Investigation of Spray Characteristics for Detonability: A Study on Liquid Fuel Injector and Nozzle Design

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript deals with the experimental characterization of water sprays generated by air-assist atomizers of four different configurations. It has no close relation to pulsed detonation engines (PDEs) and spray detonability as claimed by the authors and therefore does not fit the scope of Aerospace and must be rejected in the present form. Moreover, the manuscript contains many inconsistencies and drawbacks. To somehow help the authors in manuscript revision, I provide below my comments and recommendations.

(1) The Introduction looks inadequate and incomplete in terms of the actual material presented in the manuscript. Instead of reviewing multiple studies of air assist atomizers available in the literature, the authors discuss the issues related to PDEs, detonation, propulsion, etc. 

(2) Parameters in Table 1 are confusing. Nitrogen-gas density at normal temperature (298 K) is shown to vary from 1.15 to 14.8 kg/m3, while its pressure varies from 8 to 20 bar. This is inconsistent with true values.

(3) The use of water instead of the propulsion fuel is not grounded and justified.

(4) The measurement error analysis is absent.

(5) There is no proof that the instantaneous images in Fig.5 and density-gradient images of Figs.6 and 7 show continuous liquid films (ligaments) rather than dense spray zones.

(6) There is no definition of "liquid film" length. How and with what accuracy is it measured?

(7) Logarithmic scales in Figs. 10 and 11 indicate huge uncertainties in the results, which must be explained. 

(8) My main concern is that the authors approach is not validated at all. The authors must provide validation examples for measuring droplet SMD, in particular in the dense spray zone. Otherwise, the reader must take everything they write on faith. The authors' claim that the observed "trend ... aligns with similar findings by other researchers" (lines 350, 351) does not stand up to criticism.

(9) The authors claim that their results "allow predicting the detonability of valveless detonation systems using liquid fuel" (lines 368, 369) is too far reaching and does not correspond to reality.

(10) The relevant literature contains many correlations like Eq. (4). First, the authors do not refer to any of them. Second, there is no discussion on the origin of Eq. (4) and the reason it differs from other available correlations.

(11) Correlations (5) and (6) have the limited range of applicability. The authors must clearly indicate this range.

(12) I have strong doubts that the atomizers used by the authors could generate water droplets of 5-6-micron diameter (see Fig. 13). This size is considerably smaller than the critical drop diameter for airblast atomizers. 

(13) Finally, the Conclusions look not substantiated by the obtained results. 

Comments on the Quality of English Language

 Moderate editing of English language required

Author Response

Author's Reply to the Review Report

This manuscript deals with the experimental characterization of water sprays generated by air-assist atomizers of four different configurations. It has no close relation to pulsed detonation engines (PDEs) and spray detonability as claimed by the authors and therefore does not fit the scope of Aerospace and must be rejected in the present form. Moreover, the manuscript contains many inconsistencies and drawbacks. To somehow help the authors in manuscript revision, I provide below my comments and recommendations.

First of all, I would like to express my sincere gratitude for your review of our paper. We are delighted that the feedback provided will contribute to enhancing the quality of our work. Please review the comments provided for each section.

(1) The Introduction looks inadequate and incomplete in terms of the actual material presented in the manuscript. Instead of reviewing multiple studies of air assist atomizers available in the literature, the authors discuss the issues related to PDEs, detonation, propulsion, etc. 

This study explores the performance evaluation of atomization preceding combustion-involved detonation experiments. Therefore, it may also encompass detonation research. However, in line with your feedback, subsections have been added to specify the application of liquid fuel/oxidizer, providing a more natural flow to the content.

(2) Parameters in Table 1 are confusing. Nitrogen-gas density at normal temperature (298 K) is shown to vary from 1.15 to 14.8 kg/m3, while its pressure varies from 8 to 20 bar. This is inconsistent with true values.

>> 

The content has been revised to accurately specify the range of gas density used in the experiments.

 

(3) The use of water instead of the propulsion fuel is not grounded and justified.

>> 

 

Using water in atomization experiments may seem irrational; however, it is a common practice in current injector experiments. The mechanism of atomization is typically explained by the ratio of viscosity to surface tension, making water a rational choice due to its non-toxic nature and accessibility for experimentation. In fact, the current trend in engine development involves conducting combustion experiments after atomization experiments using water, as indicated in the paper "Some Effects of Using Water as a Test Fluid in Fuel Nozzle Spray Analysis" from 1981 (attached). The primary factor affecting the relationship between atomization and combustion experiments is not the type of liquid used in atomization experiments but rather the characteristics of pressure. While high pressure environments in atomization experiments may not significantly affect SMD, they can greatly affect the evaporation rate of the liquid during combustion. Therefore, assuming atmospheric pressure conditions for both the atomization and subsequent detonation combustion experiments is reasonable.

 

 

(4) The measurement error analysis is absent.

>> 

For SMD, being a statistical value of droplet diameter, measurement errors are not included. However, in the case of image measurements, it is recommended to include measurement errors. Therefore, the paper has been updated to reflect cases where measurement errors are not included. (Figure. 8 is added. 247-251)

(5) There is no proof that the instantaneous images in Fig.5 and density-gradient images of Figs.6 and 7 show continuous liquid films (ligaments) rather than dense spray zones.

>> 

I have attached additional data to the cover letter to further validate the content. With an increase in the mass flow rate of gas entering the injector, you can observe a decrease in the thickness of the liquid film and a shorter breakup length. These results indicate that as the gas-to-liquid flow ratio increases, the length of the liquid film decreases, and conversely, it increases when the ratio decreases. Therefore, these experiments were conducted properly and reflect the characteristics of the injector accordingly.

 

(6) There is no definition of "liquid film" length. How and with what accuracy is it measured?

>> 

The content has been updated to include information regarding measurement methods and errors, as pointed out in (4). (Figure. 8 is added. 247-251)

(7) Logarithmic scales in Figs. 10 and 11 indicate huge uncertainties in the results, which must be explained. 

>> 

As mentioned in (4), uncertainty has been added to the data in Figures 10 and 11, and this has been explained accordingly.

(8) My main concern is that the authors approach is not validated at all. The authors must provide validation examples for measuring droplet SMD, in particular in the dense spray zone. Otherwise, the reader must take everything they write on faith. The authors' claim that the observed "trend ... aligns with similar findings by other researchers" (lines 350, 351) does not stand up to criticism.

>> 

The method for measuring SMD is described in Figure 4. The difference between air-assist and airblast is detailed in the referenced material. Sections 359-363 and 367-369 have been added to specify this information more concretely.

 

(9) The authors claim that their results "allow predicting the detonability of valveless detonation systems using liquid fuel" (lines 368, 369) is too far reaching and does not correspond to reality.

>> 

The topic has already been extensively researched. Studies on detonation probability have accumulated a wealth of data from researching injectors for diesel engines to the present day. Systems for appropriately timing detonations are also being developed for GDI engines. represents a significant study on detonation engines, and research on detonability and SMD is currently underway.

(10) The relevant literature contains many correlations like Eq. (4). First, the authors do not refer to any of them. Second, there is no discussion on the origin of Eq. (4) and the reason it differs from other available correlations.

>> 

Equations similar to Eq. (4) have been appearing since the 1950s, and currently, many similar formulas have been extensively researched for predicting SMD. In the introduction, reference was made to representative formulas mentioned in the referenced literature to highlight the difference from empirical formulas. These equations, expressed as exponents of the main dimensionless numbers explaining SMD, are used to derive empirical formulas and determine the variables (primarily injector geometry variables) that have the greatest influence on SMD. Additionally, sections 359-363 and 367-369 have been added to specify this information more concretely. Through this, we identified the variables that have the greatest influence on SMD and were able to assess the presence of detonation. This study has already been conducted and submitted to an aerospace journal ("Manuscript ID: aerospace 3007344 Experimental Investigation on Pulse Detonation Combustion Characteristics by Atomizer Geometry").

(11) Correlations (5) and (6) have the limited range of applicability. The authors must clearly indicate this range.

>> 

The content has been added to sections 426-429.

(12) I have strong doubts that the atomizers used by the authors could generate water droplets of 5-6-micron diameter (see Fig. 13). This size is considerably smaller than the critical drop diameter for airblast atomizers. 

As mentioned, the critical SMD that can be achieved with an airblast atomizer is approximately 20-30 microns. However, the injector used in this experiment is an air-assist injector, which has the potential to form droplets as small as 5 microns. (Reference) Air-assist injectors exhibit characteristics similar to pressure swirl injectors, especially at high pressures (in this study, 16-20 bar), allowing for the attainment of such SMDs. As a result, as demonstrated in Fig. 13, errors in the empirical formula are evident below the critical SMD. I have attached the reference to the cover letter.

(13) Finally, the Conclusions look not substantiated by the obtained results. 

>> 

Since combustion was not performed in this experiment, it has not been verified how SMD affects detonability. However, from the perspective of atomization, understanding the influence of fuel/oxidizer supply pressure and injector geometry on SMD is meaningful. Low SMD promotes efficient vaporization, increasing detonability, while higher SMD hinders combustion with lower evaporation rates leading to continuous combustion. Verification of this experiment, as mentioned, has been submitted to an aerospace journal.

 

 

I would like to express my sincere gratitude for the helpful feedback, which has contributed to improving the quality of our work. I have attached the revised content for your review.

Sincerely

Authors

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper presents an experimental investigation into the liquid spray characteristics issuing from specially designed Venturi nozzle injectors. The study is well thought and planned, and the results sufficiently interesting and relevant for the application of fuel dispersion in any combustion system, and specifically for future application in (pulsed) detonation engines. The approach here is extremely phenomenological, but the techniques brought to bear (fast imaging and droplet number/size measurement by Fraunhofer diffraction) are complex and producing compelling results. The development of engineering-type correlations for film length, momentum flux, and droplet size from different types of injectors will be useful in the future for the design of practical devices.

 

The paper may be published in its present form, but its quality may be enhanced further if the authors address some minor points in their paper:

Correlations with respect to Reynolds, Weber, and other non-dimensional numbers have been used extensively in the past, and even though some past references are provided here, a more detailed exposition and motivation of the empirical formulas can be provided. In particular, what is new in this study, or in the past work of the authors, versus what has been know from the past literature, which results were expected, and which ones were counter-intuitive, all these are points that could be discussed further. 

 

The authors at times discuss the similarities to air-blast and air-assist devices, some of which are well-studied and documented from many decades of research in Diesel and other spray systems. A more detailed description, and 'tighter' connection could be made with those older studies, but also with the concepts presented here. The authors could explain more their analogies between their different types of injectors and the "air-assist", "air-blast", (hollow cone?), etc.     

Comments on the Quality of English Language

Some minor issues may be found in many places within the text. Meaning is never seriously compromised, yet the text would be improved by an editorial reading from a native English speaker. For example, from the abstract:

"Using the derived empirical correlation, the atomization mechanism varies according to the Venturi nozzle exit diameter ratio was found"

Author Response

Author's Reply to the Review Report

 

The paper presents an experimental investigation into the liquid spray characteristics issuing from specially designed Venturi nozzle injectors. The study is well thought and planned, and the results sufficiently interesting and relevant for the application of fuel dispersion in any combustion system, and specifically for future application in (pulsed) detonation engines. The approach here is extremely phenomenological, but the techniques brought to bear (fast imaging and droplet number/size measurement by Fraunhofer diffraction) are complex and producing compelling results. The development of engineering-type correlations for film length, momentum flux, and droplet size from different types of injectors will be useful in the future for the design of practical devices.

 

The paper may be published in its present form, but its quality may be enhanced further if the authors address some minor points in their paper:

Correlations with respect to Reynolds, Weber, and other non-dimensional numbers have been used extensively in the past, and even though some past references are provided here, a more detailed exposition and motivation of the empirical formulas can be provided. In particular, what is new in this study, or in the past work of the authors, versus what has been know from the past literature, which results were expected, and which ones were counter-intuitive, all these are points that could be discussed further. 

 

The authors at times discuss the similarities to air-blast and air-assist devices, some of which are well-studied and documented from many decades of research in Diesel and other spray systems. A more detailed description, and 'tighter' connection could be made with those older studies, but also with the concepts presented here. The authors could explain more their analogies between their different types of injectors and the "air-assist", "air-blast", (hollow cone?), etc.     

 

First of all, I would like to express my sincere gratitude for your review of our paper. We are delighted that the feedback provided will contribute to enhancing the quality of our work. Please review the comments provided for each section. Overall, by incorporating correlations from other researchers based on Reynolds, Weber, and other non-dimensional numbers, I have enhanced the comprehensibility of the paper. Consequently, I have established a tighter connection within the paper. Additionally, I have highlighted the relationship between the similar yet different air-assist and airblast injectors.

Thank you once again for your kind and insightful review. Such thoughtful feedback is greatly appreciated and immensely helpful to the authors.

Some minor issues may be found in many places within the text. Meaning is never seriously compromised, yet the text would be improved by an editorial reading from a native English speaker. For example, from the abstract:

"Using the derived empirical correlation, the atomization mechanism varies according to the Venturi nozzle exit diameter ratio was found."

The content has been revised to flow more naturally, and the paper is currently under review. Other expressions have also been corrected with the assistance of native speakers during the review process.

I would like to express my sincere gratitude for the helpful feedback, which has contributed to improving the quality of our work. I have attached the revised content for your review.

Sincerely

Authors

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper presents a highly interesting experimental investigation and a relevant discussion concerning the development of an injector utilizing liquid fuel and gas oxidizer for application in detonation engines. A new injector was fabricated for the present investigation by varying the Venturi nozzle exit diameter ratio and the geometric features of fuel injection hole. The authors have performed an extensive experimental measurement campaign and provide a detailed analysis of the results. The effect of diameter ratio of the Venturi nozzle on SMD was described in detailed for different fuel injection configurations examined in the study.

The subject of the paper is quite interesting for the present status of research in the area of new part development applied in next-generation propulsion systems. Therefore the paper is suggested for publication.

Author Response

Author's Reply to the Review Report

The paper presents a highly interesting experimental investigation and a relevant discussion concerning the development of an injector utilizing liquid fuel and gas oxidizer for application in detonation engines. A new injector was fabricated for the present investigation by varying the Venturi nozzle exit diameter ratio and the geometric features of fuel injection hole. The authors have performed an extensive experimental measurement campaign and provide a detailed analysis of the results. The effect of diameter ratio of the Venturi nozzle on SMD was described in detailed for different fuel injection configurations examined in the study.

The subject of the paper is quite interesting for the present status of research in the area of new part development applied in next-generation propulsion systems. Therefore the paper is suggested for publication.

Thank you for your kind and insightful review. Such thoughtful feedback is greatly appreciated and immensely helpful to the authors. Additional experiments reflecting the research topic have been conducted and the findings are currently being published and reviewed. Therefore, if the paper is published, the authors would be very pleased. Thank you.

 

Sincerely

Authors

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed all my comments. The manuscript could be now considered for publication in the present form