A Comparative Study of Different CFD Codes for Fluidized Beds
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsManuscript ID: dynamics-3045189
Manuscript title: A Comparative Study of Different CFD-codes for Fluidized Beds
Overall comments: The authors thoroughly simulated and compared different in-house and open source CFD solvers for simulating gas-solid two-phase flow in a bubbling fluidized bed. Specifically, the authors investigated the effects of different superficial gas velocities and numerical parameters or sub-models on the prediction of hydrodynamic behaviors, including macroscopic bed expansion ratios and pressure drop. Overall, this article can be recommended for publication after appropriately considering the following issues below.
Specific comments:
-1- In the field of CFD simulation of gas-solid fluidized beds, an important research aspect might be the use of coarse grids, specifically when the grid size exceeds three times the particle size. Coarse grid simulations employ semi-empirical drag models, which tend to overestimate the interphase drag force and, consequently, the bed expansion height, due to the failure to account for the effects of non-uniform structures on sub-grid drag (e.g., Chem Eng Sci, 2019, 204, 228). This may present a good opportunity to discuss the issue further possibly assisted by referring the relevant literature above. Additionally, many recent studies have utilized high-fidelity DNS data to develop drag correlations while it seems the authors mainly focused on the empirical drag formulations, and an extra discussion on this topic may enhance the interest of the Introduction.
-2- Figures 2 and 3: The authors performed coarse-grid simulations with approximately 20 times the particle size to compare the S-O and Gidaspow drag models. In fact, these two types of drag models do not adequately account for the impact of mesoscale heterogeneous structures on drag coefficient prediction under coarse-grid conditions in bubbling and turbulent fluidized beds (e.g., AIChE J, 2021, 67(8), e17299). The two figures clearly demonstrate this point. As the t superficial gas velocity increases, mesoscale structures such as bubbles and particle clusters gradually form in the bed. Therefore, at high gas velocities, the bed expansion becomes increasingly overestimated compared to experimental values. The authors are suggested to provide additional discussion and clarification regarding the concern issue above probably with the aid of the suggested relevant literature.
-3- For multiphase flows, the user guideline and the existing reports commonly adopted the pressure outlet choice to describe the outlet boundary condition. Please give comments and clarifications.
-4- The grid independence analysis could be more detailed.
Comments on the Quality of English LanguageMinor revision
Author Response
1. In the field of CFD simulation of gas-solid fluidized beds, an important research aspect might be the use of coarse grids, specifically when the grid size exceeds three times the particle size. Coarse grid simulations employ semi-empirical drag models, which tend to overestimate the interphase drag force and, consequently, the bed expansion height, due to the failure to account for the effects of non-uniform structures on sub-grid drag (e.g., Chem Eng Sci, 2019, 204, 228). This may present a good opportunity to discuss the issue further possibly assisted by referring the relevant literature above. Additionally, many recent studies have utilized high-fidelity DNS data to develop drag correlations while it seems the authors mainly focused on the empirical drag formulations, and an extra discussion on this topic may enhance the interest of the Introduction.
Response: We found this to be a very constructive comment and have included a grid sensitivity analysis in the paper, as well as mentioned the relevant reference.
2. Figures 2 and 3: The authors performed coarse-grid simulations with approximately 20 times the particle size to compare the S-O and Gidaspow drag models. In fact, these two types of drag models do not adequately account for the impact of mesoscale heterogeneous structures on drag coefficient prediction under coarse-grid conditions in bubbling and turbulent fluidized beds (e.g., AIChE J, 2021, 67(8), e17299). The two figures clearly demonstrate this point. As the superficial gas velocity increases, mesoscale structures such as bubbles and particle clusters gradually form in the bed. Therefore, at high gas velocities, the bed expansion becomes increasingly overestimated compared to experimental values. The authors are suggested to provide additional discussion and clarification regarding the concern issue above probably with the aid of the suggested relevant literature.
Response: We have addressed this comment by including additional discussion on the grid sensitivity analysis and referencing the suggested literature.
3. For multiphase flows, the user guideline and the existing reports commonly adopted the pressure outlet choice to describe the outlet boundary condition. Please give comments and clarifications.
Response: In line with the Fluent user guidelines, we adopted the pressure outlet choice and set zero backflow, ensuring a fully developed flow condition. In OpenFOAM, we used zero gradient, and in MFiX, we used pressureOutflow. All these settings aim to achieve fully developed flow conditions.
4. The grid independence analysis could be more detailed.
Response: We have expanded the grid sensitivity analysis to provide more detail, addressing this suggestion.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsIn the manuscript “A Comparative Study of Different CFD-codes for Fluidized Beds” the fluidized bed columns from literature were analyzed in both 2D and 3D simulations using three different CFD software codes: ANSYS 106 Fluent 2023 R3, MFiX 23.1.1 and OpenFOAM 1. The proposed computational model is correct and clearly described. The simulation results are presented on clear charts and discussed in detail. Chapter 5 conducted an extensive comparative analysis of three considered numerical codes for two models of the resistance factor.
I have only two comments on the submitted manuscript:
1. The literature review is extensive and current, individual items well described, but the Authors should include the following items:
- A CFD Comparative Study of Bubbling Fluidized Bed Behavior with Thermal Effects Using the Open-Source Platforms MFiX and OpenFOAM, Fluids 2022, https://doi.org/10.3390/fluids7010001
- CFD simulation of gas–solid fluidized bed hydrodynamics; prediction accuracy study, International Journal of Chemical Reactor Engineering 2022, https://doi.org/10.1515/ijcre-2022-0071
- Comparison of Experimental Results from Operating a Novel Fluidized Bed Classifier with CFD Simulations Applying Different Drag Models and Model Validation, Processes 2022, https://doi.org/10.3390/pr10091855
- CFD-DEM Fluidized Bed Drying Study Using a Coarse-Graining Technique, Ind. Eng. Chem. Res. 2023, https://doi.org/10.1021/acs.iecr.3c02960
- CFD simulations to study bed characteristics in gas–Solid fluidized beds with binary mixtures of Geldart-B particles: A qualitative analysis, Frontiers 2023, https://doi.org/10.3389/fenrg.2023.1059503
- Computational fluid dynamics modeling of gas–solid fluidized bed reactor: Influence of numerical and operating parameters, Experimental and Computational Multiphase Flow 2024, https://doi.org/10.1007/s42757-023-0158-x
2. Page 3, Line 130 – no explanation for the choice of gas velocities.
Author Response
1. The literature review is extensive and current, individual items well described, but the Authors should include the following items:
- A CFD Comparative Study of Bubbling Fluidized Bed Behavior with Thermal Effects Using the Open-Source Platforms MFiX and OpenFOAM, Fluids 2022, https://doi.org/10.3390/fluids7010001.
- CFD simulation of gas–solid fluidized bed hydrodynamics; prediction accuracy study, International Journal of Chemical Reactor Engineering 2022, https://doi.org/10.1515/ijcre-2022-0071.
- Comparison of Experimental Results from Operating a Novel Fluidized Bed Classifier with CFD Simulations Applying Different Drag Models and Model Validation, Processes 2022, https://doi.org/10.3390/pr10091855.
- CFD-DEM Fluidized Bed Drying Study Using a Coarse-Graining Technique, Ind. Eng. Chem. Res. 2023, https://doi.org/10.1021/acs.iecr.3c02960.
- CFD simulations to study bed characteristics in gas–Solid fluidized beds with binary mixtures of Geldart-B particles: A qualitative analysis, Frontiers 2023, https://doi.org/10.3389/fenrg.2023.1059503.
- Computational fluid dynamics modeling of gas–solid fluidized bed reactor: Influence of numerical and operating parameters, Experimental and Computational Multiphase Flow 2024, https://doi.org/10.1007/s42757-023-0158-x.
Response: We agree that references 1 and 2 are highly relevant to our paper and have included them. However, references 3, 4, 5, and 6 use different methodologies or conditions that are less relevant to our study, and therefore, we have not included them.
2. Page 3, Line 130 – no explanation for the choice of gas velocities.
Response: We have clarified in the paper that the selected gas velocities are adapted from Taghipour et al., as our experimental data reference the same.
Author Response File: Author Response.pdf
