[Objective] To meet the deep peak regulation requirements of the unit, the steam flow state and the blast heating characteristics under the condition of small volume and low flow rate after the zero-load transformation of the low-pressure cylinder are analyzed, and related research is carried out. 

[Methods] Taking a 320 MW unit steam turbine as the research object, the MSCAN global photogrammetry system is used to establish a three-dimensional 2 flow field model of the single-channel of the lower cylinder with 7 stages of single blades on one side. Eight sets of working conditions with gradually varying inlet total pressure are set. The H-O-H structured grid, CFX steady-state calculation, IAPWSIF97 water vapor model and SST k-ω turbulence model are adopted for calculation. The working performance, flow structure and forced heating characteristics of the lower cylinder under different flow rates are systematically analyzed. 

[Results] The numerical calculation results show that when the steam flow entering the low-pressure cylinder drops to 7.19% of the rated condition, the last stage begins to output negative power. When the flow continues to decrease to 2.91% o f the rated condition, the low -pressure cylinder stops outputting power and enters a zero -output state. At this time, the flow entering the low -pressure cylinder is extremely low, and the low-pressure cylinder is in a forced heating state. The steam flow p attern in the last stage blade passage includes the lower end wall separation zone, the stator blade separation zone, the last stage rotor blade vortex, and the casing circumferential vortex, etc. At the same time, the position of the top of the rotating a nd stationary gap of the last stage blade shows a strong forced heating phenomenon. The forced heating effect significantly increases with the decrease in flow. When the steam flow drops to 1.05% of the rated condition, the average temperature of the last stage stator blade surface is 312.75 K higher than the rated condition, and the average temperature of the last stage rotor blade surface is 331.88 K higher than the rated condition. The maximum temperatures on the blade surfaces of the last rotor and stat or blades reach 761.58 K and 698.99 K. 

[Conclusion] After the steam turbine enters the zero-output condition of the low -pressure cylinder, the working performance of the low -pressure cylinder significantly deteriorates, the temperature in the final stage b lade passage rises noticeably, and the working environment of the blades becomes severely deteriorated. The rapid increase in blade surface temperature can easily cause thermal fatigue damage to the materials, thereby reducing the blade strength. Therefore, power plants should strictly control the deep load -following operation of the units. During the unit maintenance period, regular inspections of the blades should be conducted to enhance the operational safety of the units.