11th European Conference on Turbomachinery Fluid dynamics & Thermodynamics
Honeycomb seals are widely used in centrifugal compressors both for the high leakage flow reduction and the enhanced rotor dynamic stability. Due to the narrow gaps and high rotational speeds the flow inside the honeycomb seals is affected by the onset of acoustic phenomena which is the subject of the present paper. A stationary test rig, composed by a honeycomb surface facing a flat plate, designed to evaluate the friction factor within such type of seals is here used to demonstrate the phenomena leading to friction factor jumps occurring at certain flow speeds. Combined use of static pressure measurements and hot wire anemometry, supported by numerical analysis, permitted to compute the characteristic Strouhal number of the flow above the cavity and to define a corrected Mach number able to represent the air velocities characterized by friction factor jumps. It was also possible to note that the friction factor enhancement occurs when the unsteady forcing due to flow-cavity interaction excites the air system eigenmodes. These findings were generalized for six different honeycomb geometries tested with seven increasing clearances each and for a range of Reynolds number comprised between 40000 and 130000.