Impact of Novel Nozzles on Atomization Flow Field and Particle Features: Simulation and Experimental Validation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this manuscript, compressible flow simulations are used to explore the details of three different designs for gas atomization nozzles. It is shown that modified designs achieve similar flow features to the baseline at reduced pressures, and in particular that one design (type III) can give similar performance at an atomization pressure of 5 MPa compared to a the baseline (type I) at 7 MPa. The approach described in the paper seems sound and the main conclusions interesting, and I think this work is of interest and benefit to the community. There are a few points that require clearer explanation or correction before publication, and these are listed below, starting with some of the more consequential points.

 

1. The analysis given in Section 2.2 describes idealized 1D flow in a nozzle, but it’s not clear whether or how these equations are used in the CFD simulations (for which I think the full flow equations from Section 2.3 are used). The important conclusion of 2.2 may be that a converging-diverging nozzle is needed to achieve supersonic flow at the nozzle outlet, but this is well understood and doesn’t require most of the details in 2.2. Is the analysis in section 2.2 needed?

 

2. Section 2.3 includes flow equations for the gas, but does not explain how the multi-fluid gas-melt flow model is implemented. More detail is needed here.

 

3. Figure 9 shows discrete particles in the flow, but discrete particles don’t make sense in a 2D axisymmetric representation (or, at least, they must be interpreted carefully). What’s the meaning of these particles in the 2D geometry?

 

4. It’s not clear what conclusions are meant to be drawn from the experimental results in Section 3.3. Is it simply that the two cases shown (NTI at 7 MPa, and NTIII at 5 MPa) lead to similar particle shapes and distributions? If that’s the case, what’s the meaning of the second-last sentence in the Conclusions section, stating that NTIII (5 MPa) demonstrates superior fragmentation performance? Is this meant to be seen in the experiments? The text in Section 3.3 is very short and doesn’t discuss any observations of a performance difference. Please clarify this.

 

Remaining points are more minor and are listed in order of appearance in the paper:

 

5. There are a few punctuation or wording errors throughout the paper; please proof-read carefully. (One example: the second-last sentence of the abstract is a fragment, and I think a colon is meant at the end of the previous sentence instead of a period.)

 

6. In Figure 1, the meaning of the inset in purple dashed lines is not quite clear; I think this is indicating that the shape of the small inlet region changes for different geometries, but this isn’t obvious, and it’s hard to see in later flow plots such as Figure 3. A zoomed-in view of one or more of the real simulation geometries would be helpful.

 

7. It’s mentioned in Section 2.1 that the NTI design employs 18 individual nozzles. How are these captured in the 2D axisymmetric geometry used for the simulations, and how does any approximation of this affect the results?

 

8. In Table 2, the thermal conductivity of the alloy is not given, but this should be an input to the model.

 

9. Just before equation (4) the text refers to the energy equation for an incompressible fluid; I think this should read “compressible.”

 

10. In equation (14), it’s confusing to use “F” for a time scale when elsewhere “F” represents forces.

 

11. In equation (15), the numerator should include u_p, not u_d.

 

12. Flow in section 3.1 is described as being in the “closed-wake” condition. I’m not familiar with this term. Can this be defined?

 

13. In Figure 6, is it possible to mark the x-axis (v = 0) locations in order to make the recirculation zones and stagnation points clearer?

 

14. In the paragraph before Figure 7, the abbreviation AP is defined along with the other abbreviations appearing in the figure, but “AP” does not appear in the figure (I understand that it appears in Table 3).

 

15. Abbreviations that appear in Figure 7 should be defined in the figure caption, not just the main text.

 

16. I believe section 3.3 should be entitled “Validation,” not “Verification,” since it involves comparison to experiments.

 

17. The labels in Fig. 10(a) and (b) seem reversed, according to the caption; (a) should be 7 MPa, and (b) should be 5 MPa.

 

Comments on the Quality of English Language

Overall the English is ok. There are a few punctuation or wording errors throughout the paper; please proof-read carefully. (One example: the second-last sentence of the abstract is a fragment, and I think a colon is meant at the end of the previous sentence instead of a period.)

Author Response

Comment 1: The analysis given in Section 2.2 describes idealized 1D flow in a nozzle, but it’s not clear whether or how these equations are used in the CFD simulations (for which I think the full flow equations from Section 2.3 are used). The important conclusion of 2.2 may be that a converging-diverging nozzle is needed to achieve supersonic flow at the nozzle outlet, but this is well understood and doesn’t require most of the details in 2.2. Is the analysis in section 2.2 needed?

Reply: We are very grateful for your suggestion regarding the overly complicated description of the nozzle principle in our manuscript. We have simplified the description of the basic formula sources and mechanisms in 2.2. Once again, we express our sincere thanks for your valuable feedback.

 

Comment 2: Section 2.3 includes flow equations for the gas, but does not explain how the multi-fluid gas-melt flow model is implemented. More detail is needed here.

Reply: Thank you for your question. Since the study in the text only focuses on the secondary atomization process of gas-melt, in this process, the large droplets after the primary atomization are simplified as discrete phase particles (i.e., DPM model), and the specific details of the DPM model have been provided in Section 2.4.

 

Comment 3: Figure 9 shows discrete particles in the flow, but discrete particles don’t make sense in a 2D axisymmetric representation (or, at least, they must be interpreted carefully). What’s the meaning of these particles in the 2D geometry?

Reply: Thank you very much for your question. We have provided a more detailed explanation of the suggestions you put forward and made the corresponding revisions in the text. The specific changes are as follows: In the gas-liquid two-phase flow field, the gas jet impacts the discrete phase particles. After the flow field stabilizes (about 40 ms), the particle images of NTI (7 MPa) and NTIII (5 MPa) are compared. In the particle images, red particles represent the largest particles (approximately 160-200 μm), yellow and green particles represent medium-sized particles (approximately 60-160 μm), and blue particles represent smaller particles (approximately 10-60 μm). Through image comparison, after the secondary recirculation zone breaks the particles, NTI (7 MPa) has more medium-sized particles (measured by the width of the green part). The number of large particles that are not fully broken in NTI (7 MPa) is also greater than that in NTIII (5 MPa) (indicated by the yellow dashed circles, i.e., the red particles within the circles). In the second half of the breaking process, most of the large red particles in both NTI (7 MPa) and NTIII (5 MPa) are broken into medium and small particles. However, compared with NTIII (5 MPa), NTI (7 MPa) still contains a small number of large particles and relatively more medium-sized particles (indicated by the red circles). This observation indicates that the breaking effect of NTIII (5 MPa) is better than that of NTI (7 MPa). These descriptions have been added in Section 3.2.

 

Comment 4: It’ not clear what conclusions are meant to be drawn from the experimental results in Section 3.3. Is it simply that the two cases shown (NTI at 7 MPa, and NTIII at 5 MPa) lead to similar particle shapes and distributions? If that’s the case, what’s the meaning of the second-last sentence in the Conclusions section, stating that NTIII (5 MPa) demonstrates superior fragmentation performance? Is this meant to be seen in the experiments? The text in Section 3.3 is very short and doesn’t discuss any observations of a performance difference. Please clarify

Reply: As pointed out, we realized that the description in this part was unclear and the introduction was insufficient. Therefore, we have made revisions and additions in the article and provide the following explanations: The experimental results presented in this paper illustrate the similar particle shapes and distributions through the two cases shown (7 MPa in NTI and 5 MPa in NTIII). The penultimate sentence in the conclusion section, that is, NTIII (5 MPa) shows better crushing performance, can be observed in the experiment. We will provide a detailed explanation below: (i) The conclusion drawn from the gas single-phase flow field (Table 3) is that NTIII demonstrates a greater advantage in minimizing energy losses. (ii) In the simulation of the two-phase field, when NTI (7 MPa) and NTIII (5 MPa) both reach a stable and identical time, it can be observed from the two-dimensional particle diagram (yellow and red circles) that NTIII (5 MPa) has a better crushing effect, with a more dense and relatively concentrated particle distribution. (iii) Through SEM images, it is observed that NTI (7 MPa) has a small amount of extremely irregular morphology (filaments or short rods), while the particles in NTIII (5 MPa) are generally spherical (with small satellite structures), which can also preliminarily indicate that NTIII (5 MPa) has a better crushing effect. Additionally, the median particle size of NTI (7 MPa, d50 = 52.5 μm) and NTIII (5 MPa, d50 = 52.2 μm) can also show that the d50 value of NTIII (5 MPa) is smaller, indicating that NTIII (5 MPa) exhibits better crushing performance.

As required, we have a detailed descriptions about Section 3.3. That is stated as “From the SEM images, it can be observed that at NTI (7MPa), most of the powder par-ticles are spherical, but there are also a few irregular particles in shapes such as drawing, Satellite organization, and short bar. However, at NTIII (5MPa), these irregular powder particles are mostly in the shapes of ellipse and satellite (There are fewer extremely ir-regular shapes.). ------------------- On the whole, NTⅢ (5MPa) has a certain advantage in generating smaller particles after crushing.”

 

Comment 5: There are a few punctuation or wording errors throughout the paper; please proof read carefully. (One example: the second-last sentence of the abstract is a fragment, and I think a colon is meant at the end of the previous sentence instead of a period.)

Reply: Accepted, we have made the necessary revisions and we are very appreciative of this comment and suggestion.

 

Comment 6: In Figure 1, the meaning of the inset in purple dashed lines is not quite clear; I think this is indicating that the shape of the small inlet region changes for different geometries, but this isn’t obvious, and it’s hard to see in later flow plots such as Figure 3. A zoomed-in view of one or more of the real simulation geometries would be helpful.

Reply: The opinion is extremely valuable and we have made the necessary revisions. The original model diagram has been replaced with a real gas nozzle diagram and the enlarged diagram of NTIII has been added to Figure 1(d).

 

Comment 7: It’s mentioned in Section 2.1 that the NTI design employs 18 individual nozzles. How are these captured in the 2D axisymmetric geometry used for the simulations, and how does any approximation of this affect the results?

Reply: Many thanks for addressing this issue. The 18 discrete nozzles cannot be directly reflected in the two-dimensional symmetrical graph; they are merely the type of nozzles used in industrial practice. We are very grateful for your valuable suggestion, which can be taken as our subsequent research. Regarding this, we have summarized the relevant literature. Alexander Ariyoshi Zerwas et al. studied the impact of the gas atomizer nozzle configuration on metal powder production for additive manufacturing. The article makes a comparison between the ring-hole and ring-slot type nozzles [1]. Tong Mingming et al. explored the differences between annular slit nozzles and discrete gas nozzles by simulating two-dimensional symmetrical flow fields. However, due to the change in the relative positions of the gas inlets to the tip of the liquid delivery tube, the discrete nozzles could not be presented in the same way as in three-dimensional modeling in the article [2].

Ref 1: Zerwas AA, Silva FC, Fritshing U. Impact of the gas atomizer nozzle configuration on metal powder production for additive manufacturing. Powder Technol. 2024, 443, 119974. doi: 10.1016/j.powtec.2024.119974.

Ref 2: Tong MM, Browne DJ. Modelling compressible gas flow near the nozzle of a gas atomiser using a new unified model. Computers & Fluids. 2008, 38, 1183-1190. doi: 10.1016/j.compfluid.2008.11.014.

 

Comment 8: In Table 2, the thermal conductivity of the alloy is not given, but this should be an input to the model.

Reply: Thank you for your suggestion. We only discussed the evolution of the flow field in the two-phase field, mainly using the DPM model. We assumed that the large droplets after the first breakage were ideal discrete phase particles as the injection initial positions for the second breakage. In this model, the input conditions of the discrete phase particles only involved parameters such as particle density, specific heat, viscosity and surface tension. Thank you again for your suggestion.

 

Comment 9: Just before equation (4) the text refers to the energy equation for an

incompressible fluid; I think this should read“compressible.”

Reply: We are really thankful for your guidance and what you said is correct. To optimize the overly complicated derivation process in Section 2.2 and regarding Comment 1, we have now made the corresponding revisions to this part.

 

Comment 10: In equation (14), it’s confusing to use “F” for a time scale when elsewhere“F” represents forces.

Reply: Thank you for highlighting the discomforting aspect here. We have received and corrected it. Change the original time scale to a more precise drag force.

 

Comment 11: In equation (15), the numerator should include u_p, not u_d.

Reply: Thank you very much for your careful reading. It has now been corrected.

 

Comment 12: Flow in section 3.1 is described as being in the “closed-wake” condition. I’m not familiar with this term. Can this be defined?

Reply: Dear Reviewer, we are deeply appreciative of the questions you have raised. In response to the inquiry regarding "closed-wake," we provide the following explanation: Ting et al. provided explanations for the open wake and closed wake states and investigated the influence of open wake (with only one recirculation zone) and closed wake on the flow field (A secondary circulation zone) has emerged. and the final experimental results. They also observed the superiority of the closed vortex phenomenon in the process of metal powder preparation. After the closed vortex phenomenon occurred, the change in the median particle size (D50) of the powder could reach 42% [1-2]. Motaman et al. investigated the single-phase gas flow behavior of closed-coupled gas atomization by using four different melt nozzle head designs and two different gas nozzles. The critical value WCP for the transition from open wake to closed wake was the focus of the study [3].

Ref 1: Ting J, W M, Eisen WB. The effect of wake-closure phenomenon on gas atomization performance Mater. Sci. Eng. A. 2002, 326, 110-121. doi: 10.1016/S0921-5093(01)01437-X.

Ref 2: Ting J, Anderson IE. A computational fluid dynamics (CFD) investigation of the wake closure phenomenon. Mater. Sci. Eng. A. 2004, 379, 264-276. doi: 10.1016/j.msea.2004.02.065.

Ref 3: Motaman S, mullis AM, Borman DJ. ANumerical and Experimental Investigations of the Effect of Melt Delivery Nozzle Design on the Open to Closed-Wake Transition in Closed-Coupled Gas Atomization. Metall. Mater. trans B. 2015, 46, 1990-2004. doi: 10.1007/s11663-015-0346-6.

 

Comment 13: In Figure 6, is it possible to mark the x-axis (v = 0) locations in order to make the recirculation zones and stagnation points clearer?

Reply: We are grateful for your valuable suggestions and improved the images in the original text and is presented in Figure 6.

 

Comment 14:In the paragraph before Figure 7, the abbreviation AP is defined along with the other abbreviations appearing in the figure, but “AP” does not appear in the figure (I understand that it appears in Table 3).

Reply: Thank you very much for your detailed comments and suggestions. We have added the distribution ranges of the three types of AP to Figure 7.

 

Comment 15: Abbreviations that appear in Figure 7 should be defined in the figure caption, not just the main text.

Reply: Thank you for your suggestion. We have made the modifications and added the details in the original text. Re-presented as Figure 7 again.

 

Comment 16: I believe section 3.3 should be entitled “Validation,” not “Verification,” since it involves comparison to experiments.

Reply: Thank you very much. Your suggestion is extremely valuable. We have already changed “Verification,” to “Validation,”.

 

Comment 17: The labels in Fig. 10 (a) and (b) seem reversed, according to the caption; (a) should be 7 MPa, and (b) should be 5 MPa.

Reply: Thank you very much for pointing out the errors or defects. We have already corrected them.

Reviewer 2 Report

Comments and Suggestions for Authors

General Comments

Understanding the happenings during gas atomization process with the aid of simulations such as CFD is beneficial in the provision insight for process control to attain customized properties and  distribution of atomized particles as well as resource savings. 

The authors reviewed the significance and influence of atomization parameters such as gas pressure and nozzle type /design on particle size distribution. Although investigations and concept of designing and testing new nozzles is generally not new, the authors present their novel designs which according to their results improves resource efficiency.

 

Questions

I however have a few comments and questions for the authors 

 

Line 224 -  Please state the purity of the N₂ gas used in your experiment

Line 228 - 230 - How was the particle size distribution obtained, with what instrument ?

Line 228 - 230 - Here you state that microstructure analysis was performed however, from your results, only the morphological images are presented. Otherwise, i dont see any analysis performed

Line 444 -446 - Can you comment on what effect the different nozzle design have on formation satellite formation and other defects ?

 

Author Response

Comment 1: Please state the purity of the N2 gas used in your experiment.

Reply: Dear Reviewer, thank you for your question. The nitrogen gas we used in the experiment was regular nitrogen with a purity of 99.9%.

 

Comment 2: How was the particle size distribution obtained, with what instrument ?

Reply: We appreciate the questions you have raised. We sieved the particles ranging from 0 to 100 micrometers, took scanning electron microscope (SEM) images, and then measured the particle size using ImageJ software. Eventually, we obtained the particle size distribution.

 

Comment 3: Here you state that microstructure analysis was performed however, from your results, only the morphological images are presented. Otherwise, i’don’t see any analysis performed.

Reply: Dear Reviewer, the question you raised is very insightful, as you are concerned about conducting a detailed analysis of the experimental results. According to your suggestions, we have made corrections to the previous manuscript, and the specific corrections are as follows: From the SEM images, it can be observed that at NTI (7MPa), most of the powder par-ticles are spherical, but there are also a few irregular particles in shapes such as drawing, Satellite organization, and short bar. However, at NTIII (5MPa), these irregular powder particles are mostly in the shapes of ellipse and satellite (There are fewer extremely ir-regular shapes.). By comparing the two, it can be concluded that at NTI (7MPa), most of the powder is broken into regular spherical shapes, with only a few irregular particles, and these are extremely irregular, short bars or even drawn out. However, at NTIII (5MPa), there are fewer extremely irregular particles. The reasons for the appearance of these morphologies lie in the fact that both NTI (7MPa) and NTIII (5MPa) have rela-tively high gas pressures, increasing the probability of droplet collisions. This causes small droplets to adhere to the surface of large droplets, forming satellite structures. However, NTIII utilizes its contraction-expansion characteristics, has a lower pressure at the gas outlet but a higher velocity. This results in droplets not being immediately broken into fine short bars (due to the lower pressure), but the interaction between gas and liquid is enhanced (due to the higher velocity), leading to a higher degree of sub-division. In addition, the particle size distribution of both conforms to a normal distribution, and the median particle size of NTⅢ (5MPa, d50 = 52.2μm) is smaller than that of NTⅠ (7MPa, d50 = 52.5μm). On the whole, NTⅢ (5MPa) has a certain advantage in generating smaller particles after crushing. These descriptions have been added in Section 3.3.

 

Comment 4: Can you comment on what effect the different nozzle design have on formation satellite formation and other defects?

Reply: Thank you very much for raising this question. To be honest, this work does not explore the issue of powder defects. However, we believe that by changing the angles of different nozzles, the size of the circulation zone can be altered. The smaller the circulation zone, the lower the probability of particle collisions, and thus fewer satellite structures will be produced.