Investigating the effect of aerodynamic damping on an axial flow fan blade

Basson, Nico Raymond (2015-12)

Thesis (MEng)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Axial flow fans used in air-cooled condensers experience a variation in blade loading due to distorted inlet flow patterns and obstacles in close proximity to the fan blades. The fan blades vibrate at their natural frequencies when excited. Observations made during the operation of these fans have shown that the vibrational response exhibited by the blades varies with angle of attack. This phenomenon is suspected to be due to aerodynamic damping. The purpose of this study was to investigate the influence of aerodynamic damping on the vibrational behaviour of a generic flat plate axial flow fan blade. The blade's structural vibrational properties were determined at vacuum conditions in a purpose designed and built vacuum chamber. Experiments were subsequently conducted in a large wind tunnel at various flow velocities, with the blade's setting angle altered between data sets. The blade's measured damped frequency and damping ratio was shown to be proportional to air density. The total damping was further shown to be proportional to the airflow velocity at which the blade was excited, with a greater increase noted at large setting angles. Phenomena noted during the investigation were validated numerically using a dynamic Euler-Bernoulli beam model. The model performed well for instances of static atmospheric conditions, but was unable to replicate results obtained from wind tunnel experiments. Trends noted during the vacuum chamber experiments were successfully replicated. Examination of the trends exhibited by the experimental results suggest a complex fluid interaction with the blade. It is therefore recommended that a fluid-structure-interaction analysis be performed.

AFRIKAANSE OPSOMMING: Aksiaalwaaiers wat gebruik word in lugverkoelde kondensors ondervind wisselende lembelasting a.g.v. verwronge invloei-patrone en hindernisse naby die waaierlemme. Die waaierlemme vibreer teen hulle natuurlike frekwensies wanneer hulle opgewek word. Waarnemings tydens die bedryf van die waaiers het gewys dat die vibrasie-reaksie by die lemme verskil soos wat die invalshoek verander. Hierdie verskynsel is moontlik a.g.v. aërodinamiese demping. Die doel van hierdie studie is om die invloed van aërodinamiese demping op die vibrasie-gedrag van 'n generiese plat-plaat aksiale waaierlem te ondersoek. Die lem se strukturele vibrasie-eienskappe word bepaal onder vakuumtoestande in 'n doel-geboude vakuumkamer. Eksperimente is hierna gedoen teen verskeie vloei-snelhede in 'n groot windtonnel met die lem se monteerhoek as veranderlike in die datastelle. Die lem se gemete dempings-frekwensie en dempings-verhouding is in verhouding met lugdigtheid. Die totale demping is verder eweredig aan die lugvloei-snelheid waarteen die lem opgewek raak, met 'n hoër verhoging teen 'n groter invalshoek. Verskynsels wat waargeneem is tydens die ondersoek is numeries geëvalueer deur gebruik te maak van 'n dinamiese Euler-Bernoulli-balkmodel. Die model het goed presteer tydens statiese atmosferiese toestande, maar die resultate van die windtonnel eksperimente kon nie dupliseer word nie. Tendense waargeneem tydens die vakuumkamer eksperimente is suksesvol gedupliseer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97885
This item appears in the following collections: