Sources of Noise in High-Temperature Centrifugal Fans

Sources of Noise in High-Temperature Centrifugal Fans

The noise generated by high-temperature centrifugal fans primarily stems from rotational noise, vortex noise, and exhaust noise. By analyzing the structure of these fans, the department examines the noise at the outlet, inlet, motor housing, and noise radiated through the ductwork. So, how is this noise generated, and what are the underlying principles? Noise Sources in High-Temperature Centrifugal Fans Rotational noise is caused by the pressure of the surrounding gas generated by the blades uniformly distributed on the impeller. Rotational noise is proportional to the 10th power of the impeller’s circumferential speed. When air flows over the blades, a boundary layer forms on the blade surface—particularly on the suction side—where the thickness increases, generating more vortices. At the rear of the high-temperature centrifugal fan, the boundary layers from the suction and pressure sides converge into what is known as the wake region. The pressure and velocity of the airflow in the wake region are lower than those in the main flow region. Consequently, during rotation, the airflow at the blade exit becomes highly non-uniform. This non-uniform airflow periodically acts on the surrounding gas medium, generating pressure pulsations that produce noise. The greater the non-uniformity of the airflow, the louder the noise. Vortex noise, as it is called, is primarily caused by turbulent boundary layers, vortices formed as air flows past the blade tips, and vortex shedding. Vortex noise generated by pressure pulsations at the blades is proportional to the sixth power of the impeller’s peripheral speed. Therefore, the higher the fan’s peripheral speed, the greater the noise. Exhaust noise is related to the inlet and outlet ducts of the high-temperature centrifugal fan. When the inlet duct of the high-temperature centrifugal fan is straight and free of curved sections, the noise level is lower. In the presence of curved sections, the noise is slightly higher. The same applies to the outlet of the high-temperature centrifugal fan: since the fan inlet is a closed air intake and the outlet is an open discharge to the external environment, the noise at the outlet is greater than that at the inlet. The noise generated by high-temperature centrifugal fans is primarily caused by the fan blades forcing the target gas into a vortex during operation. As the blades rotate, they create friction or impact with the air. The faster the speed, the higher the air vibration frequency, and the sharper the noise. This effect becomes more pronounced as the width or thickness of the blades increases. It becomes even more noticeable as the width or thickness of the high-temperature centrifugal fan blades increases. The noise frequency consists of multiple frequencies, all of which are related to the speed of the high-temperature centrifugal fan.