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Figuring Flow PUBLIC ACCESS

A CFD Program that Monitors Airflow In HVAC Systems Has Helped Slash Design Time for anAuto Parts Maker.

[+] Author Notes

Tim Hall and George Anderson are technical specialists at Visteon Automotive Systems in Dearborn, Mich.

Mechanical Engineering 122(02), 72-73 (Feb 01, 2000) (2 pages) doi:10.1115/1.2000-FEB-6

NASA and the US Navy are exploring magnetic fields in search of quicker, smoother, and cheaper launches. A Navy program based in Lakehurst, NJ, wants to learn if magnetic propulsion is the answer. It awarded two contracts in December for what it calls the electromagnetic aircraft launch system. One award went to Northrop Grumman of Sunnyvale, California, and the other to General Atomics in San Diego. NASA is in the second year of a three-year program to investigate magnetic launch technology. The systems under study by NASA combine linear induction, to accelerate the vehicle to launch speed, with magnetic levitation, using opposing magnetic fields to suspend the vehicle above its track. The Navy’s catapult program does not require magnetic levitation. The US Maglev Technology Assessment, written jointly by the Corps of Engineers and the Department of Transportation, points out that since ‘magnetic drag is small at high speeds, only aerodynamic drag consumes appreciable energy.

AT VISTEON AUTOMOTIVE SYSTEMS, the world’s second biggest automotive parts maker, engineers have improved the performance of automotive heating, ventilation, and air conditioning (HVAC) systems and reduced design lead time by 75 percent with the help of new computational fluid dynamics software. They’ve also caught potential flow problems much sooner in the design process than previously possible.

Visteon, launched in 1997 in Dearborn, Mich., has operations in 21 countries. Currently, Ford accounts for 92 percent of the company’s sales, although Visteon seeks to build its non-Ford business to 20 percent of sales by 2002.

In the past, Visteon engineers, using conventional physical testing methods, have taken about a year to fully develop a new vehicle’s HVAC system. With the help of the new CFD software—which provides quick feedback on the airflow performance of a proposed design—the process now takes about three months.

The CFD software uses unstructured meshes and allows for parallel processing, which makes it possible for engineers to model complex HVAC components in one day. Parallel processing, which distributes the computations of a simulation among several processors, can significantly shorten the time it takes to work out a problem. Engineers at Visteon now can evaluate five typical concept designs in one overnight run. In the past, they could evaluate only one design in that time. The design engineers have found that their ability to evaluate many alternate concepts at low cost early in the design process often leads to significant improvements in system performance.

Tim Hall and George Anderson are technical specialists at Visteon Automotive Systems in Dearborn, Mich.

Before the widespread use of computer flow simulations, the only way design engineers could evaluate design of a proposed air handling system was to build a prototype and test it in the laboratory. Engineers placed air handling components on a test stand, supplied conditioned air at the inlet, then measured airflow and temperature distribution at critical points in the system. The method took a lot of time and required engineers to construct expensive prototypes. In addition, it provided the engineers little or no understanding of why a design performed the way it did. This sort of testing didn’t show a detailed map of the circulating air. It also didn’t detect air turbulence, temperature stratification, constrictions that adversely affected performance, or pressure loss within the system.

CFD Analysis in Use

These drawbacks led Visteon engineers to begin experimenting with CFD a few years ago in order to get detailed information on flow fields and pressure distribution within the air handling system without building a prototype.

A CFD simulation shows fluid velocity, pressure values, and temperature throughout an air handling system. The systems often consist of complex geometries, such as ducts that expand and contract, cross-sections that change from round to square, and ducts that move around complex curves with many branches and internal walls. The CFD simulations are capable of dealing with these geometries and, what’s more, the simulations can depict what’s termed the boundary conditions of the system—such as a particular inlet velocity or the airflow rate. As part of the analysis, designers can change the system’s geometry and boundary conditions, then view the resulting effect on fluid flow patterns.

Although flow through the complex geometries used in air handling systems can be accurately simulated, it doesn’t mean the process is easy. The simulations require accurate geometric representation to correctly show system performance, which means engineers or specialists often labored over building these simulations. Also, designers usually need to evaluate system performance in many different configurations. For example, they sometimes need to evaluate the air handling system in four or five different modes of operation—such as vent, floor, defrost, and mixed — and at each of eight different temperature control points, which, again, required an extensive time investment.

In fact, all these computer modeling difficulties meant that in the past it took Visteon designers as long to build a CFD model as it did to build a prototype. And the CFD software they had available to them used a structured-mesh approach in which block structures must be defined by hand before the computer could generate a volume mesh for the entire system. It took designers weeks or months to produce a grid analysis of complex air handling system components using this approach. Also, the modeling tools available were in many cases too difficult for use by design engineers, who generally didn’t have a chance to become familiar with a particular software the way a full-time user of that software would. Typically, then, Visteon would delegate CFD analysis to CFD specialists inside the company or to consultants. The delegation further increased HVAC design lead time.

With CFD software, Visteon engineers air circulation in a blower subsystem before producing the subsystem itself (photo at far right).

Grahic Jump LocationWith CFD software, Visteon engineers air circulation in a blower subsystem before producing the subsystem itself (photo at far right).

However, that lead time has been slashed since Visteon moved to an unstructured-mesh CFD software tool, which makes it possible for the design engineer to perform the entire modeling process in much less time.

The software, Fluent/UNS, from Fluent Inc. in Lebanon, N.H., generates an unstructured tetrahedral volume mesh from a triangular surface mesh. The mesh generation is completely automated. The software creates a hybrid mesh consisting of prism layers in near-wall regions and tetrahedral cells in the remainder of the domain. Prism elements are more suitable for resolving boundary layers. The result is better accuracy without the time-consuming task of building an all-hexagonal mesh.

Visteon design engineers have found that sometimes the software automation fails because of issues regarding the surface mesh. When that happens, the engineers have to modify the surface mesh manually. But they’ve found they can manually mesh the ungridded areas in much less time than they spend doing it with the former software. This makes it possible, in almost every case, to complete the CFD model in one day.

The auto-parts manufacturer has also begun using computer-aided-design solid modeling software to create a series of designs, which helps to streamline the CFD analysis process. The CAD software is from Structural Dynamics Research Corp. in Milford, Ohio. It is used to generate a universal file output that can be translated to the CFD software. The CAD software lets the engineers create a series of models with slightly different dimensions for testing purposes. When each model is translated, the CFD software need only remesh the areas of the solid model that have changed from one version to the next.

Visteon has also taken advantage of improvements in solver technology. Previous solvers could analyze a complicated air handling system’s geometry overnight. While that is a reasonable time for evaluating a single geometry, it presents difficulties for the engineer who needs to analyze 15 different models to choose the best design among three variables for one part of the design and among five for a second part of the design. The CFD software allows use by multiple processors. Visteon uses a computer server from Silicon Graphics of Mountain View, Calif, and clusters of workstations for parallel processing of CFD models.

To take advantage of this parallel processing capability, engineers distribute problems for nighttime analysis at the same workstations used by HVAC designers during the day. This saves the company money—effectively doubling computer power—because engineers are using workstations during a down time, when the machines would otherwise be turned off.

Another recent improvement comes from a new feature of the CFD software, which allows a fan performance curve to be input to the model. Using this technique, engineers can easily determine what type of fan is required to meet airflow requirements within the vehicle—normally 158 cubic feet per minute for heating and 300 cubic feet per minute for cooling.

Without this fan-performance feature, the user has to guess at the flow within the enclosure, calculate the pressure using CFD, and see if it matches the fan’s characteristics. If the pressure doesn’t match, then the user must make another guess and repeat the process until one guess results in pressure that matches fan characteristics. Normally, at least three iterations are required to make a match. The CFD software lets the engineer enter the fan curve directly into the model.

These tools within the CFD analysis package have helped Visteon engineers not only to reduce HVAC system design lead time by 75 percent, but make substantial performance improvements in vehicle HVAC systems.

In the past, problems with a design became apparent only late in the design cycle after full vehicle prototypes had been built and tested. Using CFD, Visteon engineers can identify and solve nearly all problems before the first prototype is even built.

Copyright © 2000 by ASME
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