Numerical Analysis of a Novel Twin-Impeller Centrifugal Compressor
, Amélie Danlos 1,†, Florent Ravelet 1,†, Michael Deligant 1,†, Moises Solis 1,†, Sofiane Khelladi 1,† and Farid Bakir 1,†Round 1
Reviewer 1 Report
In this article, a novel with twin-impellers powered by autonomous systems is presented, which extend the operating range by about 22% toward the lower flow rate. And CFD study is conducted to reveal the mechanisms by analyzing flow morphology and thermodynamic characteristics inside compressor. The writing is fluent and the analyses are rigorous. I recommend publishing this paper in Computation after minor revision.
Three specific comments are as follows:
- The two paragraphs in section 1 (Introduction) is too long for reading friendly. I suggest they should be divided into several paragraphs with clear logical structure.
- The authors explain the reason why twin-impellers with different rotating speed can improve the operating range in section 2.2. I think it’s better if this explain can be shown by velocity triangle.
- Generally, the CFD simulation efficiency is higher than experimental results. But the results shown in Figure 4 is conflicting to this general knowledge. What do authors think about this result?
Author Response
Dear Reviewer ,
First of all, thank you very much for accepting to review our paper. Your comment helps us to improve the quality of our work. We would like to answer your comments as following:
- The two paragraphs in section 1 (Introduction) is too long for reading friendly. I suggest they should be divided into several paragraphs with clear logical structure.
We divide these two paragraphs into 4 paragraphs for better logical structure.
- The authors explain the reason why twin-impellers with different rotating speed can improve the operating range in section 2.2. I think it’s better if this explain can be shown by velocity triangle.
We add the velocity triangle of two operating mode in section 2.2 (figure 4) to better explain the surge improvement. The following phase has been added to this section: “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 β1 and β3 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.”
3. Generally, the CFD simulation efficiency is higher than experimental results. But the results shown in Figure 4 is conflicting to this general knowledge. What do authors think about this result?
The CFD results depend on numerous parameters such as: type of model (simplified model or completed model), quality of mesh, turbulence model, boundary conditions, gap between rotor and casing, etc. In our model, we used a completed model comprised the gap between the rotor and casing. Furthermore, in the region of low-pressure ratio (<1.2) the CFD result is slightly underestimate compared to the experimental data.
We hope that our replies satisfy you. Thank you again for your contribution
Authors
Author Response File: 
Author Response.pdf
Reviewer 2 Report
In this paper an active control method has been introduced with a twin impeller centrifugal compressor in order to expand its operating range; a CFD method has been conducted to analyse the flow structure inside the compressor. The topic is very interesting, with a useful engineering application; the paper is quite well organized with a good quality in the complex. However some suggestions to further improve the quality are reported below.
The introduction is quite poor, you should add some other references on the unstable phenomena that occurs in centrifugal compressors. For example, you should report the existing method to detect the surge; so you could add these refs:
- Cravero, C.; Marsano, D. “Criteria for the stability limit prediction of high-speed centrifugal compressors with vaneless diffuser. Part I: flow structure analysis”, Proceedings of the ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition, Virtual Conference, 21-25 September 2020, ASME paper GT2020-14579.
- Cravero, C.; Marsano, D. “Criteria for the stability limit prediction of high-speed centrifugal compressors with vaneless diffuser. Part II: the development of prediction criteria”, Proceedings of ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition, Virtual Conference, 21-25 September 2020, ASME paper GT2020-14589.
In these papers for the case of centrifugal compressors analyse the phenomena responsible of the unsteadiness (as the tip leakage vortex at low velocity or the counter pressure of the volute at high speed) by using full annulus model, but also simple model with only a single channel (to save computational resources). Moreover some criteria have been developed to predict the instability operating range, these models are accurate and they can be used for the optimization strategy.
The experimental setup has been described and used to validate the numerical model with a good accordance. Also the numerical model with the setup of the mesh and of the models is reported, however is a bit disorganized: the figure are not announced in the text and are not ordinated. Maybe some tables could help the reader as in the case of the boundary conditions to have a better comprehension on the work. Moreover could you report some other geometrical characteristic of the compressor?
The results and analysis section is quite good organized and well described, however the contours must be improved as quality and dimension. The conclusion should better resume the scope of your work that is not too clear.
Author Response
Dear Reviewer,
First of all, thank you very much for accepting to review our paper. Your contribution helps us to improve the quality the paper. Concerning to your comments, we would like to answer as following:
- The introduction is quite poor, you should add some other references on the unstable phenomena that occurs in centrifugal compressors. For example, you should report the existing method to detect the surge; so you could add these refs:
- Cravero, C.; Marsano, D. “Criteria for the stability limit prediction of high-speed centrifugal compressors with vaneless diffuser. Part I: flow structure analysis”, Proceedings of the ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition, Virtual Conference, 21-25 September 2020, ASME paper GT2020-14579.
- Cravero, C.; Marsano, D. “Criteria for the stability limit prediction of high-speed centrifugal compressors with vaneless diffuser. Part II: the development of prediction criteria”, Proceedings of ASME Turbo Expo 2020: Turbomachinery Technical Conference and Exposition, Virtual Conference, 21-25 September 2020, ASME paper GT2020-14589.
In these papers for the case of centrifugal compressors analyse the phenomena responsible of the unsteadiness (as the tip leakage vortex at low velocity or the counter pressure of the volute at high speed) by using full annulus model, but also simple model with only a single channel (to save computational resources). Moreover some criteria have been developed to predict the instability operating range, these models are accurate and they can be used for the optimization strategy.
Your suggestions are very interesting, these paragraphs added in the introduction: “Recently, a new method for detecting the instability phenomenon in a vaned diffuser compressor [9] and vaneless diffuser compressor [10] were introduced. In this method, some criteria have been developed to predict the instability operating range such as the tip leakage vortex at low velocity or the counter pressure of the volute at high speed by using full annulus model or simplified model (single passage). These models are accurate, and they can be used for the optimization strategy in an initial design phase.”
- The experimental setup has been described and used to validate the numerical model with a good accordance. Also the numerical model with the setup of the mesh and of the models is reported, however is a bit disorganized: the figure are not announced in the text and are not ordinated. Maybe some tables could help the reader as in the case of the boundary conditions to have a better comprehension on the work. Moreover could you report some other geometrical characteristic of the compressor?
We add table 4 to summarize the boundary condition of the model. Some geometrical characteristics of the compressor are also described in table 1 and 2.
- The results and analysis section is quite good organized and well described, however the contours must be improved as quality and dimension. The conclusion should better resume the scope of your work that is not too clear.
We add the following phase in the conclusion: ” Experimental results of Twin-Impeller Centrifugal Compressor show the interesting results in instability region when varying the speed of each impeller independently. The surge phenomenon of this compressor can be greatly improved by active control method. This method allows to improve surge margin by about 22% compared to conventional compressors. The CFD simulation has been conducted to further explain the fluid flow characteristics inside compressor”
We hope that our replies satisfy you. Thank you again for your contribution.
Authors,
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
All my requestes and suggestions have been answered and added in the revised paper. Now, the work has the quality for a publication.
