Centrifugal fans are widely used for air-conditioning equipments. Demands for air conditioners have arisen for quiet considered the indoor condition, energy saving associated with the global warming, and which have made it important to develop high performance fans. In order to meet these demands, the technology was studied to reduce leakage flow for a high efficiency and low noise centrifugal fan in this research. Leakage flow occurs when a part of air discharged from fans flows into a gap between the bellmouth and the shroud. This flow is the pressure loss because it returns from the high-pressure side to the low-pressure side wastefully. Furthermore, the high noise is caused, because this leakage air can’t flow along the shroud. As the final successful technology to reduce leakage flow, we have developed for the new bellmouth with multiple ribs on the bellmouth plane placed opposite fan. By using the new bellmouth, the centrifugal fan’s motor input for the 4-direction ceiling-embedded cassette type air conditioner has been reduced by 5.4[%] and the noise by 1.2[dBA]. We analyzed the leakage flow structures of the centrifugal fan were analyzed by the numerical simulation and LDV measurements. It has been found that the leakage flow is decreased, since the partial leakage flows back in an opposite direction to a gap between the bellmouth and the shroud. Additionally, it has been found that the turbulence intensity of outlet airflow is decreased by the new bellmouth.

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