introduction
In recent years, the development of high-efficiency and wide flow range high-pressure ratio centrifugal compressor has become the focus of researchers' research.
In modern turbomachinery design, pneumatic skimming technology is an effective way to reduce blade tip leakage loss and achieve higher performance, skimming has been widely used in axial compressor design, and has achieved good research results, but in the application of centrifugal compressor, the research on skimming blade is still very limited.
In order to expand the application of grazing technology to the research of shunt blades, this paper designs the leading edge of the shunt blade of a transonic centrifugal compressor, studies the influence of grazing blades on their aerodynamic performance, and explores the potential mechanisms and methods of improving flow by grazing characteristics.
Computational models and numerical simulation methods
1. Research object
A small transonic centrifugal compressor is composed of a semi-open impeller and an airfoil diffuser, the impeller contains 9 main blades, 9 shunt blades, the inlet diameter is 59.7mm, the outlet diameter is 108.8mm, the blade top gap is evenly distributed, the height is 0.2mm, the diffuser is 16 blades, and the compressor design speed is 111700r/min.
The initial compressor was designed using ConceptsNREC software, the blades were non-grazing, and the meridian flow surface and three-dimensional models were shown in Fig. 1(a) and Fig. 1(b).
Figure 1 Study object and grazing angle definition
On the basis of which the initial design scheme is named case_0, zero means that the number of graze angles is 0, the leading edge of the shunt blade is designed to be skimmed to study the influence of the grazing angle α of different shunt blade leading edge on the performance and internal flow of the transonic centrifugal compressor.
The design range of the leading edge of the shunt blade and the grazing angle are shown in Fig. 1(c) and Fig. 1(d), and the leading edge of the blade is bent in the skimm, keeping the angle and thickness of the front end unchanged.
A total of 8 different leaf types of shunt blades, including forward sweep and backsweep, were named case__SF5, case__SF10, case__SF15 and case__SF20, and sweep was named case_SB5, case_SB10, case_SB15 and case_SB20. The front sweep angle is positive, the back sweep angle is negative, and the design sweep angle range is -20°~20°; The design scheme is exactly the same except for the geometry of the leading edge of the main blade, and the other geometric dimensions and the number of impeller blades.
2. Meshing and numerical methods
The compressor impeller and diffuser grids are generated using NUMECA-Autogrid software, and the first layer grid is spaced 1×10-6m to ensure y+<10.
In order to determine the influence of grid number and grid density on the calculation results, and comprehensively consider the required computing resources, the grid independence of the case_0 was verified, and the calculation results of 700,000, 1.01 million and 1.32 million mesh numbers were compared respectively, as shown in Figure 2, it was found that the results of 1.01 million grids were not much different from 1.32 million grids, so 1.01 million was selected as the number of computing meshes.
Figure 2: Grid independence verification
The specific impeller grid distribution is 26 (circumferential) ×57 (spreading direction) ×95 (flow direction), and 20 layers of grid are arranged in the leaf top gap; The diffuser mesh is distributed as 43 (circumferential) × 57 (spreading) ×91 (flow direction), encrypted near the wall, and the divided mesh is shown in Figure 3.
Figure 3: Centrifugal compressor calculation grid
The numerical simulation calculation uses NUMECA-Fine/Turbo software to solve the Naiver-Stokes equation, and the turbulence model adopts the Spalart-Allmaras equation turbulence model, which is based on the second-order accuracy center difference format. Select a blade channel as the calculation domain and set the periodic boundary condition.
The inlet is axial intake, given a total temperature of 298K and a total pressure of 100kPa; The solid wall is an insulated non-slip boundary; Given the average static pressure at the outlet, the flow field of different working conditions is obtained by changing the boundary conditions of the outlet.
Analysis of calculation results
1. Overall performance analysis
The flow-efficiency (η) characteristic curves of the compressor at different sweep angles of the leading edge of the shunt blade at different design speeds are shown in Figure 4.
Figure 4 Compressor performance characteristic curve
As can be seen from Figure 4, the shunt blade skimming hardly changes the blockage flow of the compressor.
The blocking flow rate is related to the throat area of the impeller, while the leading edge and blade shape of the main blade of the impeller remain unchanged, and the change of grazing angle of the leading edge of the shunt blade has little effect on the laryngeal area, so the blocking flow rate remains basically unchanged.
The near-stall flow rate of the case_0 is 0.3945kg/s, the stall margin is 21.7%, compared with the case_0, the forward sweep of the shunt blade in Figure 4(b) reduces the mass flow near the stall point, the stall boundary is extended, and the working range of the compressor is extended, the smallest case_SF20 mass flow is 0.3869kg/s, the widest working range, the stall margin is 23.1%, and the forward sweep improves the overall performance of the compressor, and the efficiency is case_ SF10 reaches maximum.
The sweep in Figure 4(a) slightly increases the mass flow near the stall point and reduces the compressor operating range. With the increase of α, the flow rate near the stall point of the compressor increases, and the working range decreases.
2. Analysis of the highest efficiency working conditions
The change of grazing angle of the leading edge of the shunt blade affects the overall performance of the compressor, and the difference in performance is most obvious near the highest efficiency working point.
Compared with the case_0, the performance of the impeller at the highest efficiency working point changes with the grazing angle as shown in Figure 5, the pressure ratio of the case_0 at the highest efficiency working point is 6.346, the efficiency is 78.04%, when the α gradually increases from -20°, the compressor efficiency continues to rise, reaching the maximum in the case_SF10, increasing by 0.77% and the pressure ratio by 0.91%; When the α continues to increase, the efficiency decreases and the pressure ratio does not change much.
Figure 5: Variation of point compressor performance with grazing angle
The maximum difference in efficiency between different grazing angles is 1.58%, and the difference in pressure ratio is 3.01%. Backsweep degrades performance and frontsweep improves performance, so case__SB20, case__SF10 and case_0 are selected to represent backswept, forwardswept and non-sweeping types, and their internal flow fields are analyzed in detail.
The distribution of high static pressure coefficient between the main blade and the shunt blade at 90% is shown in Figure 6, and it can be seen from Figure 6(a) that the shunt blade grazing has a certain influence on the static pressure load distribution of the main blade.
Fig. 690% distribution of high static pressure coefficient of leaves
Compared with the case_0, the static pressure coefficient of the pressure surface case_SF10 the middle of the main blade is reduced, the suction surface is almost unchanged, the pressure difference between the two sides of the blade is reduced, and the transverse pressure gradient in the channel is weakened, which helps to prevent the lateral flow of fluid in the channel, reduce the gap leakage loss, and is less likely to cause stalls. However, case_SB20 the static pressure coefficient of the pressure surface increases, the suction surface decreases, and the pressure difference between the two sides of the blade is large, which will increase the gap leakage loss.
In Figure 6(b), the shunt blade skimming changes the axial chord length of the blade top, so the starting position of the static pressure coefficient is different, but the static pressure coefficient on both sides of the front section of the blade does not change much; At the tail of the blade, the static pressure coefficient of the case_SB20 pressure surface and the suction surface are reduced, and the pressurization effect is weakened.
Figure 7 is the static pressure distribution cloud and limit streamline of the suction surface of the main blade, and the distribution diagram shows that the load at the front end of the main blade is almost unchanged.
Fig. 7 Static pressure distribution cloud and limit streamline distribution of main blade suction surface
The middle load of 50% leaf height to the top of the leaf changed significantly, and the middle load of the case_SB20 decreased and the case_SF10 load increased, indicating that the influence of the shunt blade grazing on the main leaf was concentrated at the top of the middle leaf of the blade, which also corresponds to the static pressure coefficient distribution in Figure 7(a).
At the same time, it can be seen from the distribution of the limit streamline that the radial migration improvement effect of the shunt blade on the low-energy fluid at the front end of the main blade is not obvious because the load at the front end of the main blade is almost unchanged.
Fig. 8 shows the relative Mach number distribution at 95% leaf height, and compared with the case_0, the intensity of the oblique shock wave at the front edge of the suction of the main blade of the case_SB20 and case_SF10 is basically unchanged, and the range of the high Mach number region of the case_SB20 increases.
Figure 8 95% leaf height relative to Mach number distribution cloud
In Figure 8(a), due to the sweep back of the shunt blades, case_SB20 the axial chord length of the top of the blades decreases, which cannot better suppress the separation flow in the middle of the channel, and produces a large range of low-velocity areas before the shunt blades, which affects the flow situation;
However, in Figure 8(c), the case_SF10 shunt blade swept forward, and the axial chord length of the blade top increased, which better improved the separation flow in the middle of the channel, suppressed the low-velocity zone on the pressure surface side, and reduced the low-velocity zone on the suction surface side, and the overall flow was better.
Figure 9 is 95% leaf high entropy distribution cloud diagram, for convenience, the flow channel on both sides of the shunt blade is called the left channel and the right channel, from Figure 9 (b) can be seen, case_0 in the front edge of the main blade suction has a small range of entropy increase area, which is the leakage loss caused by the leakage flow of the main blade gap, the leakage flow of the suction surface of the leading edge of the shunt blade is mixed with the leakage flow of the main blade, forming a large range of entropy increase area in the right channel, and the corresponding loss is also large, and the entropy value is the largest at the tail edge of the main blade pressure surface near the impeller exit. The corresponding losses are also the largest.
Figure 9 95% leaf high entropy distribution cloud
Compared with the case_0, the entropy value of the front edge of the case_SB20 suction of the main blade in Fig. 9(a) increased, indicating that the leakage flow increased, the loss intensified, and the entropy value of the right channel decreased, but the left channel had a high entropy increase zone, indicating that part of the leakage flow was mixed with the main stream in the left channel, which further increased the flow loss.
In Figure 9(c), the entropy value of the front edge of the case_SF10 suction of the main blade decreases, the leakage loss decreases, the entropy increase zone of the right channel increases but the entropy value decreases, and the high entropy region at the downstream tail edge decreases, so the total loss decreases.
3. Analysis of blocking flow conditions
The blockage flow is the maximum flow rate in the compressor performance characteristic line, and the best performance under the blockage flow is taken, that is, the calculation result when the average static pressure at the outlet is 400kPa is the blocking flow condition point, then the performance parameters of the compressor in each design scheme are shown in Table 1.
Table 1
Compared with case_0, the inlet flow of case_SB20 and case_SF10 remained basically unchanged, and the case_SB20 performance decreased the most, the efficiency was reduced by 0.568%, and the compression ratio was reduced by 0.209%. case_SF10 efficiency and compression ratio also decreased slightly.
Figure 10 shows the distribution of 95% leaf height relative to Mach number under blockage conditions, and in addition to oblique shock waves, there are strong channel shocks in the flow channel, case_0 Mach number 1.67 before the wave.
Figure 10 95% leaf height relative to Mach number distribution cloud
Compared with the case_0, the shock intensity of the case_SF10 channel in Figure (c) is increased, the Mach number is 1.72, and a weaker shock wave is generated at the front edge of the suction of the shunt blade, so the decrease in performance is mainly due to the increase of shock loss.
However, the channel shock intensity of the case_SB20 in Figure 10(a) is weakened, and the Mach number in front is 1.64, but due to the decrease of the axial chord length of the leaf top, the separation flow in the middle of the channel increases in a wide range, and a large amount of low-energy fluid is generated before the shunt blade, which seriously affects the flow situation and leads to a decrease in performance.
conclusion
The performance of transonic centrifugal compressors with different shunt blade skimming patterns was obtained by numerical method, and the internal flow field was analyzed in detail, and the results showed that the shunt blade skimming almost did not change the blockage flow of the compressor, and had little effect on the throat area of the compressor.
The forward sweep of the shunt blade has a tendency to extend the working range of the compressor, increase the stall margin, and improve the performance. The sweep results in a slight reduction in the compressor's operating range and stall margin, as well as a decrease in performance.
The influence of the shunt blade sweep on the main blade is concentrated in the middle of the blade, and the radial migration improvement effect of the low-energy fluid at the front end is not obvious, and the pressure difference in the middle of the main blade is reduced, which weakens the transverse pressure gradient in the channel, which helps to prevent the lateral flow of the fluid, reduce the gap leakage loss, and is less likely to cause stalls. The sweep increases the pressure difference between the middle sides of the main vane, increasing the leakage loss.
The oblique shock wave of the shunt blade grazing on the suction surface of the main blade has little effect, and the effect on the low-energy fluid in the middle of the channel and its two sides is obvious.
The axial chord length of the top of the blade is increased by sweeping forward of the shunt blade, which better improves the separation flow in the middle of the channel, inhibits the development of low-energy flow mass on the pressure surface side, and weakens the low-energy flow group on the suction surface side, and the overall flow situation is greatly improved.
The backsweep reduces the axial chord length of the top of the blade, although the low-energy flow mass on the suction surface side is greatly improved, but the separation flow in the middle of the channel is greatly increased, and the low-energy flow mass accumulates with the low-energy flow mass on the pressure surface in front of the shunt blade, which aggravates the energy loss.