Influence of Pressure, Velocity and Fluid Material on Heat Transport in Structured Open-Cell Foam Reactors Investigated Using CFD Simulations

Round 1

Reviewer 1 Report

This review concerns the manuscript of Sinn et al. "Influence of Pressure, Velocity and Fluid Material on Heat Transport in Structured Open-Cell Foam Reactors Investigated Using CFD Simulations". The work is a purely numerical CFD study of fluid/heat flow in fully resolved foam structures strongly based on authors' previous works ([15,19]) - a reader is referred to other publications for the details of the model and validation tests. Although the present study can be considered as incremental, some may find the results useful enough to warrant publication. 

I have a few remarks that should be addressed by the authors:

1. The influence of certain phenomena (like e.g. gravity) on the heat flow is analysed but the authors do not make any attempt to estimate the relative importance of these phenomena using characteristic non-dimensional numbers (Reynolds, Peclet, Froude etc.). Maybe proper analysis of Froude criterion would make CFD study of gravity influence superflous?

2. Total heat source was selected as S=12.5 W (line 138). This value is not motivated. How general are the obtained results and how the shown trends depend on the particular value of S?

3. Grid independence study (Appendix A) is not particularly convincing as it seems that the studied parameters are not sensitive to the number of cells. It is better to show some parameter which is sensitive to grid density and reaches limiting value for sufficiently dense grid.

Author Response

Answers in italic

I have a few remarks that should be addressed by the authors:

We thank the reviewer for the positive feedback on our manuscript. The point-by-point answers follow:

1. The influence of certain phenomena (like e.g. gravity) on the heat flow is analysed but the authors do not make any attempt to estimate the relative importance of these phenomena using characteristic non-dimensional numbers (Reynolds, Peclet, Froude etc.). Maybe proper analysis of Froude criterion would make CFD study of gravity influence superflous?

• The reviewer certainly has a point. So far natural convection in open-cell foams has only been studied under conditions that are not comparable to the ones that are standard in heterogeneous catalysis (i.e., temperature above 500K, heat production/consumption, flow rates, pressure, etc.). However, some studies stated the great impact of nat. convection on the overall heat transport of the foam. Natural convection, however, significantly depends on the geometry of the hot surface. Hence, classic ranges for Froude, Rayleigh or Grashof numbers are not applicable. Furthermore, with the induced heat production in the solid, significant temperature differences throughout the solid foam and fluid occur. Therefore, we could not use an a-priori criterion and simulated the effect of gravity. We have add thus added an explanation on page 6 (l. 191).

2. Total heat source was selected as S=12.5 W (line 138). This value is not motivated. How general are the obtained results and how the shown trends depend on the particular value of S?

• This is a good point. I our previous work [1], we estimated the heat production of the CO₂ methanation to be approx. S = 50 W for a 25 x 25 mm cylindrical foam. It was further shown for a range of heat source intensities (5 – 150 W), that specific heat flows and temperature increase collapse regardless of the applied temperature. To make this clearer, we added some explanation (p4., l. 104; p. 5, l. 152)
• [1] C. Sinn, G.R. Pesch, J. Thöming, L. Kiewidt, Coupled conjugate heat transfer and heat production in open-cell ceramic foams investigated using CFD, Int. J. Heat Mass Transf. 139 (2019) 600–612. https://doi.org/10.1016/j.ijheatmasstransfer.2019.05.042.

3. Grid independence study (Appendix A) is not particularly convincing as it seems that the studied parameters are not sensitive to the number of cells. It is better to show some parameter which is sensitive to grid density and reaches limiting value for sufficiently dense grid.

• This is a good suggestion. We extended the grid independence study by additional mesh generations (see p. 8 Appendix B1).

Reviewer 2 Report

This manuscript uses the numerical method to study the heat transport in a structured open-cell foam reactors.  The influence of pressure, velocity, and fluid properties have been investigated in the study.  Overall this is a well-organized paper with clear narration.

My main concern is about the mesh setup in the simulations.  It was stated that the polyhedral mesh was used for all the computational domain.  It would be helpful to show a picture of the generated mesh.  Is the boundary layers near the cell and structured walls are refined?  Without solving the boundary layers using refined cells, the captured physics can have a large uncertainties.

There is no validation of the developed model in the manuscript.  The would diminish the reliability of the developed model.  Some benchmark cases with existed experiment/simulation data would be important to validate the developed model in this work. 

Author Response

Answers in italic

This manuscript uses the numerical method to study the heat transport in a structured open-cell foam reactors.  The influence of pressure, velocity, and fluid properties have been investigated in the study.  Overall this is a well-organized paper with clear narration.

We like to thank the reviewer for the supportive comments on our manuscript. The answer follows:

1. My main concern is about the mesh setup in the simulations.  It was stated that the polyhedral mesh was used for all the computational domain.  It would be helpful to show a picture of the generated mesh.  Is the boundary layers near the cell and structured walls are refined?  Without solving the boundary layers using refined cells, the captured physics can have a large uncertainties.

• This is a helpful comment. We added some depictions of the meshes to the Appendix (p. 8). The polyhedral mesh is in line with literature on validated catalytic reaction in foams [2].

2. There is no validation of the developed model in the manuscript.  The would diminish the reliability of the developed model.  Some benchmark cases with existed experiment/simulation data would be important to validate the developed model in this work. 

• The observation of the reviewer is correct as there is no actual validation in this manuscript. Our previous model is virtually identical and was validated and verified. In this study, mainly the geometry was exchanged. In order to make this more clear to the reader we added a sentence (p. 5, l. 139): In our previous studies, similar models were validated against pressure drop correlations, heat transfer correlations and verified against CFD data [15,19]. Here, the principial changed model property is the CAD-created geometry. We therefore omit to validation as the models are virtually identical.

[2]      Y. Dong, O. Korup, J. Gerdts, B. Roldán Cuenya, R. Horn, Microtomography-based CFD modeling of a fixed-bed reactor with an open-cell foam monolith and experimental verification by reactor profile measurements, Chem. Eng. J. 353 (2018) 176–188. https://doi.org/10.1016/j.cej.2018.07.075.

Round 2

Reviewer 2 Report

The authors addressed my question properly.  I would recommend it for publication at ChemEngineering.