Thermal Transport in Metallic Porous Media 199 lsw () r Ja cTT   . (57) The local heat transfer coefficient and Nusselt number along the x direction can be obtained in Eqs. (59) and (60):    ee ll ws ws 0 0 1 / y y kk TT hx TTy xTT yx            . (58)   e x Nu x h x k  . (59) The condensate film Reynolds number is expressed as follows:    ll 4hx Re x cJa     . (60) In the region outside condensation layer, the domain extension method is employed, where special numerical treatment is implemented during the inner iteration to ensure that velocity and temperature in this extra region are set to be zero and T s , and that these values cannot affect the solution of velocity and temperature field inside condensation layer. The governing equations in Eqs. (52)-(54) are solved with using SIMPLE algorithm (Tao, 2005). The convective terms are discritized using the power law scheme. A 200×20 grid system has been checked to gain a grid independent solution. The velocity field is solved ahead of the temperature field and energy balance equation. By coupling Eqs. (52)-(55), the non-linear temperature field can be obtained. The thermal-physical properties in the numerical simulation, involving the fluid thermal conductivity, fluid viscosity, fluid specific heat, fluid density, fluid saturation temperature, fluid latent heat of vaporization, and gravity acceleration are presented in Table 4. Parameter Unit Value Liquid density l  -3 kg m 977.8 Vapor density v  -3 k g m 0.58 Liquid kinematic viscousity l  Pa s  2.825×10 -4 Liquid thermal conductivity l k 11 Wm K    0.683 Liquid heat capacity at constant pressure l c 11 Jk g K   4200 Saturation temperature s T C 100 Latent heat r -1 Jk g  297030 Gravity acceleration g 2 ms   9.8 Table 4. Constant parameters in numerical procedure of film condensation Heat Transfer – Engineering Applications 200 For a limited case of porosity being equal to 1, the present numerical model can predict film condensation on the vertical smooth plate for reference case validation. The distribution of film condensate thickness and local heat transfer coefficient on the smooth plate predicted by the present numerical model with those of Nusselt (Nusselt, 1916) and Al-Nimer and Al- Kam (Al-Nimer and Al-Kam, 1997) are shown in Fig. 19. It can be seen that the numerical solution is approximately consistent with either Nusselt (Nusselt, 1916) or Al-Nimer and Al- Kam (Al-Nimer and Al-Kam, 1997). The maximum deviation for condensate thickness and local heat transfer coefficient is 14.5% and 12.1%, respectively. 0.0 0.2 0.4 0.6 0.8 1. 0 0.0 5.0x10 -5 1.0x10 -4 1.5x10 -4 2.0x10 -4 2.5x10 -4 3.0x10 -4 3.5x10 -4  (m) y (m) numerical solution Al-Nimer and Al-Kam, 1997 Nusselt, 1916 Fig. 19. Distribution of condensate thickness for the smooth plate (  =0.9, 10 PPI) Figure 20(a) exhibits the temperature distribution in condensate layer for three locations in the vertical direction ( x/L=0.25, 0.5, and 0.75) with porosity and pore density being 0.9 and 10 PPI, respectively. Evidently, the temperature profile is nonlinear. The non-linear characteristic is more significant, or the defined temperature gradient   l //Tyx      is higher in the downstream of condensate layer since the effect of heat conduction thermal resistance of the foam matrix in horizontal direction becomes more obvious. 0.0 0.2 0.4 0.6 0.8 1.0 65 70 75 80 85 90 95 100 x/L=0.75 x/L=0.5 T (℃) y/ x/L=0.25 Fig. 20. Temperature distribution in condensate layer for different x (  =0.9, 10 PPI) Effects of parameters involving Jacobi number, porosity, and pore density are discussed in this section. Super cooling degree can be controlled by changing the value of Ja. The effect of Ja on the condensate layer thickness is shown in Fig. 21(a). It can be seen that condensate Thermal Transport in Metallic Porous Media 201 layer thickness decreases as the Jacobi number increases. This can be attributed to the fact that the super cooling degree, which is the key factor driving the condensation process, is reduced as the Ja number increases, leading to a thinner liquid condensate layer. For a limited case of zero super cooling degree, condensation cannot occur and the condensate layer does not exist. The effect of porosity on the condensate film thickness is shown in Fig. 21(b). It is found that in a fixed position, increase in porosity can lead to the decrease in the condensate film thickness, which is helpful for film condensation. This can be attributed to the fact that the increase in porosity can make the permeability of the metallic foams increase, decreasing the flow resistance of liquid flowing downwards. The effect of pore density on the condensate film thickness is shown in Fig. 21(c). It can be seen that for a fixed x position, the increase in pore density can make the condensate film thickness increase greatly, which enlarges the thermal resistance of the condensation heat transfer process. The reason for the above result is that the increasing pore density can significantly reduce metal foam permeability and substantially increase the flow resistance of the flowing-down condensate. Thus, with either an increase in porosity or a decrease in pore density, condensate layer thickness is reduced for condensation heat transfer coefficient. 0.00.20.40.60.81. 0 0.0 1.0x10 -4 2.0x10 -4 3.0x10 -4 4.0x10 -4 5.0x10 -4 (m) x ( m ) Ja=2 Ja=7 Ja=14 Ja=35 0.0 0.2 0.4 0.6 0.8 1. 0 0.0 1.0x10 -4 2.0x10 -4 3.0x10 -4 4.0x10 -4 5.0x10 -4 6.0x10 -4 (m) x ( m ) =0.80 =0.85 =0.90 =0.95 0.0 0.2 0.4 0.6 0.8 1.0 0.0 2.0x10 -4 4.0x10 -4 6.0x10 -4 8.0x10 -4 1.0x10 -3 1.2x10 -3 1.4x10 -3 1.6x10 -3 (m) x(m) 5 PPI 20 PPI 40 PPI 60 PPI (a) (b) (c) Fig. 21. Effects of important parameters on condensate thickness distribution: (a) effect of Jacobi number (  =0.9, 10 PPI); (b) effect of porosity (10PPI); (c) pore density (  =0.9) 5. Conclusion Metallic porous media exhibit great potential in heat transfer area. The characteristic of high pressure drop renders those with high porosity and low pore density considerably more attractive in view of pressure loss reduction. For forced convective heat transfer, another way to lower pressure drop is to fill the duct partially with metallic porous media. In this chapter, natural convection in metallic foams is firstly presented. Their enhancement effects on heat transfer are moderate. Next, we exhibit theoretical modeling on thermal performance of metallic foam fully/partially filled duct for internal flow with the two- equation model for high solid thermal conductivity foams. Subsequently, a numerical model for film condensation on a vertical plate embedded in metallic foams is presented and the effects of advection and inertial force are considered, which are responsible for the non- linear effect of cross-sectional temperature distribution. Future research should be focused on following areas with metallic porous media: implementation of computation and parameter optimization for practical design of thermal application, phase change process, turbulent flow and heat transfer, non-equilibrium conjugate heat transfer at porous-fluid Heat Transfer – Engineering Applications 202 interface, thermal radiation, experimental data/theoretical model/flow regimes for two- phase/multiphase flow and heat transfer, and so on. 6. Acknowledgment This work is supported by the National Natural Science Foundation of China (No. 50806057), the National Key Projects of Fundamental R/D of China (973 Project: 2011CB610306), the Ph.D. Programs Foundation of the Ministry of Education of China (200806981013) and the Fundamental Research Funds for the Central Universities. 7. References Alazmi, B. & Vafai, K. (2001). Analysis of Fluid Flow and Heat Transfer Interfacial Conditions Between a Porous Medium and a Fluid Layer. 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The simple structure of power cables turn to quite complex structure by increased heat, environmental and mechanical strains when voltage and transmitted power levels are increased. In addition, operation of existing systems at the highest capacity is of great importance. This requires identification of exact current carrying capacity of power cables. Analytical and numerical approaches are available for defining current carrying capacity of power cables. Analytical approaches are based on IEC 60287 standard and there can only be applied in homogeneous ambient conditions and on simple geometries. For example, formation of surrounding environment of a cable with several materials having different thermal properties, heat sources in the vicinity of the cable, non constant temperature limit values make the analytical solution difficult. In this case, only numerical approaches can be used. Based on the general structure of power cables, especially the most preferred numerical approach among the other numerical approaches is the finite element method (Hwang et al., 2003), (Kocar et al., 2004), (IEC TR 62095). There is a strong link between current carrying capacity and temperature distributions of power cables. Losses produced by voltage applied to a cable and current flowing through its conductor, generate heat in that cable. The current carrying capacity of a cable depends on effective distribution of produced heat from the cable to the surrounding environment. Insulating materials in cables and surrounding environment make this distribution difficult due to existence of high thermal resistances. The current carrying capacity of power cables is defined as the maximum current value that the cable conductor can carry continuously without exceeding the limit temperature values of the cable components, in particular not exceeding that of insulating material. Therefore, the temperature values of the cable components during continuous operation should be determined. Numerical methods are used for calculation of temperature distribution in a cable and in its surrounding environment, based on generated heat inside the cable. For this purpose, the conductor temperature is calculated for a given conductor current. Then, new calculations are carried out by adjusting the current value. Heat Transfer – Engineering Applications 206 Calculations in thermal analysis are made usually by using only boundary temperature conditions, geometry, and material information. Because of difficulty in identification and implementation of the problem, analyses taking into account the effects of electrical parameters on temperature or the effects of temperature on electrical parameters are performed very rare (Kovac et al., 2006). In this section, loss and heating mechanisms were evaluated together and current carrying capacity was defined based on this relationship. In numerical methods and especially in singular analyses by using the finite element method, heat sources of cables are entered to the analysis as fixed values. After defining the region and boundary conditions, temperature distribution is calculated. However, these losses are not constant in reality. Evaluation of loss and heating factors simultaneously allows the modeling of power cables closer to the reality. In this section, use of electric-thermal combined model to determine temperature distribution and consequently current carrying capacity of cables and the solution with the finite element method is given. Later, environmental factors affecting the temperature distribution has been included in the model and the effect of these factors to current carrying capacity of the cables has been studied. 2. Modelling of power cables Modelling means reducing the concerning parameters’ number in a problem. Reducing the number of parameters enable to describe physical phenomena mathematically and this helps to find a solution. Complexity of a problem is reduced by simplifying it. The problem is solved by assuming that some of the parameters are unchangeable in a specific time. On the other hand, when dealing with the problems involving more than one branch of physics, the interaction among those have to be known in order to achieve the right solution. In the future, single-physics analysis for fast and accurate solving of simple problems and multi- physics applications for understanding and solving complex problems will continue to be used together (Dehning et al., 2006), (Zimmerman, 2006). In this section, theoretical fundamentals to calculate temperature distribution in and around a power cable are given. The goal is to obtain the heat distribution by considering voltage applied to the power cable, current passing through the power cable, and electrical parameters of that power cable. Therefore, theoretical knowledge of electrical-thermal combined model, that is, common solution of electrical and thermal effects is given and current carrying capacity of the power cable is determined from the obtained heat distribution. 2.1 Electrical-thermal combined model for power cables Power cables are produced in wide variety of types and named with various properties such as voltage level, type of conductor and dielectric materials, number of cores. Basic components of the power cables are conductor, insulator, shield, and protective layers (armour). Conductive material of a cable is usually copper. Ohmic losses occur due to current passing through the conductor material. Insulating materials are exposed to an electric field depending on applied voltage level. Therefore, there will be dielectric losses in that section of the cable. Eddy currents can develop on grounded shield of the cables. If the protective layer is made of magnetic materials, hysteresis and eddy current losses are seen in this section. Coupled Electrical and Thermal Analysis of Power Cables Using Finite Element Method 207 Main source of warming on the power cable is the electrical power loss (R·I 2 ) generated by flowing current (I) through its conductor having resistance (R). The electrical power (loss) during time (t) spends electrical energy (R·I 2 ·t), and this electric energy loss turns into heat energy. This heat spreads to the environment from the cable conductor. In this case, differential heat transfer equation is given in (1) (Lienhard, 2003). θ ( θ) ρkWc t       (1) Where; θ : temperature as the independent variable ( o K), k : thermal conductivity of the environment surrounding heat source (W/Km), ρ : density of the medium as a substance (kg/m 3 ), c : thermal capacity of the medium that transmits heat (J/kg o K), W : volumetric heat source intensity (W/m 3 ). Since there is a close relation between heat energy and electrical energy (power loss), heat source intensity (W) due to electrical current can be expressed similar to electrical power. dxd y dzPJE (2) Where J is current density, E is electrical field intensity; dx.dy.dz is the volume of material in the unit. As current density is J =   E and electrical field intensity is E = J/, ohmic losses in cable can be written as; 2 1 dxd y dz σ PJ (3) Where  is electrical conductivity of the cable conductor and it is temperature dependent. In this study, this feature has been used to make thermal analysis by establishing a link between electrical conductivity and heat transfer. In equation (4), relation between electrical conductivity and temperature of the cable conductor is given as; 00 1 σ ρ (1 α(θθ))    (4) In the above equation ρ 0 is the specific resistivity at reference temperature value θ 0 (Ω·m); α is temperature coefficient of specific resistivity that describes the variation of specific resistivity with temperature. Electrical loss produced on the conducting materials of the power cables depends on current density and conductivity of the materials. Ohmic losses on each conductor of a cable increases temperature of the power cable. Electrical conductivity of the cable conductor decreases with increasing temperature. During this phenomenon, ohmic losses increases and conductor gets more heat. This situation has been considered as electrical-thermal combined model (Karahan et al., 2009). In the next section, examples of the use of electric-thermal model are presented. In this section, 10 kV, XLPE insulated medium voltage power cable and 0.6 / 1 kV, four-core PVC insulated low voltage power cable are modeled by considering only the ohmic losses. However, a model with dielectric losses is given at (Karahan et al., 2009). Heat Transfer – Engineering Applications 208 2.2 Life estimation for power cables Power cables are exposed to electrical, thermal, and mechanical stresses simultaneously depending on applied voltage and current passing through. In addition, chemical changes occur in the structure of dielectric material. In order to define the dielectric material life of power cables accelerated aging tests, which depends on voltage, frequency, and temperature are applied. Partial discharges and electrical treeing significantly reduce the life of a cable. Deterioration of dielectric material formed by partial discharges particularly depends on voltage and frequency. Increasing the temperature of the dielectric material leads to faster deterioration and reduced cable lifetime. Since power cables operate at high temperatures, it is very important to consider the effects of thermal stresses on aging of the cables (Malik et al., 1998). Thermal degradation of organic and inorganic materials used as insulation in electrical service occurs due to the increase in temperature above the nominal value. Life span can be obtained using the Arrhenius equation (Pacheco et al., 2000). a B E k θ dp Ae dt   (5) Where; dp/dt : Change in life expectancy over time A : Material constant k B : Boltzmann constant [eV/K] θ : Absolute temperature [ o K] E a : Excitation (activation) energy [eV] Depending on the temperature, equation (6) can be used to estimate the approximate life of the cable (Pacheco et al., 2000).  a Bii E Δθ k θθΔθ i ppe        (6) In this equation, p is life [days] at  temperature increment; p i is life [days] at  i temperature;  is the amount of temperature increment [ o K]; and  i is operating temperature of the cable [ o K]. In this study, temperature distributions of the power cables were obtained under electrical, thermal and environmental stresses (humidity), and life span of the power cables was evaluated by using the above equations and obtained temperature variations. 3. Applications 3.1 5.8/10 kV XLPE cable model In this study, the first electrical-thermal combined analysis were made for 5.8/10 kV, XLPE insulated, single core underground cable. All parameters of this cable were taken from (Anders, 1997). The cable has a conductor of 300 mm 2 cross-sectional area and braided copper conductor with a diameter of 20.5 mm. In Table 1, thicknesses of the layers of the model cable are given in order. [...]... coefficient h is computed from the following empirical equation (Thue, 1999) 210 Heat Transfer – Engineering Applications Material Copper conductor XLPE insulation Copper wire screen PVC outer sheath Soil Thermal Conductivity k (W/K.m) 400 1/3.5 400 0.1 1 Thermal Capacity c (J/kg.K) 385 385 385 385 89 0 Density  (kg/m3) 87 00 1 380 87 00 1760 1600 Table 2 Thermal properties of materials in the model h  7.371... temperature distributions were determined 222 Heat Transfer – Engineering Applications Copper conductor PVC insulation PVC filler Steel wire armour PVC outer sheath Fig 18 View of 0.6/1 kV, 3 x 35/16 mm2, PVC insulated power cable Cable Components Phase conductors (r1) Neutral conductor (r2) Filling material (r3) Armour (r4) Outer sheath (r5) Radius (mm) 3 .8 2.6 11.5 12.5 14.5 Table 5 Radiuses of the... considering the thermal strength of PVC material of 70oC Thermal conductivity [W/Km] Isil iletkenlik [W/mK] 0.7 0. 68 0.66 0.64 0.62 0.6 0. 58 0 20 40 60 Sicaklik [C] Temperature [C] 80 Fig 22 Variation of thermal conductivity of the water with temperature 100 226 Heat Transfer – Engineering Applications This value is the value of the current carrying capacity where all of the cable is immersed in the water... when three pieces of cables are laid side by side, the cable in the middle heats up more because of both not being able to transmit its heat easily and getting heat from the side cables This also lowers the current carrying capacity of the 2 28 Heat Transfer – Engineering Applications center conductor To reduce this effect it is necessary to increase the distance between cables In the study that was conducted... of temperature of the cable insulation with wind velocity 2 18 Heat Transfer – Engineering Applications In order to see the borders of this effect, cable life has been calculated for both cases by using the equation (6) and the results are indicated in Fig 13 and Fig 14 Activation energy of 1.1 eV for XPLE material, Boltzmann constant of 8. 617·10-5 eV/K was taken for the calculations and it is assumed... equi-temperature lines are shown in Fig 20 and Fig 21, respectively 224 Heat Transfer – Engineering Applications Fig 20 Temperature distribution Fig 21 Equi-temperature lines Temperature distribution during the balanced loading of the cable can be seen from the figures In this case, there will be no current on the neutral conductor and the heat produced by currents passing through to three phase conductors... parameters are the parameters used in the heat transfer equation (1) Heat sources are defined according to the equation (3) After geometrical and physical descriptions of the problem, the boundary conditions are defined The temperature on bottom and side boundaries of the region is assumed as fixed (15oC), and the upper boundary is accepted as the convection boundary Heat transfer coefficient h is computed... expected to heat up more because of two adjacent cables at both sides, as shown in Fig 7(a) In this case, current carrying capacity of the middle cable will be reduced Table 4 indicates the change in temperature of the middle cable depending on the distance between cables and corresponding current carrying capacity, obtained from the numerical solution 214 Heat Transfer – Engineering Applications. .. climate-changing parameter When the cable is laid in the soil with moisture more than normal, it is easier to disperse the heat generated by the cable If the heat produced remains the same, according to the principle of conservation of energy, increase in dispersed heat will result in decrease in the heat amount kept by cable, therefore cable temperature drops and cable can carry more current Thermal conductivity... of the cable components and the surrounding environment are defined as given in Table 6 Cable Material Conductor (copper) Insulator (PVC) Armour (steel) Air Density ρ (kg/m3) 87 00 1760 785 0 1.205 Thermal Capacity c (J/kg·K) 385 385 475 1005 Thermal Conductivity k (W/K·m) 400 0.1 44.5 k_air() Table 6 Thermal parameters of the cable components Thermal conductivity of air varies with temperature As shown . of Heat and Mass Transfer, Vol.45, No. 18, (August 2002), pp. 3 781 –3793, ISSN 0017-9310 Heat Transfer – Engineering Applications 204 Popiel, C.O. & Boguslawski, L. (1975). Heat transfer. Uniform Suction Velocity. Journal of Heat Transfer, Vol. 86 , (1964), pp. 481 - 489 , ISSN 0022-1 481 Jamin Y.L. & Mohamad A.A. (20 08) . Natural Convection Heat Transfer Enhancements From a Cylinder. Density  (kg/m 3 ) Copper conductor 400 385 87 00 XLPE insulation 1/3.5 385 1 380 Copper wire screen 400 385 87 00 PVC outer sheath 0.1 385 1760 Soil 1 89 0 1600 Table 2. Thermal properties of