A Numerical Study on Blade Design and Optimization of a Helium Expander for a Hydrogen Liquefaction Plant
, Jeongmin Seo 1, Mooryong Park 1, Bumseog Choi 1, Junyoung Park 1, Jesung Bang 1, Donghyun Lee 2, Byungock Kim 2, Soowon Kim 1, Youngchul Lim 1 and Adrian Alford 3Round 1
Reviewer 1 Report
The reviewed manuscript is at a good level when it comes to the originality of the presented solutions and formulated final conclusions and will probably meet with the interest of readers, and its results can be used in the design of industrial installations. The manuscript presents numerical research on blade design and optimization of the helium expander for a hydrogen liquefaction plant.
The manuscript has both a clear descriptive form, and numerical study on the design and optimization of the helium expander blade is also clearly presented.
However, the presented manuscript requires some changes, primarily to improve its readability, i.e. additions and corrections should be made to the presented text, especially in Chapter 3 – Expander Design.
Below are selected detailed remarks:
- It is recommended to introduce a list of abbreviations,
- Nomenclature should be significantly expanded to include the quantities appearing in the text, and which are missing in the nomenclature
- Nomenclature lacks units for individual quantities
- It is proposed to move the text (lines 87 – 92) along line 97
- It is proposed to move the text (lines 201 – 211) after Figure 4
- More detailed description is required by the boundary conditions, mixing plane and inlet conditions used.
Author Response
Please see the attachment. Thank you very much.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors presented aerodynamic deign and optimization of the blade of a helium expander for a hydrogen liquefaction plant. The design process of the expander was provided. In addition, the aerodynamic optimization design of the expander for improvement aerodynamic performance was conducted. This paper is recommended to be accepted until the authors response the following comments. Comments: (1) The authors should add the grid independence test of the aerodynamic performance analysis of the designed expander. (2) The authors should explain the small difference between the referenced deign and optimized design.Author Response
Please see the attachment. Thank you very much.
Author Response File: Author Response.pdf
This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.
Round 1
Reviewer 1 Report
The paper deals with the design and optimization of cryogenic expanders for liquefaction processes. Typical calculation steps are described and a common optimization method is applied to optimize certain geometrical parameters. The procedure is in principle comprehensible, but important references to formulas and models used are missing. Furthermore, a critical analysis of the methods used and a differentiation from alternative numerical calculation and optimization methods would have been very valuable for the scientific claim.
Please review the following issues:
- You have 26 items in your reference list but only refer to the first 8 items in your paper. This does not appear to be simply an oversight, but possibly a rough draft of your paper is present here instead of the final version. Please explain.
- You are using commercial software code to analyse the expander efficiency which is fine. However, it would be of good scientific practice not just to mention the options chosen in the used software but to give some details on why the methods are suitable in your case. What are the limits and what are the main assumptions? Just referring to the user manual in ref. 8 is not enough.
- For optimization you apply Box-Behnken design. How do you compute the metamodel, i.e. the regression analysis for the RSM? Is it also a commercial code or did you implement the procedure yourself? If yes, which programming environment do you use?
- Can you provide information on how much the 1.98% increase in efficiency you achieved is worth compared to other research in the field? It is not easy to put this into perspective based on your paper.
Author Response
- You have 26 items in your reference list but only refer to the first 8 items in your paper. This does not appear to be simply an oversight, but possibly a rough draft of your paper is present here instead of the final version. Please explain.
- Answer : As pointed out, references not mentioned in the text have been deleted. The significance and contribution of previous studies mentioned in text are listed in the reference.
- You are using commercial software code to analyse the expander efficiency which is fine. However, it would be of good scientific practice not just to mention the options chosen in the used software but to give some details on why the methods are suitable in your case. What are the limits and what are the main assumptions? Just referring to the user manual in ref. 8 is not enough.
- Answer : Applied turbulence model was additionally introduced in section 3.4 The boundary conditions applied in the calculation are explained in section 3.4
The reason for performing the Steady calculation was explained and its limitations were mentioned.
For the reader’s understanding, section 3.3 is divided into two sections. That is, Section 3.3 three-dimensional shape geometry and Section 3.4 Numerical analysis.
- For optimization you apply Box-Behnken design. How do you compute the metamodel, i.e. the regression analysis for the RSM? Is it also a commercial code or did you implement the procedure yourself? If yes, which programming environment do you use?
- Answer : Section 4 Optimization has been supplemented. In particular, design parameters and metamodels were presented in Table 5.
- Can you provide information on how much the 1.98% increase in efficiency you achieved is worth compared to other research in the field? It is not easy to put this into perspective based on your paper.
- Answer : In a hydrogen liquefaction system, improving component efficiency directly affects the performance and economics of the system.
An efficiency improvement of 1.98% in the expander system can have a significant impact on performance improvement from an overall system perspective.
The expander developed in this study is for the 0.5TPD system, but if the 10TPD class expander to be developed in the future is developed using the optimization technique applied in this paper and the system is operated for a long time, the benefit will be great.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The paper does not contain any new insights or research results. It describes a standard expander design procedure and is hence not of interest for the scientific or engineering community.
Possibly interesting aspects - such as the impact of the real gas model - are not at all adressed.
The study lacks proper information. It is not mentioned which real gas model has been used, nor is there any statement on assumed mechanical losses. Tip speed is limited to 211 m/s without any statement why. The title is misleading, possibly to spark interest. It should be mentioned that the paper is dealing with a helium expander.
The numerical study is not satisfactory. No grid independency study is mentioned. Apart from showing colorful pictures, there is no explanation whatsoever why efficency is improved in the course of the optimization. From the pictures it seems that both pitch/chord ratio and aspect ratio have been changed in the course of the optimization.
In Fig. 1 the compression process (10 -> 1) occurs at decreasing entropy. How can this be achieved?
Author Response
- Possibly interesting aspects - such as the impact of the real gas model - are not at all adressed.
The study lacks proper information. It is not mentioned which real gas model has been used, nor is there any statement on assumed mechanical losses.
- Answer : How to apply real gas properties for expander design was additionally explained in section 3.4. The assumed mechanical losses are the losses in the volute and the losses in the rotor back disk, and related details are further explained in section 3.4.
- Tip speed is limited to 211 m/s without any statement why.
- Answer : In centrifugal turbines design, the relationship between specific speed(Ns) and specific diameter(Ds) is generally Ns•Ds = 2.
In the preliminary design stage, Ns•Ds =2 was set and the design was carried out. So if Ns is determined, then Ds is determined.
Since U_tip has the relation between rotation speed and radius, Ns•Ds=2 is determined, so U_tip is a fixed value.
Related content added to Section 3.1.
- The title is misleading, possibly to spark interest. It should be mentioned that the paper is dealing with a helium expander.
- Answer : The title and keywords have been revised to reflect your comments.
- The numerical study is not satisfactory. No grid independency study is mentioned.
- Answer : For grid independence study, 6 types of grids were checked. In section 3.4, grid independency was added.
- Apart from showing colorful pictures, there is no explanation whatsoever why efficency is improved in the course of the optimization.
- Answer : When the entropy flow field of the original model and the optimized model was compared, it was confirmed that the entropy of the optimized model was smaller than that of the original model.
Further research is needed to find the cause of the entropy difference.
However, in this study, the efficiency was improved through the optimized design of the cryogen expander designed by applying real gas properties, and it was confirmed that the size of entropy was reduced in terms of the flow field.
- From the pictures it seems that both pitch/chord ratio and aspect ratio have been changed in the course of the optimization.
- Answer : During the optimization process, the span of the mid-chord has changed, but the pitch does not.
It seems that there was a difference in scale in the process of expressing the flow field of the original model and the optimized model for the same legend.
- In Fig. 1 the compression process (10 -> 1) occurs at decreasing entropy. How can this be achieved?
- Answer : Figure 1 has been modified to reflect the points you mentioned.
There is an inter cooler inside the compressor. The omitted part in the T-S diagram is indicated by a dotted line.
Author Response File: Author Response.docx
Reviewer 3 Report
The authors try to introduce a design process for increasing the efficiency of the cryogenic expanders. The design parameters were optimized by using the response surface method (RSM). The research is innovative and of high practical value. However, there are still some issues that need to be carefully revised and added to.
- The subject of the article is more specific and a schematic diagram of its operation can be given to help the reader understand it.
- The format of the graphs and tables is not standardised. For example, what are the horizontal and vertical coordinates mean of Figure 2 and what are their units. What is DP meaning in Figure 6?
- The equations or principles of performance prediction need to be added in the text at section 3.3.
- How CFD are done? The computational domain, mesh, turbulence model, and boundary conditions need to be further explained in the text at section 4.
- How to prove the conclusion that the optimized expander’s efficiency increased by 1.98% compared to that of the original model? Test data needs to be provided to verify the credibility of the method.
- Nomenclature part didn’t list all the symbols used in the text.
Author Response
1. The subject of the article is more specific and a schematic diagram of its operation can be given to help the reader understand it.
- Answer : The title has been revised to be more specific in reflection of the comments.
Schematic is added for reader's understanding in Figure 1(a).
2. The format of the graphs and tables is not standardised. For example, what are the horizontal and vertical coordinates mean of Figure 2 and what are their units. What is DP meaning in Figure 6?
-Answer : The horizontal and vertical coordinates of Figure 2 were corrected.
A design point (DP) was added to the nomenclature.
3. The equations or principles of performance prediction need to be added in the text at section 3.3.
-Answer : As you mentioned, the definition of efficiency was explained in sec 3.2 and 4.2.
4. How CFD are done? The computational domain, mesh, turbulence model, and boundary conditions need to be further explained in the text at section 4.
-Answer : The section 3.3 was divided into two sections. In Section 3.4, explanations for the domain, mesh, turbulent model, and BC for numerical analysis have been added.
5. How to prove the conclusion that the optimized expander’s efficiency increased by 1.98% compared to that of the original model? Test data needs to be provided to verify the credibility of the method.
-Answer : This paper is about the optimization of cryogenic expander, and compares the original design result with the optimized design result.
As you mentioned, the proof of the performance difference between the two models will be shown through later experiments.
6. Nomenclature part didn’t list all the symbols used in the text.
-Answer : As mentioned, the nomenclature was modified.
Author Response File: Author Response.docx
Reviewer 4 Report
The authors presented a global optimization exercise for a radial expander. Both the impeller and the meridional contours are allowed to vary in the opt process. With a small number of samples, the authors achieved a nearly 2% improvement for the efficiency. However, it would be better if the power-expansion ratio can also be included in the results for comparison.
Author Response
It would be better if the power-expansion ratio can also be included in the results for comparison.
-Answer : As pointed out, the power-expansion ratio of the original model and the optimized model is shown in fig 17(c).
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Thank you for answering my questions and considering my comments.
