An experimental vibration study for a small centrifugal fan unit is presented in this work. The flow features in this kind of units are always of unsteady nature due to the geometrical constraints. As a consequence of such unsteady features, the vibrations and noise become often a serious design problem and may cause the unit bad functioning. With the use of fast response accelerometers, a fault diagnosis protocol is introduced in order to measure the extent of signal variations for different and expected failures. The study is based on the experimental measurements of two perpendicularly placed sensors which capture the acceleration signals for three different flow rates and three different operations, namely, the normal (or no failure) operation, the obstructed channel operation and the unbalance shaft operation. A squirrel cage or Sirocco fan is used for the study. It is a kind of centrifugal turbomachine with forward curved blades, often characterized by their small size. Its low cost and size make them be ideal for residential heating and cooling of vehicles. One of the important characteristics, the small chord of the rotor blades and the high number of them often used has been previously reported in the bibliography. Typically, most common damages or faults come from the moving blades, bearings, fixed blades, rotor and sealing. In general, the vibration signals indicate malfunction symptoms, one symptom may be due to two different faults, and the same fault can produce two different symptoms. The main goal of the study is the application of a fault diagnosis technique as the purified orbit diagram (POD) to reveal the limit range values for some standard failures. The finding of such limits is determinant to implement a fault diagnosis protocol that will decrease the maintenance costs and increase the operation efficiency of the unit. Such protocol would become straightforward from the presented data. The Purified Orbit Diagram is obtained by harmonic reconstructing corresponding harmonic components of two perpendicular directions in time domain. Three different flow rates, namely nominal and two off-design conditions are tested to be able to compare the different POD components as a function of the flow rate.

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