Mesoscale Insights into Convective Heat Transfer in Concentric Cylinder Systems
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe manuscript presents a numerical study of convective heat transfer in concentric cylinders at mesoscopic scales using finite element simulations. The topic is timely and relevant to micro and mesoscale thermal management research, and the paper is generally well written. The study successfully compares numerical predictions to established analytical models, providing insight into the limitations of conventional correlations. However, several areas require clarification, additional validation, and improved discussion.
-The manuscript lacks sufficient quantitative validation of the FEM results.
-Consider providing mesh sensitivity analysis to ensure numerical accuracy, given that the smallest mesh elements reach the sub nanometer scale.
-The definition of “mesoscale” is somewhat ambiguous in this context. The specific range of characteristic dimensions that defines “mesoscale” in the study, and how continuum assumptions remain valid at these scales.
-I recommend to provide a more detailed discussion of the transition mechanism from conductive to convective regimes, possibly supported by local Rayleigh number distributions or flow visualization metrics.
-The conclusion that convection becomes relevant at smaller scales than predicted analytically is intriguing but needs stronger justification. Could numerical artifacts such as low Grashof numbers or meshing constraints influence this earlier onset of convection?
-The definition of happ(ri, ro) (Eq. 13) and its physical meaning need clarification.
-Equation (13) defines an “apparent thermal transfer coefficient,” but its relation to conventional heat transfer coefficients or Nusselt numbers is unclear.
-I recommen to define all symbols in the nomenclature.
-The study’s potential impact on MEMS or nanowire thermal measurements is only briefly mentioned.
-Figures 3 (b) would benefit from clearer maker and unclear line indications.
-Pleaase consider including more recent studies on micro/nanoscale convection for broader context. They are too old literature. Then update the introduction including recent literature.
-The conclusion that “a cylinder becomes free-standing when ro ≈ 200ri” is based on a single parameter set (ri = 100 µm, ΔT = 100 K). How general is this ratio for different fluids or temperature differences?
-The analysis focuses on keff/k but does not present Nusselt (Nu) or Rayleigh (Ra) number correlations. The use of keff/k > 1 as an indicator of convection is unconventional and lacks physical clarity.
-Can this 2D assumption realistically capture 3D buoyancy-driven flow patterns such as plumes?
-The conclusion claims that the results can guide identification of conduction-dominated regimes “for future mesoscale systems,” yet only air and laminar steady-state conditions were studied. Would similar behavior be expected for other gases or under turbulent or transitional conditions?
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript presents a systematic numerical investigation of heat transfer in coaxial annuli, focusing on the transition from conduction‐dominated to convection‐dominated regimes under different temperature gradients and geometrical scales. The topic is relevant and well structured, and the results could be useful for micro- and meso-scale thermal design. Overall, the paper is clearly written and of potential interest for publication after minor revision.
1. A proper mesh-independence proof should be provided rather than simply stating that the “Finest” mesh in COMSOL was used and “works well.” Please demonstrate grid convergence by comparing at least two or three levels of mesh refinement and showing that key parameters (e.g., total heat flux or keff/kk_\mathrm{eff}/k) vary negligibly. This short validation will enhance the numerical credibility.
2. The current figures convey limited physical insight. It is recommended to enrich the visualization—for example, adding comparative temperature and velocity contour plots for conduction- and convection-dominated regimes—to clearly illustrate the mechanism transition. Improving the figure clarity and density would greatly strengthen the discussion and make the conclusions more convincing.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe revised manuscript is now excellent. All previous concerns of the 1st manuscript are fully and convincingly resolved. This is an original, high-quality contribution showing that natural convection in air becomes significant at much smaller scales than classical correlations predict, with clear implications for microwire/MEMS thermal experiments. I am happy to recommend acceptance as is no further changes needed. Congratulations on a great paper~!.
