Research on Energy Dissipation Mechanism of Hump Characteristics Based on Entropy Generation and Coupling Excitation Mechanism of Internal Vortex Structure of Waterjet Pump at Hump Region

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Manuscript has novelty and physical interpretations have been provided. However please add information about quality of the mesh and Y+ value.

Also add information about under relaxation values selected and about pressure velocity coupling selected. 

Author Response

Feb 15, 2025

Mr Long Yun, Ph. D

China National Research Center of Pumps

Jiangsu University

301 XueFu Road, JingKou, Zhenjiang

China, Jiangsu, 212013

 

Dear editor and reviewer,

Thank you for your review of the manuscript.

We are very grateful to the reviewer for his positive opinion on our work. We have carefully followed his/her comments and addressed all queries listed below. All corrections are marked in red in the revised version.

 

Comments 1: Manuscript has novelty and physical interpretations have been provided. However please add information about quality of the mesh and Y+ value.

Also add information about under relaxation values selected and about pressure velocity coupling selected. 

Response :

The Y+ values for each component are shown in the table below. When generating the mesh, the boundary layer was created by controlling the Y+ value of the first-layer mesh. In this case, the first-layer mesh lies within the viscous sublayer, meeting the computational requirements of the SST k-ω turbulence model.

Table 2 Wall Y+ values of each component

Unit	Wall shear	Wall minimum	Wall maximum	Wall average	Component average
Inlet pipe	Pipe-wall	1.083	15.882	4.110	4.151
	Propeller cap	1.168	17.727	6.549
Impeller	Hub	0.038	4.739	2.004	3.504
	Blade	0.051	15.673	4.212
	Wheel Rim	0.131	10.202	3.198
Diffuser	Hub	0.049	7.150	2.075	3.714
	Blade	0.041	10.330	4.035
	Wheel Rim	0.131	10.202	3.937
Outlet pipe	Axis	5.495	12.737	8.000	7.518
	Pipe-Wall	5.323	9.857	7.368

The solver used in this study is CFX. The relaxation factors were kept as default without any modifications, and the pressure-velocity coupling scheme adopted was the second-order upwind scheme.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

It is certainly a very interesting research topic, comprising numerical and experimental studies. However, I feel some lack of objectivity and physical interpretation of the results as well. Let me explain below.

Usually the study of pumps starts from basic definitions of the measured variables (e.g. head, shaft power and efficiency), and the ideal performance of the pump is obtained from the mass, angular momentum and energy balances using the velocity triangles at pump's inlet and outlet.

It is difficult to understand the behaviour of any pump without evaluating its efficiency at different flow rates and rotation speeds on the light of momentum and energy balances. Instead, the authors work with the concept of entropy production, without defining it and without showing how it is related to the efficiency of the pump.

As a second comment, has the observed cavitation during the experiments also been obtained in the numerical study or associated to Cp values? I couldn't see this connection.

This study seems a bit confuse to me. I do not recommend its publication in the present form.

 

Author Response

Feb 15, 2025

Mr Long Yun, Ph. D

China National Research Center of Pumps

Jiangsu University

301 XueFu Road, JingKou, Zhenjiang

China, Jiangsu, 212013

 

Dear editor and reviewer,

Thank you for your review of the manuscript.

We are very grateful to the reviewer for his positive opinion on our work. We have carefully followed his/her comments and addressed all queries listed below. All corrections are marked in red in the revised version.

Comments 1: It is certainly a very interesting research topic, comprising numerical and experimental studies. However, I feel some lack of objectivity and physical interpretation of the results as well. Let me explain below.

Usually the study of pumps starts from basic definitions of the measured variables (e.g. head, shaft power and efficiency), and the ideal performance of the pump is obtained from the mass, angular momentum and energy balances using the velocity triangles at pump's inlet and outlet.

It is difficult to understand the behaviour of any pump without evaluating its efficiency at different flow rates and rotation speeds on the light of momentum and energy balances. Instead, the authors work with the concept of entropy production, without defining it and without showing how it is related to the efficiency of the pump.

As a second comment, has the observed cavitation during the experiments also been obtained in the numerical study or associated to Cp values? I couldn't see this connection.

This study seems a bit confuse to me. I do not recommend its publication in the present form.

Response:

We are thankful to the reviewer for this valuable suggestion. Section 3.1 has been added, with supplementary details on the method and formulas. The pressure coefficient is a dimensionless parameter that measures the difference between the local pressure and the reference pressure. The pressure coefficient varies in different cavitation stages, which also indirectly indicates the relationship between the cavitation phenomenon observed in the experiments and the pressure coefficient. However, in the current work, we have not yet conducted a simulation study on this aspect. The focus of the simulation has been on the entropy generation analysis method.

For details, please refer to the attachment

 

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript desribes a study of the performance of a water-jet pump at two operating regimes in the hump region of the flow-head curve : the peak regime and the valley (or trough)  regime. The comparison of numerical and experimental overall performance is first presented, then the numerical results for details of the flow is shown and finally the high speed photography is used to characterize the effect of cavitation. This is a complete work which deserves publication in jmse. Below are my remarks on details.

Lines 2-3 : charac-teristics

Line 10 : define mixed flow and flow-head curve

Line 22 : Trough means Valley

Line 26 : Keywords: Waterjet

Line 31 : rephrase : are widely were conducted using

Line 35 : sys-tems (Heo at al. , 2016)

Line 47, Figure 2: define EnD and QnD and flow-head curve in caption

Line 56 slope (Yun, 2018)

Same corrections as lines 35, 56 for all references

Line 74, Figure 3 : small characters, difficult to read

Lines 100, 101, Figure 5 : why a booster pump ?

Line 152, Figure 9 : Frozen Rotor means diffuser or guide vanes

Line 166 : , as shown

Line 221 : trough means valley

Line 267 :  of all leaves means blades

Line 309 : conditions of valley and peak operating conditions

Figures 22-23 : difference between falling cloud cavitation and cloud cavitation. What is the unit °, rotation angle ?

Lines 499, 523, 541, 545 : no need of capital letters in these 4 references (4, 13, 18, 20).

Author Response

Feb 15, 2025

Mr Long Yun, Ph. D

China National Research Center of Pumps

Jiangsu University

301 XueFu Road, JingKou, Zhenjiang

China, Jiangsu, 212013

 

Dear editor and reviewer,

Thank you for your review of the manuscript.

We are very grateful to the reviewer for his positive opinion on our work. We have carefully followed his/her comments and addressed all queries listed below. All corrections are marked in red in the revised version.

This manuscript desribes a study of the performance of a water-jet pump at two operating regimes in the hump region of the flow-head curve : the peak regime and the valley (or trough) regime. The comparison of numerical and experimental overall performance is first presented, then the numerical results for details of the flow is shown and finally the high speed photography is used to characterize the effect of cavitation. This is a complete work which deserves publication in jmse. Below are my remarks on details.

Comments 1: Lines 2-3 : charac-teristics

Response: We are thankful to the reviewer for this valuable suggestion. The revisions have been made.

 

Comments 2: Line 10 : define mixed flow and flow-head curve

Response: We are thankful to the reviewer for this valuable suggestion.

Mixed Flow

Mixed flow refers to a type of fluid flow where both axial and radial flow components are present. In pumps, mixed flow occurs when the fluid flows through the impeller in both axial (parallel to the axis of rotation) and radial (perpendicular to the axis of rotation) directions. This flow type is characteristic of certain types of pumps, such as mixed-flow pumps, which are designed to handle a combination of both centrifugal and axial forces. The flow in such pumps has a moderate degree of both axial and radial components, making it different from purely axial flow or purely radial flow.

Flow-Head Curve

The flow-head curve (also known as the pump performance curve) is a graphical representation that shows the relationship between the flow rate (Q) and the head (H) produced by a pump. It typically illustrates how the head changes with varying flow rates and is used to assess the pump’s performance under different operating conditions.

Head (H) refers to the energy per unit weight of the fluid, often expressed in meters or feet, and indicates how much the pump raises the fluid against gravity.

Flow Rate (Q) is the volume of fluid that passes through the pump per unit of time, often expressed in liters per second (L/s) or gallons per minute (GPM).

The curve generally shows:

At low flow rates, the head produced by the pump is high.

At high flow rates, the head typically decreases, reflecting the pump's efficiency at different operating conditions.

Comments 3: Line 22 : Trough means Valley

Response: We are thankful to the reviewer for this valuable suggestion. The entire text has been proofread to ensure the consistent use of the term "Valley."

 

Comments 4: Line 26 : Keywords: Waterjet

Response: We are thankful to the reviewer for this valuable suggestion. The revisions have been made.

 

Comments 5: Line 31 : rephrase : are widely were conducted using

Response: We are thankful to the reviewer for this valuable suggestion. The revisions have been made.

The superior performance of mixed-flow pumps, including high flow capacity, operational efficiency, and resistance to cavitation, makes them a critical component in applications across shipbuilding, agriculture, water management, and power sectors.

 

Comments 6: Line 35 : sys-tems (Heo at al. , 2016)

Response: We are thankful to the reviewer for this valuable suggestion. The revisions have been made.

 

Comments 7: Line 47, Figure 2: define EnD and QnD and flow-head curve in caption

Response: We are thankful to the reviewer for this valuable suggestion. The explanation for the dimensionless number has been added to the text, and the new content is as follows:

The flow-head curve is a graph that describes the relationship between the pump head (H) and the flow rate (Q), typically obtained through experiments or numerical simulations. The unusual bend in the flow-head curve termed the hump characteristics, is illustrated in Fig 2, the horizontal axis represents Q_nD ​, a dimensionless parameter used to characterize the pump's flow performance. It is typically defined as the ratio of the pump flow rate (Q) to a reference flow rate. The vertical axis represents E_nD ​, a dimensionless parameter used to characterize the pump's energy performance. It is usually defined as the ratio of the pump head (H) to a reference energy value.

 

Comments 8: Line 56 slope (Yun, 2018) Same corrections as lines 35, 56 for all references

Response: We are thankful to the reviewer for this valuable suggestion. The suggestion to reduce the self-citation rate has been taken into account, and the reference in this section has been removed. Thank you for your suggestion; the remaining references have also been proofread.

 

Comments9: Line 74, Figure 3 : small characters, difficult to read

Response: We are thankful to the reviewer for this valuable suggestion. The font in the image has been enlarged, and the modified image is shown below.

 

Comments 10: Lines 100, 101, Figure 5 : why a booster pump ?

Response: We are thankful to the reviewer for this valuable suggestion. Reasons for The Role of a Booster Pump

1. Providing Sufficient Inlet Pressure
   A water-jet pump typically relies on the kinetic energy of high-speed water flow to drive the fluid. Therefore, a certain level of inlet pressure is required to ensure that the water jet achieves a sufficiently high velocity to generate the necessary suction effect. If the supply pressure in the experiment or system is insufficient, a booster pump can increase the inlet pressure, allowing the water-jet pump to function properly.
2. Maintaining Stable Flow Conditions
   In the hump characteristics region, the flow may become unstable, and cavitation may even occur. Using a booster pump can regulate the inlet pressure, reducing flow separation and turbulence effects, thereby maintaining more stable flow conditions, improving experimental control, and enhancing data reliability.
3. Enhancing Experimental Flexibility
   In experimental research, a booster pump allows for flexible adjustment of the inlet pressure, enabling the study of a water-jet pump’s performance under different operating conditions. For instance, when investigating the hump characteristics region, researchers can vary the booster pump’s pressure to control the flow rate and pressure entering the water-jet pump, allowing observation of flow characteristics under different conditions.

Comments 11: Line 152, Figure 9 : Frozen Rotor means diffuser or guide vanes

Response: We are thankful to the reviewer for this valuable suggestion. Frozen Rotor refers to the stationary-rotating interface between the impeller and the diffuser. The updated image is shown below.

 

Comments 12: Line 166 : , as shown

Response: We are thankful to the reviewer for this valuable suggestion. The revisions have been made.

 

Comments 13: Line 221 : trough means valley

Response: We are thankful to the reviewer for this valuable suggestion. The entire text has been proofread to ensure the consistent use of the term "Valley."

 

Comments 14: Line 267 : of all leaves means blades

Response: We are thankful to the reviewer for this valuable suggestion. The revisions have been made.

 

Comments 15: Line 309 : conditions of valley and peak operating conditions

Response: We are thankful to the reviewer for this valuable suggestion. The revision has been made. The modified version is:

"To further analyze the pressure distribution and flow characteristics on different blade surfaces and within the flow passage at the conditions of valley and peak operating conditions, the pressure distribution contour maps at different spanwise heights within the impeller and guide vane regions are extracted, as shown in Figure 18."

 

Comments 16: Figures 22-23 : difference between falling cloud cavitation and cloud cavitation. What is the unit °, rotation angle ?

Response: We are thankful to the reviewer for this valuable suggestion. The unit of rotation angle is: degrees.

The difference between cloud cavitation and detached cloud cavitation:

1. Cloud cavitation
   Cloud cavitation refers to the formation of a group of bubbles or a bubble cloud in the fluid. These bubbles are widely distributed in the fluid, typically accompanied by vortices or a sudden drop in local pressure. In this state, the bubbles do not rapidly detach but remain relatively stable.

1. Falling cloud cavitation
   Detached cloud cavitation refers to the detachment of certain bubble groups from cloud cavitation. That is, the originally stable bubble cloud partially separates due to changes in fluid velocity, pressure variations, or local disturbances. After detachment, the bubble group quickly disappears or moves, causing impact on the surrounding area.

 

Comments 17: Lines 499, 523, 541, 545 : no need of capital letters in these 4 references (4, 13, 18, 20).

Response: We are thankful to the reviewer for this valuable suggestion. The incorrect references have been updated.

 

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The paper presents a comparative study on the operation of a mixed-flow pump under unstable conditions, specifically in the region known as the hump region. The experimental study is conducted using a test rig developed by the authors’ research group, which appears to be well-suited for the present investigation. The computational analysis is performed using commercial software, and the methodology is well-documented, providing sufficient details on key aspects such as the turbulence model used, grid resolution, and the minimum size of the wall-adjacent grid, which is critical for turbulence modeling.

The authors claim that their results align well with existing knowledge on the hump instability characteristics of mixed-flow pumps. While the conclusions are largely qualitative, they provide a clear and appropriate summary of the study’s findings.

Overall, the paper contains a satisfactory level of technical detail and is suitable for publication in its current form. However, the authors are encouraged to proofread the manuscript to correct minor grammatical errors.

Author Response

Feb 15, 2025

Mr Long Yun, Ph. D

China National Research Center of Pumps

Jiangsu University

301 XueFu Road, JingKou, Zhenjiang

China, Jiangsu, 212013

 

Dear editor and reviewer,

Thank you for your review of the manuscript.

We are very grateful to the reviewer for his positive opinion on our work. We have carefully followed his/her comments and addressed all queries listed below. All corrections are marked in red in the revised version.

Comments 1: The paper presents a comparative study on the operation of a mixed-flow pump under unstable conditions, specifically in the region known as the hump region. The experimental study is conducted using a test rig developed by the authors’ research group, which appears to be well-suited for the present investigation. The computational analysis is performed using commercial software, and the methodology is well-documented, providing sufficient details on key aspects such as the turbulence model used, grid resolution, and the minimum size of the wall-adjacent grid, which is critical for turbulence modeling.

The authors claim that their results align well with existing knowledge on the hump instability characteristics of mixed-flow pumps. While the conclusions are largely qualitative, they provide a clear and appropriate summary of the study’s findings.

Overall, the paper contains a satisfactory level of technical detail and is suitable for publication in its current form. However, the authors are encouraged to proofread the manuscript to correct minor grammatical errors.

Response : We are thankful to the reviewer for this valuable suggestion. Thank you for your high evaluation of this paper. We have thoroughly proofread the grammar throughout the article. Please refer to the revised version for the specific changes.

Author Response File: Author Response.pdf

Reviewer 5 Report

Comments and Suggestions for Authors

More specific comments are needed per chapter on the following questions:

 

1. Introduction
2. What is the main research question addressed?

Describe how the internal flow structure and energy dissipation mechanism, especially related to entropy generation and vortex dynamics, contribute to the “hump” phenomenon observed in the performance curve of a waterjet pump.

Explain how waterjet pumps sometimes exhibit an unexpected “hump” (a ridge or plateau) on their performance curve (usually a plot of voltage versus flow). Is this hump assumed to be related to specific flow patterns (vortices) and energy losses within the pump, and uses entropy generation as a way to quantify these losses. Is it intended to relate the internal fluid dynamics of the pump to the external performance characteristic (the hump).

 

2. Hydraulic performance testing and numerical simulation methods
3. Do you think the topic is original or relevant to this field?

Yes, the authors believe that the topic is both original and relevant to the field of fluid dynamics and pump engineering.

Is the originality in that the general study of pump performance and internal flow is well established and focusing specifically on the hump phenomenon and its connection to entropy generation and the coupling of vortex structures excitation adds a layer of novelty. Hump is a less common and less understood behavior, and using entropy generation as a tool for analyzing the associated energy losses provides a specific and potentially insightful approach

Understanding and mitigating the hump phenomenon is relevant to improving the efficiency and stability of water jet pumps. Hump can lead to unpredictable performance and potentially limit the operating range of the pump. By understanding the underlying mechanisms, engineers can design pumps that are less susceptible to this problem. Furthermore, using entropy generation as a diagnostic tool can be valuable for optimizing pump design and identifying areas of inefficiency. This has practical implications for various applications where water jet pumps are used.

 

3. Hump characteristics analysis
4. Does it address a specific gap in this field?

Also, explain why this is/is not the case.

Focus on an analysis that is not widely covered in the existing literature. The combination of the three elements: the hump phenomenon, entropy generation, and coupled vortex dynamics suggests a level of detailed analysis that is not widely covered in the existing literature. If the hump phenomenon were fully understood, there would be less need for this type of in-depth research. Similarly, while entropy generation is a useful tool, its specific application to the hump phenomenon in waterjet pumps, especially in relation to the analysis of coupled vortex structures, represents a more focused and original contribution

 

4. Experimental study on cavitation structure in hump region
5. What does it add to the subject area compared to other published material?

The authors should point out in their discussion of other published material:

Combined approach: The combination of focusing on hump, using entropy generation, and analyzing coupled vortex structures is the main strength of the paper. This multi-pronged approach offers a more comprehensive understanding of the problem than studies that consider only one or two of these aspects.

Potential for practical application: By understanding the mechanisms behind hump, the research could lead to practical improvements in pump design. This could include modifications to the impeller, diffuser, or other components to minimize hump and improve overall efficiency and stability. The research provides design guidelines or insights, and this would be a significant advantage over purely descriptive studies.

 

5. Conclusion
6. Are the conclusions consistent with the evidence and arguments presented and do they address the main question posed?

The conclusions should be complementary and consistent with the evidence and arguments mentioned in the title: hump, entropy generation, and vortex dynamics. They should represent a logical progression of the research: observe the hump, analyze energy losses using entropy generation, identify flow structures responsible for these losses, and potentially suggest ways to mitigate the problem.

Author Response

Feb 15, 2025

Mr Long Yun, Ph. D

China National Research Center of Pumps

Jiangsu University

301 XueFu Road, JingKou, Zhenjiang

China, Jiangsu, 212013

 

Dear editor and reviewer,

Thank you for your review of the manuscript.

We are very grateful to the reviewer for his positive opinion on our work. We have carefully followed his/her comments and addressed all queries listed below. All corrections are marked in red in the revised version.

 

Comments 1:

More specific comments are needed per chapter on the following questions:

 

1. Introduction
2. What is the main research question addressed?

Describe how the internal flow structure and energy dissipation mechanism, especially related to entropy generation and vortex dynamics, contribute to the “hump” phenomenon observed in the performance curve of a waterjet pump.

Explain how waterjet pumps sometimes exhibit an unexpected “hump” (a ridge or plateau) on their performance curve (usually a plot of voltage versus flow). Is this hump assumed to be related to specific flow patterns (vortices) and energy losses within the pump, and uses entropy generation as a way to quantify these losses. Is it intended to relate the internal fluid dynamics of the pump to the external performance characteristic (the hump).

 

1. Hydraulic performance testing and numerical simulation methods
2. Do you think the topic is original or relevant to this field?

Yes, the authors believe that the topic is both original and relevant to the field of fluid dynamics and pump engineering.

Is the originality in that the general study of pump performance and internal flow is well established and focusing specifically on the hump phenomenon and its connection to entropy generation and the coupling of vortex structures excitation adds a layer of novelty. Hump is a less common and less understood behavior, and using entropy generation as a tool for analyzing the associated energy losses provides a specific and potentially insightful approach

Understanding and mitigating the hump phenomenon is relevant to improving the efficiency and stability of water jet pumps. Hump can lead to unpredictable performance and potentially limit the operating range of the pump. By understanding the underlying mechanisms, engineers can design pumps that are less susceptible to this problem. Furthermore, using entropy generation as a diagnostic tool can be valuable for optimizing pump design and identifying areas of inefficiency. This has practical implications for various applications where water jet pumps are used.

 

1. Hump characteristics analysis
2. Does it address a specific gap in this field?

Also, explain why this is/is not the case.

Focus on an analysis that is not widely covered in the existing literature. The combination of the three elements: the hump phenomenon, entropy generation, and coupled vortex dynamics suggests a level of detailed analysis that is not widely covered in the existing literature. If the hump phenomenon were fully understood, there would be less need for this type of in-depth research. Similarly, while entropy generation is a useful tool, its specific application to the hump phenomenon in waterjet pumps, especially in relation to the analysis of coupled vortex structures, represents a more focused and original contribution

 

1. Experimental study on cavitation structure in hump region
2. What does it add to the subject area compared to other published material?

The authors should point out in their discussion of other published material:

Combined approach: The combination of focusing on hump, using entropy generation, and analyzing coupled vortex structures is the main strength of the paper. This multi-pronged approach offers a more comprehensive understanding of the problem than studies that consider only one or two of these aspects.

Potential for practical application: By understanding the mechanisms behind hump, the research could lead to practical improvements in pump design. This could include modifications to the impeller, diffuser, or other components to minimize hump and improve overall efficiency and stability. The research provides design guidelines or insights, and this would be a significant advantage over purely descriptive studies.

 

1. Conclusion
2. Are the conclusions consistent with the evidence and arguments presented and do they address the main question posed?

The conclusions should be complementary and consistent with the evidence and arguments mentioned in the title: hump, entropy generation, and vortex dynamics. They should represent a logical progression of the research: observe the hump, analyze energy losses using entropy generation, identify flow structures responsible for these losses, and potentially suggest ways to mitigate the problem.

Response：We are thankful to the reviewer for this valuable suggestion. Thank you very much for your help in the publication of this article. We have made revisions based on your suggestions. Please refer to the revised version for specific changes. Once again, we express our sincere gratitude.

Author Response File: Author Response.pdf