Inlet Flux Influence to Aerodynamic Performance of Steam Turbine Vane Cascade

In order to improve the aerodynamic performance and the performance of off-design working condition, it is necessary to research the inlet flow rate influence to overall performance of steam turbine. This study uses CFD technology to numerical simulation the flow field of nozzle cascades passage at 80, 100, 120% flow rate, respectively. The simulation results indicates that the various flow loss of nozzle have a little change at 100% design working condition, have decreased at 80% design working condition and have increased at 120% design working condition.


INTRODUCTION
In recent several decades, our steam turbine industry had fast development.With the fast and stable development of our economic the power demand was increased year by year, so improving the total aerodynamic performance of steam turbine were aim of the engineers of turbomachinery (Karamanis et al., 1999;Wang et al., 1994).At the same time, the periodic change of electricity consumption was bigger than before, such as the seasonal demand of enterprise or the fluctuation of electricity consumption by users, the stability requirement of steam turbine's off-design working condition was more and more rigorous than before.So we must research the off-design working performance of steam turbine cascades.Every country engineering had conducted many work to improve the cascade efficient (Murt et al., 1994;Yamamoto et al., 1994).This study mainly researches the influence to cascade aerodynamic performance by inlet flow rate change and the results can provide the theoretical support to improve the aerodynamic efficient of cascade.

NUMERICAL SIMULATION METHOD
Sixty MW super-critical steam turbine nozzle cascade was selected as the numerical simulation geometric model, the CFD simulation software was ANSYS-CFX.The blade geometric model was shown in Fig. 1.The mesh grid model was shown in Fig. 2. The total mesh sum was 420000 and the minimum Fig. 1: Geometry model mesh angle was 36.47.It was indicated the orthogonality was fine.The maximum length-width ratio was 127.73 that also reached the demand of CFX and there were no negative mesh.Maximum expansion ratio is 2.56.The mesh grid model was reached the demand of CFD simulation of the flow field of nozzle cascade.The boundary was as follow; inlet total pressure was 103100 Pa, inlet total temperature was 316 k, outlet static pressure was 0.1 MPa, inlet flow rate were 80, 100 and 120%, respectively design flow rate.The turbulence model was S-A model.that there is adverse pressure gradient at the 80% relative axial position, the adverse pressure gradient is very short that can weaken the secondary flow.In the three flow rate workingaveraged static pressure isolines distribution are nearly similar to each other and are mainly parallel with each other.It indicates that the ar flow with little flow loss.After 80% relative axial position, passage flow begin to be turbulence with bigger flow loss because there gradient.As Fig. 3 shows that 100% design flow rate, (c) 120% design At the middle of blade height, the static pressure coefficient decreases as the flow rate increasing.The static pressure coefficient is decreased from 0.45 to 0.15 with 80 to 100% design flow rate and is decreased from 0.1 to -0.3 with 100 to 120% design flow rate.

Pitch
Total pressure loss coefficient distribution along axial direction: As Fig. 5 shows that before 60% relative axial position, the total pressure loss coefficients of 80, 100 and 120%, respectively design flow rate have the similar change law and little difference.At 120% design working condition, the total pressure loss coefficient is 7.8%, its efficient is 92.2%.At 100% design working condition, the total pressure loss coefficient is 6.3%, its efficient is 93.7%.At 80% design working condition, the total pressure loss coefficient is 3.7%, its efficient is 96.3%.It is obvious that the flow loss of 80% design flow rate is smaller than the other two working condition, but the smaller flow rate will not provide enough steam to the next cascade stage.

Pitch-averaged total pressure loss coefficient distribution along spanwise at cascade outlet:
The pitch-averaged total pressure loss coefficient distribution along spanwise at cascade outlet is shown in the Fig. 6.Comparing with the three curves from Fig. 6, it is clear that there are three regions, root of blade, middle of blade and tip of blade, from root to tip blade.The total flow loss curve can be divided into five regions, there is a high flow loss region at the up and down endwalls, it is mainly induced by the frictional loss with boundary layer flow.There is also a high flow loss region near the up and down endwalls about 10% relative blade height, it is mainly induced by the up and down passage vortexes.The passage flow loss of most middle blade region is smaller than other region, it is mainly boundary layer loss on the blade walls, this part of flow loss is smaller than other region but occupies more region.In the nozzle cascade, the main flow loss is boundary layer flow loss, the other flow loss are made of passage vortex loss and wake fall off vortex loss and secondary flow loss.
The cross-section total pressure loss isoline at cascade outlet is shown in Fig. 7.It is can be seen from Fig. 7 that the down fields of three flow rate have the similar total pressure distribution, the waking flow fields have the similar geometric type, the high flow loss are all located in the waking flow region and endwall region, the three fields are all typical

CONCLUSION
The CFD simulation of nozzle cascade at offdesign and design working condition indicates; at the design working condition, all kinds of aerodynamic performance parameters are all good for the nozzle cascade and all kinds of flow loss are not large, it indicates the prototype blade cascade has excellent aerodynamic performance.At smaller flow rate working condition, the boundary layer flow and secondary flow loss are all decreased, but too low flow rate will not provide enough demand steam for the next cascade stage and will influent the power of steam turbine.At bigger flow rate working condition, the boundary layer flow loss and secondary flow loss are all increased and will outstanding decrease the total system efficient.
By the analysis and research of all kinds of aerodynamic performance parameters in the cascade flow field at different flow rate, in the 80~120% design working condition region, no air flow block up phenomenon is founded, the up and down passage vortexes have small change, the three waking fall off vortex have similar constructer, it is indicated that the blade cascade adopted by this study has excellent aerodynamic performance in off-design working condition.

Fig. 4 :
Fig. 4: Influence of variant flow rate to static pressure coefficient distribution at middle blade profile