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Research Papers: Gas Turbines: Turbomachinery

Numerical and Experimental Investigation of Axial Gap Variation in High-Pressure Steam Turbine Stages

[+] Author and Article Information
Juri Bellucci

Department of Industrial Engineering,
University of Florence,
via di Santa Marta, 3,
Florence 50139, Italy
e-mail: juri.bellucci@arnone.de.unifi.it

Filippo Rubechini, Andrea Arnone

Department of Industrial Engineering,
University of Florence,
via di Santa Marta, 3,
Florence 50139, Italy

Lorenzo Arcangeli, Nicola Maceli

GE Oil & Gas,
via Felice Matteucci, 2,
Florence 50127, Italy

Berardo Paradiso, Giacomo Gatti

Dipartimento di Energia,
Politecnico di Milano,
via Lambruschini, 4,
Milan 20158, Italy

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received August 4, 2016; final manuscript received August 31, 2016; published online January 4, 2017. Editor: David Wisler.

J. Eng. Gas Turbines Power 139(5), 052603 (Jan 04, 2017) (9 pages) Paper No: GTP-16-1390; doi: 10.1115/1.4035158 History: Received August 04, 2016; Revised August 31, 2016

This work aims at investigating the impact of axial gap variation on aerodynamic performance of a high-pressure steam turbine stage. Numerical and experimental campaigns were conducted on a 1.5-stage of a reaction steam turbine. This low speed test rig was designed and operated in different operating conditions. Two different configurations were studied in which blades axial gap was varied in a range from 40% to 95% of the blade axial chord. Numerical analyses were carried out by means of three-dimensional, viscous, unsteady simulations, adopting measured inlet/outlet boundary conditions. Two sets of measurements were performed: steady measurements, from one hand, for global performance estimation of the whole turbine, such as efficiency, mass flow, and stage work; steady and unsteady measurements, on the other hand, were performed downstream of rotor row, in order to characterize the flow structures in this region. The fidelity of computational setup was proven by comparing numerical results to measurements. Main performance curves and spanwise distributions have shown a good agreement in terms of both shape of curves/distributions and absolute values. Moreover, the comparison of two-dimensional maps downstream of rotor row has shown similar structures of the flow field. Finally, a comprehensive study of the axial gap effect on stage aerodynamic performance was carried out for four blade spacings (10%, 25%, 40%, and 95% of S1 axial chord) and five aspect ratios (1.0, 1.6, 3, 4, and 5). The results pointed out how unsteady interaction between blade rows affects stage operation, in terms of pressure and flow angle distributions, as well as of secondary flows development. The combined effect of these aspects in determining the stage efficiency is investigated and discussed in detail.

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Figures

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Fig. 1

Spanwise distribution of S1 and R exit flow angle

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Fig. 2

Spanwise distribution of S1 and R nondimensional exit static pressure

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Fig. 3

The low speed closed-loop test rig. Technical scheme and main components: (a) turbine stage, (b) torque sensor, (c) axial compressor, (d) DC motor, (e) centrifugal blower, (f) Venturi nozzle, and (g) heat exchanger.

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Fig. 4

The test section and the measurement planes of FRAPP probe (plane P2) and rakes (planes P0 and P3). The tangential traversing is obtained by rotating the external casing.

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Fig. 12

Static pressure distribution at S1/R interface at midspan: effect of blade spacing (instantaneous CFD results, AR = 3)

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Fig. 13

Absolute exit flow angle distribution at S1/R interface at midspan: effect of blade spacing (instantaneous CFD results, AR = 3)

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Fig. 5

Computational grids for the large axial gap

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Fig. 14

Entropy contours at stator/rotor interface (S1/R): instantaneous numerical results for two extreme axial gap

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Fig. 15

Static pressure contours at stator/rotor interface (S1/R): instantaneous numerical results for two extreme axial gap

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Fig. 16

Spanwise distribution of absolute exit flow angle downstream S1 and R row: effect of blade spacing (time-averageCFD results, AR = 3)

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Fig. 17

Stage total-to-total efficiency gain for several axial gapand aspect ratio: CFD results near peak efficiency for Re = 0.3 × 106

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Fig. 18

Stage total-to-total efficiency gain for several axial gap and Reynolds number: CFD results near peak efficiency for AR = 1.0 and AR = 3.0

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Fig. 11

Relative Mach number contours downstream rotor row: instantaneous numerical and experimental results for three stator pitch (large gap)

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Fig. 10

Static pressure contours downstream rotor row: instantaneous numerical and experimental results for three stator pitch (large gap)

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Fig. 9

Spanwise distribution of absolute exit flow angle downstream rotor row: time-average numerical and experimental results (large gap)

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Fig. 8

Spanwise distribution of total temperature and pressure at plane P3: time-average numerical and experimental results (near peak efficiency)

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Fig. 7

Power output versus mass flow. Numerical (lines and filled symbols) and experimental results (open symbols) for nominal and large axial gap.

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Fig. 6

Efficiency versus work coefficient. Numerical (lines and filled symbols) and experimental results (open symbols) for nominal and large axial gap.

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