Application of a Performance-Improvement Method for Small-Size Axial Flow Turbines

Round 1

Reviewer 1 Report (New Reviewer)

Comments and Suggestions for Authors

The authors conducted a research that how to improve the aerodynamic performance of gas turbines under part load conditions. The topic is interesting, especially wide operating range required.  Before publishment, the followings should be addressed.

1) In abstract, the background should be simplified. Now there are most half lines of abstract. The research method needs be added in it. 

2) In main blade passages, ideal air is used?  In section of Materials and Methods, the authors wrote the ideal air is used in the second paragraph.  Why not gas from combustion?

3) A meridional figure should be used to show overall impression.

4) In Table 1, "k" in "KW" or "Kg/s" should be small letter.

5) What is tip clearance of rotor blades?

6) Fig. 7 is an incomplete display.

7) The author showed the total pressure and velocity in passages under with or without injection. But static pressure is better to show the interaction between main flow and jet flow. So the authors should add some Fig.s and content to show them, such as streamlines, static pressure distribution along blade.

8) The injection angle, injection position and injection velocity (or injection flow rate) are bound to impact on turbine performance improvement. But they are missing.

Author Response

The authors conducted a research that how to improve the aerodynamic performance of gas turbines under part load conditions. The topic is interesting, especially wide operating range required.  Before publishment, the followings should be addressed.

We would like to thank the reviewer for the time dedicated to our paper. We understood the comments and modified the paper accordingly. This helped us improve the quality of the paper.

1) In abstract, the background should be simplified. Now there are most half lines of abstract. The research method needs be added in it. 

The abstract has been updated to simplify the background and add the research methods.

2) In main blade passages, ideal air is used?  In section of Materials and Methods, the authors wrote the ideal air is used in the second paragraph.  Why not gas from combustion.

Yes, as was mentioned in the paper, air was used and considered an ideal gas for these simulations. This hypothesis was used at this stage of research to ease the complexity of the numerical study. The use of combustion gases and its interaction with the injected air is a topic for future research.

3) A meridional figure should be used to show overall impression.

A meridional figure has been considered but the information given by this figure was found not relevant.

4) In Table 1, "k" in "KW" or "Kg/s" should be small letter.

The paper has been updated with these changes.

5) What is tip clearance of rotor blades?

The tip clearance of the rotor blades is 0.25 mm. The paper has been updated to include this information

6) Fig. 7 is an incomplete display.

Figure 7 has been updated.

7) The author showed the total pressure and velocity in passages under with or without injection. But static pressure is better to show the interaction between main flow and jet flow. So the authors should add some Fig.s and content to show them, such as streamlines, static pressure distribution along blade.

Figure 5 was added to present static pressure distribution before and after injection.

8) The injection angle, injection position and injection velocity (or injection flow rate) are bound to impact on turbine performance improvement. But they are missing.

The injection angle is 90 degrees as was mentioned in page 4 (perpendicular injection). The injection position is described by the parameter ”axial distance” which is 0.2 as was mentioned in Table 3. Injection mass flow is also mentioned in Table 3, 2.5% of total mass flow.

 

Reviewer 2 Report (New Reviewer)

Comments and Suggestions for Authors

Nice work and very interesting. However, I have some comments and suggestions. First of all, I would strongly recommend to mention at the very beginning of the paper that the fluid that is injected is air taken from the compressor.

Secondly, please double check table 1. Vane and rotor diameters are mentioned to be shroud diameters. 47.5 mm is very small and from the figures and my experience it shoulf be the hub diameter. I think ~80 mm shroud diameter would be a reasonable value as shroud diameter.

I would also suggest to add a zoom-in view to figure 4. That view should show the vicinity of the injection sections. I think that can support the text. 

In chapter 3.1, page 6, line 180: dethatching is most likely the wrong word. Maybe detaching is better? On page 9, line 233 I think the authors wanted to refer to figure 7 and not figure 8. Please double check it.

Can the authors please go into more detail, why the method is working. One would assume that for a well-designed turbine the mixing losses due to injection should produce a large amount of pressure loss, reducing the turbine output power. The method assumes that the change of flow angle (imporiving inflow condition of the turbine) is that much that the turbine is able to overcome that pressure loss and produce more power. For a well designed turbine the method should reduce the power output in the nominal operating point. Maybe the authors can comment on this in the paper.

Author Response

Nice work and very interesting. However, I have some comments and suggestions.

We would like to thank the reviewer for the time dedicated to our paper. We understood the comments and modified the paper accordingly. This helped us improve the quality of the paper.

First of all, I would strongly recommend to mention at the very beginning of the paper that the fluid that is injected is air taken from the compressor.

The paper has been updated and this is now mentioned in the abstract.

Secondly, please double check table 1. Vane and rotor diameters are mentioned to be shroud diameters. 47.5 mm is very small and from the figures and my experience it shoulf be the hub diameter. I think ~80 mm shroud diameter would be a reasonable value as shroud diameter.

The paper has been updated; the values mentioned in the table are for the radius, not the diameter.

I would also suggest to add a zoom-in view to figure 4. That view should show the vicinity of the injection sections. I think that can support the text. 

A zoom-in view was added in figure 4.

In chapter 3.1, page 6, line 180: dethatching is most likely the wrong word. Maybe detaching is better? On page 9, line 233 I think the authors wanted to refer to figure 7 and not figure 8. Please double check it.

Detaching is the better word.  It is figure 7 not figure 8 (figure 8 in the end as a new figure was added). The paper has been updated.

Can the authors please go into more detail, why the method is working. One would assume that for a well-designed turbine the mixing losses due to injection should produce a large amount of pressure loss, reducing the turbine output power. The method assumes that the change of flow angle (imporiving inflow condition of the turbine) is that much that the turbine is able to overcome that pressure loss and produce more power. For a well designed turbine the method should reduce the power output in the nominal operating point. Maybe the authors can comment on this in the paper.

The method is developed only for use at partial regimes. Regardless of how well the turbine is designed at nominal regimes, at partial loads, due to a fixed geometry, the flow in the turbine passages is not optimal and can be improved. The increase turbine power resulted after injection is caused by the increase vane outlet velocities due to a smaller flow section. The power increase due to higher velocities overcame the losses resulting form fluid interaction in the vicinity of injection sections. From a certain regime onwards the injection system will not lead to a power increase and will most likely lead to a reduction in rotor generated work. This trend can be observed in Figure 7.

The paper has been updated in this regard.

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

Attached, please find my comments

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English language needs a major overhaul.

Author Response

We would like to thank the reviewer for the time dedicated to our paper. We understood the comments and modified the paper accordingly. This helped us improve the quality of the paper.

We consider that the paper is relevant as the method is applied to a different type of turbine a new analysis is performed at engine level.

1. A reference was added for the first paragraph of the introduction. Additional a reference was added for the statements regarding previous work

2. The version of ANSYS CFX has been added. A reference for the turbulence model has been added. The mesh size is specified as being 0.5 mm with lower values near the walls and injection sections.

3. The paper has been updated to improve the text.

4. The performance in this context refers to rotor generated power.

5. The tables were introduced to describe each step of the logical scheme, thus all are considered relevant.

6. The conclusions section has been updated.

Round 2

Reviewer 3 Report (New Reviewer)

Comments and Suggestions for Authors

Attached, please find my comments

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Extensive editing of English language is required

Author Response

We would like to thank the reviewer again for the time dedicated to our paper. We have reviewed the comments and made the necessary changes.

The last paragraph has been rewrite to be supported by the results from the manuscript. The second to last paragraph has been moved to the introduction section.

All suggested language issues has been reviewed and the text modified accordingly.

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

Comments and Suggestions for Authors

1. In the abstract, the following is presented as the main result: “the results obtained show that in all partial modes, the turbine power can be increased.” It is not clear why the increase of power at partial modes is useful? It should be what the consumer needs. We can increase the engine power by increasing fuel consumption.

2. It is not clear how the influence of the diameter of the holes through which the injected air is supplied on pressure losses was taken into account. An analysis of previous publications also did not reveal any information about how the flow in holes is modeled.

3. The purpose of the work is not clearly presented as a test of the previously proposed system on two turbine projects. It is necessary to describe specifically what and why is checked and what are the criteria for these checks. And now it looks something like this: we proposed a method for increasing the power of the turbine and showed its performance by calculation. So, the novelty of the present study is doubtful.

4. As stated in the introduction (lines 54-56), the main difference of the proposed method is to increase the jet velocity. It is clear that if the applied energy is increased, then the power of the turbine will increase. But the main question is what happens to the economy? On partial modes, not additional power is needed, but an increase in efficiency at a given power. And with this it is not clear: by increasing the speed, we increase the losses. In addition, there will be pressure losses of the injected air along the path from the compressor to the turbine, there will be a flow distortion in the interblade channel due to air injection at an angle, there will be losses due to mixing gas and air flows with different impulses and energy. Thus, the utility the study is questionable.

5. It is unclear the contradiction that arises between the information given in lines 54-58 and 144-146. First, it is written that the main feature of the proposed injection system is the minimal impact on the flow. Obviously, for this it is necessary to inject parallel to the flow. And then it is written that only perpendicular injection is considered, since it provides the best performance.

6. The flow in front of the turbine and behind the turbine is non-uniform (along the radius and circumference in each section). In this regard, I would like to understand how the efficiency of the turbine was determined (given, for example, in Table 6). There are many methods for averaging the parameters of inhomogeneous flows (by area, by flow, by enthalpy, by entropy, by momentum, ...). What method did the authors use?

7. Explanations are needed on how the angular position of the line on which the injection holes are located is determined (see Fig. 3). It is written that this line is parallel to the line of critical sections. Why then does the line of critical sections have such a slope?

8. Neither in this article, nor in the previous articles of the authors, the overall thermodynamic effect of injection has been evaluated. For example, in this article, an uncooled turbine is considered. Thus, the use of injected air reduces the work of the cycle. But increasing the efficiency of the turbine increases the operation of the cycle. Which process is dominant? Is there any positive effect at all? If it is, where is the optimum?

General conclusion:

The article illustrates the method considered in previous publications of the authors on two examples; these examples do not differ significantly (in terms of the nature of the flows and emerging phenomena) from the previously considered turbine with a capacity of 1351 kW.

It does not contain new results in either theoretical or practical areas.

The thermodynamic efficiency of the considered method is not substantiated. It may be a subject of research worthy of publication.

The paper is well written and easy to read, but its revision will require a long time. In this way, we propose to present this study as a new paper with much better explanation of its novelty and the utility of the proposed method.

Comments on the Quality of English Language

English is satisfactory

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presented steady state numerical analysis of two small turbines. The simulations were done for the cases with and without injection while the injection method has been presented previously in another publication of the authors showing the applicability of the method for part load regimes.

Although the paper is well structured and written, it does not have enough novelty and contribution to the knowledge, beyond the scope of the previously published articles of the authors. Also, the paper does not present a new insight into the method or flow physics.

Therefore, I strongly recommend perform the suggestions made in the last paragraph of the paper (Line 303-316) as part of this paper and resubmit. 

Here are just a few comments for the authors on the current version.

-Line 103, Stage conditions-->probably it is stage mixing condition.

-Section2.1. and 2.2., Why are the regime 81%-100% for turbine 1 and 85%-100% for turbine 2 not considered in this study?

-Line 230-236, This is just a qualitative statement which needs to be proved by quantitative data. Comparing the averaged velocity at the rotor outlet plane would help.

-Line 261-269, Why power increase is more pronounced at higher regimes? 

-The reference section does not follow a unique style, compare ref.1 and  ref.2.

Comments on the Quality of English Language

The level of English language is acceptable.

There are some typos and grammatical errors, e.g. in line numbers: 9,28,33,168,211,233,253