Page 185 - gas transport in porous media

CHAPTER 10

NATURAL CONVECTION GAS TRANSPORT
IN POROUS MEDIA

K. KHANAFER AND K. VAFAI
Department of Mechanical Engineering, University of California, Riverside, CA 92521, USA

10.1 INTRODUCTION

Natural convection is the study of heat transport processes by fluid motion carrying
energy with it as a result of the temperature difference between the fluid and the
solid. Convection heat transfer consists of two main mechanisms due to both diffusion
(random molecular motion) and bulk motion of the fluid.
Convection heat transfer can be classified according to the nature for the flow
as forced convection and free convection. Forced convection investigates the heat
transfer between a moving fluid and a solid surface. The fluid flow is caused by
an external means such as a fan, a pump, or atmospheric winds. Thus the flow
has a nonzero streaming motion in the farfield away from the solid surface. There
are various types of forced convection such as duct flows and bodies immersed in
a uniform stream.
In contrast, a free convection flow field is a self-sustained flow driven by the pres-
ence of a temperature difference, which is opposed to a forced convection where
external means are used to cause the flow. Natural convection fluid motion is due
solely to buoyancy force caused by the density differences as a result of the tempera-
ture difference. This force is a strong function of the temperature difference between
the solid and the fluid. As such the buoyancy force will induce a flow current due to
the gravitational field and the variation in the density field. Convection above a hot
surface occurs because hot air expands, becomes less dense, and rises. In general,
natural convection heat transfer is usually smaller compared to a forced convection
heat transfer.
Natural convection can be divided in two main branches namely external and
internal natural convection. External natural convection may occur along different
geometries such as free convection along vertical walls, inclined walls, and horizontal

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C. Ho and S. Webb (eds.), Gas Transport in Porous Media, 179–200.
© 2006 Springer.