2-Maxwell-两相导热率比例0.5比204.docx

Izmir Composites Science and Technology 63 (2003) 113117 www.elsevier.com/locate/compscitech Thermal conductivity of particle lled polyethylene composite materials . Dilek Kumlutas¸ a,*, Ismail H. Tavmana, M. Turhan C¸ obanb aDokuz Eylu¨l University, Mechanical Engineering Department, Bornova, . 35100, Turkey bTUBITAK Marmara Research Cent., Energy Syst. Environmental Res. Inst., Gebze, Turkey Received 17 January 2002; received in revised form 28 August 2002; accepted 28 August 2002 Abstract In this study, the e ective thermal conductivity of aluminum lled high-density polyethylene composites is investigated numeri- cally as a function of ller concentration. The obtained values are compared with experimental results and the existing theoretical and empirical models. The thermal conductivity is measured by a modied hot-wire technique. For numerical study, the e ective thermal conductivity of particle-lled composite was calculated numerically using the micro structural images of them. By identi- fying each pixel with a nite di erence equation and accompanying appropriate image processing, the e ective thermal conductivity of composite material is determined numerically. As a result of this study, numerical results, experimental values and all the models are close to each other at low particle content. For particle content greater than 10%, the e ective thermal conductivity is expo- nentially formed. All the models fail to predict thermal conductivity in this region. But, numerical results give satisfactory values in the whole range of aluminum particle content. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Composite material; Thermal conductivity; Image processing; Finite di erence 1. Introduction Knowing physical properties of the composite mate- rials has gained signicant importance in the design of new systems. For many materials applications, infor- mation is needed on their thermal properties. Deter- mining the thermal conductivity of composite materials is crucial in a number of industrial processes. The tem- perature elds in composite materials cannot be deter- mined unless the thermal conductivities of the media are known. Despite the importance of this material prop- erty and the considerable number of studies that have been carried out, the determination of e ective thermal conductivity of a composite material is partially under- stood. The e ective thermal conductivity of a composite material is a complex function of their geometry, the thermal conductivity of the di erent phases, distribution within the medium, and contact between the particles. * Corresponding author. Tel.: +90-232-3883138-121; fax: +90- 232-3887868. E-mail address: dilex.kumlutasdeu.edu.tr (D. Kumlutas¸ ). Numerous theoretical and experimental approaches have been developed to determine the precise value of this parameter. Maxwell 1 studied the e ective thermal conductivity of heterogeneous materials. By solving Laplaces equa- tion, he determined the e ective conductivity of a ran- dom suspension of spheres within a continuous medium. The model developed by Maxwell 1 assumes that the particles are su ciently far apart that the potential around each sphere will not be inuenced by the presence of other particles. Russell 2 developed one of the early model systems using the electrical analogy, assuming that the discrete phase is isolated cubes of the same size dispersed in the matrix material and that the isothermal lines are planes. Based on Tsaos 3 prob- abilistic model, Cheng and Vachon 4 assumed a para- bolic distribution of the discontinuous phase. Lewis and Nielsen 5 derived a semi-theoretical model by a modi- cation of the Halpin-Tsai equation 6,7 to include the e ect of the shape of the particles and the orientation or type of packing for a two-phase system. Baschirow and Selenew 8 developed the equation for the case when 0266-3538/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII : S0 266 -3 538 (0 2)001 94 -X 114 D. Kumlutas¸ et al. / Composites Science and Technology 63 (2003) 113117 2 m the particles are spherical and the two phases are iso- tropic. In real composite material, the isothermal sur- and for series conduction model: 1 1 Þ faces present a very complex shape and cannot be analytically determined. The models used to calculate thermal conductivities are thus highly simplied models kc ¼ km þ kf ð of the real media. Veyret et al. 9 characterized con- ductive heat transfer through composite, granular, or In the case of geometric mean model, the e ective thermal conductivity of the composite is given by: brous materials by using the nite element method. ð1 Þ Terada et al. 10 generated the nite element model by identifying each pixel with a nite element and accom- panying appropriate image processing. Deissler and Boegli 11 carried out the numerical studies. They pro- posed a solution to Laplaces equation for a cubic array of spheres presenting a single point of contact. Deiss- lers 11 works were extended by Wakao and Kato 12 for a cubic or orthorhombic array of uniform spheres in contact. Shonnard and Whitaker 13 have investigated the inuence of contacts on two-dimensional models. They have developed a global equation with an integral method for heat transfer in the medium. In this study, the e ective thermal conductivity of aluminum lled high-density polyethylene composites is investigated numerically as a function of ller con- centration. The obtained values are compared with experimental results and the existing theoretical and empirical models. kc ¼ kf :km ð3Þ Tsao 3 derived an equation relating the two phase solid mixture thermal conductivity to the conductivity of the individual components and to two parameters which describe the spatial distribution of the two pha- ses. By assuming a parabolic distribution of the dis- continuous phase in the continuous phase (Fig. 1), Cheng and Vachon 4 obtained a solution to Tsaos 3 model that did not require knowledge of additional parameters. The constants of this parabolic distribution were determined by analysis and presented as a function of the discontinuous phase volume fraction. Thus, the equivalent thermal conductivity of the two phase solid mixture was derived in terms of the distribution func- tion, and the thermal conductivity of the constituents.: For, kf > km The e ective thermal conductivity of high-density 1 1 polyethylene containing particulate llers is obtained kc ¼ pCk k k Bk k numerically at several ller concentrations. A numerical approach was used to determine the e ective thermal conductivity of particle composites in this study. The ð f m Þ½ m þ ð f B m Þ ð4Þ e ective thermal conductivity of the material was determined using the Laplace equation, as were the ln pðkmþ Bðkf kmÞ þ 2 pCð k f k mÞ B 1 B þ k temperature and ux elds within the control volume. A nite di erence method was used in this study. Calcu- lation is carried out on two-dimensional geometric spaces. The results obtained from this calculation were compared to results found in prior literature. pðkm þ Bðkf kmÞ 2 pCð k f k mÞ where for both equations: B ¼ p3 = 2 C ¼ 4p2 = ð 3 Þ 2. Thermal conductivity models In this section are listed several models and a brief description of their basis. Many theoretical and empirical models have been proposed to predict the e ective ther- mal conductivity of two-phase mixtures. Comprehensive review articles have discussed the applicability of many of these models that appear to be more promising 14,15. For a two-component composite, the simplest alter- natives would be with the materials arranged in either parallel or series with respect to heat ow, which gives the upper or lower bounds of e ective thermal con- ductivity. For the parallel conduction model: kc ¼ð1 Þ:km þ :kf c : composite; m : matrix; f : filler; : volume fraction of filler ð1Þ Lewis and Nielsen 5 modied the Halpin-Tsai equa- tion 6,7 to include the e ect of the shape of the parti- Fig. 1. Parabolic distribution of the discontinuous phase for the Cheng and Vachon 4 model. D. Kumlutas¸ et al. / Composites Science and Technology 63 (2003) 113117 115 6 2=3 þ 2=3 5 m cles and the orientation or type of packing for a two- phase system. 1 þ AB Assuming the pores are cubes of the same size and the isothermal lines are planes, Russell 2 obtained the conductivity using a series parallel network: kc ¼ km 1 B ð5Þ 2 kc ¼ km 6 km 31 kf 7 7 ð7Þ where kf 1 B ¼ km ¼ 1 þ 1 m 6 4 2=3 þ km 1 2=3 7kf þ kf A 2 km þ The values of A and m for many geometric shapes and orientation are given in the Tables 1 and 2. Maxwell 1, using potential theory, obtained an exact solution for the conductivity of randomly distributed and non-interacting homogeneous spheres in a homo- geneous medium: 3. Experimental study The matrix material is a high-density polyethylene in powder form, with a density of 0.968 g/cm3 and a melt index of 5.8 g/10 min. Its thermal conductivity at 36 C is 0.543 W/mK. The metallic ller is aluminum in the form of ne powder, with particles approximately spherical in shape and particle size in the range of 4080 microns for aluminum. The solid density of aluminum is 3 kf þ 2km þ 2 ðkf km Þ 2.7 g/cm and its thermal conductivity 204 W/mK. kc ¼ km Table 1 kf þ 2km ðkf km Þ ð6Þ Samples are prepared by the mold compression process. HDPE and aluminum powders are mixed at various volumetric concentrations. The mixed powder is then melted under pressure in a mold and solidied by air- Value of A for various systems Type of dispersed phase Direction of A heat ow Cubes Any 2.0 Spheres Any 1.50 Aggregates of spheres Any (2.5/ n)-1 cooling. The process conditions are molding temperature of 185 C, pressure of 4 MPa The resulting samples for thermal conductivity measurements are rectangular in shape of 100 mm length, 50 mm width, and 17 mm thick- ness because of the measuring probe. Homogeneity of the samples is examined using a light microscope. Aluminum particles are found to be uniformly distributed in high- Randomly oriented rods Aspect ratio=2 Randomly oriented rods Aspect ratio=4 Randomly oriented rods Aspect ratio=6 Randomly oriented rods Aspect ratio=10 Randomly oriented rods Aspect ratio=15 Any 1.58 Any 2.08 Any 2.8 Any 4.93 Any 8.38 density polyethylene matrix with no voids in the structure. 4. Numerical modeling of the problem Two-dimensional numerical analysis was carried out for the conductive heat transfer in the composite mate- rial (Fig. 2). The temperature eld in the composite Uniaxially oriented bers Parallel to bers Uniaxially oriented bers Perpendicular to bers 2L/D 0.5 material was dened by solving Laplaces equation numerically using a nite di erence formulation. The Table 2 Value of m for various systems Shape of particle Type of packing m Spheres Hexagonal close 0.7405 Spheres Face centered cubic 0.7405 Spheres Body centered cubic 0.60 Spheres Simple cubic 0.524 Spheres Random close 0.637 Rods or bers Uniaxial hexagonal close 0.907 Rods or bers Uniaxial simple cubic 0.785 Rods or bers Uniaxial random 0.82 Rods or bers Three dimensional random 0.52 Fig. 2. Two-dimensional model of the composite material. 116 D. Kumlutas¸ et al. / Composites Science and Technology 63 (2003) 113117 X y Laplaces equation was solved by imposing the follow- ing boundary conditions: 1. The horizontal sides perpendicular to the direc- tion of the heat ow are isothermal at the entrance to and the exit from the cell. 2. The vertical sides parallel to the direction of the heat ow are adiabatic. The heat ow moving into or out of the cell reaches its peak at the center of the ller particles. For an ele- mentary two-dimensional cell with the dimensions of Lx (along the x axis) and Ly (along the y axis), the thermal conductivity is determined using the following relation: from the numerical analysis are compared with the results obtained from the experimental study, it is seen that the numerical results are higher. It can be said that, since the quality of the pictures used in image processing carried out on the picture les in numerical study are k Lx Tcell c Ly Ti ¼ kixi ð8Þ i with Pixi ¼ Lx and ki=(km in the continuous phase, kf in the inclusions) In the considered heat conduction problem the tem- peratures at the nodes along the boundaries y=0 and y=Ly are prescribed and known as T1 and T2, but the temperatures at the nodes in the interior region and on the adiabatic boundaries are unknown. Therefore, the problem involves many unknown temperatures. The equations needed for the determination of these tem- peratures are obtained by writing the appropriate nite- di erence equation for each of these nodes. 5. Results and discussion In this study, a numerical approach is used to deter- mine the e ective thermal conductivity of particle lled composite materials. The temperature eld for the composite material is dened by solving Laplaces equation using a nite di erence formulation. The tem- perature eld for the composite material consisting of 4% by volume aluminum particles lled high-density polyethylene is shown in Fig. 3. It may be seen from this gure that the isotherms are deformed at the location of aluminum particles represented by white inclusions. The ux lines are directed toward inclusions due to the much higher thermal conductivity value of aluminum (204 W/ mK) with respect to high-density polyethylene (0.543 W/mK). In Fig. 4, the thermal conductivity values obtained from the experimental study for aluminum l- led high-density polyethylene are compared with several thermal conductivity models and with the numerical results obtained in this study. As seen from this gure, Russel 2 and Cheng and Vachon 4 models predict fairly well thermal conductivity values up to 10% by volume of aluminum particles whereas beyond 10% of particle content all models underestimate the thermal conductivity of the composite. If the results obtained Fig. 3. Isothermal curves for the composite material composed of 4% aluminum lled high-density polyethylene. Fig. 4. Comparison of the numerical results, models and experimental values for aluminum ller. D. Kumlutas¸ et al. / Composites Science and Technology 63 (2003) 113117 117 not so good, the procedure was not so successful. Objects dened as noise in the picture will be accepted as Al particles added into the high-