Investigation Of Transient Flow Field And Pressure Pulsation In An Axial Flow Fan

The variable pitch axial flow fan has become one of the important auxiliaries of large thermal power unit with its advantages of compact structure, large flow rate and high efficiency. The axial flow fan generates vibration during operation which is caused by internal unsteady flow and pressure pulsation. Because of the presence of abnormal installation angle, vibration of axial flow fan improves and internal flow deteriorates which seriously affect economic and safe operation of the fan. Therefore, it is of great significant to further research on pressure pulsation and internal transient flow field in an axial flow fan which is with normal condition and single blade abnormally deviating.The pressure pulsation and transient flow field characteristic of an axial fan of type OB-84 under normal condition and single blade abnormally deviating are simulated with CFD, which consider the flow field in steady state as the initial flow field of unsteady state. The characterization capabilities of approximate entropy and sample entropy are examined which are extracted from pressure signal of single blade abnormally deviating.In addition, using Ansys finite element analysis module, the equivalent stress and total deformation of the blade with normal condition and single blade abnormally deviation are analyzed to study the effect of abnormal installation on blade static structure.The results show that, in normal condition, pressure signal of each monitor point has similar distribution with different flow coefficients and appears as periodic or quasi-periodic fluctuation. Under design flow rate, the maximum amplitudes at different monitoring points are presented at blade passing frequency, and the variation of flow rate only affects the time-frequency distribution. The average turbulent kinetic energy on 10%spanwise cross section improves with increasing flow rate, and the high pressure region at outlet cross section moves to the hub. The turbulent kinetic energy at the trailing edge and the passage between adjacent blades are enhanced with duration. The high pressure region at outlet cross section reciprocates between the casing and the hub during the rotation of impeller.After blade abnormally deviating, the periodic and quasi-periodic distribution of pressure signal are damaged. And the effect of deviation angle on time-domain distribution is diverse, and is more obvious under greater deviation angle. Pressure pulsation intensity of each monitor point is promoted with the increase of deviation. Thetime-frequency distribution of P2 is altered with abnormal deviation blade, and the maximal energy of pressure pulsation improves as deviation angle increases. At the same time, the periodic distribution of static pressure at impeller outlet, total pressure at 90%spanwise cross section and time-averaged static pressure at middle stream surface are destroyed. Both approximate entropy and sample entropy can be used to identify abnormal deviation blade, but sample entropy is better. In addition, the maximum of equivalent stress and total deformation are enlarged due to the increase of deviation angle,but the effect of abnormal deviation blade on distribution of them is relatively weak. The static strength test results of abnormal deviation blade still meet the requirement of the strength.