Numerical Study on the Influence of Leakage Flow at the Top of the Moving Blades on Turbine Stage Aerodynamic Performance

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In order to reduce the loss caused by the leakage flow at the top of the moving blade, a shroud is installed on the top of the modern steam turbine blade to prevent the lateral flow of the steam across the blade tip. The tip shroud effectively reduces the losses caused by leakage flow and is therefore widely used. Due to the existence of the axial gap between the vane and the moving vane, the steam flow enters the space between the shroud and the cylinder along the gap after flowing out of the vane, and then flows out from the rear of the moving vane. The low-energy leakage fluid flowing from the end of the shroud blends with the main stream, causing some loss. In order to control the leakage flow, reduce the leakage loss, and improve the flow efficiency, the labyrinth steam seal teeth are generally installed on the top of the bucket. The study of the influence of steam seal teeth on the tip flow and loss of the Journal of Thermophysics 28 has a practical significance for understanding the leakage flow at the top of the moving blade.

Due to the difficulty of measurement, the leakage flow in the top gap of the turbine moving shroud has been studied somewhat in 1935, but it is still slow. With the development of various advanced measurement technologies, it became a hot spot in the 1990s. DentonW first established the loss model and empirical formula for the leakage flow at the top of the moving lobe, and Denton believed that the main loss occurred during the leakage flow and mainstream mixing. Peters et al. performed a leaking flow at the top of a semi-turbine bucket. The turbulence model uses the standard fc-e model. The wall of the cascade is a non-slip thermal insulation wall. The calculated boundary conditions are set at the inlet boundary to give the total inlet temperature and total pressure of the first-order semi-turbine stage, the outlet boundary is given the static pressure at the outlet, the wall surface is a non-slip thermal insulation solid wall, and the flow channel is set to a periodic boundary condition. Slip grid processing is used between the vanes and the moving leaves.

Table 1 Basic geometric parameters of the moving blade Number of blades Leaf mounting angle Length (mm) Axial pitch (mm) Axial chord length (mm) Root radius (mm) Aspect ratio 3 Results analysis is the leaf in three cases Top 3D streamline diagram. At the top without the steam seal teeth, the air flow enters the tip chamber (3 mm high) from the gap (1.5 mm) of the radial steam seal teeth. It can be seen from the figure that the airflow shrinks when passing through the gap of the steam seal (1.5 mm), enters the chamber in the form of a jet, and generates flow separation at the leading edge of the shroud, and curls to form a distinct vortex. As the fluid develops in the axial direction, the leaking fluid reattaches to the upper surface of the shroud. At the outlet of the end gap of the shroud, fluid near the upper end wall impinges on the corner of the cylinder wall, forming a recirculation zone, and pushing the leakage fluid downward. In general, the fluid in the gap can maintain a relatively short axial velocity when there is no steam seal, and the gap flows out obliquely downward under the push of the vortex in the exit corner region to develop downstream.

When there is a steam seal tooth, the leaking fluid forms a jet through the steam seal tooth gap and forms a backflow in the space behind the steam seal tooth.

It is the detailed flow diagram of the top of the moving blade when the three steam seals are designed. The velocity vector of each key part is depicted. The fluid at the inlet of the tip clearance is subjected to radial clearance suction, has a large radial velocity, and develops against the upper end wall to form a clockwise vortex. The axial velocity of the jet formed by the steam seal gap is large, and it is developed against the shroud. The cavity after the gap of the steam seal and the downstream steam seal teeth form a counterclockwise flow. In this way, when the leakage fluid flows through the tip clearance, the axial velocity changes several times, which inevitably causes the dissipation and loss of kinetic energy. When such leakage fluid flows out of the gap and is mixed with the mainstream, it has a negative effect on the flow efficiency. . After passing through the third steam seal tooth, the leaking fluid enters the main flow from the end of the shroud, its axial velocity is somewhat long, and a vortex is formed at the upper upper end wall of the lower stationary vane inlet. In this way, the leakage fluid seriously affects the flow field structure of the rotor blade outlet, which has a potential negative effect on the efficiency of the lower stage vane.

The airflow angle of attack of the lower vane leading edge caused by the mainstream and leakage flow blending is depicted. The effect of the visible leakage flow on the downstream flow field. At the 50% leaf exhibition, when the top of the blade is steam-sealed, the fluid enters the stationary blade according to the design conditions, and the airflow angle of attack is the design angle of attack. In the absence of steam seals, the leakage flow has affected the mid-leaf spread, resulting in a small negative angle of attack for the airflow, resulting in a flow separation of the airflow at the leading edge suction side. At 80% of the leaf exhibition, the negative angle of attack is very obvious for the case of no steam seal. In the case of a steam seal tooth, the effect of the leakage flow can be seen, the inlet fluid has a small negative angle of attack, and the small flow separation zone occurs at the suction front. For the case of three steam seal teeth, the direction of the air flow is also slightly changed compared with the 50% blade spread, and the air flow deviation from the design condition is not obvious, not as strong as the former two cases. At the 90% leaf exhibition, the negative angle of attack of the airflow in three cases has been more obvious. However, the size of the negative angle of attack is still the largest when there is no steam sealing, the second is the time when one steam seals the teeth, and the deviation is the smallest when the three steam seals. This also indicates that the addition of the number of teeth of the steam seal inhibits the leakage flow at the top of the shroud.

(a) 50% leaf spread (no steam seal teeth, one steam seal tooth and three steam seal teeth) (b) 80% leaf spread (no steam seal teeth, one steam seal tooth and three steam seal teeth) (c ) 9% leaf spread (no steam seal teeth, one steam seal tooth and three steam seal teeth), the streamline diagram at the different leafhopper positions on the leading edge of the vane and the circumferential angle of the vane exit of the vane inlet The distribution of the total pressure loss coefficient along the leaf height direction. It can be seen from the figure that the leakage flow changes the angular distribution of the exit airflow near the upper end wall and increases the flow loss in this area. Steaming the teeth can weaken this effect. It is the distribution of the circumferential average airflow angle at the exit of the vane along the leaf height direction. It can be seen that the leakage flow also has a great influence on the flow field of the vane, so that the angular distribution of the airflow in the upper half is significantly different from that in the ideal case without the tip clearance. When there is a leakage flow of the shroud, in the case where the two leaf tops are steam-sealed, the flow also has the characteristic of the angular distribution of the air flow when there is no gap. For the case where there is no steam seal on the top, the flow angle distribution changes greatly. The exit air flow angle / (°) The circumferential average air flow angle distribution The exit air flow angle / (°) The circumferential average vane exit air flow angle Table 2 gives three In the case of the downstream leaves of the moving blades, the leakage flow at the top of the moving leaves affects the overall performance. It can be seen that as the number of teeth of the steam seal in the tip clearance is large, the flow loss caused by the leakage fluid passing through the gap of the steam seal and the backflow occurs thereafter. However, due to the increase in the number of teeth of the steam seal, the leakage flow is significantly reduced, so that the leakage effect is suppressed to have a significant positive effect.

Table 2: No difference in the number of teeth of different steam seals and its effect on the performance of the vane. Number of teeth of the seals, dimensionless leakage, average pressure loss of the outlet, isentropic efficiency, 4 conclusions. This paper is for the first-order semi-transparent belt. The leakage flow of the tip of the moving blade was numerically simulated. The morphology of the leakage flow with different number of steam seals in the gap, the loss development, and the influence on the downstream static flow field after mixing with the mainstream were compared. The leakage flow at the top of the moving blade with the shroud has a significant adverse effect on the performance of the bucket and the performance of the downstream blade row. If the steam seal teeth are not installed, the large leakage flow not only weakens the functional force of the bucket, but also causes the mixing loss, and the flow state of the downstream flow field deviates from the design condition. The presence of leaf-top steam seals can significantly reduce leakage flow.

The number of steam seals increases, and the loss of leakage fluid is large, but the total loss is reduced due to the decrease in leakage flow.

The mixing of the leaking fluid with the main flow causes the fluid velocity in the blending zone to deflect and enter the downstream stationary vane at a negative angle of attack, resulting in an angle of attack loss.

The effect of reducing the leakage flow caused by the large number of teeth of the steam seal can significantly reduce the area of ​​the airflow deflection caused by the blending, weaken the degree of deflection, and reduce the negative angle of attack. Due to the high radial velocity of the leaking fluid, it will develop toward the mid-lobe in the vane channel, which will change the flow field structure of the upper half of the vane, and bring the angle of attack loss to the downstream bucket. An increase in the number of steam seals can weaken this effect.

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