Thermal conductivities effective

The effective thermal conductivity of a Hquid—soHd suspension has been reported to be (46) larger than that of a pure Hquid. The phenomenon was attributed to the microconvection around soHd particles, resulting in an increased convective heat-transfer coefficient. For example, a 30-fold increase in the effective thermal conductivity and a 10-fold increase in the heat-transfer coefficient were predicted for a 30% suspension of 1-mm particles in a 10-mm diameter pipe at an average velocity of 10 m/s (45). 

When an atom or molecule receives sufficient thermal energy to escape from a Hquid surface, it carries with it the heat of vaporization at the temperature at which evaporation took place. Condensation (return to the Hquid state accompanied by the release of the latent heat of vaporization) occurs upon contact with any surface that is at a temperature below the evaporation temperature. Condensation occurs preferentially at all poiats that are at temperatures below that of the evaporator, and the temperatures of the condenser areas iacrease until they approach the evaporator temperature. There is a tendency for isothermal operation and a high effective thermal conductance. The steam-heating system for a building is an example of this widely employed process. 

The heat pipe has properties of iaterest to equipmeat desigaers. Oae is the teadeacy to assume a aeady isothermal coaditioa while carrying useful quantities of thermal power. A typical heat pipe may require as Htfle as one thousandth the temperature differential needed by a copper rod to transfer a given amount of power between two poiats. Eor example, whea a heat pipe and a copper rod of the same diameter and length are heated to the same iaput temperature (ca 750°C) and allowed to dissipate the power ia the air by radiatioa and natural convection, the temperature differential along the rod is 27°C and the power flow is 75 W. The heat pipe temperature differential was less than 1°C the power was 300 W. That is, the ratio of effective thermal conductance is ca 1200 1. 

There is assumed to be no interaction between the superfluid and normal components, thus the superfluid component can diffuse very rapidly to a heat source where it absorbs energy by reverting to the normal state. It thereby produces the very high effective thermal conductivity observed in helium II. 

However, the models represent only crude approximate descriptions of the complex physical systems involved. Probably the most important phenomenon excluded is that of heat transfer. Suspended-bed operations are characterized by a high effective thermal conductivity, and thus represent a good approximation to isothermal behavior, and the above models should provide an adequate description of these systems. Fixed-bed operations will probably in many cases depart significantly from isothermal conditions, and in such cases models should be constructed that take heat transfer into... 

Van Driesen and Stewart (V4) have reported temperature measurements for various locations in commercial gas-liquid fluidized reactors for the large-scale catalytic desulfurization and hydrocracking of heavy petroleum fractions (2500 barrels per day capacity). The hydrogenation was carried out in two stages the maximum and minimum temperatures measured were 774° and 778°F for the first stage and 768° and 770°F for the second. These results indicate that gas-liquid fluidized reactors are characterized by a high effective thermal conductivity. 

Measurements of heat-transfer coefficients and effective thermal conductivity for gas-liquid fluidized beds have also been carried out by Manchanda (M2). 

The heat-transfer properties of the suspended-bed operations appear superior to those of the packed-bed operations. In particular, the former have a high effective thermal conductivity, and the development of uneven temperature distributions is therefore less likely. 

Some workers have correlated experimental data in terms of k at the arithmetic mean temperature, and some at the temperature of the bulk plasma. Experimental validation of the true effective thermal conductivity is difficult because of the high temperatures, small particle sizes and variations in velocity and temperature in plasma jets. 

The performances of the MHP array were evaluated for different methanol filling charges under different experimental conditions. The results indicated an increase in the effective thermal conductivity to about 200 W/m K under optimum conditions, equivalent to a 67% increase over an empty array. Performances are favored by reducing the input heat flux or increasing the cooling temperamre. 

Aluminum foam can be used as a porous medium in the model of a heat sink with inner heat generation (Hetsroni et al. 2006a). Open-cell metal foam has a good effective thermal conductivity and a high specific solid-fluid interfacial surface area. 

The effective thermal conductivity tensor depends on the transformation vec-... 

Effective thermal conductivity Eluid thermal conductivity... 

Shiina, Y., and Inagaki, T., 2005, Study on the efficiency of effective thermal conductivities on melting characteristics of latent heat storage capsules, Int. J. Heat Mass Transfer 48 373-383. 

Before the calculation of performances, heating and cooling loads which can be covered by the Energy Pipe System are evaluated. Total amount of heating and cooling is 230 GJ and 75.6 GJ, respectively, and peak loads are 50 kW for heating. On the other hands, average effective thermal conductivity was measured by the on-site thermal response test and it was 2.2 W/(mK). 

Fujioka, K., et al., 1998. Measurement of effective thermal conductivity of CaCl2 reactor beds used for driving chemical heat pumps, J. Chem. Eng. Japan, 31, 266-272. 

Equations (8) are based on the assumption of plug flow in each phase but one may take account of any axial mixing in each liquid phase by replacing the molecular thermal conductivities fc, and ku with the effective thermal conductivities /c, eff and kn eff in the definition of the Peclet numbers. The evaluation of these conductivity terms is discussed in Section II,B,1. The wall heat-transfer terms may be defined as... 

Effective Thermal Conductivities of Porous Catalysts. The effective thermal conductivity of a porous catalyst plays a key role in determining whether or not appreciable temperature gradients will exist within a given catalyst pellet. By the term effective thermal conductivity , we imply that it is a parameter characteristic of the porous solid structure that is based on the gross geometric area of the pellet perpendicular to the direction of heat transfer. For example, if one considers the radial heat flux in a spherical pellet one can say that... 

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