The Reattachment Process of a Lifted Jet Diffusion Flame by Repetitive DC Pulse Discharges

Round 1

Reviewer 1 Report

This paper presents a physical process revealing the enhancement of a lifted diffusion flame with the help of induced corona discharge, supported by experiments. The authors use conditioned PIV with direct flame visualization to assess the impact of using ozone on the flame base attachement.

The manuscript is well written, and the figures are clear to be read and analyzed. My main concern is related to the physics that is highlighted to built the reattachment process, and which is not proven by experiments or any complementary arguments.

Overall, I would recommend the authors to take into account the following comments to improve the quality and the robustness of the manuscript.

Major comments

The introduction lacks of clarity to describe the main motivation of your study. The pitch should be more explicit and related to specific arguments of the literature reported for the reader to clearly appreciate the novelty and originality of the contribution.

You are performing PIV measurements but little details are provided. First,  what kind of seeding particules are you using, and have you checked their influence on the discharge? Second, if you have conducted cold PIV, you are measuring a biased gas velocity (not as close as the flame front). Do you have an idea on the error you obtain?

On p3, Sec. 3.1.1, I do not see any reference to Fig.2 (might be missing or wrong reference).

p4/ Based on Fig. 3, you mention that the corona discharge influence the subsequent flame-edge location. There is no evidence (or it must be discussed)  on the fact that the a corona discharge is not igniting the upstream flow.

p4 and Fig. 2, I do not see the importance of the red stars. As I understood, whatever the PRF values, the flame height is the same (6.5mm) but with a temporal lag. What do you intend to demonstrate here?

p4-p5/ You are comparing the relative flame-base displacement. My main concern is that you have here a diffusion flame and you make a reference to a property of premixed flame.

The correlation between the flame propagating speed (Fig. 4) and the presence of ozone (Fig. 5) could be better highlighted, i.e. comparing in a single plot the ozone concentration and the corresponding flame speed. There are numerous statements that are built on only a visualization of ozone fields, but not supported.

Minor comments:

• Materials and methods
  • p2/ You should clearly state that this is a diffusion flame
  • p2/ Is there any reason for the location of the electrode with regards to the burner rim?
  • p3/ The reference 16 on the cond-PIV might be wrong since I have not found a comprehensive explanation.
  • p3/ By expanding your laser beam for the ozone measurements, you are having a line-of-sight measurement. Knowing that your process is 3D, you are obtaining an spatially integrated information. If so, this should be mentioned.
  • p3/ what is the typical values of ozone absorption in your measurements
• Results
  • p5/ I wonder how the discharge influence the instantaneous flow field? Do you have any evidence with your measurements right after the discharge.
  • p5/ How do you estimate the stoichiometric concentration line in Fig. 5?

 

 

Author Response

The introduction lacks of clarity to describe the main motivation of your study. The pitch should be more explicit and related to specific arguments of the literature reported for the reader to clearly appreciate the novelty and originality of the contribution.

Response: Thank you for reviewer’s comment. In the introduction, we have improved the description and particularly emphasize the novelty and originality of our study. The combustion enhancement by electric and plasma discharges in DC/AC and pulse forms has attracted intensive research attentions and has been studied by previous literatures, especially on the stabilization of lifted flames. It has been reported that during the electric/plasma forced reattachment of a lifted flame, induced corona discharge has been observed and corona induced ozone has also been reported to be beneficial for the flame reattachment. However, the detailed plasma assisted reattachment process and mechanism, and roles of induced corona discharge and corona-induced ozone on the reattachment process are still unclear and undocumented. These are the motivation and purpose of the present study and we have indicated these descriptions by letters in red and blue colors.

 

You are performing PIV measurements but little details are provided. First, what kind of seeding particles are you using, and have you checked their influence on the discharge? Second, if you have conducted cold PIV, you are measuring a biased gas velocity (not as close as the flame front). Do you have an idea on the error you obtain?

Response: Due to the length limit of the manuscript, we have to reduce the measurements discussion to a minimum and details of this experimental setup and related arguments of the current PIV (cond-PIV, conditioned PIV) technique as applied in the present DC pulse forced reattachment experiment were discussed in our previous work, see Ref. 16 in Combust Sci Tech, 191 (2019) 726-744 . Seeding particles used in this study was Titanium dioxide, which is insulating to electron and heat, therefore, it would not affect the discharge phenomenon. In order to accurately determine the gas velocity relative to a rapid-moving flame base, simultaneously employing a shuttered particle image velocimetry (PIV) system conditioned with respect to instantaneous flame base location using a high-speed camera is indispensable. This conditioned PIV (Cond-PIV) measurements was employed in this study. The instantaneous flame propagation speed at the leading-edge, Se, relative to upstream unburned mixture could be estimated by Se=V_g+V_F, where V_g and V_F are respectively the axial component of instantaneous gas velocity (from PIV) and the corresponding lab-coordinate absolute flame speed (from high-speed camera). PIV measurements were performed on the cold gas upstream of the reattaching flame base. This method can precisely measure the flow speed on flame leading edge and the details of this diagnostics could be referred to reference 16. Sec. 2.2., from line 100 to 110, described cond-PIV as well.

 

 

 

 

On p3, Sec. 3.1.1, I do not see any reference to Fig.2 (might be missing or wrong reference).

Response: The reference of Fig. 2 is at the line 176, 177 of p4, Sec. 3.1.2.

 

p4/ Based on Fig. 3, you mention that the corona discharge influence the subsequent flame-edge location. There is no evidence (or it must be discussed) on the fact that the corona discharge is not igniting the upstream flow.

Response: The propane diffusion flame was conducted in this study, and the electrode was positioned outside the burner rim where the electrode did not influence the jet exit flow. We have carefully checked our high-speed images to make sure that corona discharge did not ignite the flame, instead, the reattaching flame moves continuously upstream even between pulse discharge. Since there was no fuel in this area, the light spot near electrode on Fig.3 could be elucidated to be resulted from the air discharge instead of ignition, as can be seen that the light spot does not expand into flame in the following images. Photographs of nozzle on Fig. 3 can confirm that no ignition occur on the upstream.

 

p4 and Fig. 2, I do not see the importance of the red stars. As I understood, whatever the PRF values, the flame height is the same (6.5mm) but with a temporal lag. What do you intend to demonstrate here?

Response: In Fig. 2, the red stars indicate that the sudden drop of flame base heights are coincident with the appearances of corona discharge under various PRF, which implied that the propagating flame edge was interacted with corona discharge when the flame height is less than 6.5 mm. It also implies that when the reattaching flame base comes to certain distance from the electrode, say below 6.5 mm, corona discharge is automatically induced and flame reattachment is also greatly influenced and enhanced.   

 

p4-p5/ You are comparing the relative flame-base displacement. My main concern is that you have here a diffusion flame and you make a reference to a property of premixed flame.

Response: The stabilization mechanism of lifted diffusion flames is triple flame structure and it is generally recognized that the fuel stream and the ambient air are premixed or partially premixed ahead of the lifted flame base. From the references, it appears that the anchoring point of a lifted diffusion flame base located in a region where a stoichiometric composition is attained. Hence, the property of premixed flame could be reasonably referred in this study.

 

The correlation between the flame propagating speed (Fig. 4) and the presence of ozone (Fig. 5) could be better highlighted, i.e. comparing in a single plot the ozone concentration and the corresponding flame speed. There are numerous statements that are built on only a visualization of ozone fields, but not supported.

Response: In Fig. 5, the red dash line is the flame leading-edge location. Limited by the laser energy of equipment, the y-axis resolution in this figure is only about 2mm, so precise correlation between flame propagating speed in Fig. 4 and ozone concentration in Fig. 5 are hardly to be obtained. At this moment,  only “qualitative” investigation of the effect of corona induced ozone on flame stabilization can be claimed. In the future, we will improve equipment to provide higher resolution data for correlation.

 

Minor comments:

Materials and methods

• p2/ You should clearly state that this is a diffusion flame

Response: We state the diffusion flame on Sec. 2.1. line 77.

• p2/ Is there any reason for the location of the electrode with regards to the burner rim?

Response:  Electrode is detached from the burner rim to distinguish the outstanding effect of electric/plasma on the “enhancement/assistance” of the flame reattachment process, and chosen to locate at a radial position below the lifted flame base close to flow shear layer of the fuel jet where it is easy to initiate corona discharge. These descriptions have been added to page 2

• p3/ The reference 16 on the cond-PIV might be wrong since I have not found a comprehensive explanation.

Response: Description of cond-PIV was elaborated on Sec. 2.2., see also response to previous comment on cond-PIV. In order to accurately determine the gas velocity relative to a rapid-moving flame base, simultaneously employing a shuttered particle image velocimetry (PIV) system conditioned with respect to instantaneous flame base location using a high-speed camera is indispensable. This conditioned PIV (Cond-PIV) measurements was employed in this study. The instantaneous flame propagation speed at the leading-edge, Se, relative to upstream unburned mixture could be estimated by Se=V_g+V_F, where V_g and V_F are respectively the axial component of instantaneous gas velocity (from PIV) and the corresponding lab-coordinate absolute flame speed (from high-speed camera).Cond-PIV measures PIV with gas velocity and flame speed instantaneously by high-speed camera. With these data, the flow speed on flame leading-edge can be calculated accurately.

• p3/ By expanding your laser beam for the ozone measurements, you are having a line-of-sight measurement. Knowing that your process is 3D, you are obtaining an spatially integrated information. If so, this should be mentioned.

Response: As you said, our data is integrated information. However, what we concern is the effect of ozone on flame reattachment, so the results are used and discussed only qualitatively as mentioned in Sec. 2.3. from line 120 to 125. We have mentioned this in Sec. 2.3.

• p3/ what is the typical values of ozone absorption in your measurements

Response: According to Beer-Lambert law, described as that the absorbance, A, is proportional to the absorbing material concentration, C, in mole per liter and the path length in the absorption medium, L, in centimeter. It can be represented by the relation:  where I is the intensity of transmitted light, I0 the intensity of incident light, κ the molar absorption coefficient of material for the specific wavelength of incident light, which has a relation with the absorption cross section, σ, and number density of ozone can be expressed as   , As shown in Fig. 5 and 6 of the ozone density contour is typically around the value of 120-220.

Results

• p5/ I wonder how the discharge influence the instantaneous flow field? Do you have any evidence with your measurements right after the discharge.

Response: Before corona discharge appears, electrical magnetic field has on influence on flow field. As flame reattachs to specific height, corona discharge occurs. Electron stream in corona discharge will affect flow field, which is included as  the effect of corona discharge on flame.

• p5/ How do you estimate the stoichiometric concentration line in Fig. 5?

Response: The stoichiometric concentration line of the present fuel jet flow can be either numerically calculated or traced by following the natural reattachment (without electric/plasma discharge) trajectory of lifted diffusion flame when exit velocity is sufficiently reduced. The estimation of stoichiometric concentration line was described on Sec. 3.1.3. , from line 225 to 228.

Reviewer 2 Report

This is a good experimental work. This reviewer offers the following comments for authors’ consideration.

(1) They conclude “DC pulsed discharge of positive polarity with the PRF ranged from 200 Hz to 1500 Hz can effectively reduce the lift-off height, and the flame base can be reattached to the nozzle 320 rim when the PRF is beyond 600 Hz.” This statemen seems an equipment specific. For example, what will happen if the burner diameter changes?   They can use non dimensional number to describe this lift-off phenomena.

(2) The direct proof of the role of ozone on flame reattachment is to measure the concentration of ozone. Have they thought about this possibility?      

Author Response

They conclude “DC pulsed discharge of positive polarity with the PRF ranged from 200 Hz to 1500 Hz can effectively reduce the lift-off height, and the flame base can be reattached to the nozzle rim when the PRF is beyond 600 Hz.” This statemen seems an equipment specific. For example, what will happen if the burner diameter changes?   They can use non dimensional number to describe this lift-off phenomena.

Response: In the present study we concentrate on the interaction between the D. C. pulsed discharge of positive polarity and lifted diffusion flame height, therefore, we did not specifically investigate the influence of burner diameter.  The reason we used single electrode and placed electrode outside the burner rime is just to induced one-side flame base point reattachment and to avoid the effect of burner diameter as usually seen by using axisymmetric lifted flame reattachment. However, the results of PRF may be equipment specific. Therefore, we claim that this is only “qualitative” investigation of the forced reattachment process and mechanism.

 

The direct proof of the role of ozone on flame reattachment is to measure the concentration of ozone. Have they thought about this possibility? 

Response: We agree in general with the reviewer comments, however, the methods to direct measure the concertation of ozone are extremely inconvenient. Due to the short-lived and weak absorption intensity nature of the induced ozone, shot-to-shot quantitative data comparison is infeasible. Instead, the ozone image relative to unpulsed background is used to qualitatively delineate the forced reattachment phenomena and process. The UV absorption method is a trustworthy measurement. However, due to the nature of strong shot-to-shot variation, the very weak S/N, and the integrated information, as well as the difficulty of calibration, quantitative and direct measurement of the ozone concentration is still unfeasible and has not been reported in the literature.

Reviewer 3 Report

It is a nice piece of work that will be of very good interest to combustion community.

Authors investigated the effect of pulse discharge and its repetition frequency PRF on propane flame and evaluated ozone density and flame stabilization mechanism in detail.

I recommend this paper to be accepted after the following minor concerns are addressed.

 

1)

There are some minor typographical errors:

• Line 130: ln[f(x)](I/I_0) --> ln(I/I_0)
• Line 213: 3.1.3.2.D ozone … --> 1.3. 2-D ozone …?
• Line 344: …reattachment.6. Patents -->  …reattachment.
• Line 367: (200\74) --> (2004)?
• etc.

Please proofread the manuscript to correct them.

 

2)

In section 2.1, a plasma pulse is described to have 80µs width with 16kV peaks and authors states that discharge power was monitored in experiments. Since the pulse duration seems to be constant over different PRFs, I assume higher PRF will put more energy to the discharge zone. Is there any effect of the amount of discharge power on flame stabilization/reattachment similar to PRF as described in section 3.1.4 and in Fig.7?

Author Response

There are some minor typographical errors:

• Line 130: ln[f(x)](I/I_0) --> ln(I/I_0)
• Line 213: 3.1.3.2.D ozone … --> 1.3. 2-D ozone …?
• Line 344: …reattachment.6. Patents -->  …reattachment.
• Line 367: (200\74) --> (2004)?
• etc.

Response: Thanks for reviewer’s comment. All typographical errors have been modified and marked in the revised manuscript.

 

In section 2.1, a plasma pulse is described to have 80µs width with 16kV peaks and authors states that discharge power was monitored in experiments. Since the pulse duration seems to be constant over different PRFs, I assume higher PRF will put more energy to the discharge zone. Is there any effect of the amount of discharge power on flame stabilization/reattachment similar to PRF as described in section 3.1.4 and in Fig.7?

Response: We agree in general with the reviewer’s comments. Under same period, more energy was put into the discharge zone with higher PRF. However, the energy provided from each pulse was identical over different RPFs and the variation of flame stabilization location is caused by ozone produced by each corona discharge. Therefore, we did not focus on the effect of the amount of discharge power on flame reattachment processes. For various RPFs, we have measured the instant flame base speed during two pulses. The result showed as same as described in Sec. 3.1.4. In case of two pulses with equal duration, the energy provided from plasma were identical over different RPFs.

Reviewer 4 Report

The authors have investigated an important problem (liftoff) and  in the combustors and a mitigation strategy (electron discharge). How the electron discharge is supporting the reattachment phenomena is discussed. Here are a few comments:

1) It seems the authors submitted the draft along with the "track changes" the journal tool. All their comments and color text is confusing. 

2) Line 27: This statement is not correct. Low carbon fuel may not produce carbon emissions, but it can produce other emissions such as NOx. 

3) Statement in line 32 is not clear to the readers. The authors must describe what is "The effects on combustion process". What are the effects that the authors are describing?

4) The authors must define what is "E/N field" when using first time.

5) Fundamentally, flame dynamics is due to the ratio of flow timescales to the chemical timescales. The reattachment phenomena is  due to  acceleration is chemical timescale as described by the authors. Thus, it would be interesting if the authors discuss on what  specific reactions are causing the kinetic acceleration when interacting with O3.

6) How reactive is the O3 chemically with the fuel compared to O2? 

7) Also, the discharge of electrons can also increase the localized temperature of the gas. So, reattachment may be due to the accelerated kinetics and also due to increase in gas temperature due to electrode. How the authors can separate these two phenomena on the observed reattachment? The authors discussed that the PRF when placed in upstream can not ignite the mixture. But, this can be wrong. At the core of the jet, due to the momentum of the jet, the fluid has very less time to absorb the heat from the discharge plug. On the other hand, at the periphery of the nozzle, the airflow is mainly due to entrainment and velocity is lesser so that it can absorb the heat. 

Though, the experiments presented in the paper is good. I still need the authors to address my review comments to improve the result and discussion section. Thus, I can recommend this to the journal after seeing a satisfactory rebuttal of the authors. 

Author Response

We would like to express our sincere gratitude to the reviewers and the editor for their constructive comments and instructions. We have carefully perused the reviewers’ comments and used a more explicit description in the revised manuscript to improve the article's quality. The added/modified sentences last time (first revision) were presented in “red color”, added contents this time (second revision) were presented in “green color” in the revised manuscript. Sentences in “blue color” are contents related to reviewers’ comments, which do not change in this revised manuscript. “Track change” function will make color some mistakes, so please open the file without “track change”.

The process of plasma assisted flame reattachment studied in this manuscript is a very complicated continuous transition process of flame-flow-electric/plasma interaction. The process and related mechanism is still unclear and have not been studied and undocumented in the literature. Although we have tried to investigate this process using high-speed camera and cond-PIV (PIV measurement conditioned with respect to “instantaneous” flame position and velocity using high-speed camera simultaneously), see Ref. 16 in the manuscript, the results were not very successful and were not satisfactory as the electric/plasma forced flame reattachment is not a steady, axisymmetric process and the 2D planar PIV using laser sheet tends to miss to capture the flame location and the leading flame base point of the reattaching flame. In addition, the velocity measurements cannot fully explain the complicated electric/plasma-flame interaction and cannot delineate the interaction mechanism, which the induced chemical interaction, such as induced Ozone, may involve and contribute to the reattachment process. Therefore, the pitch in this experimental study is that based on previous cond-PIV results we tried to investigate this reattachment process and mechanism by using 2D line-of-sight laser absorption measurement of ozone concentration. Details of cond-PIV has been elaborated in previous publication, reference 16. In this study, we emphasize on the discussion of ozone concentration. Cond-PIV measurement is used to provide indirect supporting evidences of “exceptional” high reattaching flame velocity when corona is induced with high PRF (pulse repetition frequency).

point-by-point responses are as follows:

1) It seems the authors submitted the draft along with the "track changes" the journal tool. All their comments and color text is confusing. 

Answer: We apologize for the confusion of “track changes” of color text. In this revision, the added/modified sentences last time (first revision) were presented in “red color”, added contents this time (second revision) were presented in “green color” in the revised manuscript. Sentences in “blue color” are contents related to reviewers’ comments, which do not change in this revised manuscript. “Track change” function will mix up color text in different computers, so please open the file without “track change”

2) Line 27: This statement is not correct. Low carbon fuel may not produce carbon emissions, but it can produce other emissions such as NOx. 

(Line 27: Low carbon fuel can provide high efficiency and low pollution combustion. How-ever, it will face some problems.)

Answer: Yes, it was our mistake. We have added more description about the background on low carbon fuel and associated NOx emission in the first paragraph of the “Introduction”. We have changed the description of our concern and highlighted in “green color”.

3) Statement in line 32 is not clear to the readers. The authors must describe what is "The effects on combustion process". What are the effects that the authors are describing?

Answer: We have added the description of combustion effects in introduction from line 40 to 44 and highlighted in “green color”, which reads “Firstly, plasma can rapidly raise the mixture temperature via energy transfer from electrons to molecules. The second enhancement pathway is that high energy electrons, ions, and molecules produced from plasma lead to the subsequent production of active radicals and reactive species in the fuel and air streams. Plasma generated ionic wind and flow motion via the Coulomb and Lorentz forces provides hydrodynamic enhancement by changing local velocity”.

4) The authors must define what is "E/N field" when using first time.

Answer: We have added the definition of “E” and “N” in the article and highlighted in green color.

5) Fundamentally, flame dynamics is due to the ratio of flow timescales to the chemical timescales. The reattachment phenomena is due to acceleration is chemical timescale as described by the authors. Thus, it would be interesting if the authors discuss on what specific reactions are causing the kinetic acceleration when interacting with O3.

Answer: We have added the description of flame propagation assisted by O3 in paragraph 2 of section 3.1.4. and highlighted in “green color”, which reads “ Ozone can become a carrier of atomic oxygen and can be certainly transported to flame front at low temperature below 400 K. Previous literature [14] indicated that the ozone molecules will decompose immediately early in the pre-heat zone of flame and release O and O2

Then atomic oxygen rapidly reacts with the fuel to abstract an H atom, also, the ozone simultaneously reacts with H atom, both leading to produce abounding OH. Following these OH molecules react with the fuel and fuel fragments to form H2O and produce heat release early in the pre-heat zone, leading to temperature augmentation.

6) How reactive is the O3 chemically with the fuel compared to O2? 

Answer: As mentioned in additional description in section 3.1.4., ozone will decompose to atomic oxygen which rapidly reacts with H atom. In other words, Ozone tends to decompose to atomic oxygen to react with abstract H atom in the fuel molecule in low temperature in comparison with O2, which takes high temperature to decompose into atomic oxygen or has to wait for the attack by hydrogen atom after fuel hydrogen abstraction in the preheat zone ahead of the flame.

7) Also, the discharge of electrons can also increase the localized temperature of the gas. So, reattachment may be due to the accelerated kinetics and also due to increase in gas temperature due to electrode. How the authors can separate these two phenomena on the observed reattachment? The authors discussed that the PRF when placed in upstream can not ignite the mixture. But, this can be wrong. At the core of the jet, due to the momentum of the jet, the fluid has very less time to absorb the heat from the discharge plug. On the other hand, at the periphery of the nozzle, the airflow is mainly due to entrainment and velocity is lesser so that it can absorb the heat. 

Answer: Unlike thermal plasma of sparks, corona discharge is cold plasma, see the review by Ju and Sun in Reference 4 in the manuscript. Heat transfer to gas is slower than kinetical acceleration, so gas temperature raise is minor and contribution to flame enhancement is insignificant as compared to chemical kinetic enhancement through production of active radicals and reactive species, such as Ozone in this experiment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

This is a re-review of the manuscript processes-1086440. The authors have provided a point-by-point rebuttal to my comments. However, I believe that no substantial information are given or that the modifications made to the manuscript are very marginal and do not address my comments. Specifically:

• In my opinion, the introduction has not been substantially modified to have a clearer overview of the manuscript pitch. The first sentence is just about generality.
• The reference 16 is not providing enough details to be able to understand the experimental procedure adopted here. Even there is another article or the authors should expand the section dealing with cond-PIV measurements.
• Regarding PIV measurements, what is the resolution of your technique. I believe that if your technique is not spatially resolved, you are having a bias in determining the fresh gas velocity. 
• In order to have the lab-coordinate velocity of the flame front, what is the criterion to determine the flame height? Is this criterion sensitive on the flame velocity value?
• The additional explanations about ozone measurements make me feel that the authors are making a serious technical flaw: if PIV is 2D (a plane), whereas the flame location is captured via a line-of sight technique, there is no reason to correlate these data. Moreover, the ozone is also made in a line-of-sight fashion (laser beam expansion). Then, I am curious to see how can you correlate a 2D information with a 3D one. (in other words, which kind of PIV are you performing?).
• Regarding Figure 2, as I understood, the red star is denoting the time elapsed between the initiation of the corona discharge and the rapid flame base reattachment. How do you figure out the flame base is reattaching since the flame base height is still decreasing before this critical time?

I still see major problems with the manuscript after the second review and therefore I cannot recommend it for publication

Author Response

We would like to express our sincere gratitude to the reviewers and the editor for their constructive comments and instructions. We have carefully perused the reviewers’ comments and used a more explicit description in the revised manuscript to improve the article's quality. The added/modified sentences last time (first revision) were presented in “red color”, added contents this time (second revision) were presented in “green color” in the revised manuscript. Sentences in “blue color” are contents related to reviewers’ comments, which do not change in this revised manuscript. “Track change” function will make color some mistakes, so please open the file without “track change”.

The process of plasma assisted flame reattachment studied in this manuscript is a very complicated continuous transition process of flame-flow-electric/plasma interaction. The process and related mechanism is still unclear and have not been studied and undocumented in the literature. Although we have tried to investigate this process using high-speed camera and cond-PIV (PIV measurement conditioned with respect to “instantaneous” flame position and velocity using high-speed camera simultaneously), see Ref. 16 in the manuscript, the results were not very successful and were not satisfactory as the electric/plasma forced flame reattachment is not a steady, axisymmetric process and the 2D planar PIV using laser sheet tends to miss to capture the flame location and the leading flame base point of the reattaching flame. In addition, the velocity measurements cannot fully explain the complicated electric/plasma-flame interaction and cannot delineate the interaction mechanism, which the induced chemical interaction, such as induced Ozone, may involve and contribute to the reattachment process. Therefore, the pitch in this experimental study is that based on previous cond-PIV results we tried to investigate this reattachment process and mechanism by using 2D line-of-sight laser absorption measurement of ozone concentration. Details of cond-PIV has been elaborated in previous publication, reference 16. In this study, we emphasize on the discussion of ozone concentration. Cond-PIV measurement is used to provide indirect supporting evidences of “exceptional” high reattaching flame velocity when corona is induced with high PRF (pulse repetition frequency).

point-by-point response are as follows:

• In my opinion, the introduction has not been substantially modified to have a clearer overview of the manuscript pitch. The first sentence is just about generality.

Answer: We have added more description about the background in the beginning of the “Introduction” and elaborated the manuscript pitch, as also commented above in this “response to reviewers’ comments, in the last paragraph of introduction. Added contents were presented in “green color” sentences.

• The reference 16 is not providing enough details to be able to understand the experimental procedure adopted here. Even there is another article or the authors should expand the section dealing with cond-PIV measurements.

Answer: Details of cond-PIV were presented in previous publication, reference 16. We have added description of PIV system in section 2.2. and highlighted in “green color”. We have to admit that the previous results in Reference 16 were not very successful and were not satisfactory as the electric/plasma forced flame reattachment is not a steady, axisymmetric process and the 2D planar PIV using laser sheet tends to miss to capture the flame location and the leading flame base point of the reattaching flame. In addition, the velocity measurements cannot fully explain the complicated electric/plasma-flame interaction and cannot delineate the interaction mechanism, which the induced chemical interaction, such as induced Ozone, may involve and contribute to the reattachment process. In this article, we emphasize on the discussion of ozone concentration. Cond-PIV measurement is used to provide indirect supporting evidences of “exceptional” high reattaching flame velocity when corona is induced with high PRF (pulse repetition frequency).

• Regarding PIV measurements, what is the resolution of your technique. I believe that if your technique is not spatially resolved, you are having a bias in determining the fresh gas velocity. 

Answer: We have added the description of PIV resolution in the end of section 2.2. The PIV scattering images are captured on a progressive interline transfer CCD camera (1360 × 1024 pixels) equipped with double-exposure function. The image of the camera is found to be 28 μm/pixel in a field of view of 29.5 × 38.9 mm2 imaged by a 75 mm lens. Final interrogation window size and overlap were respectively 16 × 16 pixels and 62 %, and it corresponds to a PIV spatial resolution as smaller as 0.17 mm, which is considered to be sufficient for the present study. On the other hand, the temporal resolution for flame base velocity was limited to maxi-mum framing rate of high-speed video camera whose frame rating was 1250 Hz. A conservative estimate of the uncertainty in absolute flame speed is within 0.05 m/s.

• In order to have the lab-coordinate velocity of the flame front, what is the criterion to determine the flame height? Is this criterion sensitive on the flame velocity value?

Answer: As described in section 2.2, the instantaneous lift-off height, Hb, is defined as the axial distance between the most upstream flame-base location (leading-edge) and nozzle exit. The flame-base location was determined by high- speed image, and in contrast to the axisymmetric flame base usually found in natural flame reattachment, in the present single-electrode setup the electric/plasma forced flame reattachment occurs on one side with a sharp pointing flame base as can be seen from the flame picture. Therefore, the high-speed camera can capture the real flame base of the present forced flame reattachment.

• The additional explanations about ozone measurements make me feel that the authors are making a serious technical flaw: if PIV is 2D (a plane), whereas the flame location is captured via a line-of sight technique, there is no reason to correlate these data. Moreover, the ozone is also made in a line-of-sight fashion (laser beam expansion). Then, I am curious to see how can you correlate a 2D information with a 3D one. (in other words, which kind of PIV are you performing?).

Answer: As explained in the response to the above comment, the flame-base location was determined by high- speed image, and in contrast to the axisymmetric flame base usually found in natural flame reattachment, in the present single-electrode setup the electric/plasma forced flame reattachment occurs on one side with a sharp pointing flame base as can be seen from the flame picture. Therefore, the high-speed camera can capture the real flame base of the present forced flame reattachment to calculate the corresponding lab-coordinate absolute flame speed. In this study, 2D PIV only provided the evidence of flame propagation enhancement. We do not correlate 2D planar PIV data with integrated 3D ozone data. We mainly use the ozone concentration to analyze the flame reattachment mechanism in this research.

• Regarding Figure 2, as I understood, the red star is denoting the time elapsed between the initiation of the corona discharge and the rapid flame base reattachment. How do you figure out the flame base is reattaching since the flame base height is still decreasing before this critical time?

Answer: The red stars in Fig. 2 are denoting the instantaneous flame base locations (flame base height) at the instance of first appearance of corona discharge identified from high-speed camera images. In the revision, we have emphasized the appearance of corona discharge by red circle in the high-speed images in Fig. 3.

Author Response File: Author Response.docx

Reviewer 4 Report

I would recommend this paper for the journal publication. In the current form, this paper is archival.