1. Introduction
Due to the high single-stage pressure ratio, compact structure, and low production cost [
1], centrifugal compressors are widely used in engineering machinery, such as turbochargers, air compression systems, and small gas turbines [
2]. With the increasing application of turbochargers in the engineering field, the requirements for the performance of its core component, i.e., the centrifugal compressor, become higher. Centrifugal compressors are usually designed on the premise of a uniform gas inlet, while in practice, due to the space and layout limitations of the supercharger, bent inlet pipes are usually used, resulting in an asymmetrical inlet environment [
3]. The existence of this asymmetric inlet environment can cause a deviation from the design operating conditions, which leads to changes in the performance of the centrifugal compressor [
4,
5].
The early research on asymmetric inlets for compressors was mainly conducted for axial-flow compressors [
6,
7]. With the wide application of centrifugal compressors, investigation into a non-uniform gas inlet has gradually received more attention with the methods of experimental measurements and numerical simulations. The non-uniform flow conditions at the inlet of a centrifugal compressor are usually modelled by using distortion generators in an experiment [
8]. By changing the installation form of the inlet honeycomb to generate asymmetric flow at the inlet of a low-speed centrifugal compressor, Ariga et al. [
9] found that the inlet distortion changed the incidence angle of gas at the inlet of the impeller and exerted unfavorable influences on the efficiency. Dickmann et al. [
10] conducted research on the blade excitation of a centrifugal compressor under the condition of asymmetric inlet flow caused by a bent pipe, and the experimental results showed that the amplitude magnitude of the blade varied with the flow rate. Zemp et al. [
11] and Kammerer and Abhari [
12] also experimentally investigated the non-uniform inlet flow of a centrifugal compressor, it was found that the fluctuation amplitude of the blade load was the largest at the leading edge, and it decreased with the decrease in flow rate. In the relevant experimental study, the distortion generators are usually used to generate the distortion flow field, but this method is different from the distorted flow field in practical application [
13]. Although the experimental methods are important in the studies of centrifugal compressors, they require a long period, high expenditure consumption, and are greatly restricted by a variety of environmental conditions and testing methods [
14].
With the rapid development of the CFD (Computational Fluid Dynamics) method, it has been widely used in the research of rotating machinery due to the advantages of short application period, low implementation cost, and high functionality [
15]. Li et al. [
16,
17,
18] numerically simulated the effect of a 90-degree bend pipe on the performance of centrifugal compressors and found that the distance between the bent pipe outlet and impeller inlet leads to more deterioration on the performance of centrifugal compressors, which provided a reference for the arrangement of the inlet bent pipe for the same type of compressors. Zhang et al. [
19] investigated the performances of the centrifugal compressor with different inlet pipes by changing the length and diameter of the inlet pipe through numerical simulation, with the results indicating that different inlet pipes resulted in different loads at the compressor inlet; therefore, the performance of the centrifugal compressor changed. The research of Kim et al. [
20] showed that the secondary flow intensity of the fluid through the bent pipe was related to the curvature radius of the pipe, the secondary flow intensity showed a trend of decay with the increase in the curvature radius of the bent pipe, and some bent pipes also caused the total pressure distortion of the flow in the pipe. Zhao et al. [
21] adjusted the installation angle of the 90-degree pipe in the circumferential direction. The numerical results showed that the adjustment of the installation angle of the inlet pipe reduced the efficiency of the compressor. It is recommended to avoid the unfavorable direction of the installation angle for the inlet pipe. Wang et al. [
22] carried out experimental and numerical research on the performance changes in centrifugal compressors caused by a 180-degree inlet elbow with different installation angles. The results showed that the degree of the centrifugal compressor’s performance degradation caused by the 180-degree inlet elbow was related to its circumferential installation position, in which the inlet elbow with the circumferential installation angle of 240° led to a pressure ratio decrease of nearly 5% and an efficiency decrease of nearly 7.5% at the near-choke point condition. Wang et al. [
23] numerically investigated the influence of the distortion of air intake on the performance of the compressor by installing a plug-in plate distortion simulator; however, the diversity of the distortion flow between the distortion simulator and the practical operation conditions creates different results. Tong et al. [
24] investigated the distortion of the inlet flow field caused by sharing a single inlet manifold of the twin compressor, and the results showed that the performance of the right compressor was more likely to deteriorate than the left compressor, and it was related to the location of the intake distortion.
However, there is usually an inlet pipe structure with only a single elbow, which was established in the previous research on the non-uniform inlet flow conditions of centrifugal compressors. In practical application, it is very likely that the double elbow or more complex inlet pipe will be adopted for the centrifugal compressor [
21,
22], and the differences and variation rules of the effects between the single elbow and double elbow inlet pipes on the performance of centrifugal compressors are rarely involved in current research. The same type and different intensity of non-uniform flow conditions generally exist in reality but receive less attention in present studies. The research on the flow field of centrifugal compressors is mostly on qualitative analysis, while there is also a lack of quantitative characterization. Moreover, the inlet distortion is often simulated by setting different inlet pressure conditions at the circumferential position in the numerical simulation, which lacks authenticity for the inlet flow distortion caused by the inlet elbow in practical engineering applications [
25]. Therefore, for the centrifugal compressor with different inlet bent pipes in practical applications, the quantitative characterization of the non-uniform flow with different intensities at the inlet of the centrifugal compressor, as well as its influence on the changes in the performance of the centrifugal compressor, still needs to be further investigated.
In this paper, systematic research on the influence of different inlet bent pipes on the performance of a centrifugal compressor was carried out. The inlet distortion in practical engineering applications was considered. By analyzing the flow field changes of the centrifugal compressor with different inlet elbows, the variation laws of influence from the inlet elbows to the performance of the centrifugal compressor were obtained. Firstly, the two different inlet bent pipes, i.e., a single 90-degree bent pipe (p90) and Z-shaped bent pipe (pz), were established, which resulted in the flow distribution of the vortex flow with different intensities. Then, the quantitative characterizations of the non-uniform flow of p90 and pz were acquired and represented by the distortion degree. Finally, the quantitative relationship between the distortion degree and performance reduction degree of the centrifugal compressor was given, which can be used as reference for the selection and optimization of the inlet bent pipes, as well as for the performance prediction of the centrifugal compressor.
5. Conclusions
Based on the Fluent software, the performance of a centrifugal compressor with a straight inlet pipe under different rotational speeds was numerically simulated by solving the Reynolds Average Navier–Stokes equations; the accuracy of the numerical simulation method with the SST k-ω turbulent model was validated by experimental results. Then, the different inlet bent pipes of 90 degrees and Z-shaped bent pipe were established, and the corresponding performance of the centrifugal compressor was obtained based on the validated numerical simulation method. Finally, the flow characteristics at the outlet of inlet pipes as well as the impeller passage were analyzed, respectively. The following main conclusions were drawn:
(1) The performance of the centrifugal compressor with two inlet bent pipes was decreased compared with the straight inlet pipe, while the performance reduction degree of the centrifugal compressor with pz was higher than that of p90. The maximum pressure ratio reduction degree of the centrifugal compressor with p90 and pz reached 3.11% and 6.82%, respectively, and their maximum efficiency reduction degrees were 7.37% and 14.83%.
(2) The total pressure loss coefficient of the inlet pipes increased continuously with the increase in the flow rate. The total pressure loss coefficient of pz was higher than that of p90 under different rotational speed conditions, which reached 0.024 and 0.047 for p90 and pz at the condition with high rotational speed.
(3) The circumferential distortion degree of the plane increased with the increasing flow rate under each rotational speed condition, while that of p90 and pz reached 0.0351 and 0.0479 at the large flow rate condition. The pressure ratio reduction had a power–law relationship with the circumferential distortion degree of the plane, while the efficiency reduction degree had an exponential relationship with it.
(4) The uniform flow characteristics were disturbed after the swirling flow distribution close to symmetric vortex that formed at the outlet of p90 and pz entered the impeller passage. The distortion area of the total pressure and axial velocity at the inlet of the impeller-maintained similarity with the distortion area at the outlet of elbows, which was near 72°–144° for p90, while those of pz were close to 108°–180° and 288°–360°.
(5) Due to the higher distortion degree formed at the outlet of pz, the high turbulence kinetic energy area in the impeller passage with pz was wider, and the non-uniformity in the circumferential direction was stronger, which resulted in more flow loss. The area with high turbulence kinetic energy was similar to the distorted area at the inlet of the impeller.