Numerical Analysis and Experimental Study of Unsteady Flow Characteristics in an Ultra-Low Specific Speed Centrifugal Pump
Round 1
Reviewer 1 Report
Comments for author File: 
Comments.pdf
Author Response
Dear Editor,
We would like to thank the editors and reviewers for their time and energy in handling our research work. Thank you very much for the reviewers’ useful comments and suggestions on our manuscript. We have meticulously read your comments, and modified the manuscript accordingly. In addition, we also have optimized the content of the manuscript and greatly improved the logic and language quality of the full text. Detailed corrections are listed below point by point.
(1). The note of the average pressure should be lowercase as the one in the Eq.(4).
Response: Thanks to the reviewers for useful comments. We have corrected all similar errors in the manuscript.
(2). The sentence in the section 2.2, “The internal flow field of the centrifugal pump can be considered periodic flow.”, should be corrected as “…can be considered as periodic flow.”
Response: Thanks to the reviewers for useful comments. We have corrected this error and marked it in the manuscript in line 107, as follows: “The internal flow field of the centrifugal pump can be considered as periodic flow.”.
(3). The sentence in the section 3.2, “The calculation results of the SST k-ω turbulence model agrees well with the experimental data compared to other turbulent models.”, should be corrected as “…agree well with…”. Besides, this sentence has an extra full stop.
Response: Thanks to the reviewers for useful comments. We have corrected this error and marked it in the manuscript in line 145, as follows: “The calculation results of the SST k-ω turbulence model agree well with the experimental data compared to other turbulent models.”
(4). The pressure amplitude in Fig.4(b) is the results of the scheme G2? If this is true, the description of Fig.4(b) in the text should be moved behind the sentence “Considering the computational cost, scheme G2 was selected for numerical simulation in this study.”. Because logically, the result of Figure 4 should be used to verify the selection of scheme G2.
Response: Thanks to the reviewers for useful comments. Fig.4(b) is the result of scheme G2, we have changed the order of the sentences and marked in the manuscript in line 168.
(5). Some sentences of the description about Fig.7 are not proper enough, which need to be improved. And the difference between pressure rise coefficient and head coefficient is 0.01 but not 0.1.
Response: Thanks to the reviewers for useful comments. We have corrected this error and marked it in the manuscript in line 209, as follows: “However, the fluctuation of the impeller pressure rise coefficient is 0.01 times of the head coefficient and mainly occurs near the tongue.”
(6). The font size and font of the marks “B1, B2, B3” and “C1, C2, C3” in Fig.11 and Fig.12 should be consistent.
Response: Thanks to the reviewers for useful comments. We have changed the font size in Figure 12(a).
(7). In the description of the Fig.8, the sentences “With the increasing flow rate, pressure of inlet gradually decreases.” and “As the flow rate increases, the pressure at the inlet of the impeller also increases, causing the head to drop.”, is the description here wrong?
Response: Thanks to the reviewers for useful comments. We have corrected this error and marked it in the manuscript in line 225, as follows: “With the increasing flow rate, pressure of inlet gradually increases.”
(8). This manuscript considers flow in side chambers, https://doi.org/10.1115/1.4054138 and https://doi.org/10.1177/09544062211073023 are suggested to be cited.
Response: Thanks to the reviewers for useful comments. We have taken these two pieces of literature as references, as follows:
- Gu, Y, Li, J, Wang, P, et al. An Improved One-Dimensional Flow Model for Side Chambers of Centrifugal Pumps Considering the Blade Slip Factor. Journal of Fluids Eng. 2022; 144(9): 091207.
- Gu, Y, Li, J, Wang, P, et al. A Flow Model for Side Chambers of Centrifugal Pumps Considering Radial Wall Shear Stress. Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science. 2022;236(13).
(9). The sentence in the text of Fig.12, “Assuming that the blade has no thickness, the changes of various hydraulic properties will be monotonically decreasing during T1-T5.”, this description is not applicable to the fluctuation of the head Φ according to Fig.10(a).
Response: Thanks to the reviewers for useful comments. We have changed the description and marked in line 304 of the manuscript, as follows: “Assuming that the blade has no thickness, the changes of various hydraulic properties will be monotonic during T1-T5.”
Author Response File: 
Author Response.docx
Reviewer 2 Report
This is potentially an interesting modelling study of centrifugal pumps, but it seems incomplete. Lots of results are presented without any clear purpose.
Mathematical symbols need to be defined; a nomenclature list would be good. Some symbols seem to be used inconsistently, e.g. Q_des, Q_BEP.
Line 23: Before using the term ‘specific speed’, it should be defined. The physical meaning of ‘ultra-low specific speed’ should be explained too. Is it low flow, high pressure?
Line 72: “high-frequency dynamic pressure sensors” suggests that unsteady pressure measurements were taken, but there’s no evidence of them in the paper apart from figure 4(b). “±0.1% accuracy” is meaningless without explaining what it is relative to, and whether this takes into consideration thermal drift, which can be an issue for high-frequency dynamic pressure sensors.
Fig 3: state that the horizontal lines are the experimental values.
Figure 4(b) might be clearer as a 2D plot with appropriate scaling.
Line 114: “The existence of the second wave is due to the thickness of the blade.” Is there any evidence for this claim?
Are figures 5-8 experimental or simulated? I suspect they are simuolated, but this is not stated. It would be good to see a comparison with experiment, at least for the head coefficient. How is the shaft power coefficient defined? What are W_C1, W_C2 and W_C3?
Figure 10 is far too small and it is not clear what each plot shows. What are SS and HS?
Line 229: what is the balance hole, and why does leakage through it cause low pressure?
Line 231: I see no ‘oval marked area’ on figure 13.
The paper contains numerous contour plots of pressure, turbulent kinetic energy and eddy dissipation rate, but the purpose of or conclusions from them are not clear.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
Reviewer 3 Report
The paper is well structured, it presents the experimental aspects and the numerical simulation, but it does not clearly present, in the conclusions chapter, what is the accuracy of the numerical simulation compared to the experimental results. This would be beneficial for designers, who could make optimizations only through numerical methods, without consuming resources for experimental trials.
Author Response
Thank you for the reviewer’s recognition of our work. Thank you very much for the reviewer’s review of our manuscript in such a detailed manner. You have put forward a series of useful and reasonable comments that will greatly help us improve the manuscript. We have meticulously read your comments, and modified the manuscript accordingly. Detailed corrections are listed below point by point.
Major comments:
The paper is well structured, it presents the experimental aspects and the numerical simulation, but it does not clearly present, in the conclusions chapter, what is the accuracy of the numerical simulation compared to the experimental results. This would be beneficial for designers, who could make optimizations only through numerical methods, without consuming resources for experimental trials.
Response: Thanks for the comments of the reviewers, we have made a lot of changes to the syntax, logical structure and content of the full text. We have added the relative description to the Conclusion section, which is well revised as follows:
To study the transient fluctuation characteristics of ultra-low specific speed cen-trifugal pump, numerical simulation and experimental methods were applied. The un-steady flow pattern is studied by analyzing the hydraulic performance and pressure loss coefficient under different flow rates. According to the characteristics of the internal flow field, the mechanism of the rotor-stator interaction that causes the transient changes is discussed.
- The head of the numerical simulation presents a good agreement with that of the experimental data. And the maximum error of hydraulic head is calculated to be lower than 5% at the selected grid scheme. Thus, the numerical results in this study are fairly reliable.
- The number of cycles of all hydraulic characteristics is equal to the number of blades, and the hydraulic performance shows a similar change trend under different flow rates. But the transient performance of shaft power and head shows different changing trends due to different influencing factors. Most of the hydraulic losses occur inside the impeller, so it is the impeller flow field that determines the level of time averaged hydraulic performance. The hydraulic loss of volute is small, but it determines the fluctuation range of hydraulic performance.
- The hydraulic loss and turbulent dissipation in the impeller mainly occur near the balance hole, so it is less affected by rotor-stator interaction, about 10% of the total fluctuation range. The circumferential velocity and pressure distribution at the volute inlet show obvious changes with time. The effect of rotor-stator interaction on the flow field in the volute is more significant.
- According to the characteristics and causes, the transient hydraulic performance is divided into three stages. In the period of power increase, the rotor-stator interaction is the strongest, causing the increase of shaft power. However, due to the thickness of the blade and the structural form of the trailing edge, the loss in volute and head of pump have non-linear changes. During the power stability period, the rotor-stator interaction has little influence on the impeller, and the continuously improved flow field improves the hydraulic performance. During the power reduction period, the distance between the blade and the tongue is shortened, which makes the blade wake flow have a greater impact on the tongue, resulting in increased hydraulic loss.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
The comments on th eprevious draft have been addressed satisfactorily and the paper is now acceptable, subject to a few minor corrections:
Lines 239:240: reword to "One cycle of the above four external characteristic curves is shown in Figure 10 (a), and five typical blade rotation angles are shown in Figure 10 (b).
Figure 10(a): text is too small to read.
Figure 11: State what HS and SS mean.
Author Response
Please see the attachment.
Author Response File: Author Response.docx
