` ` Raytheon applic. for EPP
 
 

CFD DRIVES DOWN DESIGN CYCLE TIME AND COSTS

Electronic Packaging and Production Magazine, April 1999

CFD (computational fluid dynamics) is helping Raytheon E-Systems Div. shorten its design cycle time and eliminate costly and time-consuming redesign steps. Using an easy-to-learn, desktop version of CFD thermal modeling software, the military electronics leader is detecting potential thermal problems early in the development cycle when engineers have maximum flexibility in packaging design and component placement. Early detection eliminates costly redesign, which according to RaytheonĚs Dan Jones, senior principal engineer, "often accounts for most of the cost".

Raytheon E-Systems develops a variety of ruggedized, military electronics systems. The equipment includes an assortment of communications boxes, plus phased-array antenna systems, specialized computer systems and other associated equipment. Increasingly this equipment combines custom designs with commercial off-the shelf boards as part of the military's thrust to reduce program costs.

Recently Raytheon upgraded an 18 x 9 x 7 inch, rack-mounted communications package containing two power supplies drawing a total of 600 watts. When the proposed air-cooled design was analyzed using conventional, two-dimensional thermal techniques, the analysis indicated that component junction temperatures would remain comfortably at 146 deg. C. and not exceed the 150 deg. C. upper limit. However, when the same design was subjected to the scrutiny of Coolit, a 3-D desktop CFD package, from Daat Research Corp., Hanover, NH, a different scenario emerged. CFD analysis showed that the junction temperature would reach an unhealthy 176 deg. C, threatening system reliability.

"As soon as I saw the results of the Coolit analysis, I knew there was something wrong with the original calculation," declares Jones.

Thermal analysis usually involves making certain decisions about the volume being analyzed, such as assuming uniform heat dissipation or uniform heat transfer off large surfaces, or possibly neglecting the effects of thermal entry. These

Solid model of the rack mounted communications package modeled with Coolit

 

assumptions can lead to inaccurate predictions, especially if an engineer goes too far in an effort to simplify the analysis, or respond to scheduling pressures.

"The hand calculations involved some assumptions that really weren't appropriate," notes Jones, and Coolit picked up these errors."

Coolit automatically determined which assumptions were valid and then calculated the appropriate values required for the finite differences codes. When using the conventional approach, Jones had to calculate these values, by hand using empirical formulas. Coolit also solved the complete flow field, taking care of all the effects mentioned above.

Color-coded thermal plot pinpoints hot spots in power supply design. Because upper limit of temperature scale is set at 80 deg. C., all components at or exceeding that value are quickly spotted by their red color.

 

The CFD tool then pinpointed specific component issues. For example, it calculated that the power supply heat sink could not adequately dissipate the heat. Using its simulation capabilities, the tool further showed that the problem could be easily corrected by increasing the number of heat sink fins.

Coolit also helped optimize air flow. Initially, air was diverted so that it would brush both the component and heat sink sides of the power supplies. CFD analysis showed that by redirecting more flow to the heat sink side of the assembly, the heat dissipation could be dramatically improved.

"CFD analysis easily shaved a couple months off the development cycle by eliminating false design starts, " declares Jones.

CFD has saved Raytheon from a costly and time consuming redesign on other programs, as well. In one application, a very thin profile box was to be cooled by a single, very tiny fan. Using Coolit, Jones found one fan could not handle the thermal load; in fact, 5 fans were needed. Furthermore, he found the box had to be pressurized in order to provide adequate air flow across the most troublesome components.

Streamlines show airflow through power supply is unimpeded by component placement. In this view, the full temperature range of the system is displayed. Maximum temperature reaches 105.5 deg. C. and occurs in four high-power FETs that are directly mounted to the finned heat sink in the bottom of the box.Streamlines show airflow through power supply is unimpeded by component placement. In this view, the full temperature range of the system is displayed. Maximum temperature reaches 105.5 deg. C. and occurs in four high-power FETs that are directly mounted to the finned heat sink in the bottom of the box.

 

If conventional analytic techniques had been used, Jones feels the design changes mostly likely would have progressed in time consuming steps: one fan, two fans, three fans, etc. until the required 5 fans were reached. With simulation, the each change were made and the impact calculated in minutes.

"In my applications, we often have to squeeze several kilowatt transmitters into relatively small packages that can easily overheat if the cooling schemes can't dissipated the power," says Jones. "We have to be careful as to the type of cooling scheme we select. With Coolit, we can identify flow reversals, determine whether the fan sizes are adequate, spot areas of overheating---all without building costly prototypes or reworking drawings every time we want to "test" a change.

To build a CFD model, Jones first sketches the enclosure on his computer screen and then places components, heat sinks and boards inside. The shapes are selected from the software's part library, or they may be custom-created or imported from CAD software.

Component properties, such as thermal conductivity also are selected from the software library or they can be entered through a dialog box. Fan curves are drawn or entered through a table. The information can be entered in any combination of units; the software automatically handles the necessary conversions.

The output is displayed as 2D and 3D simulations that show airflow and temperature distribution. Color-coded temperature patterns make hot spots conspicuous, while velocity vectors indicate the direction and speed of air movement. The two graphics are overlaid so that the interaction between air flow and temperature is obvious. If Jones is not satisfied with the results, he repositions the components or substitutes new ones with different thermal characteristics.

Coolit also provides on-line animation that enables Jones to "inject particles" and watch as they travel through the enclosure at a speeds proportional to the local flow velocities. As the particles travel, they change colors as temperatures change. While this happens, Jones can rotate the 3-D image and view it from any angle.

Jones feels that most other CFD packages are focused toward aerodynamic applications where the user is worried about flow through a jet engine or pressure on a fuselage, and where the user is concerned with parameters, such as compressibility effects.

"Electronics engineers have a different class of problems than aerodynamicists," he points out. "They are mostly involved with fan-cooled electronics inside an enclosure, and Coolit is specifically tuned for electronics applications."

Jones has worked with CFD for over 15 years and has seen the learning curve for this engineering tool decreased exponentially over the last 10 years.

"Applying Coolit takes no longer than conventional analysis, and the user receives the benefit of a complex, 3-D thermal analysis that easily delivers what no physical test can; it identifies the temperature at every spot within a box."

A physical test is limited by the number of temperature sensors that are used and where they are placed within the enclosure. If they are put in the wrong spots, the hot spot may be missed.

Jones declares, "If a company wants to stay ahead of its competition, it needs to use CFD for its thermal analyses."



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