I am doing this work at the Marcus Wallenberg Laboratury (MWL) for Sound and Vibration Research, Department of Vehicle Engineering, KTH, in Stockhom and my academic supervisor is Professor Hans Boden.
In many machines, and also in vehicles, fans are among the most important sound sources. Examples are air coolers as used in power stations and the process industry, office machines, such as computers and copiers, household appliances such as vacuum cleaners and personal care appliances such as hair dryers. In cars, in aircraft, in passenger spaces of ships and in railway vehicles, the interior noise level is often determined by fans in ventilation and airconditioning systems. At the same time, cooling fans are among the most important sources of externally radiated noise of vehicles in certain operating conditions.
While the general principles of noise reduction for fans in ideal flow conditions are well known, these general principles are often not sufficient for designing the installation of a fan in a system. Fans in machines and in vehicles often have to operate in far-from-ideal flow conditions. It is well known that inflow disturbances and inflow turbulence increase noise production by axial fans. This makes noise prediction formulas for non-disturbed, smooth inflow conditions useless for applications with disturbed or turbulent inflow. Also, an axial fan which is optimized for ideal inflow conditions is probably not the best choice for disturbed inflow conditions. For centrifugal fans, turbulent inflow has less effect on sound production, but non-ideal flow conditions can increase noise production significantly.
The present project aims at developing tools for optimizing the combination of a fan and its environment for minimum noise production in the above mentioned applications, where fans are used in non-ideal flow conditions..
In the technical tasks, data will be generated systematically on noise production by in-duct axial fans and free axial fans in non-ideal flow conditions. Next, the test data will be analyzed and the results of tests and analysis will be integrated in design procedures for low noise fans for non-ideal flow conditions.
The acoustic source power of a family of in-duct and free axial flow fans will be determined for well defined inflow disturbances and non-uniform inflow. For in-duct fans the two load method will be used at low frequencies and an adapted version of ISO-5136 will be used at high frequencies. For the free axial fan the sound power level will be determined in a reverberant room.