heat-conduction refinements were added. Ultimately, the team reduced component junction temperatures by 40 degrees Celsius compared with the initial design. See more on thermal management and electronics enclosure designs at Backplane chassis and enclosure design trends marked by high performance, small size, and thermal management.

The Mentor Graphics FloTherm spans applications ranging from evaluating sealed electronic modules to predicting airflow in fully-loaded server racks and blade enclosures. FloVent is employed to track the flow of cooled or heated air through vehicles and buildings. The MicReD T3Ster (Trister) hardware measurement tool characterizes thermal impedances over an entire heat path -- from the semiconductor junction that generates the heat to the outermost system housings and into the ambient -- for the design of lighting and laser systems, printed circuit boards, and electronic enclosures.

The company’s FloEFD Concurrent CFD tool set spans various mil-aero applications, including modeling the flow of coolants, fuel, and other liquids and gases, as well as evaluating aerodynamic surfaces, such as aircraft wings and fuselages. It has been used in the development of a micro aerial vehicle concept, in the evaluation of “nose pod” cooling and aerodynamics for a military reconnaissance aircraft, and in evaluating a critical nitrogen-injection feature for the fuel tanks in a Bell Helicopter military rotorcraft.

Helicopter heat

Engineers at Bell Helicopter, a Textron Company in Fort Worth, Texas, used the Mentor Graphics FloEFD for helicopter inlet temperature distortion modeling. “Flight testing revealed some marginal engine inlet air temperature distortion levels, so CFD was used to try to identify the culprit,” says David H. Loe, principal engineer, Bell Helicopter Textron.

“It was assumed that either hot gas re-ingestion or inlet air heat transfer was the root cause, so we set up a helicopter model that simulated the problematic flight condition. Exhaust re-ingestion did not initially appear to be likely based on the CFD, so we added heat transfer to the model and applied some ballpark surface temperatures to the engine inlet,” Loe explains. “We quickly learned that it was highly possible that surface heat transfer to the incoming fresh air could be taking place and modified the CFD model to simulate insulation on some of the inlet surfaces. The ‘insulated’ model showed improved inlet temperature distortion levels, so the flight test aircraft was ultimately outfitted with insulation blankets on critical surfaces identified in the CFD model.” The software tool enabled the team to overcome the time constraints associated with a test program, and to model complex geometry in a short timeframe.

In another application, FloEFD aided Bell engineers in helicopter oil cooler airflow management modeling. “Space constraints forced a non-standard, blower-to-heat exchanger air duct, with rapid diffusion and some awkward, undesirable twisting of the duct,” Loe describes. “The primary concerns were: excessive total pressure losses in the ducting that would adversely affect the cooling blower airflow rate and non-uniform air distribution at the cooler core inlet face that would result in poor cooler heat rejection characteristics.

“The CFD model helped to identify the areas within the ductwork where the flow was separated from the surface, and to clarify the system total pressure losses,” Loe continues. “Some changes to the duct detailing were recommended to improve the airflow characteristics and flow uniformity at the cooler inlet face.” In the end, the simulation tool enabled the team to model complex internal airflow geometry, and to work an airflow system into an area with a high level of packaging constraints.

Chassis considerations

Effective thermal management at the chassis/system level requires detailed knowledge of the platform storage/operating environments, as well as the system power dissipation and available cooling options, Reichenfeld notes. Electronic assemblies are designed to maintain component junction and die temperatures within specified limits. The chassis/system design must consider cooling methods to dissipate the heat into the operational environment from electronics thermal convection/conduction paths, he adds.

Extreme cold- and hot-start requirements are the challenge with commercial off-the-shelf (COTS) components that are typically limited within -40 to 71 degrees C operation at the system level. “Careful consideration of the criticality of the system is necessary to determine the effects caused from a loss of heating/cooling on functions provided to the platform,” Reichenfeld says. “In some cases, redundancy and loss of cooling analysis is necessary for safety and mission-critical elements in order to prevent loss of life.”

Hybricon Products, part of Curtiss-Wright Controls Electronic Systems, forced air chassis houses conduction-cooled electronics in a current airborne application at an altitude of 50,000 feet. The modified 1-ATR-tall chassis includes a 6U, 13-slot custom Hybrid VME64x-VXS-OpenVPX backplane with a 1400-watt, MIL-grade power supply and two high-performance, MIL-grade fans. The application also required a high-performance heat exchanger due to the very high power dissipation.

Unmanned systems

Unmanned systems, whether in the air or on the ground, are delivering much needed information and soldier protection on the increasingly digital battlefield. Armed with sensitive electro-optics, such as sensors, and often venture into and persistently survey harsh environments, gathering mission-critical data while helping to keep warfighters out of harm’s way. Unmanned systems’ thermal management needs are nearly as complex and intricate as their compact electronics, however.

Curtiss-Wright Controls Electronic Systems in Santa Clarita, Calif., is designing the next-generation chassis for Northrop Grumman’s Advanced Mission Management System (AMMS) for the Broad Area Maritime Surveillance Unmanned Aircraft System (BAMS UAS) program. The BAMS UAS will provide the U.S. Navy with a persistent maritime intelligence, surveillance, and reconnaissance (ISR) system to protect the fleet and provide a capability to detect, track, classify, and identify maritime and littoral targets. The chassis has six integral cooling fans providing air flow for up to 800 watts power dissipation at 55 degrees C. The operation of the AMMS is critical to the mission objectives and protection of U.S. Navy fleets during operations, Reichenfeld describes.

TYZX in Menlo Park, Calif., has unveiled ruggedized electronics packaging on embedded vision systems in support of unmanned systems performing person-tracking, situational awareness, and system navigation tasks in harsh outdoor conditions. G3 Embedded Vision System’s all-weather, compact, and rugged packaging design enables use in harsh outdoor conditions without additional enclosures, while its 12-watt power consumption eliminates the need for active cooling components.

Dontech in Doylestown, Pa., has also recognized the need to maintain electronics at optimal operating temperatures. The company launched its Therma Klear transparent heaters to provide the warmth necessary to extend the operating temperature of liquid crystal displays (LCDs) in cold environments (from 0 degrees to below -40 degrees C) and for the anti-fog, anti-icing, and de-icing of optics and optical camera, sensor, and display assemblies.

A Therma Klear heater, composed of an electrically conductive thin-film coating on a transparent substrate, generates heat when current flows across the coating. Dontech heaters employ different types of crystalline materials (e.g., zinc sulfide or germanium), glass, acrylic, and polycarbonate substrates. Applications include avionics displays, vehicle displays, mobile computers, and handheld devices.

Heat-dissipating designs

Thermal management of military and aerospace electronics continues to be one of the foremost challenges facing design teams, admits Ivan Straznicky, principal engineer and technical fellow at Curtiss-Wright Controls Embedded Computing (CWCEC) in San Diego. “The desire to use the latest generation, multi-core processors to meet the performance needs of ever more sophisticated applications are driving power dissipation levels and densities beyond what was once considered unachievable in military and aerospace electronics.

“In order to make use of these high-performance processors in standard circuit card cooling configurations (e.g., conduction cooling), thermal engineers are painstakingly re-examining every element of thermal designs to eke out enough improvement for successful products,” Straznicky adds. “Without such improvements, the use of these processors would be limited to lower speed grades and/or more benign environmental conditions (e.g., lower allowable card edge temperatures). Even with such improvements, the cooling ability of standard configurations, such as conduction and forced air over components, is not infinite and is quickly reaching limits of the governing physics (e.g. material properties, airflows).” Continued innovation at all levels of heat removal, in particular the chassis and system levels, is required to optimize their capabilities to efficiently remove heat to ambient environments, he says.

High-density processing solutions, such as those designed and developed by CWCEC, require highly efficient cooling solutions to meet challenging mil-aero environments. Most of the company’s card-level products are designed to standard cooling configurations, such as forced air over components or conduction cooling; however, some of its cards can now dissipate approximately 4X the heat compared to a decade ago, with similar boundary conditions, Straznicky says. CWCEC engineers have also worked with customers and partners to develop and design products using such advanced cooling technologies as spray cooling, air flow through cooling, and liquid flow through (LFT) cooling. In fact, engineers at CWCEC and Parker Hannifin Corp. in Cleveland, Ohio, developed an LFT prototype capable of cooling over 650 watts of power using 55 degree C Polyalphaolefin (PAO) coolant.

Liquid cool

Parker Hannifin has expanded its portfolio of liquid thermal solutions with the acquisition of SprayCool (formerly Isothermal Systems Research or ISR) in Liberty Lake, Wash. The company’s newest mil-aero applications center on liquid-cooled electronics enclosures and two-phase cold plate solutions.

“We continue to see more demand for both conduction-cooled enclosures that are side-wall cooled with single phase liquid, such as ethylene glycol and water (EGW) or PAO, coupled to a remote Heat Rejection Unit (HRU), and SprayCool enclosures where we take advantage of direct-spray and the evaporative cooling process on the electronics inside a sealed enclosure,” explains Joe Baddeley, business development manager, Parker Aerospace, Thermal Management Systems Team.

“Sensor and image processing applications that utilize standard 6U board form factors are still leading the pack in terms of liquid cooled enclosures, but we are seeing a lot more proposals and trade studies for smaller 3U systems, especially with the release of VPX and the ability to push more power into these smaller modules,” Baddeley notes.

Two-phase cold plate solutions are being employed primarily in power electronics and radar applications. “In both applications, the trades and early development units are clearly showing the potential for not only improved thermal performance and subsequent reliability gains over air or single phase liquid cooling at the electronics level (IGBTs, power amplifiers, transmitter modules), but also the ability of pumped two-phase systems to accomplish the task without the need for larger vapor compression systems (VCS) or chillers,” Baddeley continues. “Thermal efficiency gains realized with two-phase cooling give the integrator the best chance at reducing or eliminating the need to chill down the coolant before it enters the electronics assembly.”

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