Numerical Simulation of Flow Fields in Diaphragm Gland of Steam Turbine

Transcription

1 Numerical Simulation of Flow Fields in Diaphragm Gland of Steam Turbine Zhang Hongli Shenyang Institute of Engineering, P.R.China Sheng Wei Shenyang Institute of Engineering, P.R.China Wang Shuqun Shenyang Institute of Engineering, P.R.China Abstract The leakage characteristics of the bending-teeth gland, which may be helpful for the fast start and fast stop of the steam turbine, was discussed in this paper by using ANSYS6.1 software. By compared the flow fields in straight-teeth gland and bending-teeth gland, the methods for the bending-teeth gland to decrease the steam leakage was proposed. The results show that, although the leakage of the bending-teeth gland is larger than that of the straight-teeth gland at the same number of teeth and other conditions, this will be changed if the number of the teeth increases or the leaking gap decreases for the bending-teeth gland. Introduction In the flow part of the steam turbine, with the gap between the static stator and rotating rotor, there is leakage of steam or air, which will decrease the efficiency of the steam turbine. In order to decrease this leakage, different kinds of seals are installed at the diaphragm, the top of the blade and the shaft sleeve. Since the friction between the stator and rotor for their different speed of thermal expansion and shrinkage caused by using the straight-teeth seal, which is one of the major obstacles for the fast start and stop of the steam turbine. Therefore, many research workers are interested in the development of new types of seals. Considering the bending-teeth, which are made by bending the straight-teeth, due to their bendy characteristics, if they are too close to touch the rotor, they will bend more without causing severe friction, thus steam turbine can adapt to fast start and fast stop. By using ANSYS 6.1 software, the flow fields in bending-teeth gland and straight-teeth gland were simulated, and the leakage characteristics of the bendingteeth gland was discussed. Procedure Numerical simulation condition The structure of the fourth diaphragm gland of a certain 300MW steam turbine is referenced, which has three teeth, with their length of 8mm, width of mm at the root and 1mm from the middle of the whole length to the top. The space between adjacent teeth is 8mm and the gap between the top of the teeth and the rotor is 1.6mm. While the structure of the bending-teeth gland is quite similar to that of the straight-teeth gland, which is formed by bending the straight-teeth gland to about 45 and leveled on the top, as shown in Figure 1.

2 (a) Straight-teeth gland (b) Bending-teeth gland Figure 1. Schematic of the structure of straight-teeth gland and bending-teeth gland Using FLUID141 element, two-dimensional flow fields in the straight-teeth gland and bending-teeth gland are simulated with compressible air as the simulation medium. Meshing elements for the straight-teeth gland and bending-teeth gland are shown in Figure. (a) Meshing elements in straight-teeth gland (b) Meshing elements in bending-teeth gland Figure. Meshing elements in straight-teeth gland and bending-teeth gland During the simulation, as we are focused on the leakage flow in the flow passage formed by the gap, the two-dimensional Cartesian coordinates is used neglecting the axisymmetric characteristic of the steam turbine, and the heat transfer between the fluid and the rotor or the gland is neglected, the process of the fluid flow is assumed to be well-insulated throttle. The stagnation pressure of the compressible air in the inlet is 9Mpa with its stagnation temperature of 73K, and the outlet pressure is 8.636Mpa Math model of fluid flow in the glands The fluid flow is described by the equation of mass conservation and the equation of motion. Here, the standardκ ε turbulent model is used. ( ρ V ) = 0 ( ρ VV ) = ( eff V ) p µ

3 G µ σ κ t ( ρvκ ) = κ + G ρε κ µ t ε ε Vε = ε + C1G κ Cρ σ ε κ κ ( ρ ) u v u v = µ t µ t x y y x κ Where, µ t = Cµ ρ, µ eff = µ t + µ l ε The boundary conditions in the flow fields are as follows. 1) Inlet and outlet Pressures are known ) Wall u = 0 ; v = 0 κ Criteria of convergence During the simulation, convergence monitor for each DOF, such as velocity, pressure, temperature, turbulent kinetic energy and so on, is calculated. The convergence monitor for each variable φ is defined as the following equation: convergence N φi φi i= 1 monitor = N k φ k i i= 1 k 1 In this paper, the criteria of convergence for each variable is 10-3 Analysis Results and Discussion Comparison of flow fields As shown in Figures 3 and 4, the flow trends in the straight-teeth gland and bending-teeth gland are basically similar. Due to the obstacle of the teeth, two vortexes are formed. However, the hindrance of the bending-teeth gland is comparatively smaller than that of the straight-teeth gland, thus the velocity in leaking gap of the bending-teeth gland is larger than that of the straight-teeth gland at the same condition, while the maximum velocity increases from 19.87m/s in the straight-teeth gland to 61.91m/s in the bending-teeth gland with the same number of teeth of 3, and the turbulent kinetic energy increases from 1759m /s to 744m /s (Figure 5).

4 (a) Flow field in straight-teeth gland (b) Flow field in bending-teeth gland Figure 3. Flow fields in straight-teeth gland and bending-teeth gland (a) Stream lines in straight-teeth gland (b) Stream lines in bending-teeth gland Figure 4. Stream lines in straight-teeth gland and bending-teeth gland (a) Turbulent kinetic energy in straight-teeth gland (b) Turbulent kinetic energy in bending-teeth gland Figure 5. Turbulent kinetic energy in straight-teeth gland and bending-teeth gland

5 Since the flow in the teeth is an isoenthalpy process, where 1 T + CP V = T 0 Therefore, the places with lower velocity will have comparative higher temperature, as shown in Figure 6. (a) Temperature field in straight-teeth gland (b) Temperature field in bending-teeth gland Figure 6. Temperature fields in straight-teeth gland and bending-teeth gland Comparison of leakage The leakage of the straight-teeth gland and bending-teeth gland can be evaluated by the flux of the inlet section, which is proportional to the inlet velocity. As shown in Figure 7, on the condition of 3 teeth and leaking gap of 1.6mm for both bending-teeth gland and straight-teeth gland, the leakage in the bendingteeth gland is larger for about 18.5%, while if the teeth of the bending-teeth gland are increased to 4, the leakage will be decreased a little less than that of the straight-teeth gland with 3 teeth.

6 Figure 7. Comparison of leakage with different number of teeth The simulation results show that the leaking gap between the top of the teeth and the rotor may have some influence on the leakage. Considering that the bending-teeth, which are made by bending the straight-teeth, due to their bendy characteristics, can be installed safely with comparatively smaller gap between the stator and rotor without causing severe friction. As shown in Fig 7, although at the same gap of 1.6mm, the leakage of the bending-teeth gland is larger than that of the straight-teeth gland at the same number of teeth, it will be changed for about 11% less if the gap of the bending-teeth gland decreases to 1.0mm (Figure 8).

7 Figure 8. Comparison of leakage with different leaking gap Conclusion Due to the bendy characteristics, using bending-teeth gland can adapt to the fast start and fast stop of the steam turbine. Although at the same number of teeth and other conditions, the leakage with bending-teeth gland is larger than with the straight-teeth gland, this will be changed if the number of the teeth increases or the leaking gap decreases for the bending-teeth gland. References 1) T. Thiemann, A. de. Lazzer, M Deckers, 热力透平 (Thermal Turbine)[J], 003, Vol.3 (): 86-94