What is it about?
Many porous materials contain pores of very different sizes. For example, clusters of small particles may contain tiny pores within them while leaving much larger pores between the clusters. Fluid can move differently through these two pore networks, and the temperatures within them do not necessarily remain the same. Such structures occur in applications including porous wicks used in heat pipes. In this paper, we develop a mathematical model of natural convection next to a heated vertical surface embedded in such a material. Rather than treating the porous material as a single uniform medium, the model allows fluid motion and temperature to differ between the large- and small-pore regions. Numerical calculations and mathematical analysis show how these two regions exchange heat and how their temperatures gradually become closer as the flow develops along the heated surface.
Featured Image
Photo by Spring Fed Images on Unsplash
Why is it important?
Models of heat transfer in porous materials often assume that the entire material can be represented by a single velocity and temperature. Our results show that this assumption can be inadequate for materials containing two distinctly different pore systems. Near the beginning of the heated surface, the two regions can be far from thermal equilibrium because fluid motion is much stronger in one pore network than in the other. Farther downstream, heat exchange between them gradually brings their temperatures closer together. This distinction is important when porous structures are designed for thermal management. The model identifies a small set of parameters that controls how strongly the two pore systems interact and how rapidly thermal equilibrium is reached. The results therefore provide a framework for understanding and eventually designing bidisperse porous materials for applications in which fluid flow and heat transfer occur simultaneously. The analysis also shows that the model behaves robustly when the description of momentum exchange between the two pore networks is generalized.
Perspectives
I find this work particularly interesting because it shows how adding one level of structural detail to a familiar porous-medium problem can reveal physical behavior that a conventional single-pore model cannot capture. Treating the large- and small-pore regions separately makes it possible to describe differences in both fluid motion and temperature while still reducing the problem to a relatively compact mathematical formulation. I also view this paper as an early step toward a more complete understanding of transport in multiscale porous materials. At the time of the study, the two-velocity, two-temperature model was still new, and important questions concerning experimental validation and the determination of interphase momentum-transfer parameters remained open. I hope that the framework helps motivate further experimental and theoretical studies of how pore structure controls convection and heat transfer.
Andrey V Kuznetsov
North Carolina State University
Read the Original
This page is a summary of: Vertical Free Convective Boundary-Layer Flow in a Bidisperse Porous Medium, Journal of Heat Transfer, July 2008, ASME International,
DOI: 10.1115/1.2943304.
You can read the full text:
Contributors
The following have contributed to this page
