Axial Turbine Performance Enhancement by Specific Fluid Injection
Round 1
Reviewer 1 Report
In this paper, a new method to improve the performance at partial regimes by specific fluid injection was proposed, and a numerical analyze was conducted to study the influence of different parameters on the overall performance of an axial turbine. In the opinion of this reviewer, the present work is meaningful and interesting in somewhat. I recommend the paper can be considered after major revision for publication in Aerospace. However, there are some comments needed to be considered by the authors to improve the quality of this paper.
[1] A list of acronyms is needed.
[2] The authors need emphasize clearly what are new in this paper, and explain the purpose of this study.
[3] The number of references is too small and the literature review is too scarce. More new references should be added and discussed in the introduction part.
[4] Convergence analysis is missing: how do you know that your numerical results represent approximately the solution of the PDEs and your (unspecified) boundary conditions?
[5] The quality of all the figures should be improved. The background color of the flow field diagram should be pure white background, and the curve should be drawn using special drawing software to make the figures more beautiful.
[6] It should be improved including the main novel of the manuscript.
[7] Make sure then that the Conclusions section defines clearly and unambiguously the novel findings of the paper, and that they are reflected in the Abstract.
[8] The main contribution of this work should be clearly explained in both theory and practice.
[9] How can the effectiveness of this method used by the authors to enhance the performance of the turbine be verified? The article does not have experimental results for verification. Besides, the authors should increase the discussion results compared to other researches in terms of error, computational time, the efficiency of the machine, recovered energy, feasibility of the recovery system, among others.
Comments for author File: 
Comments.pdf
Author Response
Dear Sir/Madam,
Thank you for the time you allocated for our paper. We have understood your remarks and it helped us improve the paper. The following are our responses to your remarks:
Point 1: A list has been added at the end of the paper.
Point 2: The purpose of this study is to further research the novel injection system by determining the influence of the geometrical parameters as well as the influence of the injection system at different partial regimes. The paper has been updated to clearly emphasize this.
Point 3: The use of injection to directly increase the turbine power could not be identified up to this moment, however similar systems for loss reductions have been proposed, thus the paper has been updated to present and discuss these studies.
Point 4: For the convergence criteria 3 parameters were monitored: residuals, mass flow imbalance and the rotor blade torque. The residual target was fixed at 1E-6 reaching at least 1E-4 for each case (with mass flow residuals achieving smaller values). In terms of mass flow imbalance the values were smaller than 0.1% for both stator and rotor volumes at each of the simulated cases. The blade torque represents the final parameter that was monitored, the torque curve flattened the criteria being a variation smaller that 0.2% at 300 iterations.
For the boundary conditions these were set identically for all the studied cases, verification was completed at the nominal point, the parameters at this point being similar with those for which the turbine was designed. The paper has been updated to describe the convergence analysis.
Point 5: The paper has been updated in order to improve the figures quality.
Point 6: The main novel has been updated.
Point 7: The conclusion section has been updated to better present the novel findings of the paper.
Point 8: The work presented in this paper contributes to further characterize the novel performance enhancement method by specific fluid injection. In theory, the results can be used to determine the influence variation for different parameters in order to further develop the method and achieve an optimum configuration. In practice the optimum configuration might not be technically feasible due to manufacturing limitations, thus the final configuration must take into account these limitations. The paper has been updated to better describe the main contribution.
Point 9: The effectiveness of the injection method could not be verified as the system is in the development phase and has not yet reached experimental testing. The paper has been updated to include a comparison with existing system identified in literature.
Reviewer 2 Report
It cannot be accepted in present form and needs major revision.
1. The numerical method used here should be validated by the test data.
2. The size and quality of the mesh near the wall and the injection holes should be given.
3. For the comparison between the case with injection and the one without injection, do the authors use the same inlet mass flow rate or outlet mass flow rate?
4. Actually, the improper injection would deteriorate the cooling efficiency, but the authors do not consider the cooling influence. The conclusions would be not useful for the real turbine.
5. Where is the injection flow from? The injection flow would have high pressure and it would consume power to be generated. If we consider the power consumption of the injection flow, can the power increasement be achieved?
6. For the injection holes, the inlet temperature and mass flow rate are given as the boundary condition, how to control the injection flow angels?
7.
Author Response
Dear Sir/Madam,
Thank you for the time you allocated for our paper. We have understood your remarks and it helped us improve the paper. The following are our responses to your remarks:
Point 1: The numerical method could not be verified as the system is in the development phase and has not yet reached experimental testing. Only the nominal point could be verified.
Point 2: The general mesh size is 1 mm with 0.5 mm near the walls, 0.2 mm in the vicinity of the injection orifices and 0.05 mm for injection orifices. The maximum values of the y+ parameter are below 150 with an average value of 90. The paper has been update to clarify this.
Point 3: The comparison between the two cases is possible as the mass flow at the rotor outlet is the same. The injection fluid is considered to be extracted from the engine compressor and injected in the turbine vanes, thus the rotor outlet mass flow is the same. This was achieved by using the same outlet boundary condition at the rotor outlet, fixing the outlet mass flow.
The paper has been updated to clarify this.
Point 4: We agree that the interaction between the injection flow and the cooling flow would result in a complex flow field that may deteriorate the cooling efficiency. This interaction was not the purpose of this study but represents and interesting topic and we will try to study in a future work. However, we do not agree with the fact that this system could not be used for a real turbine. Firstly, the system is designed to enhance the turbine performances at partial loads, thus the cooling efficiency is less important due to lower operating temperatures. Secondly, the system is not limited to high pressure ratio turbines (where a cooling system is needed) being most suited to low pressure turbines, where the operating temperatures are low enough, thus a cooling system is not required.
Point 5: The injection fluid is considered to be extracted from the compressor. The case presented in this paper describes a power turbine, thus the turbine inlet pressure is lower, requiring also a low injection pressure. It is true that mechanical energy is necessary to obtain the injection pressures but the comparison between the cases with and without injection suggest that the same mass flow is better used in an injection configuration (97.5% turbine inlet mass flow and 2.5% injection mass flow) than in classical configuration (100% turbine inlet mass flow). For both cases the rotor outlet mass flow being the same.
Point 6: For this paper the injection mass flow is perpendicular to the injection sections. The injection angle could be controlled by modifying the inlet boundary conditions settings, with a desired angle. However the injection angle does not represent a parameter of interest in this paper as the influence of this parameter has been determined in a previous study. The study showed that perpendicular injection determined the best results.
Reviewer 3 Report
The paper describes the effect of an additional fluid injection system on axial turbine performance. The research is the continuation of previous work of the Authors. The results seem to be reasonable but the calculation approach is not described sufficiently. I recommend to publish this work after minor revision according to comments given below:
1) The introduction does not provide a sufficient background. The Authors refers only to two positions [10][11] which are directly related to the topic of the paper.
2) Please add a reference regarding the k–ε turbulence model.
3) Please show in a figure the general view of simulated turbine (as a whole, not only separated elements). Show the distribution of orifices for selected case.
4) Please add mathematical equations to calculate the parameters like Mach number, turbine power increase. Give the relationship between the turbine power increase and coverage degree.
5) Despite some problems of direct comparison between this work and other researchers the Authors should discuss the performance of this system and other reported results.
Author Response
Dear Sir/Madam,
Thank you for the time you allocated for our paper. We have understood your remarks and it helped us improve the paper. The following are our responses to your remarks:
Point 1: The use of injection to directly increase the turbine power could not be identified up to this moment, however similar systems for loss reductions have been proposed, thus the paper has been updated to present and discuss these studies.
Point 2: The paper has been updated to include references regarding the k–ε turbulence model.
Point 3: The paper has been updated to include the turbine assembly view (Figure 2c) and the orifice distribution for selected case (Figure 11).
Point 4: A linear influence of the coverage degree on the power increase has been observed and can be expressed as a first order polynomial. However the power increase is determined by a number of parameter not only the coverage degree. In a future work a simplified analytical model, corrected with a number of empirical parameter, one of which is determined by such a polynomial, will be proposed. This model is not the purpose of the present paper.
Round 2
Reviewer 1 Report
Thanks for the modifications.
Reviewer 2 Report
All comments have been carefully replied and it can be considered for publication.
