Designing a fan blade usually involves mounting a prototype in a plenum and measuring pressure and flow rates at different rotational speeds. Such point measurements provide little information as to why designs work well or poorly. Consequently, engineers often settle for less than ideal performance because they don't have tools to optimize designs. But new computational-fluid-dynamics (CFD) models let fan designers rely less on guess work. Franklyn Kelecy, an application engineer with CFD software developer , Lebanon, N.H., compared results from a CFD simulation of a four-bladed fan, with published experimental wind-tunnel data, over a wide range of flow rates. "Results correlate closely," he says. CFD simulations estimate fluid velocity, pressure, and temperature throughout the solution domain with complex geometries and boundary conditions. Designers immediately see the effects of changes to CFD geometry or boundary conditions such as inlet velocity, flow rate, and rotational speed. In addition, simulations give more complete information than physical testing, such as color-coded graphics of flow direction, and velocity. These provide more insight as to why a design is performing as it is, which allows rapid design improvements. Wind-tunnel tests in this case were done over a range of flow rates at a fan speed of 2,000 rpm and standard atmospheric conditions. The Reynolds number, based on a 110-mm fan diameter and blade tip speed, is 1.2 X 10 . Blades are thin, cambered plates (with circular arc sections) attached to a 25-mm diameter shaft. Kelecy developed a grid for the fan and wind-tunnel domain using the Gambit CFD preprocessor. He says recent developments in meshing simplify and speed what was once a tedious task. A tetrahedral mesh around the blade with a wedge mesh at the inlet solves slightly faster than an all-tetrahedral mesh. And postprocessors integrated with solvers also speed the operations. Blade rotation
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