Yasuyuki Takatsu

Natural convection along a vertical porous surface

This paper describes an experimental study on natural convection along a vertical porous surface consisting of a bank of parallel plates with constant gaps. Compared with a smooth surface, the heat transfer for a porous surface with streamwise gaps is somewhat enhanced owing to enthalpy transport due to flow within the gaps and that with spanwise gaps is enhanced due to the leading- and trailing-edge effects of solid-phase micro surfaces. Observations show that no clear transition to turbulent flow occurs at a critical Rayleigh number for a smooth surface and that the boundary layer oscillates with a dominant frequency at higher Rayleigh numbers. The dominant frequency for a porous surface is nearly the same as that for a smooth surface. This fact obviously indicates that the oscillations of natural convection are almost independent of the porous structure of the heating surface and are mainly dependent on the behavior of the outer boundary laver.

Natural Convection in a Vertical Enclosure With Grooves

The natural convection in a vertical enclosure with grooves has been numerically studied. To identify the structure of the thermal boundary layer along the discontinuous vertical surface, the iso-flux condition is imposed to each heating (or cooling) portion of the discontinuous vertical surface, and the adiabatic condition is applied to the other solid walls including the grooves. We focus on the behavior of the thermal boundary layer along the discontinuous vertical surface, and clarify the dual structure of the thermal boundary layer. The whole thermal boundary layer develops along the discontinuous vertical surface, while the internal thermal boundary layer forms on each solid portion of the discontinuous flat surface. We can recognize from the isotherms near the solid portion that the internal thermal boundary layer develops from the leading edge to the trailing edge of the solid portion. The internal thermal boundary layer plays an important role in the heat transfer enhancement. As the thin thermal boundary layer forms at the leading and trailing edges, the heat transfer coefficient near the leading and trailing edges takes higher values.Copyright

Buoyant Plume through a Permeable Porous Layer Located above a Line Heat Source in an Infinite Fluid Space.

An experimental and analytical study is made on the effects of a horizontal porous layer on the development of the buoyant plume arising from a line heat source. In the experiments, the porous layer is simulated by an array of acrylic resin plates spaced with a small gap width according to the Hele-Shaw analogy. Visual observations and experimental temperature distributions show the expansion and contraction of the plume at the lower and upper interfaces of the permeable layer. It is then found that similarity solutions assuming their proper virtual origins can approximately predict the changes in the plume width at the interfaces. The vicinity of the interface is understood as a region of transition between the similarity solutions. The numerical analysis adopts the Beavers-Joseph slip boundary condition with the slip coefficient a estimated as α=1/√(e), where e is the prosity of the porous layer. A fairly good agreement between the experimental and numerical results confirms the validity of the slip coefficient a=1/√(e).