2.1. Experiment Facilities
The Twin-Impellers Centrifugal Compressor (TICC) has been realized in the laboratory of Fluid Engineering and Energy system (LIFSE) [
11,
12].
Figure 2 illustrates the test bench used to characterize the performance. This test bench is designed in a suction application; the compressor aspirates the air at the room temperature and blows to the atmosphere. The air is aspirated into the system through a tank, on which there is a flat plate with 120 holes drilled with a diameter of 8 mm, these holes are used to regulate the air flow into the system. Then the air passes through a long pipe with a diameter of D = 158.5 mm connects the tank to the compressor. A flow meter, a thermo-couple and four pressure taps are equipped on this pipe to measure the compressor inlet parameters. The Pt100 thermo-couple is located at 20D to measure the inlet temperature. The flow-meter includes a Pitot tube connected to an FC66 manometer with an accuracy
, at 12D, is used to measure the inlet flow rate. Four pressure taps are arranged on the circumferential inlet pipe, at the distance of 3D, and connected to an FC322 manometer with an accuracy 0.5%. This manometer measures the differential pressure between the inlet and the atmospheric pressure. Two Kistler dynamic pressure sensors are located on the pipeline at the inlet and between the impeller’s region with sensitivity
, natural frequency > 100 kHz. These sensors are used to analysis the pressure signal to detect the pressure fluctuation in the system, one of the indications of the surge phenomenon. Three Pt100 thermo-couples are installed on the circumferential outlet pipe, at the distance of 4D from the downstream compressor, to measure the outlet temperature. These thermo-couples have a range of uncertainty of 0.02 °C ÷ 0.04 °C in the range of temperature considered in this experimental study. In the case of measurements, the TICC is driven by two independent electric motors. The speed of the impellers can be changed in order to improve the surge region. Especially, the rotation direction of the first impeller can be changed in the opposite direction.
2.2. Active Control Method to Extend the Operating Range
The active control method is used in this paper to improve the surge area of the compressor thereby extending the operating range. This method is based on the pressure signal analysis data of two dynamic pressure sensors to give a signal to control the rotational speed of each impeller accordingly. The speed of each impeller will be increased or decreased depending on the desired pressure ratio and flow rate. In the surge region of the reference single-impeller compressor, for example, the twin-impellers compressor needs to reduce the upstream impeller speed and increase the downstream impeller speed to maintain the same pressure ratio. As the flow rate decreases, the differential incidence angle in the upstream impeller increases, the flow separation starts in the blade passage, hence the eddies are formed. The reducing upstream impeller speed minimizes the increase in the incidence angle, thereby eliminating the formation of vortex regions, which cause surges phenomenon at low flow rate.
For clearly demonstrating the improved surge region of the novel twin-impellers compressor, a Single-Impeller Centrifugal Compressor (SICC) which has the identical size was selected as the reference compressor. Based on the experimental curve of the reference compressor at the speed selected of 11k rpm, this curve is reconstructed on the novel compressor by controlling its impellers speed. The speed of each impeller is adjusted corresponding to the desired pressure ratio, based on the characteristic curve of the reference compressor.
Figure 3 shows a comparison between the two experimental performance curves, the blue line depicts the single-impeller compressor curve at 11k rpm, the red line represents the reconstructed twin impeller compressor based on the reference compressor one.
The TICC performance curve is constructed with almost the same pressure ratio of SICC one by fixing the downstream impeller speed at 10k rpm, and then the speed of upstream impeller is controlled to obtain the identical pressure ratio. The first point starts with the speeds of the upstream and downstream impeller respectively equal to −12k rpm and 10k rpm in counter-rotating mode. When the mass flow rate diminishes, the first impeller speed is reduced to archive the same pressure ratio of SICC. The Best Efficiency Point (BEP) attains when the speed of each impeller is 10k rpm at the mass flow rate of 0.543 kg/s. This point is shifted slightly to the left than the best performance point of SICC at 0.648 kg/s. The next points at lower mass flow rate, the speed of the upstream impeller is reduced gradually to meet the same performance of the reference compressor. When the performance curve approaches the near surge point of SICC (, kg/s), the speed of the two impellers is controlled simultaneously to maintain the pressure ratio constant.
The Near Surge Point (NSP) is the limit point in counter-rotating mode, where the speed of the upstream and downstream impeller are respectively −6240 rpm and 10,920 rpm. This point is moved slightly to the left compared to the limit point of SICC. When the mass flow rate is further diminished, the upstream impeller speed decreases rapidly to eliminate the instability phenomenon. This speed drops even to zero and then reverses rotation to further eliminate surge phenomenon at lower flows. The compressor works in co-rotating mode with two impellers rotate in the same direction. The basic principle of this method is controlling the incidence angle at the inlet of the impellers by varying the speed of the upstream-impeller. Indeed, the incidence angle increases when the mass flow rate decreases. If it reaches a limit value, the flow separation will take place inside the impeller and create vortex cells causing instability.
Figure 4 depicts the velocity triangles of TICC in the two modes of operating: counter-rotating-rotating mode (
Figure 4a) and co-rotating mode (
Figure 4b). It is clear that the velocity components are also change if the upstream-impeller speed is varied. The relative velocity components in co-rotating mode diminish considerably lead to the change in the
and
compared to the counter-rotating mode. Of course, the enthalpy of the compressor in the co-rotating mode reduce so that the speed of the downstream-impeller needs to increase to compensate the energy loss of the upstream-impeller used to settle the instability phenomenon. In the co-rotating mode, the compressor can operate at the lower flow rate, and the surge limit of the compressor is shifted significantly to the left compared to the reference compressor.
The Extended Surge Point (ESP) is the new limit point where the compressor works at a low mass flow rate of 0.08 kg/s. This means that the surge margin of the compressor has been extended by about 22% toward the lower flow rate. The upstream and downstream impeller speeds are 6240 rpm and 12k rpm, respectively. In order to achieve the maximum extension of operating range, the upstream and downstream impeller speeds are 6240 rpm and 12k rpm respectively, in other words, the upstream impeller speed reaches of 50% downstream impeller speed. Furthermore, the polytropic efficiency of TICC is always higher that SICC, the peak efficiency of TICC reaches while that of SICC is . Besides, the TICC efficiency at the extended region drops rapidly because the temperature rises quickly.
Experimental results demonstrate the remarkable effect of using double impeller in expanding the operating range of centrifugal compressors. However, in order to better understand the flow structure inside the compressor in both counter-rotating and co-rotating modes, numerical simulation studies are conducted and presented in the following section.