Numerical Study On Unsteady Aerodynamic Characteristics Of Ultralow Load Multi-Stage Wet Steam Turbine

The existing diversified power generation methods require flexible operation of the steam turbine unit,which results in the long-term operation of the low-pressure cylinder under low-load conditions,and the flow field structure presents a very complicated three-dimensional flow,especially the flow in the last stage of long blade cascade,will cause off-flow,backflow and other phenomena,and even enter the “ventilation” operation,so that the economic and safety of the unit is reduced,so it is of great engineering significance to study the low-flow cylinder low-flow operating conditions.According to the above problem,this paper establishes a low-pressure multi-stage analysis calculation model based on the parameters of a typical steam turbine provided by Harbin Turbine Company Limited.and uses commercial software ANSYS-CFX to perform numerical calculations,study the conditions of extremely small flow conditions.The flow field structure inside the lower and lower pressure cylinders,as well as the unsteady aerodynamic and vibration characteristics of the last-stage long blades,The flow field structure inside the low-pressure cylinder and the unsteady aerodynamic and vibration characteristics of the last stage long blade under the condition of extremely small flow were studied,which provided theoretical support and technical reference for the long-term low-load depth peak-adjusting operation of supercritical and ultra-supercritical heating units.Firstly,the six-stage turbine simplified model of the low-pressure cylinder is subjected to steady calculation.the low-pressure cylinder wet steam flow under different flow conditions was investigated by analyzing meridional flow field,flow field in S1 flow surface,last-stage moving blade surface flow,temperature field and humidity distribution.The results show that the flow state of the low-pressure cylinder of the steam turbine is relatively good before entering the ventilation condition.When operating under the condition of extremely small flow,the structure of the flow field is extremely complicated,especially the flow inside the last stage cascade,appears bakeflow vortex,separation vortex,rotor-stator clearance vortex successively.The interaction of these vortex structures allows the pressure distribution on the surface of the last-stage moving blade to be reconstructed,and develops from zero pressure zone to a completely negative pressure zone.Secondly,using the six-stage turbine steady calculation results as the initial field,the unsteady numerical simulation was performed using the unsteady calculation method TBR-TT of the turbomachinery in CFX.studied the method feasibility to simulate internal flow field of multistage wet steam turbine under ultra-low load condition.The unsteady numerical simulation was carried out under multiple inlet flow conditions,and the unsteady aerodynamic characteristics of the last-stage long blade and the corresponding mechanism of the flow field vortex structure were studied.The results show that the method can converge faster and better even under the condition of low flow rate when the calculation is based on k-e turbulence model.The unsteady fluid excitation force of the last stage moving blade decreases first and then increases with the decrease of the inlet mass flow rate,and a sudden increase phenomenon occurs when it is less than 2.83kg/s.The study found that this variation of the unsteady excitation force is mainly affected by the unsteady flow field structure at the top of the last-stage rotor blade.Finally,using the traditional unsteady calculation method,based on the SST turbulence model,the transient simulation is performed in the low-pressure cylinder end two-stage simplified calculation model with a modeling channel ratio of 3:5:3:4.The results show that under extremely small flow conditions,the static pressure monitoring signal of the last stage moving blade surface not only contains the last stage stator blade passing frequency and its doubling frequency,but also generates peaks and peak groups of different amplitudes in the low frequency region.The vortex identification analysis found that the vortex structure in the root and the middle region of the blade was relatively stable,while the blade tip region showed obvious unevenness in the circumferential direction due to the gap vortex and other factors.At the ventilation condition point,there is a rotation instability phenomenon in the area of the tip of the last stage moving blade propagating in the circumferential direction.