Round 1

Reviewer 1 Report

Reviewers' comments:

This manuscript presents the investigations on the effect of various concentrations of TiO₂ nanoparticle on the thermal properties, pressure drop and heat transfer of the TiO2/R1234yf nana-enhanced refrigerants. The investigation shows relative good creation and has certain reference significance for the application of nano-enhanced TiO₂/R1234yf refrigerant.

Thus, the reviewer believes that this manuscript can be published in the Sustainability after revised. Here are some more detailed comments that might be considered for the revision.

(1)   In section 2.1, authors provide the TiO2 Nanoparticle’s Properties. Can the author provide the performance of R234yf?

(2)   In section 2.2, the author should outline the meaning of each symbol under the formula. Such as formula (1) ~ (8).

(3)   Are the conditions for grid independence simulations consistent with those in Figure 2?

(4)   In the section 2.3.2, the References should be added to the theoretical formula.

(5)   In line234: “40mm” should be “40nm”.

(6)   In section 3.1, authors only described the effects of different nanoparticle volume fractions and temperature on thermophysical properties, and the reasons for this phenomenon should be explained and added. Including Fig 5 ~ Fig 8. The same problem appears in sections 3.1 and 3.2.

(7)   In the section 3.2, “Numerical Result” and “Simulation Result” may mislead readers, and it is recommended that numerical results be modified to “predict results”. Same problem in Table 5.

Author Response

Dear Reviewer,

Thank you for taking your time and giving your valuable comments for our paper. We had the opportunity to resubmit the paper after a deep revision of the manuscript following the comments of the reviewers. Therefore we followed punctually your precious remarks and we have improved the manuscript. As a matter of fact, in the paper we have highlighted the changes made in Yellow color, pertaining to your comments. Specifically, below are the answers to your point-to-point concerns.

This manuscript presents the investigations on the effect of various concentrations of TiO₂ nanoparticle on the thermal properties, pressure drop and heat transfer of the TiO₂/R1234yf nana-enhanced refrigerants. The investigation shows relative good creation and has certain reference significance for the application of nano-enhanced TiO₂/R1234yf refrigerant.

Thus, the reviewer believes that this manuscript can be published in the Sustainability after revised. Here are some more detailed comments that might be considered for the revision.

• Comment 1: In section 2.1, authors provide the TiO2 Nanoparticle’s Properties. Can the author provide the performance of R1234yf?

Response 1: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. The properties of R1234yf are added in the revised manuscript.

Table 2. Properties of POE lubricant oil and R1234yf refrigerant [36]

T (0C)	Thermal Conductivity (W/mK)		Density (kg/m3)		Specific heat (J/kgK)		Viscosity (mPa.s)
	R1234yf	POE	R1234yf	POE	R1234yf	POE	R1234yf	POE
10	0.0713	0.1467	1144	1026.21	1293	1745	0.194	56.375
15	0.0693	0.1457	1128	1022.14	1312	1756	0.182	42.673
20	0.0672	0.1447	1111	1018.08	1332	1769	0.171	33.491
25	0.0652	0.143	1094	1014.04	1354	1783	0.161	26.875
30	0.0631	0.1423	1075	1010	1379	1796	0.152	21.929
35	0.0609	0.1416	1057	1005.97	1406	1808	0.143	18.067
40	0.0586	0.1408	1037	1001.94	1437	1821	0.134	15.25

 

• Comment 2: In section 2.2, the author should outline the meaning of each symbol under the formula. Such as formula (1) ~ (8).

Response 2: Considering the valuable suggestion made by the reviewer, the terms in the equation is defined in the modified article.

• Comment 3: Are the conditions for grid independence simulations consistent with those in Figure 2?

Response 3: Authors are thankful to the reviewer for the query about the consistency of the conditions for grid independence simulations. It is consistent with the figure 2. The independence test is done for optimizing the grid size and quality.

                “In order to generate grids that perform the most effectively, it is necessary to take into account the grids' shape, quality, and number. A component that affects the overall processing cost and the precision of the outcomes of simulation analysis is the number of grids. A considerable spatial discretization error is produced by coarse grids, which lowers the accuracy of the analysis's findings [42]. Due to this the selection of optimum grid number is crucial. Based on the evaluation of various grid conditions, the grid independence analysis is a procedure used to identify the best grid condition that has the smallest grids without generating a difference in the numerical results.” (2.3.1 Grid Independency analysis)

• Comment 4: In the section 2.3.2, the References should be added to the theoretical formula.

Response 4: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. The references are added to the theoretical formula in the revised manuscript.

• Comment 5: In line234: “40mm” should be “40nm”.

Response 5: Authors are thankful to the reviewer for pointing out such a typo error committed. The mistake is now rectified in the modified manuscript.

• Comment 6: In section 3.1, authors only described the effects of different nanoparticle volume fractions and temperature on thermophysical properties, and the reasons for this phenomenon should be explained and added. Including Fig 5 ~ Fig 8. The same problem appears in sections 3.1 and 3.2.

Response 6: Authors are thankful for your valuable suggestions. The physics behind the effect of different nanoparticle volume fractions and temperature on thermophysical properties and flow characteristics are included in the modified manuscript.

            “The thermal conductivity of the nano-refrigerant is one of the key thermo-physical characteristics to enhance the thermal performance of the system. The heat transfer rate of the system greatly affects on the thermal conductivity of the nanoparticles doped in the refrigerant [46].”

            “The thermal conductivity of the nano-refrigerant enhanced with the doping of TiO₂ nanoparticle. This is due to the Brownian motion effect of the nanometer sized materials in the base working fluid. Photons are used to transmit heat by propagating lattice vibrations in crystalline nanoparticle immersed in base fluid. This kind of phonon oscillates erratically. Some ballistic phonon events might lead to advances in thermal conductivity. If the ballistic phonons that originated in a particle may travel through the fluid and reach neighbouring particles, a significant increase in thermal conductivity is occurred [47]. Due to Brownian motion, heat can travel between particles more efficiently the closer they are to one another. Meanwhile, the thermal conductivity is declined with the temperature. When the temperature is raises the mean free path between the molecules also increases. This reduces the collision between them [48]. While the separation between the nano-particle grows the coulomb interaction (i.e., near – field radiation) degrades. This reduces the thermal conductivity [49].”

            “Viscosity is another vital property, which influences the pressure drop of the refrigeration system [50].”

            “The high surface to volume area ratio of nanoparticles can be attributed as the main cause of the increase in viscosity with nanoparticles. Because nanoparticles have a larger surface area, there is more friction, which raises viscosity. On the other hand, it decreases when the temperature rises since it is hypothesised that the entropy increases, thereby the thermal energy increases. High viscosity is not preferred for refrigeration systems, as is well known. It should be remembered that at higher temperatures, the enhanced viscosity with the nanoparticle will diminish and get closer to that of the base fluid. Therefore, for such systems, the slight increase in viscosity caused by the addition of nanoparticles may be disregarded [51].”

            “The density of the nano-refrigerant is increased by the addition of nanoparticles to the base fluids. The increment in density for the resulting nano-refrigerant will become increasingly apparent as the doping fraction of nanoparticle rises. It is caused by the significant variations in density between materials in the solid and liquid phases. As the temperature rises, the volume of the fluid increased. Hence, the density of the nano-refrigerant is decreases with the temperature rise [51, 52].”

            “Generally, the specific heat of nano-refrigerant is reduced with increasing the concentration of nanoparticles. This is because the specific heat of nano oxide materials especially TiO₂ is less [53, 54]. However, the specific heat of nano-enhanced refrigerants improved with temperature. As the temperature rises, the liquid molecules are forced to fluctuate in their equilibrium value to a greater degree, resulting in a rise in the specific heat [55]. The results indicated that the nano-enhanced refrigerants required less amount of heat to raise the temperature than the base refrigerant R1234yf.” (3.1 Thermo-physical properties of TiO₂/R1234yf refrigerant)

            “The outcomes of the study indicate that the concentration of metal particles at the nanoscale affects how well nano-refrigerant transfers heat. The boundary layer's height is reduced and the flow boiling HTC of the nano-refrigerant is increased as a result of the molecular adsorption layer that forms on the surface of nanoparticles and their breakup [41].”

            “Despite the fact that the mass flow rate of the heat transfer fluid was kept constant for the purposes of this investigation, the dispersion of nano-sized particles to the pure refrigerant improves the pressure decrease. When nanoparticle concentration increases, there may be more particle-to-wall collisions, which results in a greater pressure drop for the nano-refrigerant than for the base refrigerant. The high-pressure drop of the nano-refrigerant has an effect on a refrigeration system's capacity to pump. As the nanoparticles loading rises, frictional pressure loss also rises, necessitating a significant amount of pumping energy for circulating the nano-refrigerant throughout the system.”(3.3 Pressure drop and Heat transfer of TiO₂/R1234yf refrigerant)

• Comment 7: In the section 3.2, “Numerical Result” and “Simulation Result” may mislead readers, and it is recommended that numerical results be modified to “predicted results”. Same problem in Table 5.

Response 7: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. The numerical results are modified to predict results in the revised manuscripts.

The authors would like to thank the reviewer for suggesting the very important and vital points that helped us to improve the quality of the work reported in this manuscript.  The authors have made a very genuine attempt to give utmost importance to address those points in the best possible manner. 

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Author Response File: Author Response.pdf

Reviewer 2 Report

 

1.       The authors should explain the symbols used in the equations used in the paper.

2.        In addition, a flow chart must be attached for the numerical model.

3.       The authors validated the numerical model results with the experimental results. So, authors, why not verify the simulation results with the experimental results? Comparing the numerical results and simulation results with each other is not sufficient to prove the accuracy of the developed numerical and simulation models. The authors should definitely address this topic in the paper. What are the assumptions made for the numerical model and simulation?

4.       What are the limitations of both theoretical approaches? To what extent do they both approach the experimental approach?

5.       The predictive model and ANSYS simulation approaches should be explained in more detail. It is not enough to just give the equations used.

6.        Contours of the simulation results, such as temperature, pressure, and speed, are required.

7.       Deep and mechanistic discussions are required to explain the results obtained. In the section "Results and Discussion," the study's comparative analysis with others must be thoroughly discussed. The findings of the paper should be described with quantitative analysis results, such as percent or numerical results.

8.       Can the authors shed some light on why the difference between the numerical result and the simulation result increases with the addition of TiO₂?

 

9.       Finally, the conclusion section is missing some perspectives related to future research work.

 

1.       The authors should explain the symbols used in the equations used in the paper.

2.        In addition, a flow chart must be attached for the numerical model.

3.       The authors validated the numerical model results with the experimental results. So, authors, why not verify the simulation results with the experimental results? Comparing the numerical results and simulation results with each other is not sufficient to prove the accuracy of the developed numerical and simulation models. The authors should definitely address this topic in the paper. What are the assumptions made for the numerical model and simulation?

4.       What are the limitations of both theoretical approaches? To what extent do they both approach the experimental approach?

5.       The predictive model and ANSYS simulation approaches should be explained in more detail. It is not enough to just give the equations used.

6.        Contours of the simulation results, such as temperature, pressure, and speed, are required.

7.       Deep and mechanistic discussions are required to explain the results obtained. In the section "Results and Discussion," the study's comparative analysis with others must be thoroughly discussed. The findings of the paper should be described with quantitative analysis results, such as percent or numerical results.

8.       Can the authors shed some light on why the difference between the numerical result and the simulation result increases with the addition of TiO₂?

 

9.       Finally, the conclusion section is missing some perspectives related to future research work.

Author Response

Dear Reviewer,

Thank you for taking your time and giving your valuable comments for our paper. We had the opportunity to resubmit the paper after a deep revision of the manuscript following the comments of the reviewers. Therefore we followed punctually your precious remarks and we have improved the manuscript. As a matter of fact, in the paper we have highlighted the changes made in Yellow color, pertaining to your comments. Specifically, below are the answers to your point-to-point concerns.

• Comment 1: The authors should explain the symbols used in the equations used in the paper.

Response 1: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. The equations are cited in the revised manuscript.

• Comment 2: In addition, a flow chart must be attached for the numerical model.

Response 2: Considering the valuable suggestion made by the reviewer, a flow chart for the numerical model is included in the modified manuscript.

Figure 2. Flow chart for simulation

“The flow of the simulation analysis is shown in the figure 2. The initial stage of the simulation is modelling of geometry. The geometry of the evaporator tube is modelled as per the dimension given in the figure 3. Then generate the meshes and check the mesh quality. If the mesh quality is good, then it is proceed to the specify boundary conditions. After that, chose the computation algorithm (SIMPLE) and discretization method. Then check the independency of the grid and visualize the flow.” (2.3 Simulation Method)

• Comment 3: The authors validated the numerical model results with the experimental results. So, authors, why not verify the simulation results with the experimental results? Comparing the numerical results and simulation results with each other is not sufficient to prove the accuracy of the developed numerical and simulation models. The authors should definitely address this topic in the paper. What are the assumptions made for the numerical model and simulation?

Response 3: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. The validation with experimental result and the assumptions made for the numerical model and simulation are added in the revised manuscript.

            “The predictive models are utilized to identifying the thermo-physical properties of the TiO2/R1234yf nano-refrigerant such as thermal conductivity, viscosity, specific heat and density. And also to predict the heat transfer coefficient and pressure drop of the evaporator tubes. The major assumptions considered for the predictive analysis are:

• The nanoparticles are spherical in nature
• The nano-refrigerant is treated as homogeneous
• No surfactant is used
• Single phase approach” (2.2 Predictive Models)

            “To solve the two-phase mathematical model that was used to simulate the flow boiling in the evaporator tube, some hypotheses are taken into account. The flow field is considered as two dimensional and turbulent. And also the heat flux is uniform along the length of the test section. The fluid is considered as incompressible and negligible force vectors.”(2.3 Simulation Method)

            “The results obtained from the experiment analysis done by Sun & Yang [44] are used for validation of software tool. They were determined the HTC of the CuO nanoparticle added on R141b refrigerant. The average size of the nanoparticle for this study is 30nm. The vapour quality ranges from 0.3 to 0.8 and the nanoparticle mass fraction ranges between 0.1 and 0.3% of weight They utilized a copper tube having 1mm thickness, 10mm inner diameter and 1400mm length. The table 4 shows the variation of flow boiling heat transfer coefficient is obtained by simulation and experiment analysis. The maximum deviation is observed as around 7%, which indicates the reliability of the present analysis.” (2.3.3 Software tool’s validation)

Table 4. Validation of software tool

Vapor Quality	Flow boiling HTC (W/m2K) of CuO/R141b (0.3 wt.%)
	Experiment by Sun & Yang [44]	Simulation	Deviation (%)
0.3	2255.85	2346.08	3.99
0.4	2579.3	2713.42	5.2
0.5	2845.85	2983.89	4.62
0.6	3374.94	3574.06	5.9
0.7	3847.2	4108.8	6.79
0.8	4448.3	4750.78	6.8

 

• Comment 4: What are the limitations of both theoretical approaches? To what extent do they both approach the experimental approach?

Response 4: Considering the valuable suggestion made by the reviewer, the limitations and the future scopes are included in the revised manuscript.

                “The numerical and simulation techniques used in this study. The identification of nano refrigerant performance and energy efficiency in actual cooling systems requires experimental verification, which is outside the focus of this work. Only lower concentrations and smaller sizes of nanoparticles are covered by the current research. Future analyses could expand it to include higher concentrations, different nanoparticle shapes, and varied nanoparticle sizes in the refrigerant. According to recent research the TiO₂/R1234yf nano-enhanced refrigerant has the potential to improve the performance of the system. It could be used to human comfort as well as to residential refrigeration systems and vehicular air conditioning systems. The performance of nano-enhanced refrigerants and their energy efficiency, however, must be experimentally demonstrated and must be compatible with real-time refrigeration systems. In conclusion, nano-enhanced refrigerants will soon be a key component of environmentally responsible and energy-efficient refrigeration systems.” (Conclusion Section)

• Comment 5: The predictive model and ANSYS simulation approaches should be explained in more detail. It is not enough to just give the equations used.

Response 5: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. In the modified manuscript, detail explanations are included. (2.3 Simulation method)

• Comment 6: Contours of the simulation results, such as temperature, pressure, and speed, are required.

Response 6: Considering the valuable suggestion made by the reviewer, the contours of the simulation results are added in the modified manuscript.

            “The contours of velocity and pressure obtained by the simulation analysis are depicted in the figures 10 and 11 respectively. The flow pattern along the axial length of the test section is visualized at the heat flux of 4020W/m², 1 vol.% of TiO₂ nanoparticle concentration and 200 kg/m²s mass flux.

Figure 10. Velocity contour

            The velocity is varied between 0 and 0.389 m/s. The maximum velocity is observed at the center and the minimum velocity is at the walls of the test section. At wall the velocity is zero due to the no-slip condition. It is also observed that the velocity is varies in axial as well as radial directions.

Figure 11. Pressure Contour

            The pressure variation along with evaporation section is depicted in the figure 11. It indicates the pressure decreased from 681 to 670kPa.” (3.2 Flow Patterns)

• Comment 7: Deep and mechanistic discussions are required to explain the results obtained. In the section "Results and Discussion," the study's comparative analysis with others must be thoroughly discussed. The findings of the paper should be described with quantitative analysis results, such as percent or numerical results.

Response 7: Authors are thankful for your valuable suggestions. The physics behind the effect of different nanoparticle volume fractions and temperature on thermophysical properties and flow characteristics are included the modified manuscript.

            “The thermal conductivity of the nano-refrigerant is one of the key thermo-physical characteristics to enhance the thermal performance of the system. The heat transfer rate of the system greatly affects on the thermal conductivity of the nanoparticles doped in the refrigerant [45].”

            “The thermal conductivity of the nano-refrigerant enhanced with the doping of TiO2 nanoparticle. This is due to the Brownian motion effect of the nanometer sized materials in the base working fluid. Photons are used to transmit heat by propagating lattice vibrations in crystalline nanoparticle immersed in base fluid. This kind of phonon oscillates erratically. Some ballistic phonon events might lead to advances in thermal conductivity. If the ballistic phonons that originated in a particle may travel through the fluid and reach neighbouring particles, a significant increase in thermal conductivity is occurred [46]. Due to Brownian motion, heat can travel between particles more efficiently the closer they are to one another. Meanwhile, the thermal conductivity is declined with the temperature. When the temperature is raises the mean free path between the molecules also increases. This reduces the collision between them [47]. While the separation between the nano-particle grows the coulomb interaction (i.e., near – field radiation) degrades. This reduces the thermal conductivity [48].”

            “Viscosity is another vital property, which influences the pressure drop of the refrigeration system [49].”

            “The high surface to volume area ratio of nanoparticles can be attributed as the main cause of the increase in viscosity with nanoparticles. Because nanoparticles have a larger surface area, there is more friction, which raises viscosity. On the other hand, it decreases when the temperature rises since it is hypothesised that the entropy increases, thereby the thermal energy increases. High viscosity is not preferred for refrigeration systems, as is well known. It should be remembered that at higher temperatures, the enhanced viscosity with the nanoparticle will diminish and get closer to that of the base fluid. Therefore, for such systems, the slight increase in viscosity caused by the addition of nanoparticles may be disregarded [50].”

            “The density of the nano-refrigerant is increased by the addition of nanoparticles to the base fluids. The increment in density for the resulting nano-refrigerant will become increasingly apparent as the doping fraction of nanoparticle rises. It is caused by the significant variations in density between materials in the solid and liquid phases. As the temperature rises, the volume of the fluid increased. Hence, the density of the nano-refrigerant is decreases with the temperature rise [50, 51].”

            “Generally, the specific heat of nano-refrigerant is reduced with increasing the concentration of nanoparticles. This is because the specific heat of nano oxide materials especially TiO₂ is less [52, 53]. However, the specific heat of nano-enhanced refrigerants improved with temperature. As the temperature rises, the liquid molecules are forced to fluctuate in their equilibrium value to a greater degree, resulting in a rise in the specific heat [54]. The results indicated that the nano-enhanced refrigerants required less amount of heat to raise the temperature than the base refrigerant R1234yf.” (3.1 Thermo-physical properties of TiO₂/R1234yf refrigerant)

            “The outcomes of the study indicate that the concentration of metal particles at the nanoscale affects how well nano-refrigerant transfers heat. The boundary layer's height is reduced and the flow boiling HTC of the nano-refrigerant is increased as a result of the molecular adsorption layer that forms on the surface of nanoparticles and their breakup [40].”

            “Despite the fact that the mass flow rate of the heat transfer fluid was kept constant for the purposes of this investigation, the dispersion of nano-sized particles to the pure refrigerant improves the pressure decrease. When nanoparticle concentration increases, there may be more particle-to-wall collisions, which results in a greater pressure drop for the nano-refrigerant than for the base refrigerant. The high-pressure drop of the nano-refrigerant has an effect on a refrigeration system's capacity to pump. As the nanoparticles loading rises, frictional pressure loss also rises, necessitating a significant amount of pumping energy for circulating the nano-refrigerant throughout the system.”(3.3 Pressure drop and Heat transfer of TiO₂/R1234yf refrigerant)

• Comment 8: Can the authors shed some light on why the difference between the numerical result and the simulation result increases with the addition of TiO₂?

Response 8: Authors are thankful to the reviewer for the query. This analysis is not done in this time. We will include this also in future works.

• Comment 9: Finally, the conclusion section is missing some perspectives related to future research work.

Response 9: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. In the revised manuscript, the future scope of research work is added.

            “The numerical and simulation techniques used in this study. The identification of nano refrigerant performance and energy efficiency in actual cooling systems requires experimental verification, which is outside the focus of this work. Only lower concentrations and smaller sizes of nanoparticles are covered by the current research. Future analyses could expand it to include higher concentrations, different nanoparticle shapes, and varied nanoparticle sizes in the refrigerant. According to recent research the TiO2/R1234yf nano-enhanced refrigerant has the potential to improve the performance of the system. It could be used to human comfort as well as to residential refrigeration systems and vehicular air conditioning systems. The performance of nano-enhanced refrigerants and their energy efficiency, however, must be experimentally demonstrated and must be compatible with real-time refrigeration systems. In conclusion, nano-enhanced refrigerants will soon be a key component of environmentally responsible and energy-efficient refrigeration systems.” (Conclusion Section)

The authors would like to thank the reviewer for suggesting the very important and vital points that helped us to improve the quality of the work reported in this manuscript.  The authors have made a very genuine attempt to give utmost importance to address those points in the best possible manner. 

Author Response File: Author Response.pdf

Reviewer 3 Report

Authors compared findings of the  simulation approach  with numerical results, and the predictive models are validated. The article is good. This work is original, novel and important to the field. The paper could be published after major revision

 

- Write the full form of TiO2/R1234yf in its first appearance.

- Add few more recently published paper from the year 2021/2022/2023.

- The outcomes observed from the predictive technique and the simulation approach  had an average absolute variation of 9.91%? How average absolute variation of 9.91% has been calculated? Add more reference for section 2.4 Average absolute deviation estimation.

- Explain Figure 1. SEM image of TiO2 nanoparticle?

- Provide appropriate citation for equations.

-  From the property analysis, it is observed  that the thermal conductivity, viscosity and density are enhanced by 7.49%, 2.54% and  2.87% respectively, at the temperature of 250C and volume concentration of 1%. However in the same condition, the specific heat shows 1.84% reduction. Why  specific heat shows reduction? 

Mention in suitable place.

- Why authors focused on calculation average absolute variation in the research?

- Researchers looked at how vapour compression systems behaved while using R1234yf 73 as the working fluid?

Check this type of sentence?

- Mention the significance of Nanoparticle Impact Factor?

- provide the reason why author mentioned that  that the average size of nanoparticle is 30nm? What is the significance of providing average size?

- Reconstructed the conclusion part.

- Need more explanation on Figure 4. Dependency of Grids.

-

 

 

 

Moderate editing of English language required.

Researchers looked at how vapour compression systems behaved while using R1234yf 73 as the working fluid?

Check this type of sentence?

 

Author Response

Dear Reviewer,

Thank you for taking your time and giving your valuable comments for our paper. We had the opportunity to resubmit the paper after a deep revision of the manuscript following the comments of the reviewers. Therefore we followed punctually your precious remarks and we have improved the manuscript. As a matter of fact, in the paper we have highlighted the changes made in Yellow color, pertaining to your comments. Specifically, below are the answers to your point-to-point concerns.

            Authors compared findings of the simulation approach with numerical results, and the predictive models are validated. The article is good. This work is original, novel and important to the field. The paper could be published after major revision

• Comment 1: Write the full form of TiO₂/R1234yf in its first appearance.

Response 1: Considering the valuable suggestion made by the reviewer, the terms in the equation is defined in the modified article.

• Comment 2: Add few more recently published paper from the year 2021/2022/2023.

Response 2: Considering the valuable suggestion made by the reviewer, more recently published papers are included in the revised manuscript.

• Comment 3: The outcomes observed from the predictive technique and the simulation approach had an average absolute variation of 9.91%? How average absolute variation of 9.91% has been calculated? Add more reference for section 2.4 Average absolute deviation estimation.

Response 3: Authors are thankful to the reviewer for giving suggestion to improving the quality of article.

                “The average deviation of results indicates the level of prediction. It is the deviation of the results obtained from simulation to the prediction analysis. The level of prediction is tabulated in the table 5. The AAD (average absolute deviation) is expressed by [45]:”

 

• Comment 4: Explain Figure 1. SEM image of TiO₂ nanoparticle?

Response 4: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. Explanation is given the modified article.

            ‘The scanning electron microscopy (SEM) image is revealed the size of the nanoparticle, which is shown in figure 1. The SEM is carried by Quanta device and with the magnification of x120000. It indicated that the average size of nanoparticle is 30nm and spherical in shape.” (2.1 Materials)

• Comment 5: Provide appropriate citation for equations.

Response 5: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. The equations are cited in the revised manuscript.

• Comment 6: From the property analysis, it is observed that the thermal conductivity, viscosity and density are enhanced by 7.49%, 2.54% and 2.87% respectively, at the temperature of 25⁰C and volume concentration of 1%. However in the same condition, the specific heat shows 1.84% reduction. Why specific heat shows reduction? 

Response 6: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. The reason for reduction in specific heat is added in the revised article.

“Generally, the specific heat of nano-refrigerant is reduced with increasing the concentration of nanoparticles. This is because the specific heat of nano oxide materials especially TiO₂ is less [52, 53]. However, the specific heat of nano-enhanced refrigerants improved with temperature. As the temperature rises, the liquid molecules are forced to fluctuate in their equilibrium value to a greater degree, resulting in a rise in the specific heat [54]. The results indicated that the nano-enhanced refrigerants required less amount of heat to raise the temperature than the base refrigerant R1234yf.” (3.1 Thermo-physical properties of TiO₂/R1234yf refrigerant)

Mention in suitable place.

• Comment 7: Why authors focused on calculation average absolute variation in the research?

Response 7: Authors are thankful to the reviewer for giving suggestion to improving the quality of article.

                “The average deviation of results indicates the level of prediction. It is the deviation of the results obtained from simulation to the prediction analysis. The level of prediction is tabulated in the table 5. The AAD (average absolute deviation) is expressed by [45]:”

Table 5. Level of prediction

Prediction Levels	AAD
Excellect	Upto ± 5%
Very Good	± 5 to 15%
Good	± 15 to 25%
Satisfactory	± 25 to 35%
Overpredication	above ± 35%

 

• Comment 8: Researchers looked at how vapour compression systems behaved while using R1234yf 73 as the working fluid? Check this type of sentence?

Response 8: Authors are thankful to the reviewer for pointing out such a grammatical error committed. The mistake is now rectified in the modified manuscript.

• Comment 9: Mention the significance of Nanoparticle Impact Factor?

Response 9: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. “The nanoparticle impact factor, or NIF, is influenced by the particle concentration, the base refrigerant's and nano-materials' transport characteristics, the mass flow, and the vapour quality.”

• Comment 10: Provide the reason why author mentioned that that the average size of nanoparticle is 30nm? What is the significance of providing average size?

Response 10: Authors are thankful to the reviewer for query. “The average size of nanoparticle chose below 30nm is to avoid the sedimentation of the particle in the base fluid.”

• Comment 11: Reconstructed the conclusion part.

Response 11: Authors are thankful to the reviewer for giving suggestion to improving the quality of article. In the revised manuscript, the future scope of research work is added.

            “The numerical and simulation techniques used in this study. The identification of nano refrigerant performance and energy efficiency in actual cooling systems requires experimental verification, which is outside the focus of this work. Only lower concentrations and smaller sizes of nanoparticles are covered by the current research. Future analyses could expand it to include higher concentrations, different nanoparticle shapes, and varied nanoparticle sizes in the refrigerant. According to recent research the TiO₂/R1234yf nano-enhanced refrigerant has the potential to improve the performance of the system. It could be used to human comfort as well as to residential refrigeration systems and vehicular air conditioning systems. The performance of nano-enhanced refrigerants and their energy efficiency, however, must be experimentally demonstrated and must be compatible with real-time refrigeration systems. In conclusion, nano-enhanced refrigerants will soon be a key component of environmentally responsible and energy-efficient refrigeration systems.” (Conclusion Section)

• Comment 12: Need more explanation on Figure 4. Dependency of Grids.

Response 12: Authors are thankful to the reviewer for the query about the grid independence simulations

                “In order to generate grids that perform the most effectively, it is necessary to take into account the grids' shape, quality, and number. A component that affects the overall processing cost and the precision of the outcomes of simulation analysis is the number of grids. A considerable spatial discretization error is produced by coarse grids, which lowers the accuracy of the analysis's findings [42]. Due to this the selection of optimum grid number is crucial. Based on the evaluation of various grid conditions, the grid independence analysis is a procedure used to identify the best grid condition that has the smallest grids without generating a difference in the numerical results.” (2.3.1 Grid Independency analysis)

The authors would like to thank the reviewer for suggesting the very important and vital points that helped us to improve the quality of the work reported in this manuscript.  The authors have made a very genuine attempt to give utmost importance to address those points in the best possible manner. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

After careful revision by the author, the quality of the paper has been greatly improved and can be published.

Reviewer 2 Report

The authors have substantially improved their manuscript and answered most of my previous concerns. Its technical and scientific value improved fairly. It now has the quality, in my opinion, to be published in its current form.

 

I accept the publication of the revised manuscript, sustainability-2509122, in Sustainability Journal.

English language fine. No issues detected

Reviewer 3 Report

The paper is now accepted.