| It has always been a persistent aim of the industry both at ho me and aboard to improve the operat ion efficiency o f turbomachinery. In HIP cylinder of large power steam turbine, the working media steam has high-qualit y energy, and will be converted into useful power when the steam flows through series of stages co mposed o f stationary and rotat ing blade rows. Energy loss beco mes inevitable due to the irreversible process o f flow, including profile loss, secondary flow loss,mixing loss and leakage loss, are collect ively called power dissipat ion of working media. For HIP cylinder, the stat ist ics show the power dissipation caused by working media leakage from clearance between stat ionary parts and rotat ing parts accounts for 51% of the total power dissipation. Therefore, in the research field of steam turbine, it is one o f the main subjects of the researchers to investigate new high-performance seal, balance hole as well as the matching design between the balance ho le and the corresponding seal devices, to reduce the leakage flow and it s interference to main stream and to improve the aerodynamic performance of the flow path o f steam turbine.At present, labyrinth seal is the tradit ional sealing device widely used for steam turbine and gas turbine. Wit h long term engineering pract ices, the professio nals have mastered the design features, sealing principles and it’s matching rules with the balance holes. In this thesis, numerical simulat ion is conducted for the well-proven design o f labyrinth seal for HP stages of actual steam turbine to achieve the leakage flow, related parameters, axial force and frictio n torque under same condit ions,which is be used as the basis for co mparison, meanwhile to understand the flow characteristics of flow field inside the sealing device and the matching desig n principle between balance ho le and the corresponding seal, so as to o ffer new concept for investigat ion on applicat ion new leaf seal in turbine stages.CFX co mmercial so ftware is used for numer ical simulation in this thesis. In considerat ion of the characteristics o f sealing device, such as low steam flow, hig h variat ion ratio of flow path area, abrupt changes of flow parameters and local structure, co mputat ion model suitable for capturing detailed informat ion o f flow field is established by regio nal meshing o f flow do main wit h co mbined structured mesh and unstructured mesh method, while ignoring minor features having litt le influence on flow, such as chamfers.Numerical simulations are carried out on the flow field of HP diaphragm respect ively wit h labyrinth seal and leaf seal, and co mparat ive analysis is made forthe working pr inciple, sealing effect and operation characteristics o f these two types of seals. The flow process o f labyrint h seal is composed o f a series of gradually strengthened throttling processes wit h the same number as seal teeth. The steps on rotor and the cavit ies are much larger than teeth tip clearance, which can deflect the flow for several t imes to form complex vortices, convert the kinet ic energ y dissipat ion into heat energy, and consequent ly increase the resistance to leakage fluid and reduce the leakage flow. However, labyrint h seal is non-contact t ype seal and it is hard to control the clearance between the teeth t ip and the rotor, especially as the steam turbine unit passes through the crit ical speed during start and stop,where the large vibrat ion of rotor will increase the clearance and consequent ly increase leakage loss. The flow in leaf seal is composed of so me parallel throttling processes alo ng circumferent ial direct ion, with the same number as leaf, and the leakage fluid flows through the clearance o f the adjacent leaf in laminar state. The drive force due to pressure difference is less than the viscous resistance o f the leaf,and the fluid densit y decreases and flow velocit y varies a litt le. Under same condit ions, the leakage flow of leaf seal is 1/3 of that o f labyrint h seal, meanwhile frict ion torque o f rotor is reduced. Especially, it requires much smaller axial space for the same sealing effect, provides good self-adaptat ion capacit y, and can adjust machining and installation errors by itself. During operation, the leaf will be raised by the aerodynamic force and a clearance will form between it and rotor. The higher it is raised, the better the sealing effect will be. Even if it rubs with rotor, necessar y clearance can form by means of leaf wearing without any damage to rotor.Invest igations are conducted on the matching design principle between labyrint h seal and balance ho le by co mparatively analyzing the numerical simulat io n and co mputation results of three schemes of 1.5 stage HP turbine, i.e. scheme wit h balance ho le, scheme wit hout balance ho le and scheme with only flow path. For the1.5 stage turbine, stepped labyrint h seal wit h steps on rotor is used for the diaphragm and rotating blade shroud and simple seal wit h boss at wheel rim and recessio n at diaphragm is set at the axial clearance in the stage, and balance hole is set for the wheel. The numerical simulat ion results show that, by considering the pressure difference before and after each leakage posit ion and the flow direct ion,this design of tradit ional well-proven labyrinth seal and balance ho le makes the axial clearance before and after rotat ing blade take in litt le fluid from main stream,and the leakage from subsequent diaphragm seal is slight ly larger than the sum o f leakage fro m balance hole and that fro m after-stage axial clearance, which will not only reduce the leakage loss o f the stage, but alto restrain the interference to leakage flow wit h main stream.The development of the matching design of leaf seal, balance hole and other sealing structures is the key to make full use o f the advantages of leaf seal.Numerical simulat ions are conducted to find the influence on aerodynamic performance of the stage when leaf seal is used to replace the labyrinth seal for diaphragm and shroud of 1.5 stage turbine. The simulation results reveal considerable leakage of main stream from the clearance before the wheel rim to axial clearance, and as a result, the leakage from the balance hole o f the stage is not reduced, instead, the interference to main stream after rotat ing blade is increased.Wit h the integrat ion o f the matching design principles of labyrint h seal and balance ho le as well as the revelat ion from this case, five-teeth stepped labyrinth seal is set in the axial clearance before the rim, and the leakage fro m subsequent diaphrag m leaf seal is adjusted to a value slightly larger than the sum of leakage from balance ho le and after-stage axial clearance. The simulatio n results show the sealing devices with such matching design o f leaf seal makes the leakage of stage reduced by 2.5times compared with the labyrinth seal, and the interference o f leakage flow to the main stream is restrained. Thus, the isentropic efficiency of stage is increased by nearly 1% and the axial force is reduced by 7.6%. |
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