Until recently, fans were designed using essentially the same methods as 50 years ago. Typically, this involved hand calculations using airfoil design methods to estimate the performance of various blade design concepts. The problem with these calculations was that they required many simplifying assumptions, thus limiting their accuracy in practical applications. The result was that most real-world designs relied upon building and testing prototypes. Prototypes, however, take a considerable period of time to build and test and are also expensive. Moreover, the point measurements used to evaluate a prototype provided little information as to why a particular design performed well or poorly. Consequently, engineers were often forced to settle for less than ideal performance because they didn't have the tools to shoot for a truly optimized design.
CFD BASICSRecently, fan designers have turned to CFD simulation, which provides fluid velocity and pressure and temperature values throughout the solution domain with complex geometries and boundary conditions. As part of the analysis, a designer may change the geometry of the system or the boundary conditions such as inlet velocity, flow rate, rotational speed, etc., and view the effect on fluid flow characteristics.
CFD is an effective tool for generating detailed parametric studies, making it possible to evaluate far more design alternatives than the build and test method and thereby providing opportunities for optimization. Another advantage of CFD is that it provides more complete information than physical testing, including color-coded graphics that depict flow direction and velocity in all relevant locations. This helps designers gain more insight into the reasons why a design is performing as it is, which enables rapid design improvements.
Fan designers have adopted CFD methods later than many other design disciplines due to a historically heavy dependence on testing coupled with the knowledge of highly-experienced fan designers. Now as computer hardware becomes more powerful and affordable, fan designers are beginning to use CFD software which contains tools for modeling rotating flows.
DESIGNING VARIABLE PITCH FANAn excellent example of the use of CFD technology in fan design is provided by Hartzell Fan Inc. of Piqua, Ohio. Hartzell air moving products include fans for general and process ventilation with a complete line of axial, centrifugal fans and blowers, a fiberglass product line, make-up air and heating equipment, and a range of options and accessories. Hartzell engineers used computer simulation to develop a unique variable pitch fan which can be field adjusted to provide high efficiency and low noise in a wide range of applications. The idea behind this new variable pitch fan is to maximize airflow while minimizing sound and power consumption regardless of the application.
Engineers developed an original concept design using the SolidWorks computer aided design system. This model was subsequently analyzed using Fluent CFD software. Their initial design could be run at any pitch angle between 15 and 35 degrees so they created 21 variations of the model to represent each possible pitch angle. They wanted the ability to evaluate different propeller designs without having to build prototypes and also to gain the global design information that can be obtained from CFD analysis.
Changing the pitch of a fan significantly changes its performance. For example, as the pitch is raised, the horsepower requirement increases and sound characteristics change. The task facing Hartzell Fan engineers was to develop a design that would provide excellent performance at any pitch angle, which is comparable to the task of designing 21 different fans. Engineers built a prototype of their initial design to validate their CFD model. The CFD model included both the fan and the complete test chamber. Engineers took advantage of the symmetry of the design to model only a one-quarter rotational section of the fan.
Hartzell engineers then proceeded to the heart of the actual design process, modifying the CFD model and evaluating the results in order to improve the performance of the fan, particularly in the area of noise reduction. The CFD results gave them complete airflow patterns throughout the test chamber, providing far more insight into the performance of each design alternative than could be obtained by physical testing. Most of the design changes were made in SolidWorks and then exported back to the CFD program. This step provided them with complete design documentation for each design iteration and was enabled by the ease of importing a CAD file and generating a grid by the CFD software.
Hartzell engineers steadily improved their design and by the time they were done had made a 30 percent reduction in noise compared to their original design.
CONCLUSIONSComputer simulation is an idea whose time has come for fan design. New modeling techniques provide engineers with the ability to accurately model the performance of design concepts without having to build a prototype. This makes it possible to evaluate many more designs, usually resulting in a substantial improvement in performance. At the same time, the lower cost and shorter lead times of simulation provide faster time to market and reduced development costs. When you add the fact that CFD simulation provides even more design data than physical testing, you end up with an unbeatable proposition for engineers involved in fan design.
Thomas Gustafson is vice president of engineering, Hartzell Fan, Piqua, Ohio, and Walter Schwarz, Ph.D., Member ASHRAE, is HVAC&R industry manager, Fluent Inc., Lebanon, N.H. For more information, contact Fluent Inc., 10 Cavendish Court, Centerra Resource Park, Lebanon, NH 03766; 603-643-2600; 603-643-3967 (fax); www.fluent.com.
Publication date: 02/13/2006