[分享]Combining ANSYS and SysNoise Analyses

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　　Combining ANSYS and SysNoise Analyses Gives Trane Chillers a Sound Improvement

　　The Trane Company has recently redesigned its Series R helical rotary liquid chillers. Sound was a primary design parameter as were other major goals, including reliability, cost, and efficiency. In order to meet the aggressive goals for both overall noise and tonal quality, Trane's strategy has been to develop timely computational techniques to integrate acoustics into the design process.

　　Those goals were met resulting in a design tool that fits the design cycle. "The tool we developed is based on computer-aided engineering (CAE) software including packages for design optimization and verification as well as acoustic analysis," said George C. Wan, Ph.D., principal engineer, Acoustics and Mechanics Technology in Trane's Engineering Technology & Quality organization, LaCrosse, Wisconsin. "The result is water chillers with reduced sound levels and improved tonal quality."

　　Trane's helical rotary liquid chillers are water-cooled systems driven by screw compressors for commercial cooling applications. With capacities of up to 400 tons, they are popular with shopping malls, large commercial buildings, schools, etc.

　　"Screw compressors generally radiate tonal noise at their meshing frequencies and harmonics," Wan explained. "Compared to direct-drive centrifugal compressors, screw compressors can be more annoying due to this tonal content. Thus, general noise improvements and smoothing of the tonal radiation pattern are very desirable." In redesigning Trane's RTHC model, the noise goal was 85 decibels (dBA), which was about eight dBA quieter than a first-concept chiller. The sound measurement method used is the Air-Conditioning and Refrigeration Institute's ARI 575 testing standard, which averages the sound levels measured at one meter away from the chiller.

　　The 85 dBA target was selected because Trane customers perceive it as a threshold. In addition, the Occupational Safety & Health Administration (OSHA), a unit of the U.S. Department of Labor, sets 85 dBA as the mandatory ear protection level. "Thus, 85 dBA has become a criterion for typical mechanical room installations," Wan said, although adding "no one is likely to work near a chiller for eight hours." The final sound level achieved was 83 dBA; two dBA better than the goal. "RHTC's final design was a ten dBA improvement over the original concept machine," Wan said. "Because the decibel scale is logarithmic, this is a very significant reduction, about 90 percent, of sound energy."

　　The acoustics of a water-cooled chiller are determined by its compressor and system layout, specifically the compressor design and the chiller system/component design. For a major portion of the acoustics design work, two software packages were employed: ANSYS, a design optimization and verification package from ANSYS Inc., Canonsburg, PA, a leading developer of software for finite element modeling and analysis (FEM/FEA), and SysNoise. SysNoise is a computational vibro-acoustic analysis package from LMS Numerical Technologies, Leuven, Belgium.

　　The process was (and remains) to import the engineers' design geometry as solid models into ANSYS software and then into SysNoise. Most solid modeling of screw compressors at Trane is done with Pro/ENGINEER from Parametric Technology Corp., Waltham, Massachusetts.

　　Compressor housing models were read into ANSYS software from Pro/ENGINEER as solid tetrahedron elements; geometry is imported from Pro/ENGINEER whenever possible. Transferring geometry from ANSYS to SysNoise was straightforward. SysNoise reads ANSYS .cdb files to generate elements and nodes.

　　Prior to addressing acoustics, ANSYS had been Trane's general purpose FEA software. Its most common use was for structural dynamics and modal analyses with ANSYS Mechanical, ANSYS/PrepPost, and ANSYS Structural modules. "ANSYS was extended to structural acoustics as a way to provide robust structural dynamics predictions for SysNoise," said Wan. "The compressor models are quite complex geometrically," he continued. "Nevertheless, using ANSYS software for both modal analysis and forced-response analysis, Trane was able to accurately predict compressor vibration." SysNoise read in the vibration data predicted by ANSYS and performed noise radiation analysis.

　　These analyses enabled Trane engineers to design a compressor that met its acoustics goal while still hitting the targets for reliability, efficiency, and cost. As noted, the acoustics goals included an improvement in tonal quality, which can be very subjective and difficult to quantify. "Tonal improvements were measured using sound-quality metrics," Wan said. "However, the listening experiences of a jury of people familiar with the product were also used. Previous-generation chillers were used to help define the tonal improvement. To obtain good tonal quality, noise radiation from different chiller components had to be balanced around the machine so that no hot spots were left."

　　Although computational acoustics is based on rigorous mathematical formulation, the general outlines of its application philosophy to chiller acoustics design is easy to comprehend. "The acoustics of water-cooled chillers are determined by the compressor and its interaction with the rest of the system," Wan explained. "Therefore the two major aspects of the chiller acoustics design are the compressor design and the system/component design. Optimizing compressor noise and vibration is fundamental. It is also essential to minimize acoustic energy transmission to other chiller components and minimize their noise radiation." Acoustical fluid-structural interactions were analyzed for several critical components and the interior acoustics of chambers and cavities were also designed.

　　"The compressor housing was first modeled in ANSYS solid finite elements," he continued. "Then its surface was 'skinned' by creating shell elements over the existing solid elements. The shell elements use the nodes of the solid elements on compressor surface. In SysNoise, boundary elements are created by importing these shell elements," he pointed out.

　　A similar approach was applied to other chiller components

　　including condensers, evaporators, and pipes. Analysis also included enclosed acoustic spaces such as the discharge cavities in compressors, interior acoustics of containers such as the oil separators, and pipes.

　　Element orientation is always important in a boundary element analysis (BEA). "At each nodal location of the boundary elements, the triaxial vibration predicted in ANSYS analysis was read by SysNoise," Wan reported. "Then the nodal vibration component in the surface normal direction was calculated as the boundary conditions for the BEA. To overcome a numerical difficulty known as the 'non-uniqueness problem' in BEA, internal fictitious nodes or elements have to be added for the direct or indirect BEA, respectively," he said.

　　Prior to applying ANSYS and SysNoise to these problems, acoustics design was guided by qualitative design guidelines that were derived from experience. This isn't simply "playing it by ear," Wan said. "Without a quantitative design capability, Trane was forced to spend tremendous effort in acoustics performance experiments and fine-tuning in the final stage before product release."

　　Computational acoustics technology provides quantitative design guidance and is, therefore, an ideal way to ensure the product is designed to meet customers' acoustic requirements. "Trane switched to ANSYS and SysNoise when we anticipated that CAE for acoustics prediction had become mature enough to be integrated into fast-paced product development projects such as this," Wan said. "The switch became a key part of Trane's continuous effort to maintain its leadership role in air conditioning."

　　Trane's Acoustics and Mechanics Technology Group began pioneering the ANSYS/SysNoise analyses in 1993. ANSYS and SysNoise were benchmarked on several generic acoustics and vibration problems. "Then work with LMS Numerical Technologies was validated in the interface between ANSYS and SysNoise" Wan recalled. "In addition, training the engineers in the new design methodology was important in obtaining general acceptance and support."

　　Using ANSYS and SysNoise implied significant changes in the way Trane developed new products, so strong management and development team support was secured. "As with any new technology implementation, this proved to be a key contributor to the success of the process," Wan said. "Managers and engineers welcomed the new process as it helped them work closely with acoustics engineers and take acoustics performance into consideration in product design. Better cooperation quickly resulted and acoustics design has now been formally integrated into the product development process. An interdepartmental acoustics team was formed to facilitate that process."

　　In the new RTHC models, CAE analyses along with state-of-the-art testing for hardware and software, were applied from concept design through final product qualification. The ANSYS model was validated with work done on the structural modal aspect including natural frequencies, mode shapes, and frequency response functions. "We found that SysNoise vibration can be compared with ANSYS directly without any testing," Wan pointed out. "Most of the experimental data is analyzed and managed by the LMS Cada-X system. This made validating ANSYS and SysNoise easy. Built-in interfaces with correlation and update functions in Cada-X streamlined the model's confirmation and updates."

　　Combined ANSYS and SysNoise analyses are now Trane's standard method of designing good acoustics into products and in solving general structural acoustics problems. "Use of ANSYS software is a sure way to predict vibration," Wan said. "Both ANSYS and SysNoise results can be summarized in spreadsheets for engineering review. As a result, ANSYS and SysNoise fit the overall Trane information-management strategy nicely," Wan concluded. "And, at the same time, their use gave Trane a quieter and more competitive product."

　　

[此贴子已经被作者于2005-12-17 15:19:50编辑过]

Tony0Hung

全是英文，好懒得看啊！！！