Top > Application Examples > National Defense Academy of Japan

Application Examples

National Defense Academy of Japan
Combining Tests and CFD Analysis to Solve Long Standing Issues

The configuration of cooling fans inside electronic devices is complex. This frequently results in generating large amounts of data when trying to represent the configuration accurately for fluid analysis calculations. To deal with this, engineers use the concept of the pressure-flow-rate (PQ) characteristics to simplify the calculation, although it has been pointed out that this simplification often becomes the source of errors. Professor Hajime Nakamura, from the National Defense Academy of Japan, has been exploring these issues pertaining to fan models for electronic devices and has discovered a more accurate model.

Professor Hajime Nakamura
National Defense Academy of Japan
Mechanical Engineering Dept.

 Professor Nakamura is an expert in Heat Transfer Engineering, which investigates how materials can be effectively heated and cooled. Fluid flow plays a significant role in heat transfer; for example, air can be induced to dissipate heat from the silicon chips inside electronic products. The problem is, air-flow is extremely complicated, often generating small eddies and unpredictable turbulence. Professor Nakamura, whose research specializes in investigating fluid flows, explains; “CFD software can now be used for more diverse fields compared to before, but still, there are uncertainties to the analysis. My objective is to clarify the mechanisms of such phenomena that cannot easily be analyzed, by undertaking actual tests.”

 Professor Nakamura started investigating fan models when the RC 181 Research Project was founded under the Japan Society of Mechanical Engineers. He learned about the issues associated with fan models for the first time at the RC 181 meeting. As high-density packaging techniques were common at the time, engineers wanted to use CFD software to evaluate how to best allocate the device parts to dissipate heat. When applying a fan to dissipate heat from an electronic device, the engineers usually ignored the detailed configuration of impeller and its rotation by using simplified fan models for CFD analysis, and specifying approximate PQ characteristics to determine the flow rate based on the pressure difference of upstream and downstream of a fan. However, some engineers have repeatedly pointed out that analysis results were often inaccurate, even when PQ characteristic values provided by manufacturers were used. The challenge was clear – they needed an accurate fan model for highly accurate CFD analysis. This motivated Professor Nakamura, who had previously been studying forced convection, to address this issue.

Identifying the Cause of Errors by Thorough Testing

 Professor Nakamura started by using the benchmarking model provided by RC 181 to carry out tests. One of the reasons for the inaccuracy was thought to be the varying definitions of pressure difference used by the fan manufacturers and the CFD software developers. However, no proof existed at that time to validate the theory. Pressure values were measured far away from the fans by the fan manufacturers, whereas CFD software calculated the fan pressure difference using values measured just upstream and downstream of fans. Professor Nakamura suspected that investigating the surrounding area of the fan would lead to the solution, and closely examined the velocity and pressure near the fan.

Figure 1: Simplified diagram and actual picture of an experimental apparatus.
Click to enlarge.

 Another problem was the measurement standard used for fan performance. At that time, only a few standards existed, such as ISO 5801 and JIS B 8330 for large to mid-scale industrial fans. But such standards did not exist for small fans used in miniaturized electronic devices. Followed by the establishment of JBMS-72-2003 (Acoustics — Method for the measurement of airborne noise emitted by micro-fan) in 2003 that standardized measurement methods for smaller fans with lower air-flows, Professor Nakamura designed an apparatus to inspect fan performance according to the standard. The apparatus was about 3m in length, and enabled engineers to measure the capability of smaller fans without separating them from their chassis (Figure 1). This encouraged engineers to measure PQ characteristics on their own and actively undertake further tests.

 Over a number of evaluations, Professor Nakamura became aware of three major problems that needed to be solved. First, the definitions of pressure employed by the fan manufacturer and the CFD software developer differed. Second, as the packaging density increased, the variation in the PQ characteristic values also increased. Third, the velocity distribution of the air-flow generated by the fans could not be predicted from the PQ characteristics. In search of CFD software that could be used to solve these problems, Professor Nakamura came to Cradle software. He started using SC/Tetra in 2007 to compare the results from fan tests and CFD analysis.

Figure 2: Dynamic pressure correction on PQ curve of fan model. Click to enlarge.

SC/Tetra to Adjust PQ Characteristics

  Professor Nakamura explains that the first problem was solved quickly soon after introducing SC/Tetra. As the air-flow contraction occurs at upstream of a fan, the velocity increases and the pressure decreases. Yet, manufacturers did not count for this pressure drop because they used a chamber system to measure the pressure far upstream of the fan. In comparison, the downstream pressure is almost the same wherever it is measured.

  In short, the difference between the fan measurements and fan models arose from the pressure drop near the air supply. Professor Nakamura thought that adding the dynamic pressure generated by the accelerating flow near the air supply to the PQ curve would properly adjust the values. For example, when ρ is the density of the air and u is the average velocity of the air supply, the dynamic pressure can be expressed as 1/2 ρu2. When Q is the flow rate, and A is the cross-sectional area of the flow path, the air velocity, u, can be derived from Q/A, which is known from the PQ curve and the fan configuration. Professor Nakamura applied the dynamic pressure to the fan model PQ table of SC/Tetra and found that the generated values matched to the experimental results (Figure 2). “This correction made it easier for engineers to make the necessary adjustments when working with fans,” says Professor Nakamura. He also noted that he frequently introduces this example at seminars and academic conferences to explain the research results, and is often amazed at the number of questions and amount of interest from the audience.

* All product and service names mentioned are registered trademarks or trademarks of their respective companies.
* Contents and specifications of products are as of June 1, 2013 and subject to change without notice. We shall not be held liable for any errors in figures and pictures, or any typographical errors.

Biography



 

Professor Hajime Nakamura,
National Defense Academy of Japan
Mechanical Engineering Dept.
Thermal Engineering Laboratory
Research interests Thermal Engineering, Fluid Engineering
Education Graduated Mechanical Science and Engineering from Tokyo Institute of Technology, and attained M.S. and Ph.D. degrees in Energy Science from Tokyo Institute of Technology
Doctorate Ph.D. in Engineering

Download

PDF

This article is also available in pdf

Download

Seminars

Featured Software

products

products
General Purpose Structured Mesh Thermal-fluid Analysis Software
More Details

products

products
3D Structured Mesh Thermal-fluid Analysis System for Electronics Cooling
More Details

products

products
General Purpose Unstructured Mesh Thermal-fluid Analysis System
More Details

Visitors also read

photo

Azbil Corporation

Achieving Size Miniaturization and Noise Reduction of Control Valves

  • SCRYU/Tetra
  • CADthru
photo

Aomori Prefectural Industrial Technology Research Center

- Thermal Analysis is Beneficial for the Design of Food Contaminant Scanners - Usability was a Deciding Factor in Choosing HeatDesigner for the Analysis Tool

  • 熱設計PAC
photo

CaptiveAire Systems, Inc.

Using CFD to Measure Humidity Evacuation Efficiency for a Kitchen Exhaust Hood System

  • STREAM
photo

ORIENTAL MOTOR CO., LTD.

Performing Analysis including Fans and Utilizing Results in Many Areas

  • SCRYU/Tetra

Inquiry

Contact us from the inquiry form below for any inquiry regarding this article.

製品に関するお問い合わせ

TOP