Radial pressure distributions in an air-riding face seal

Abstract

International audienceNon-contacting face seals rely on high pressures induced in a thin air-film between stationary and rotating faces. They offer ultra-low leakage and very low wear compared to contacting seals in aircraft engines. Large axial and radial movements and high temperature gradients can cause excessive distortion of the sealing faces which may become amplified at large radii, high differential pressures and rotational speeds. Such distortions alter the geometry of the gap thereby affecting the seal's performance. This paper presents an extensive investigation into the air-film behaviour of a face seal under convergent and divergent engine representative coning distortions = 0.5-2 degrees, gap = 50-300 m, and operating pressure differences =70-350 kPa. The investigation approach is both numerical and experimental. Experimental tests allowed the introduction of a known distortion onto the static face of the seal. Arrays of static pressure tappings in the primary sealing gap were used to measure the radial and circumferential variations. The experimental data are used to validate a 3D CFD model of the primary leakage path. The CFD model was generated using ANSYS ICEM and solved using ANSYS FLUENT. The models were run at the full range of operating pressures and geometries. Results show that converging coning provides the largest air-film pressures and hence the largest opening force while a diverging coning provided the least. At higher pressure ratios divergent gaps exhibited expanding supersonic flow but with unexpected levels of pressure recovery within the diverging duct. The pressure loss at the entrance to the gap was observed to be significant, particularly where entry gaps are larger. This effect was partially captured by CFD. The most significant discrepancies between CFD predictions and experiments were for the converging gap cases where the increased air-film pressure causes the disc to deform under pressure resulting in the CFD model over-predicting the pressure in the gap