Numerical Modeling of Thermally-Induced Vibration in Rotor Caused by Light-Rub Against Brush Seal

Clearance is of paramount importance for turbomachinery manufacturers to meet today’s aggressive power output, efficiency, and operational life goals. To minimize leakages, there are various seal types used, and new sealing concepts are in development. Because of their inherent flexibility and compliance, brush seals are capable of significantly reducing the leakage, and allow sufficient geometrical margins to accommodate design and operational variations of turbomachines. Brush seals can be assembled at very tight or zero radial clearance or even with interference on the rotor to minimize the leakage. This means that the risk of contact between the rotor and the seal bristles exists, especially in case of zero clearance or interference. If the contact occurs, a hot-spot develops on the rotor and this may cause the vibration to diverge, resulting in a synchronous instability, the so-called Newkirk effect. The friction forces generated by rotor-to-stator rubs often cause a shaft thermal bow whose main effect on the machine dynamic behavior is a progressive change of the synchronous (1X) vibration. The development of analytical tools able to model this phenomenon is therefore important to assess the rotordynamic stability during the design phase and avoid excessive vibrations which may have severe impact on the operability and on the mechanical integrity of the machine. The objective of this paper is the development of a numerical model to analyze the dynamic behavior of real turbomachines subject to thermally-induced vibration caused by light-rub of the rotor against brush seals. The model developed in the paper is based on the work of Bachschmid et al. [1]: the dynamics is analyzed in the frequency domain using the standard rotordynamic model, whereas the heat transfer analysis, to calculate the temperature distribution and the associated thermal bow, is studied in the time domain. The contact analysis has been deeply revised, aiming at estimating suitable normal and tangential force and the friction heating generated by the contact. The bow determined by the thermal conditions has been reproduced using suitable bending moments, which have been applied to the beam elements in the rotordynamic model.