Effect of Blockage Inside Holes on Film Cooling Performance on the Suction Side of a Turbine Guide Vane

Round 1

Reviewer 1 Report

The paper investigates the effect of blockage ratio in film cooling holes caused due to TBC deposition on film cooling effectiveness. The work can be considered for publication with minor corrections

1.  In the introduction section, enough literature were reviewed on effect of blockage ratio on film cooling effectiveness in general. But, as this paper focuses specifically on measuring film cooling effectiveness from the suction side, literature on that front should be mentioned which is missing.
2. How does measuring film cooling effectiveness from the suction side give a better perspective on the effect of blockage ratio when compared to conventional measurements? The explanation is missing
3. In figure 2, photo of wind tunnel setup, test section where the blade is mounted is not clear. It should be indicated. 
4. In figure 3, Test section exit dimension is selected as 2P (Pitch). Why?
5. In discussion section, emphasis is required on effect of blockage ratio on span wise film effectiveness. 

Overall, the paper addresses a specific problem with well designed experiments. Minor revision is required as mentioned above.

 

Author Response

Dear Reviewer,

We wish to express our gratitude for all of your comments. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in the paper. The main corrections in the paper and the responds to your comments are as following:

Point 1: In the introduction section, enough literature were reviewed on effect of blockage ratio on film cooling effectiveness in general. But, as this paper focuses specifically on measuring film cooling effectiveness from the suction side, literature on that front should be mentioned which is missing. 

Response 1: Thank you for pointing out this problem. As per your request, we have added literature about measuring film cooling effectiveness from the suction side to make the introduction section more complete. The contents in quotation marks are additions.

“Due to the flow between real vanes was significantly different from the conventional flat plate, and the suction side is a typical position of the vane. In recent years, the research on film cooling has shifted more and more from conventional flat plates to real vanes, among which a large number of researchers have studied the film cooling on the suction side. The cooling performance of cylindrical holes on the suction side were studied from the beginning [21], and then the compound angle cylindrical holes on the suction surface were further studied [22]. Meanwhile, the research on shaped holes was gradually carried out on the suction side [23]. Liu et al. [24] conducted a comparative study on film cooling performance of multiple rows of cylindrical holes and multiple rows of shaped holes on the suction side. And Gao et al. [25, 26] simultaneously studied the film cooling characteristics of multiple rows of shaped holes and multiple rows of compound angle shaped holes on the suction side. In addition, film cooling researches on the suction side under rotation conditions increase the influence of rotation, which were more consistent with the actual working condition. Li et al. [27-29] carried out studies on the cooling characteristics of multiple rows of holes on the suction side and multiple rows of compound angle holes on the suction side under rotating conditions, as well as the cooling characteristics of cylindrical holes and shaped holes on the suction side under rotating conditions. The above studies about film cooling on the suction side have made great contributions to the development of turbine vanes cooling.” (lines 63~81, page 6)

Point 2: How does measuring film cooling effectiveness from the suction side give a better perspective on the effect of blockage ratio when compared to conventional measurements? The explanation is missing.

Response 2: Thank you for this question. Compared with conventional flat plate measurements, the flow between real vanes was significantly different from that on the flat plate. The most obvious was that the flow velocity on the vane surface was not constant, but changed with position. Therefore, the results of conventional flat plate measurement could not represent the results of vane surface. As for why to choose the suction side of the vane. This was because in the three typical positions of vane suction side, pressure side and leading edge, the film coverage of pressure side and leading edge was very short, and the film coverage of suction side was the longest. Larger coverage was more likely to reflect the subtle effects of blockage. In addition, the cooling area of single-row holes on the suction side was very large. Once blockage occurs, the consequences would be the most serious. Due to the above reasons, the suction side was selected for measurement.

Point 3: In figure 2, photo of wind tunnel setup, test section where the blade is mounted is not clear. It should be indicated.

Response 3: Thank you for pointing out this problem. As per your request, the test section was indicated in figure 2 in the revised manuscript.

 

Figure 2. Photo of wind tunnel setup.

Point 4: In figure 3, Test section exit dimension is selected as 2P (Pitch). Why?

Response 4: Thank you for the question. In figure 3, test section inlet was on the left and the exit was on the right. And the dimensions of them were both selected as 2P. In order to help us explain why the size was 2P, we drew the picture below. The distance between the two vanes was P. As shown in the figure, the test section we designed consists of three vane profiles and two cascade channels. So, the dimensions of test section inlet and exit were 2P.

 

Point 5: In discussion section, emphasis is required on effect of blockage ratio on span wise film effectiveness.

 

Response 5: Thank you for your valuable advice. As per your request, we have add some words in the results and discussion of the revised manuscript to illustrate the effect of blockage ratio on spanwise averaged film effectiveness. The contents in quotation marks are additions.

 

“When the blockage ratio was 0.2, there was only a small decrease of 0.04 and a maximum relative decrease of 17% compared with the case without blockage, and there was almost no decrease beyond 15D. When the blockage ratio was 0.5, the decrease of the averaged film effectiveness became obvious, with a maximum decrease of 0.11 at the near-hole region and the maximum relative decrease of more than 25%.” (lines 266~270, page 10)

 

“At a blockage ratio of 0.5, the decrease at the near-hole region is less than the case of small blowing ratio, the maximum decrease at the region within 10D is 0.1. However, the decrease at the region beyond 10D increases, with the maximum decrease exceeding 0.11 and the maximum relative decrease approaching 35%. At the big blockage ratio of 0.8, the decrease of spanwise averaged film effectiveness is still the most serious.” (lines 295~301, page 11)

Author Response File: Author Response.docx

Reviewer 2 Report

This study deals with the correct implementation of thermal barrier coating deposition through a hydrodynamic study applied to gas turbines. In my opinion the procedure offers good consistency, the fluid motion simulation is well posed, so the performance can be improved in such a way by following this methodology but with well known restrictions. This is a key issue in the design of such type of devices, so authors must explain it in deep focusing on the real fluid effects.

A deeper description of the computational model used is missed and the error treatment (so called accuracy) is not well developed. With all of this into consideration, its contribution could be considered acceptable. The paper can be considered original and updated including the References.

Comments for author File: Comments.pdf

Author Response

Dear Reviewer,

We wish to express our gratitude for all of your comments. Those comments are all valuable and very helpful for revising and improving our paper, as well as the important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. Revised portion are marked in the paper. The main corrections in the paper and the responds to your comments are as following:

Point 1: A more detailed definition for Figure 3 should be provided regarding the feasible areas implemented.

 Response 1: Thank you for your valuable advice. As per your request, the figure 3 was redefined in the revised manuscript.

 

Figure 3. Test section.

Point 2: Figures 6, it must be clarified the number of pressure measurement points selected.

Response 2: Thank you for pointing out this problem. As for why the number of pressure measurement points was selected in this way, the reasons are as follows. Firstly, in order to accurately reflect the pressure distribution on the vane profile, the number of pressure measurement points should not be too little. But at the same time, it should not be too much. Because the pressure measurement points would affect the pressure on the vane profile, and too many pressure measurement points would cause a large deviation between the measured value and the real value. Referring to other scholars' experimental schemes and our previous experimental experience, 27 pressure measurement points was an ideal number. Secondly, in the layout of pressure measurement points, the leading edge were more dense, and the other positions were relatively sparse. This was because on turbine vanes, film holes are most dense at the leading edge, while other positions are relatively sparse. In order to carry out further research on fully cooled vanes by film in the future, this law was adopted in the spatial distribution of pressure measurement points.

Point 3: The data provided in table 1 must be considered the accuracy of the different dimensions shown apart from the information provided in section 2.3.

Response 3: Thank you for your valuable advice. As per your request, we added the accuracy of the data in table 1 in the revised manuscript.

Table 1. Vane cascade parameters.

Parameter	Value
Chord length (L)[m]	0.407 ± 0.0005
Pitch of vane (P)[m]	0.28 ± 0.0005
Vane height [m]	0.3 ± 0.0005
Inlet velocity [m/s]	10 ± 0.05
Inlet Reynolds number, Re	(3 ± 0.012)×105
Inlet turbulence intensity	1%~2%
Inlet angle of mainstream (β1) [°]	90 ± 0.1
Exit angle of mainstream (β2) [°]	18.2 ± 0.1
Mainstream temperature [K]	293 ± 1
Secondary flow temperature [K]	313 ± 1
Density ratio ()	0.936 ± 0.0005
Blowing ratio M	0.7, 1.05, 1.4

Point 4: Sentence in lines 176-179 on page 6 is not clear: “As shown…”. Please clarify this as the final results have a great influence on this statement.

Response 4: Thank you for pointing out this problem. As per your request, we clarified this statement in the revised manuscript.

“As shown in the figure 8 (a), the spanwise coverage of the film is widest without blockage inside the hole. At the hole exit, the vane surface is almost wholly covered by the region of high effectiveness in the spanwise direction, covering about 12D downstream from the hole, and the spanwise coverage width decays slowly.” (lines 195~198, page 7)

Point 5: This section results confuse for the reader. In this case it should be re-written in order to assess properly the objectives of the work developed here with the effective data obtained. Special effort must be done regarding the validation of the models developed here.

Response 5: Thank you for your valuable advice. As per your request, the conclusions were rewritten in the revised manuscript. The new conclusions were as follows:

The distribution of film effectiveness on the suction side of a turbine guide vane was measured by an infrared camera in this study. Film cooling performance degradation trends were analyzed based on variations in film effectiveness across vanes with various blockage ratios.

(1) The coverage of secondary flow and the cooling performance of vanes are both perfect in the case of without blockage. However, the film effectiveness would be reduced and its spanwise inhomogeneity would be increased due to the blockage. This effect was not obvious when the blockage ratio was less than 0.2, but with the increase of the blockage ratio, the cooling performance of the vane deteriorate rapidly because of the flow separation caused by the blockage.

(2) The degradation of spanwise averaged film effectiveness was very little when the blocking ratio was small, but it became serious when the blocking ratio was large. Moreover, the degradation of spanwise averaged film effectiveness was most significant just within 10D, and the attenuation also became serious with the increase of blowing ratio at the region beyond 10D.

(3) The degradation trend of surface averaged film effectiveness due to blockage was the same as that of spanwise averaged film effectiveness. At the blowing ratio of 0.7, the surface averaged film effectiveness under different blockage ratios decreased by 7%, 24% and 60%, respectively.

(4) To ensure the safe operation of the engine, special attention should be paid to the control of blockage inside film holes during the preparation of thermal barrier coating and vanes operation. In addition, the optimal blowing ratio should be reduced as much as possible in the design process thus to avoid the large blow ratio which cooling performance was most obviously affected by the blockage.

Author Response File: Author Response.docx