Study on Transient Characteristics of New-Type Series-Parallel Emergency Drainage Pump During Unexpected Shutdown Process

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This paper developed a new type of high-flow drainage pump capable of operating in series, parallel, and variable-speed modes to meet diverse drainage demands during emergency operations. The results are interesting; we recommend making the following major corrections before publication:

• In the abstract, please add information about the numerical model verification. Please add the information about the operation's design and inlet/outlet boundary conditions.
• In the introduction, the authors listed from 5 to 36 and only described what they did without any explanation about their limitations; please revise. Moreover, some references are not related to your pump type, “high-head turbines” and “nuclear power pump startup transitions.” Different pump types were mentioned without any comparison between their transient behaviors. Please check the citation format through the manuscript “incorrect format line 107 “[25],[26]”.
• Lines 152–153, the authors stated, “Rotational equilibrium exists between the impeller and the surrounding fluid,” while the shutdown system is not in equilibrium. Please explain.
• Please recheck the definition of equation variables through the manuscript.
• In lines 166-167, the authors stated, “Following the unexpected shutdown of the series-parallel pumps, the load torque is 166 assumed to be zero.” However, this assumption is physically inaccurate due to bearing losses and hydraulic load torque; pumped fluid resistance is not zero. Please explain in detail.
• Using the Euler scheme (equ. 2) can be unstable for a stiff transient torque system; also, the authors did not justify the time step size, and there was no error estimation. Please clarify
• Please add more information about the time step size, solver type, convergence criteria, mesh deformation, and temporal scheme order.
• Please justify why the SST k-ω turbulence model was selected over others such as SAS, LES for transient shutdown.
• In line 198 “the height of the first layer of grid was set to 0.01 mm”; however, the authors did not mention the value of y+ (SST requires y+ to be less than one, which is essential for accuracy). Please clarify.
• Why the growth ratio is 1.4, which is considered too large, and the recommended is between 1.15-1.2. Please explain
• The authors stated, “Both Mesh Scheme 4 and Mesh Scheme 3 exhibited negligible changes (less than 1%) in head coefficient and hydraulic 207 efficiency”; however, no check was made on turbulence quantities. Tip vortex structure, no grid stretching and sensitivity analyses of wall near the
• Extensive English language editing and formatting are required.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

• Extensive English language editing and formatting are required.

Author Response

Comments 1: In the abstract, please add information about the numerical model verification. Please add the information about the operation's design and inlet/outlet boundary conditions.

Response 1: Thank you for your comment. The relevant information regarding the verification of the numerical model has been added to the Abstract to clarify the validation process. In addition, the operational design, as well as the inlet and outlet boundary conditions used in the simulations, have been supplemented in the Boundary Conditions section to provide a more complete description of the computational setup.

 

Comments 2: In the introduction, the authors listed from 5 to 36 and only described what they did without any explanation about their limitations; please revise. Moreover, some references are not related to your pump type, “high-head turbines” and “nuclear power pump startup transitions.” Different pump types were mentioned without any comparison between their transient behaviors. Please check the citation format through the manuscript “incorrect format line 107 “[25],[26]”.

Response 2: Thank you for your comment. I have added analyses of the limitations of the citations in line 137.What’s more, I have further screened and streamlined the citations and some pump studies that are not relevant to this research have been deleted. The format error has been corrected.

Comments 3: Lines 152–153, the authors stated, “Rotational equilibrium exists between the impeller and the surrounding fluid,” while the shutdown system is not in equilibrium. Please explain.

Response 3: Thank you for your comment. Here, "rotational equilibrium" refers to a dynamic and stable working state, rather than static balance. In the "rotational equilibrium" state, all parameters of the system - rotational speed, flow rate, head, and pressure distribution - remain constant or only pulsate within a very small range.  Although there is a rotational balance between the impeller and the fluid. In fact, during transient processes, equilibrium is dynamic, but the system as a whole is not in a state of equilibrium.

 

Comments 4: Please recheck the definition of equation variables through the manuscript.

Response 4: Thank you for your comment. We have carefully checked the formula and the definitions of the variables and made the necessary modifications.

 

Comments 5: In lines 166-167, the authors stated, “Following the unexpected shutdown of the series-parallel pumps, the load torque is assumed to be zero.” However, this assumption is physically inaccurate due to bearing losses and hydraulic load torque; pumped fluid resistance is not zero. Please explain in detail.

Response 5: Thank you for your comment. The statement refers to the external driving torque, which becomes zero immediately after the motor is cut off during an unexpected shutdown. This does not mean that the impeller experiences no resistance. The hydraulic load torque and mechanical losses are still present and are automatically computed in CFX through the pressure and shear stresses acting on the impeller surfaces.

 

Comments 6: Using the Euler scheme (equ. 2) can be unstable for a stiff transient torque system; also, the authors did not justify the time step size, and there was no error estimation. Please clarify

Response 6: Thank you for your comment. The Euler scheme in Equation (2) is only a simplified representation of the discretized rotational equilibrium equation. In the actual computation, CFX solves the impeller motion using its fully implicit Rigid Body solver, which is unconditionally stable for stiff torque–speed coupling and avoids the numerical instability associated with explicit Euler schemes. We have added a discussion of the time-step selection and stability considerations in Section 2.2.3.

 

 

Comments 7: Please add more information about the time step size, solver type, convergence criteria, mesh deformation, and temporal scheme order.

Response 7: Thank you for your comment. The relevant information regarding the time-step size, solver type, convergence criteria, mesh deformation, and temporal scheme order has been added to Section 2.2.3.

 

Comments 8: Please justify why the SST k-ω turbulence model was selected over others such as SAS, LES for transient shutdown.

Response 8: Thank you for your comment. The SST k-ω turbulence model was selected because it combines the advantages of the k-ω model in the near-wall region and the k-ε model in the free stream, providing accurate predictions of flow separation and adverse pressure gradients. This is particularly important for the transient shutdown process where flow separation and reverse flow occur. Although scale-resolving simulations such as SAS and LES can capture more detailed turbulent structures, they require significantly more computational resources. Given the complexity of the series-parallel pump system and the long transient time (1 s), the SST k-ω model offers a good compromise between accuracy and computational cost.

 

Comments 9: In line 198 “the height of the first layer of grid was set to 0.01 mm”; however, the authors did not mention the value of y+ (SST requires y+ to be less than one, which is essential for accuracy). Please clarify.

Response 9: Thank you for your comment. We have added the relevant information to the revised manuscript for clarification.

 

Comments 10: Why the growth ratio is 1.4, which is considered too large, and the recommended is between 1.15-1.2. Please explain

Response 10: Thank you for your comment. The growth ratio of 1.4 was selected mainly due to the complex geometry of the full-channel series–parallel pump system. Using a smaller growth ratio (1.15–1.20) would lead to a substantial increase in the total cell count, making the long-duration transient simulation (1 s) computationally prohibitive. Importantly, the near-wall resolution required by the SST k-ω model is controlled primarily by the first-layer thickness, and in this study y+<1is ensured for all blade and hub surfaces. Thus, although the growth ratio is relatively larger, the boundary-layer accuracy is maintained.

 

Comments 11: The authors stated, “Both Mesh Scheme 4 and Mesh Scheme 3 exhibited negligible changes (less than 1%) in head coefficient and hydraulic 207 efficiency”; however, no check was made on turbulence quantities. Tip vortex structure, no grid stretching and sensitivity analyses of near-wall region

Response 11: Thank you for your comment. While a dedicated transient mesh-independence study focusing on turbulence quantities and tip-vortex structures was not performed, the numerical results show good agreement with the experimental measurements in terms of key hydraulic performance. This consistency indicates that the adopted mesh resolution is sufficient for capturing the main flow characteristics relevant to the transient shutdown analysis.

 

Comments 12: Extensive English language editing and formatting are required.

Response 12: Thank you for your comment. We have corrected the English logical expression and format of the entire text.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript addresses an important topic related to transient hydraulic behavior in high-flow drainage pumps operating under series and parallel configurations during unexpected shutdown events. The study is relevant for emergency drainage systems and contributes to understanding the transient performance of pumps using entropy production theory and pressure pulsation analysis. The paper is generally well structured and clearly written, with comprehensive simulation and experimental validation. However, several issues should be addressed to improve the scientific quality and presentation.

My Comments:

• While the topic is relevant, the novelty of the study compared to prior works (e.g., Ren et al. 2023; Yang et al. 2020; Trivedi et al. 2014) is not clearly highlighted. The authors should more explicitly explain what new insights or modeling improvements their work provides beyond existing transient pump studies.
• The manuscript should clarify whether the “new-type” series–parallel pump has structural innovations or whether the novelty lies mainly in the analysis method.
• The CFD modeling section lacks sufficient detail about mesh quality metrics (e.g., y+, skewness, convergence residuals). Please provide quantitative evidence that the grid and time-step independence studies ensure accuracy.
• The boundary conditions and turbulence model selection (SST k–ω) are appropriate, but the justification for using the Rigid Body module should be discussed further — especially how it compares to coupling with dynamic mesh methods in similar studies.
• The experimental setup should include more information about uncertainty propagation and how it affects the validation results.
• Figures 9–14 are informative but would benefit from quantitative comparisons between simulation and experiment (e.g., torque, flow rate, pressure amplitude values).
• The entropy production analysis is interesting; however, it remains largely descriptive. The authors should interpret how entropy generation correlates with specific flow phenomena such as vortex formation or boundary layer separation.
• The pressure pulsation analysis (Figures 16–19) could be enhanced by including Fast Fourier Transform (FFT) spectra for clarity, rather than only wavelet-based representations.
• Many figures (e.g., Figs. 11–15) have very small labels, making them difficult to read. Please improve figure resolution and enlarge fonts.
• Each figure should have a more descriptive caption that clearly identifies what is being compared and under which conditions.
• Correct minor typos (e.g., “series-paral- lel” → “series-parallel”; “volute-type series-parallel mixed-flow pump demonstrates” → “The volute-type series-parallel mixed-flow pump demonstrates”).
• Ensure consistent numbering and spacing in equations (e.g., Eq. (1)–(14)).
• Consider reordering the Results and Discussion section to present performance evolution (speed, flow, torque) before detailed entropy and pressure analyses.

My Recommendations:

• Clearer articulation of novelty and scientific contribution,
• More quantitative comparison between simulations and experiments,
• Improved figure quality and clarity, and
• Enhanced discussion connecting results with physical interpretation.

Comments on the Quality of English Language

• The English is generally good, but several sentences are long and complex. Shorter sentences would improve readability.
• For example, lines 370–383 and 540–554 contain repetitive phrases that could be condensed for clarity.
• Units and symbols (e.g., W/K, rpm, m³/h) should be consistently formatted throughout the text.

Author Response

Comments 1: While the topic is relevant, the novelty of the study compared to prior works (e.g., Ren et al. 2023; Yang et al. 2020; Trivedi et al. 2014) is not clearly highlighted. The authors should more explicitly explain what new insights or modeling improvements their work provides beyond existing transient pump studies.

Response 1: Thank you for your comment. The novelty of this work has been further clarified in the final paragraph of the Introduction. In particular, this study investigates a newly designed series–parallel pump system, whose structural configuration differs from conventional pump layouts reported in previous transient studies.

 

Comments 2: The manuscript should clarify whether the “new-type” series–parallel pump has structural innovations or whether the novelty lies mainly in the analysis method.

Response 2: Thank you for your comment. In this study, the term “new-type” series–parallel pump refers primarily to the unique system configuration and operating mode of the emergency drainage unit, rather than structural innovations of an individual pump. Relevant clarifications have been added to the revised manuscript.

 

Comments 3: The CFD modeling section lacks sufficient detail about mesh quality metrics (e.g., y+, skewness, convergence residuals). Please provide quantitative evidence that the grid and time-step independence studies ensure accuracy.

Response 3: Thank you for your comment. We have added the relevant supplementary information to Section 2.2.3.

 

Comments 4: The experimental setup should include more information about uncertainty propagation and how it affects the validation results.

Response 4: Thank you for your comment. The experimental measurements of head and efficiency were carried out in accordance with standard procedures, and the main sources of measurement uncertainty— including flow rate, pressure, and torque—were kept within the allowable ranges of the test instruments. Although a detailed uncertainty propagation analysis was not performed, the measurement accuracy is sufficient for validating the global hydraulic performance, and the numerical results show good agreement with the experimental data. A brief clarification has been added to the revised manuscript.

 

Comments 5: The boundary conditions and turbulence model selection (SST k–ω) are appropriate, but the justification for using the Rigid Body module should be discussed further — especially how it compares to coupling with dynamic mesh methods in similar studies.

Response 5: Thank you for your comment. The justification for using the Rigid Body module, including its comparison with dynamic mesh methods, has been added and discussed in the revised manuscript.

 

Comments 6: Figures 9–14 are informative but would benefit from quantitative comparisons between simulation and experiment (e.g., torque, flow rate, pressure amplitude values).

Response 6: Thank you for your comment. Experimental measurements of transient torque, instantaneous flow rate, and pressure amplitudes during unexpected shutdown are not currently available for this series–parallel pump system, as capturing such rapid unsteady processes requires specialized test facilities and high-frequency instrumentation. We acknowledge the value of such comparisons, and they will be considered in our future experimental work. In the current study, Figures 9–14 provide detailed quantitative predictions from the numerical model to illustrate the transient evolution of these key parameters.

Comments 7: The entropy production analysis is interesting; however, it remains largely descriptive. The authors should interpret how entropy generation correlates with specific flow phenomena such as vortex formation or boundary layer separation.

Response 7: Thank you for your comment. We have added brief explanations in the revised manuscript to clarify how the regions of high entropy production correspond to key flow phenomena during the transient shutdown.

 

Comments 8: The pressure pulsation analysis (Figures 16–19) could be enhanced by including Fast Fourier Transform (FFT) spectra for clarity, rather than only wavelet-based representations.

Response 8: Thank you for your comment. In this work, wavelet-based time–frequency analysis was selected because the pressure pulsations during transient shutdown exhibit strongly non-stationary characteristics. Wavelet analysis provides good temporal–spectral localization for such rapidly evolving signals. Although FFT is useful for stationary spectra, the wavelet representation already captures the key frequency features needed for the present study. For these reasons, we have retained the wavelet-based analysis.

 

Comments 9: Many figures (e.g., Figs. 11–15) have very small labels, making them difficult to read. Please improve figure resolution and enlarge fonts.

Response 9: Thank you for your comment. We have made improvements to the image resolution and icon size.

 

Comments 10: Each figure should have a more descriptive caption that clearly identifies what is being compared and under which conditions.

Response 10: Thank you for your comment. The figure captions have been revised to provide more detailed and descriptive information, including clearer identification of the compared quantities and the corresponding operating conditions.

 

Comments 11: Correct minor typos (e.g., “series-paral- lel” → “series-parallel”; “volute-type series-parallel mixed-flow pump demonstrates” → “The volute-type series-parallel mixed-flow pump demonstrates”).

Response 11: Thank you for your comment. All identified typos have been corrected in the revised manuscript.

 

Comments 12: Ensure consistent numbering and spacing in equations (e.g., Eq. (1)–(14)).

Response 12: Thank you for your comment. The numbering and spacing of all equations have been checked and corrected to ensure consistency throughout the revised manuscript.

 

Comments 13: Consider reordering the Results and Discussion section to present performance evolution (speed, flow, torque) before detailed entropy and pressure analyses.

Response 13: Thank you for your comment. We appreciate the reviewer’s consideration of result organization. In the present study, the Results and Discussion section was intentionally structured as performance evolution → rotational speed and pressure characteristics → entropy production → pressure pulsation. This sequence follows the physical progression of the transient shutdown process and provides a coherent logical flow from global performance changes to detailed flow-loss and unsteady-pressure mechanisms. Therefore, the original order has been retained.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have responded to the necessary comments, and the work is eligible for publication following English language editing and formatting.

Comments on the Quality of English Language

• English language editing and formatting are required.

Author Response

Comments 1: The authors have responded to the necessary comments, and the work is eligible for publication following English language editing and formatting.

Response 1: Thank you for your comment. The manuscript has been revised accordingly, including English polishing and formatting improvements.