Development of a Rotating Test Facility for the Experimental Characterisation of Turbomachinery Shaft Seals

Abstract

In gas turbines, seals that reduce the leakage between high and low pressure regions are critical for improved performance. Damaging rubs between the rotating and stationary parts of turbomachinery shaft seals occur due to thermal and assembly misalignments, and rotordynamic vibration during engine start-up and shut-down transients. These rubs lead to increased seal leakage and hence to reduced overall turbine efficiency and life span. In recent years, compliant seals that allow for variable clearances and a reduced frequency of seal rubs have been developed. The Film Riding Pressure Actuated Leaf Seal (FRPALS) is a non-contacting compliant seal design that maintains a tight clearance between rotating and non-rotating parts, throughout the transient conditions experienced in engines.
The FRPALS concept has been defined and its application formulated in previous reports. Preliminary tests in a two-dimensional model of the seal have also been carried out, demonstrating that the concept works as intended. This thesis presents the research performed in order to advance the new sealing technology towards a system closer to be deployed in industrial applications. The specific milestones achieved during this research are as follows: (i) design and manufacturing of a high-speed rotating test facility for the development of turbomachinery shaft seals, (ii) validation of the test rig and experimental methodology via the characterisation of a four-cavity labyrinth seal, (iii) experimental investigation of a Rayleigh-step annular seal and prediction of the pressure distribution in the clearance of the seal by solving the Reynolds equation for lubrication, and (iv) measurements of the FRPALS blow-down process and leakage performance under stationary conditions.
A novel high-speed rotating test facility for the performance characterisation of turbine shaft seals has been developed. The rig features a 254 mm diameter rotor, capable of rotating at speeds of up to 15,000 rpm (equivalent to rotor surface speeds up to 200 m/s). Pressure drops of up to 3.5 bar can be achieved. One of the main parameters to be measured with the rig are the rotordynamic coefficients of the testing seal. For this, a vibration test is performed to the seal by exciting the casing to which it is attached with an electromagnetic shaker. The rig is also capable of measuring the leakage performance of the seal; the leakage flow is collected downstream of the seal and measured by means of a thermal mass flow meter.
Labyrinth seals are a well-established sealing technology that are normally used in research work as a reference for the assessment of the leakage performance of new sealing technologies under development. Additionally, labyrinth seals have been also widely studied from a stability standpoint. For these reasons, a labyrinth seal has been chosen to perform the first test in the new design rig, validate its capabilities, and debug the experimental methodology used to calculate the rotordynamic coefficients. The results of the labyrinth seal have value by themselves as it was found that no published data were available for the rotordynamic coefficients of labyrinth seals with less than five cavities. For pressure drops of up to 3.3 bar and rotational speeds of up to 14,600 rpm, the labyrinth seal was found to have an overall stable behaviour with negative cross-coupled stiffness and positive direct damping coefficients. In general terms, increments in pressure drop translated into increments of stiffness and damping, whereas the coefficients decreased as rotational speed increased.
A Rayleigh-step annular seal featuring the same clearance geometry as the FRPALS prototype under study has been characterised. Results of rotordynamic coefficients showed that the cross-coupled stiffness is positive, which has a destabilising effect on the behaviour of the seal. However, the direct damping was found to be large enough to outweigh this effect for large values of rotational speed. In any case, the stability of the Rayleigh-step annular seal was found to be poorer than that of the labyrinth seal for the range of rotational speeds tested. Calculation of discharge coefficients from mass flow rate measurements showed that the Rayleigh-step annular seal had a discharge coefficient twice as large as that of the labyrinth seal, indicating that the Rayleigh step was less effective in restricting the flow.
The steady-state Reynolds equation for gas lubrication has been solved in order to predict the pressure distribution in the clearance of the Rayleigh-step annular seal and the FRPALS. The predictions have been shown to be in good agreement with the experimental pressure data, except for the regions in which the geometry of the clearance changes abruptly. These pressure predictions can be used to inform the design process of the FRPALS. Additionally this is a stepping stone towards the solution of the upgraded transient Reynolds equation which will provide a model for the rotordynamic coefficients.
The measurements performed to investigate the blow-down process of the FRPALS at zero rotational speed are presented. The opening and closing translations of the leaves have been measured using eddy current displacement probes targeting the movable parts of the seal. The seal clearance has been shown to remain constant for a range of applied pressure drops, which indicates the stable operation of the seal, though resulting in contact with the rotor at a pressure drop of 2 bar. Mass flow leakage measurements have also demonstrated the sealing performance of the FRPALS. Comparison of the effective clearance of the FRPALS with that of the labyrinth seal has shown that the FRPALS leaks three times more than the labyrinth seal. However the prototype tested has an interleaf area that can be restricted in order to improve its leakage performance. These measurements show the potential of the seal to film ride subject to design modifications to maintain a more uniform film thickness.

Date of Award	24 Jun 2020
Original language	English
Awarding Institution	University of Bath
Sponsors	Cross Manufacturing Co (1938) Ltd
Supervisor	Carl Sangan (Supervisor), James Scobie (Supervisor) & Patrick Keogh (Supervisor)