Research On Deposition And Low Flow Operation Of Steam Turbine

As thermal power units generally participate in peak shaving,salt in the steam is deposited in the high-pressure cylinder of the steam turbine in advance,the deposition in the steam turbine passage become serious.At the same time,under the condition of deep peak regulation,the high-pressure cylinder is also faced with the danger of windage condition during the low flow operation.The research on the performance and flow field characteristics of the stage under deposition and low flow operations of HP passage of steam turbine is made by using numerical simulation method.First,a single-pass stage model is established basing on a high pressure reaction stage of the steam turbine.Analyze the three-dimensional flow in the stage and the secondary flow structure in the cascade passage.Distribution of the flow parameters along the circumferential and blade height direction is also obtained.The result shows that the numerical simulation results are consistent with the design parameters and the results are reliable.The blade profile loss mainly caused by the suction surface boundary layer,which can limit the secondary flows in the order of aft-loading blade profile.The horseshoe vortex,passage vortex,corner vortex and wake vortex are captured,and the suction side and pressure side branches of horseshoe vortex merged with passage vortex in the downstream development process,forming a highloss area at the cascade outlet.The circumferential distribution of cascade outlet parameters is affected by the wake region.The pressure and temperature in the wake region are higher than the mainstream,while the velocity is lower than the mainstream.The height distribution of parameters is affected by centrifugal force and vortex.The outlet pressure,enthalpy and flow angle of stator increase along the height,while the outlet velocity decreases along the height.The outlet pressure,relative velocity and absolute flow angle of rotor increased along the blade height.Then,for the deposition problem of the HP of steam turbine,blade surface roughness and additional blade thickness are set to simulate deposition condition,and roughness was set at different positions to study the influence of scaling at different positions.Through the analysis of the performance and the flow field of the cascade under deposition condition,the loss mechanism caused by scaling is obtained.The result shows that the roughness of blade surface increases the blade profile loss,and the stage efficiency decreases with the increase of blade surface roughness.For reaction stage,the influence of stator blade surface roughness is similar to that of rotor blade surface roughness on stage efficiency decline,and the influence of stator blade surface roughness on stage performance is greater than that of rotor blade.The increase of roughness makes the flow separation of endwall worse and the secondary flow loss is increased.The roughness of the blade surface thickens the boundary layer on the suction surface,causing additional profile loss.The surface of suction surface is more sensitive to roughness.Blade thickness reduces stage efficiency and massflow.The effect of blade thickeness on performance is less than that of blade roughness.The influence of blade thickness and surface roughness on stage flow performance is the greatest.Finally,numerical simulation is carried out for the low volume flow condition of the highpressure stage in the two-stage cascade model.By increasing the back pressure and reducing the mass flow rate,the flow field and performance of the stage in windage condition are analyzed.The results show that,under the condition of low volume flow,vortex appears in the top clearance in front of the rotor blade,and its range expands to the top area of the rotor blade with the decrease of flow rate.The upstream flow affects the downstream flow,and the decrease of mass flow produces negative attack angle,which generates separation flow in the passage.The separation flows exist in the whole blade height passage and block the main flow.In the rotor passage,the separation flow starts from the leading edge of the blade root and develops from the bottom to the tip trailing edge;in the stator passage,the separation flow starts from the tip and develops from the tip to the root.The second stage enthalpy drop of 50% design flow is close to the critical value of transition condition,and is about to enter the transition condition,which is consistent with the result of theoretical calculation.The steam turbine has entered the windage condition under the design flow rate of 40%.In order to improve the peak shaving capacity of the steam turbine,it is necessary to optimize the profile of the blade.Under the windage condition,the exhaust steam temperature of the high-pressure cylinder increases,and the highest temperature appears at the top of the last rotor blade.Under the 20% design flow rate,the exhaust steam temperature is 30 K higher than that under the design condition.