In the operation of a centrifugal compressor of turbocharger, instability phenomena such as rotating stall and surge are induced at a lower flow rate close to the maximum pressure ratio. In this study, the compressed air at the exit of centrifugal compressor was re-circulated and injected to the impeller inlet by using two injection nozzles in order to suppress the surge phenomenon. The most effective circumferential position was examined to reduce the flow rate at the surge inception. Moreover, the influences of the injection on the fluctuating property of the flow field before and after the surge inception were investigated by examining the frequency of static pressure fluctuation on the wall surface and visualizing the compressor wall surface by oil-film visualization technique.
In recent years, the regulations of the exhaust gas and the fuel consumption of automobile become more severe with the years from the viewpoint of global warming prevention. In Europe, the technology termed the supercharged downsizing is developing and regarded as a part for environmental regulations, in which the displacement reduced by the downsizing of engine aiming to minimize its mechanical loss is supplemented by the equipment of a turbocharger for the engine to keep the torque. Therefore, the application of a turbocharger to various types of vehicle is developing rapidly. On the other hand, the centrifugal compressor, which is a main component of turbocharger, possesses higher pressure ratio in a single stage than that of axial one. But, the performance of compressors at lower flow rate is characterized by the occurrence of unsteady flow phenomena such as surge and rotating stall. Such instabilities cause critical operating conditions with strong dynamical loading on the blades and can therefore not be tolerated during compressor operation. The surge may generate enough intense vibration and noise to destroy the whole pipeline system including a compressor. As a result, the stable operation range is inevitably restricted. Therefore, the extension of the stable operation range and the improvement of the supercharging characteristics of a centrifugal compressor are required dramatically. Several investigations [
In this study, the compressed air at the exit of centrifugal compressor was re-circulated and injected to the impeller inlet by using two injection nozzles in order to suppress the surge phenomenon. In addition, the optimum circumferential relative position of two injection nozzles, which most effectively reduced the flow rate for the surge inception, was investigated for the compressor at rotational speed of 40,000 rpm. Moreover, the influences of the injection on the fluctuating property of the flow field before and after the surge inception were investigated by examining the frequency of static pressure fluctuation on the wall surface and visualizing the compressor wall surface by oil-film visualization technique.
The experimental apparatus used in this study is shown in
Then, the remaining air at the exit of the compressor scroll was discharged through the delivery duct. In the following, the case with the injection nozzle is named “Injection” and that without the injection nozzle is named “Normal”. The specifications and the configurations of centrifugal impeller are shown in
| Inlet diameter | D1 | 37.7 |
|---|---|---|
| Outlet diameter | D2 | 52.0 |
| Number of blades (Main + Splitter) | Z | 10 (5 + 5) |
| Inlet blade height | b1 | 20.0 |
| Outlet blade height | b2 | 4.0 |
tongue portion and takes positive value in the impeller rotational direction as shown in
The experiments were performed to find out the optimum circumferential position of the second nozzle for the fixed first one, which is able to most effectively reduce the flow rate of surge inception. The rotational speed of the tested impeller was 40,000 rpm. In this study, the surge inception was detected by the observation of the pressure fluctuation caused by the surge, which was suddenly increased by gradually closing the valve by its minimum revolution. The corrected flow rate G and the pressure ratio πt were defined by the following equations:
G = G 0 P a 0 P a T 1 T 0 [ kg / s ] (1)
π t = P t P a (2)
where G0 is the measured flow rate, Pa0 is the standard atmospheric pressure, Pa is the measured atmospheric pressure, T1 is the measured temperature, T0 is the standard temperature, and Pt is the measured total pressure at compressor outlet. In order to investigate the unstable phenomena, the wall static pressure fluctuation was measured at 50 mm downstream of the compressor exit. The frequency characteristics of the wall static pressure fluctuation were analyzed by performing FFT. Flow sensor was installed at the bypass pipe to measure the flow rate ejected from the injection nozzle. The oil flow visualizations were performed at the shroud and the hub side walls in the diffuser by using the oil mixture with titanium oxide.
Normal and Injection at rotational speeds of 40,000 rpm.
In order to assess the effect of injection on the reduction of critical flow rate for surge inception and on the increase of operating range of the compressor, the improvement rate of surge margin IRSM and the improvement rate of operating range IROR were defined by the following equations, respectively:
I R S M = G N − G I G N × 100 [ % ] (3)
I R O R = G I max − G I G N max − G N × 100 [ % ] (4)
where GN and GI mean the critical flow rate for surge inception in Normal and Injection, respectively, and GNmax and GImax are for the maximum flow rate.
indicates the injection position. The measurements for IP(0,+30), IP(+30, +30), IP(+60, +30), IP(+180, −135), IP(−150, −135), IP(−120, −135) and IP(−90, −135) could not be performed because the holders of two injection nozzles interfered with each other at these region of injection positions. The IRSM exhibits the improvement rate of about 40% for every IP. The effect of the difference in the position of the fixed nozzle on IRSM was not observed. In the previous study [
In order to evaluate the ratio of the injected mass flow rate GR to that discharged
from the delivery duct G, the re-circulation ratio RRI was defined by the following equation:
R R I = G R G × 100 [ % ] (5)
In order to examine the effects of the injection on the steady flow field in the diffuser, the oil flow visualizations were also conducted on the shroud and the hub side walls in the diffuser. The visualization results of the critical flow rate for surge inception (G = 0.00245 kg/s) of Normal are shown in
and the impeller counter-rotational directions are observed for Normal. This phenomenon indicates the appearance of the reverse flow over the whole shroud and hub side wall surfaces in the diffuser. On the other hand, the undisturbed oil painting region where the shearing force of the flow hardly influences is observed for Injection, which is surrounded by the solid line in each
The appearance of this undisturbed region for Injection is considered to suggest the suppression of the reversed flow by the interaction with the injected flow. The circumferential position of the undisturbed region migrates depending to the injection position IP as shown in Figures 10-15. Moreover, the total areas of the undisturbed region are almost same for every IP. Although the undisturbed region slightly appears at the hub side for each Injection, the radial component of the reversed flow is reduced compared with that in Normal. Therefore, it is considered that the injection on the inner wall of the suction pipe of the compressor would reduce the reversed flow region distributed on the hub side wall from the diffuser exit up to the impeller inlet which appears before the surge inception, and there is the relationship between the no changes in the improvement rate of surge margin due to the difference of the fixed nozzle position and in the area of undisturbed region. However, the behavior of the flow in the diffuser is still unclear whether the undisturbed region for Injection of the oil flow visualization is formed. This phenomena will be investigated more detail in the near future.
The following conclusions were obtained by the present study:
1) The present double nozzle injection system extends the stable operating range of the compressor without significant decrease in the maximum flow rate as well as in the total pressure ratio.
2) The improvement rates of surge margin of the tested compressor were not influenced by the circumferential position of the second injection nozzle because of the relatively low critical flow rate for the surge inception at the condition without the injection.
3) The independency of the improvement rate of surge margin on the circumferential position of the first injection nozzle would be related to the independency of the circumferential position of the second injection nozzle.
Hirano, T., Ogawa, T. and Tsujita, H. (2017) Effect of Double Air Injection on Instability Phenomena in Centrifugal Compressor. Open Journal of Fluid Dynamics, 7, 462-474. https://doi.org/10.4236/ojfd.2017.73031