Gözenekli ortamlarda taşınım olayı

Transport phenomena in porous media has received much attention in recent years because of its important role in a large variety of engineering and technical applications which span from the transport processes in biomechanical systems, such as blood flow in the pulmonary alveolar sheet, to the large scale circulation of brine in a geothermal reservoir. There is no doubt that this new branch of convective heat transfer keeps attracting engineering and scientists from diversified disciplines such as mechanical engineering, chemical engineering, civil engineering, nuclear engineering, aerospace engineering, bio engineering, food science and geothermal physics. Porous materials are encountered everywhere in everyday life, in technology, and in nature. A material can be defined as a porous medium if the material has the following properties, Dullien (1992): a) The material must contain relatively small spaces, called pores or voids, imbedded in the solid or semi-solid matrix. The pores usually contain some fluid, such as air, water, etc., or a mixture of different fluids, b) The fluids should be able to penetrate through one face of the material and emerge on the other side. Building materials, such as bricks, concrete, limestone, sand stone, soil, lungs and bones are examples of porous materials encountered in practice. All macroscopic properties of porous media are influenced by the pore structure. Macroscopic pore structure parameters represent average behaviour of a sample containing many pores and the some important pore structure parameters are the porosity, the tortuosity and the permeability. The porosity and the tortuosity are the characteristics of a porous medium; the permeability is the mass transfer property of the porous media. Porosity, e, is the fraction of the bulk volume of the porous material that is occupied by the pore space. Depending on the type of the porous medium, the porosity may vary from near zero to almost unity. Measurement of porosity is made by using several techniques, such as imbibition, mercury injection and gas injection methods give an effective porosity value. The porosity is the most important property of a porous medium and it affects most of the physical properties of the medium. For a homogeneous porous medium, the porosity may be a constant. But in general, the porosity is space dependent. Each void in the porous medium is connected to more than one other pore (interconnected), connected only to one other pore (dead end), or not connected to any other pore (isolated) and fluid flows through the interconnected pores. Tortuosity is used for the two-dimensional simulation of the porous medium. It is not a physical constant and depends on other porous media characteristics, such as porosity, pore diameter, channel shape, etc. Also, tortuosity depends on processes occurring during mass transfer and on the kind of material being transferred. In the simplest situation a porous medium can be considered as a bundle of capillaries. In this case, tortuosity is the ratio of the pore length to the porous medium thickness. It is difficult to determine tortuosity experimentally and in general, tortuosity is calculated by using the porosity and the effective diffusion coefficient or from the Kozeny coefficient, see Mota et al. (1999). The permeability, K, is the measure of the flow conductance of the porous medium and it is defined by the Darcy law and can be written by using porosity as Equation (9), see Ergun (1952). The permeability is independent of the nature of the fluid but it depends on the geometry of the porous medium and the measurement of the permeability can be achieved in the case of an isotropic media. Both liquids and gases have been used to measure permeability. Liquids sometimes change the pore structure and so it affects to the permeability. Keywords: Poroius media, porosity, permeability, Darcy's law, conservation equations.Gözenekli ortamlar günlük hayatımızda her sahada karşımıza çıkmaktadır. İçinden bir akışkanı geçirebilen gözenekli bir ortamda enerji geçişi ve akışkan akışı konusu bilim ve mühendisliğin çok değişik alanlarını ilgilendirmektedir. Bugüne kadar gözenekli ortamlar konusunda çok sayıda çalışma yapılmıştır. Ancak, günümüzde halen gözenekli ortamlarda hesaplanamayan ya da ölçülemeyen akış ve ortam özelliklerine yönelik çalışmalar yoğun olarak sürmektedir. Gözenekli ortamlardaki çalışmalar esas olarak Fransa'da Henry Darcy tarafından 1856 yılında bir hastaneye temiz su getirme projesi kapsamında yapılan çalışmaların daha sonra başka bilim adamları tarafından incelenip gözenekli ortamlarda akışı tanımlayan bir genel denkleme dönüştürülmesi ile başlar. Bu çağrılı çalışmada gözenekli ortamlarda kullanılan genel tanım ve temel denklemlerin tanıtılması esas alınmıştır. Bu çalışma gözenekli ortamlarla ilgili genel tanımlar ve gözenekli yapı içinde bir boşluk ve kati iskeletin olması sebebi ile oluşan mikroskobik boyuttaki içyapı düzensizliğinin çözümünü sağlayan Temsili Temel Hacim (TTH) kavramı tanımı ile başlamaktadır. Ortalama alarak mikroskobik seviyede taşınım olayının tanımı, Temsili Temel Hacim kavramı ile makroskobik ve sürekli ortam tanımlaması yapılarak elde edilmiştir. Bu çalışmada, gözenekli ortamlarda taşınımla ısı geçişi ve akısın matematiksel ve fiziksel temelleri, bu konuda iyi bilinen Darcy yasasından başlayarak ve onun yıllar içerisinde yeni düzenlemeleri He geliştirilerek anlatılmıştır. Bunlara ilaveten, bu çalışmada gözenekli ortam içinde kütle, momentum ve enerji geçişinin sürekli ortamlar yaklaşımı yaparak modellenmesi açıklanmaktadır. Çalışmanın sonunda gözenekli ortamlarda akışkan akışı ve ısı geçişi konusunda son yıllarda yayınlanan makaleler, kitaplar ve incelemeler hakkında bilgi verilmiştir. Anahtar Kelimeler: Gözenekli ortamlar, gözeneklilik, geçirgenlik, Darcy yasası, korunum denklemleri

Experimental study of foam layout effects on NO emission inside a porous burner with three porous media layers

258-260The effect of foam layout on NO emission inside a a porous burner with three porous media layers has been experimentally studied. The experimental set-up included two zones, combustion and preheating. At the combustion zone, there are five layers of porous media with different pore densities; at the preheating zone, there are alumina spheres as porous media. Numerical solutions show that permutation of porous foams with different pore densities has an effect on emission rates. The purpose of this experimental study is to investigate effect of porous media permutation and equivalence on emission rates. Results indicate that when the foam density is decreased from combustion zone to exit, NO emission is declined

Experimental Investigation of Interfacial Conditions between Fluid and Porous Layer Formed by Periodic Arrays of Circular and Non-Circular Cylinders

In this experimental study, a flow through a two-dimensional channel partially containing porous media is investigated. A two-layer structure comprising of a saturated porous layer with an overlaying fluid flow layer in a rectangular horizontal channel is designed for the experiments. Flow characteristics at the interface between clear fluid and porous layer are investigated. The porous layer consists of cylindrical rod bundle placed horizontally on the side walls of the channel in arranged square arrays. In the experiments, water white oil is used as the working fluid to match the refractive index of the cylindrical rods made of Plexiglas. Visualizations and measurements have been acquired by digital particle image velocimetry system for the velocity profiles which help us to evaluate the interface velocity and slip coefficient at the interface region. The measurement of interface velocity profile is repeated for circular, square, and 45^{°} rotated square cylindrical rods to understand the effects of the structure of the interface region. It has been observed that dimensionless slip or interface velocity depends significantly on the surface structure at the interface region when cylindrical rods each with circular, square, and 45^{°} rotated square cross-sections are used as porous medium. The volumetric flow rate can be changed according to the cross-sections of cylindrical rods. The permeability for the different arrangements of cylindrical rods is computed by an analytical study. The dimensionless slip velocity, slip coefficient, particle image velocimetry images, experimental and numerical velocity vector maps, and velocity profiles at the interface are presented