Rotordynamic Behavior of Leaf Seals: Measurement of Frequency-Dependent Force Coefficients for Varying Inlet Parameters

Abstract

While brush seals can be found in various applications for turbomachines today, leaf seals are a further development in compliant seal technology and have a lower level of maturity. Among the purported advantages are greater axial rigidity when subject to higher pressure differences and the potential for noncontacting operation due to lift-up. However, especially their rotordynamic behavior is little investigated in the literature so far. In this paper, measured rotordynamic force coefficients of a leaf seal are presented for varying inlet pressures, preswirl velocities, and excitation frequencies. The leaf pack of the tested leaf seal has zero rotor cold clearance, and its coverplates are designed for facilitating a lift-up effect when pressurizing the seal. Experiments were performed on a dynamic test rig with whirling rotor using active magnetic bearing (AMB) technology and evaluated in the frequency domain based on the impedance method. Test results for the leaf seal reveal positive direct stiffness and an advantageous rotordynamic behavior due to significant levels of direct damping and negative cross-coupled stiffness throughout the operating parameter range. Leaf seal results are compared to brush and labyrinth seal data from previous studies for varying inlet pressures and preswirl velocities. Additional computational fluid dynamics (CFD) simulations were carried out to predict the leaf deflection moment, which supports the findings regarding hydrostatic lift-up from the experimental results.