Some aspects of fluid flow and heat transfer in porous media
Flows in porous media can exhibit time-dependent, chaotic, and turbulent behaviors, despite the highly dissipative effects of viscosity and thermal conductivity. Since the complicated flow paths in porous media cannot be directly followed in general, the description of the phenomena depends on certain simplifications based on proper averaging techniques, taking the characteristic spatial and time scales specifying the phenomena into consideration. This paper discusses some basic effects of the porous structures on fluid flow and heat transfer, including the examination of the chaotic behaviors in porous media. The presence of the porous matrix brings about both the macroscopic and microscopic (particle-diameter level) effects on fluid flow and heat transfer characteristics basically through the short-distance interaction of fluid with the adjacent inner walls, which exist everywhere in porous media. It is shown that even the concentrated inhomogeneities of permeability and of thermal conductivity modify the action of the buoyant force and the flow resistance, which yields the various modes of convection and the nonlinear stabilities of three-dimensional flow modes, much less for convection with distributed inhomogeneities. Next, the basic aspects of chaotic behaviors of thermal convection in porous media are discussed, concentrating on the finite Prandtl-number effect. The inertia force causes the flow against the action of buoyant force, which will introduce the flow-reversal mechanism and brings about the instability of the flow direction in large-scale convective motions, while the instability of the thermal boundary layers dominates in the limit of Pr {r_arrow} {infinity}. That is, for thermal convection in porous media or in Hele-Shaw cells, if the condition of 1/pr = 0 is relaxed, the chaotic behaviors of different nature are produced. It is also seen that the chaotic behaviors can affect the entropy generation rate through the mechanism of the instabilities which causes chaos. The spatial structures of chaos is discussed from the distribution of the numerical Lyapunov exponent in comparison with numerical and experimental flow patterns. Finally it is noted that the small-scale vortices of the order of the characteristic dimensions of the porous structure dominate the turbulent behaviors for the high Reynolds-number regime. It is hoped that the future studies reexamine the macroscopic governing equations, considering microscopic and unsteady natures of the chaotic and turbulent behaviors.
- Research Organization:
- Kyushu Univ., Fukuoka (JP)
- OSTI ID:
- 20034211
- Resource Relation:
- Conference: 5th ASME/JSME Thermal Engineering Joint Conference, San Diego, CA (US), 03/14/1999--03/19/1999; Other Information: 1 CD-ROM. Operating system required: Windows i386(tm), i486(tm), Pentium (R) or Pentium Pro, MS Windows 3.1, 95, or NT 3.51, 8 MB RAM, MacIntosh and Power MacIntosh with a 68020 or greater processor, System software version 7.1, 3.5 MB RAM (5 MB for PowerMac) 6 MB available hard-disk space, Unix; PBD: 1999; Related Information: In: Proceedings of the 5th ASME/JSME thermal engineering joint conference, [3600] pages.
- Country of Publication:
- United States
- Language:
- English
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