Abstract
Introducing a negative preswirl at the upstream of annular gas seals has been considered as an effective way to improve the system stability. This paper demonstrates a stability enhancement approach for a short labyrinth seal using positive preswirls. The static and dynamic characteristics of the labyrinth seal with various blade numbers (5, 10, 15), inlet preswirl ratios (–0.3, –0.15, 0, 0.15, 0.3) were studied. Results show that the inlet preswirl ratio has a dramatic effect on the circumferential location of the high-pressure spot for each seal cavity, particularly for the first cavity. The inlet preswirl ratio has opposite effects on the system stability due to the difference of high-pressure spot locations between the first cavity and the others. An increasing positive inlet preswirl could improve the system stability for the labyrinth seal with fewer blades (e.g. 5 blades). Its characteristics is mainly dominated by the first seal cavity. For the labyrinth seal with 10 blades, the system characteristics shows slight dependency on the inlet preswirl ratio. For the labyrinth seal with more blades (e.g. 15 blades), the negative inlet preswirl still increases the system stability, which agrees with the conventional conclusion. The paper provides a deeper understanding on the stability improvement of the labyrinth seal.
Highlights
Labyrinth seals are used extensively in turbomachines to minimize the leakage flow rate by separating high pressure fluid from entering into a low-pressure region
In order to investigate the effect of mesh densities for an accurate prediction of the rotordynamic coefficients, coarse and fine mesh grids were tested for the labyrinth seal
For the labyrinth seal with 5 blades, the tangential seal reaction force decreases with the increasing preswirl ratio, and corresponding cross-coupled stiffness coefficient k becomes to be negative
Summary
Labyrinth seals are used extensively in turbomachines to minimize the leakage flow rate by separating high pressure fluid from entering into a low-pressure region. Circumferential flows [12, 15, 16] due to swirl brakes, shunt injection, circumferential entry velocity, and rotational shaft surface at the seal inlet have a significant effect on the system stability It can be usually categorized into zero, positive, negative preswirl in terms of the circumferential entry direction. In 1980, Benckert and Wachter [12] firstly reported that the inlet flow direction of the seal in the same direction of shaft rotation would destabilize the rotor-seal system, while the reversing preswirl at the seal inlet can effectively improve the stability by offsetting the circumferential flow. The static and dynamic force coefficients are further investigated to evaluate the stability characteristics
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