1. Introduction
Centrifugal compressors are used in various fields such as power generation, chemistry, and environmental plants, and therefore compressors with varying performance (pressure and flow rate) are required [
1,
2]. Designing the compressors required for each field requires the efforts and time of engineers [
3,
4,
5] and significant investments by compressor manufacturers [
6,
7]. Therefore, it is desirable to design a new compressor by modifying the impeller for a compressor that has already been designed and proven in performance. It is also necessary to design compressors as a group within a specific flow range by combining impeller modification methods.
There are two advantages to designing the impeller as a group within a certain range. The first is that compressors with the performance required for each field be manufactured quickly and at a low cost, and the design success rate can be increased. This is because only the impeller is modified in the design of compressors with different performances, so the casings, shafts, and bearings of proven compressors can be used. Second, all compressors can be operated stably in the operating range with a sufficient surge margin because the impeller modification method can shift the performance curve to match the operating range of the compressors required for each field.
The most popular impeller modification methods are the flow cut and axial trim methods [
8,
9]. The flow cut method reduces the flow rate by decreasing the impeller passage height. When this method is applied, the performance curve is shifted in the direction of decreasing flow rate. The axial trim method decreases the impeller passage height in the axial direction, which decreases impeller exit width (B2) and the total pressure. This method is generally used to reduce the total pressure in impellers with a backswept angle. In addition, the flow lift and axial lift methods increase the impeller passage height, unlike the flow cut and axial trim methods. When the flow lift method is applied, the flow rate is increased because the impeller passage height is increased, and when the axial lift is applied, the total pressure is increased because B2 is increased. However, when the flow lift and axial lift methods are applied to the impeller, the size and weight of the aerodynamic components of the compressor are increased, and more power is required. Therefore, these methods are not generally applicable because it is necessary to redesign the casings, shafts, and bearings and reselect the motor.
The impeller modification methods such as the flow cut and axial trim methods have long been used by most centrifugal compressor manufacturers to design new compressors, and in 2001, Rogers first introduced the flow cut method in the paper [
10]. Rogers recommended that the flow cut method should be applied only at a nondimensional specific speed (Ns) between 0.5 and 1.2. Tim David (2006) and Donghui Zhang (2010) applied the flow cut method by up to 75% (a flow fraction of 0.25 compared to the design flow rate of the base compressor) to the impeller of an industrial compressor with a nondimensional specific speed (Ns) of 0.06. However, Tim David and Donghui Zhang recommended applying only 50% of the flow cut to the impeller [
11,
12]. Daniel Swain (2014) applied the flow cut and axial trim methods to different types of impellers such as a transonic impeller with a relative Mach number between 0.8 and 1.2 at the impeller inlet tip, a closed impeller without tip clearance, and an impeller without a splitter [
13]. For most impellers, the ratio of the reduced flow rate and the reduced passage area was not 1:1 after applying the flow cut method. Daniel Swain also noted that applying the axial trim method increases the relative velocity at the impeller exit and reduces the total pressure. Shin et al. (2015, 2017) designed various industrial compressors using the flow cut and scaling methods. Shin et al. noted that the pressure at the target flow rate was different from the pressure at the design flow rate after applying the flow cut method because the work coefficient changed [
14,
15].
In previous studies, after applying the flow cut method to the impellers, the ratio of the passage area reduction and the flow rate reduction was not 1:1. In most impellers, the performance curve was shifted more to the left, which means that the target flow rate is shifted to near the choke and the relative velocity is increased at the impeller exit of the target flow rate. In the centrifugal impeller with a backswept angle, an increased relative velocity at the impeller exit causes a reduction of the total pressure [
16,
17]. The reason for this reduction is that the increased relative velocity reduces the angle of the absolute velocity (Meridional angle convention). This reduction means that the tangential velocity is reduced and the work coefficient is reduced, which results in a reduction in the total pressure [
18,
19]. Therefore, the relative velocity at the impeller exit must be maintained at the level of the impeller of the base compressor to design an impeller for which the total pressure at the target flow rate is similar to the total pressure at the design flow rate.
Designing an impeller for which the total pressure at the target flow rate is similar to the total pressure at the design flow rate is not easy using the impeller modification method with only a flow cut applied. The reason is that it is very difficult to establish a 1:1 match between the reduced passage area and the reduced flow rate. However, if the axial lift method is used in combination with the flow cut method, this problem can be solved because after the flow cut is applied, the reduced total pressure at the target flow rate can be increased to a total pressure that is similar to the design flow rate by applying the axial lift method.
The main objectives in this study are to analyze the influences of the flow cut and axial lift of the impeller on the aerodynamic performance of a transonic centrifugal compressor and to design transonic impellers by combining these methods. A NASA CC3 transonic centrifugal impeller with a backswept angle of −50° (Meridional angle convention) is selected to carry out this study, as shown in
Figure 1a. The flow cut, axial lift, and combined flow cut and axial lift methods are applied to the impeller, as shown in
Figure 1b–d. Three-dimensional, compressible, steady Navier–Stokes equations were solved to investigate the aerodynamic performance of each impeller.