Transverse Thermal Conductivity Research Articles

Forced Convection in the Entrance Region of a Packed Channel With Asymmetric Heating

The problem of a thermally developing forced convective flow in a packed channel heated asymmetrically is analyzed in this paper. The flow in the packed channel is assumed to be hydrodynamically fully developed and is governed by the Brinkman–Darcy–Ergun equation with variable porosity taken into consideration. A closed-form solution based on the method of matched asymptotic expansions is obtained for the axial velocity distribution, and the wall effect on pressure drop is illustrated. The energy equation for the thermally developing flow, with transverse thermal dispersion and variable stagnant thermal conductivity taken into consideration, was solved numerically. To match the predicted temperature distributions with existing experimental data, it is found that a wall function must be introduced to model the transverse thermal dispersion process in order to account for the wall effect on the lateral mixing of fluid. The variations of the local Nusselt number along the streamwise direction in terms of the appropriate parameters are illustrated. The thermal entrance length effect on forced convection in a packed channel is discussed.

Prediction and measurement of the thermal conductivity of composite materials

The transverse thermal conductivity of continuous fibre composite materials has been described in terms of a number of models which yield analytic expressions that are simple to evaluate but limited in application. A more general solution to the two-dimensional, steady-state heat flow equation is obtained by using the finite difference method. This involves extensive computing but this method has the advantage that a rigorous solution is obtained. The results of some experimental measurements are considered and a solution to a measurement problem for composites with fibres at any angle to the imposed temperature gradient is discussed. Heat flow in anisotropic composite materials in directions at any angle to the axes of symmetry is discussed.

Heat Transfer with LHe II Gorter-Mellink Flow In Porous Composite Materials

This paper is concerned with the characteristics of heat transfer with superfluid helium Gorter-Mellink flow in porous composite materials (diameter of channel 80µm, diameter of wire 15µm). The mathematical model is established and the results of calculation have been sorted out by dimensionless numbers. Experimental investigations were made by realizing stable transverse heat transfer with superfluid helium Gorter-Mellink flow and the transverse effective thermal conductivity is measured. This work is of great significance for the cooling of large superconductive magnets as well as for deep space information technology.

High thermally conductive communications equipment panel module for communications satellite

A new communications equipment panel module for satellite use has been developed to obtain a high thermal conductivity across the thickness in transversal direction of the panel. As the conventional communications equipment panel has a sandwich structure with aluminum alloy face sheets and a honeycomb core, the transversal thermal conductivity is relatively small mainly due to epoxy resin adhesive layers between the face sheet and the honeycomb core. Therefore, in order to dissipate high-heat flux towards space, a complicated thermal control system becomes necessary. The new panel module consists of a carbon-fiber-reinforced aluminum-alloy face sheet and a titanium-alloy core. A metallic bonding is used as the connection between the face sheet and core to enhance thermal conductivity of the panel is 1 × 10 −1 (W/K · mm) which is approx. 1000 times larger than that of the conventional panel.

Effective properties of fiber-reinforced materials: I—Bounds on the effective thermal conductivity of dispersions of fully penetrable cylinders

Third-order and fourth-order bounds on the effective transverse thermal conductivity k e of fiber-reinforced materials (statistically isotropic in the transverse plane) due to Silnutzer and to Milton, respectively, are evaluated for a distribution of fully penetrable cylinders in a matrix. Both of these bounds involve a multidimensional integral which depends upon the three-point matrix probability function of the model. For nontrivial values of the fiber volume fraction φ 2 (i.e. 0 < φ 2 < 1) in the fully penetrable-cylinder model, the third-order bounds always improve upon the second-order bounds due to Hashin and the fourth-order bounds are always more restrictive than third-order bounds. The fourth-order bounds are sharp enough for 0 ⩽ φ 2 ⩽ 0.3 to give quantitatively useful estimates of k e even when the conductivity of the fiber phase k 2 is as much as two orders of magnitude greater than the matrix conductivity k 1. For 0.8 ⩽ φ 2 ⩽ 1, the Milton bounds are sufficiently restrictive to provide reasonable approximations of k e for an insulating fiber phase such that 0.01 ⩽ α ⩽ 1, where α = k 2 k 1 . The results obtained in this study for k e translate immediately into equivalent results for the axial shear modulus of fiber-reinforced materials.

Effective Properties of Fiber-Reinforced Materials

A material consisting of a matrix in which aligned fibers of uniform circular cross section are embedded is considered. A method is developed for computing the effective axial shear modulus and transverse thermal conductivity (which are mathematically analogous) for such materials. The method, an approximate technique based on pairwise interactions, introduces a new consistency concept uniquely appropriate for this problem, and is universal in the sense that it is readily and quickly applicable to any fiber arrangement. It overcomes an apparent in consistency in other interaction methods.

Thermal conductivity of transposed multistrand flat superconductor

Longitudinal and transverse thermal conductivities are calculated for a rectangular superconductor consisting of transposed multistrand wires.

Bounds on the Transverse Effective Conductivity of Computer-Generated Fiber Composites

Using trial functions that are motivated by single-body calculations, we have derived bounds on the effective transverse thermal conductivity of a fiber composite. These bounds incorporate both fiber area fraction information and some information of the configurational statistics. Simplified expressions for the bounds are obtained for the limits of widely differing conductivity values for the constitutent phases, and of a dilute suspension. The bounds are made specific for a given computer-generated fiber composite and these specific bounds are compared with the best available bounds that require area fraction information alone. The conclusions reached are that configuration statistics are significant for effective property calculations for moderately dense composites for component conductivity values that differ by some one to two orders of magnitude, or greater. Further, the bounds based on the single-body calculation are reasonably close for component conductivity values that differ by some two orders of magnitude, or less.

Concerning the effective transverse conductivity of a two-dimensional two-phase material

A two-phase material in which the phase boundaries are cylindrical surfaces is considered (fibrereinforced material). We consider such a material for which the precise details of phase geometry are not known, and investigate to what extent knowledge of the effective transverse thermal conductivity for one pair of phase conductivities determines the effective conductivity for a geometrically identical material with different phase conductivities. Also considered is whether or not one can hope to obtain narrow bounds on effective conductivity in terms of three-point correlation functions.

An experimental study of hot plasma in contact with a cold wall

The heat transfer and wall surface damage of a hot (5*106K), dense (1016 particles cm-3) deuterium plasma containing a strong magnetic field (104 G) in contact with a cold wall is studied experimentally and compared with classical theory predictions. The energy flux and transverse thermal conductivity are in agreement with classical theory and the wall surface damage was in reasonable agreement with that predicted due to sputtering for copper, but was greater than that predicted for 304 austenitic stainless steel.

Thermal conductivity of some heat pipe wicks

Effects of tortuosity and dispersion on the effective thermal conductivity of fluid-saturated porous media are investigated analytically with help of a volume averaging theory. Firstly, a general expression for the effective stagnant thermal conductivity has been derived using a unit cell model, which consists of rectangular solids with connecting arms in an in-line arrangement. The validity of the expression for the stagnant thermal conductivity has been confirmed comparing the present results with available experimental and theoretical data for packed beds, porous foams and wire screens. Secondly, an general expression for the thermal dispersion conductivity has been sought with help of the two energy equations for solid and fluid phases, derived on the basis of a volume averaging theory. It has been shown that the interstitial heat transfer between the solid and fluid phases is closely associated with the thermal dispersion. The resulting expressions for the longitudinal and transverse thermal dispersion conductivities agree well with available experimental data and empirical correlations.

Molecular orientation effects on thermal conductivity of polydimethylsiloxane under shearing strain

AbstractExperimental results for thermal conductivity and flow birefringence on liquid poly (dimethyl siloxane), PDMS, under a shearing strain are discussed. An increase in thermal conductivity with increasing shear rate is attributed to an increased degree of orientation in a direction at 45° to the flow direction for simple shear. A mathematical model is set up. It is based on the assumption that the macromolecules do not change their time‐average shape and otherwise behave as rigid, thin cylinders which can be influenced by convection and external fields and are endowed with rotational Brownian motion. The model predicts that thermal conductivity will reach a maximum value at moderate rates of strain and begin to decrease eventually reaching a minimum value (the transverse thermal conductivity of the basic element) at very high rates of strain when the molecules are completely aligned in the flow direction. Liquids studied were a 12,500 centipoise PDMS in the pure state and in solution in methyl ethyl ketone, MEK, as well as a 65,000 centipoise solution in MEK.

Untersuchungen im System TiO-TiN-TiC

Hot pressed solid solutions within the pseudoternary system TiO−TiN−TiC have been examined in relation to their physical properties. Following parameters have been measured: transverse rupture strength, hardness, density, thermal conductivity and resistance to scaling.

The influence of nuclear spins on the transport properties of molecular gases and application to HD

Coupling between the angular momentum J and the nuclear spins of a molecule modifies those contributions to the transport properties of a molecular gas which arise from the macroscopic orientational polarization. This (low pressure) effect is studied in detailfor molecular HD at low temperature (J = 1 states). Because the rotational quantum numbers are low (J = 1), a quantum treatment is essential and moreover be classical motion of J differs significantly from simple precession: the “nutation” of J being also important. This nutation gives rise to significant phase randomization of the angular-momentum polarizations at zero field, and to a field effect on the axial polarizations which are in addition to the usual field effects on nonaxial polarizations.The complicated dependence of the microscopic transport coefficients on the magnetic field and pressure reflects not only the various intramolecular couplings involved, but also the discreteness of the energy-level structure. In fact, it is possible to resolve effects of individual energy off-diagonalities in the density matrix, which persist at energy level crossings. These contribute adsorption and dispersion peaks respectively to the field dependence of the even and odd transport coefficients. The collisionally uncoupled model used in the calculation leads to good agreement both with the experimental transverse thermal-conductivity coefficient, and with the experimental thermomagnetic torque.

Interaction of Intense Submillimeter Radiation with Plasmas

Recent advances in the development of high power submillimeter lasers have opened up new areas of investigation in the study of laser plasma interactions. These areas include studies of laser-induced gas breakdown and plasma heating at cyclotron resonance, laser-induced breakdown effects in solids, and studies of Iaser-generated parametric instabilities in arc plasmas. In addition, high power submillimeter lasers can be used for important diagnostic measurements in Tokamak plasmas. These measurements include the determination of ion temperature by Thomson scattering and the measurement of transverse thermal conductivity by resonant local heating.

ORRIBLE–A Computer Program for Flow and Temperature Distribution in 19-ROD LMFBR Fuel Subassemblies

A computer program named ORRIBLE was written to predict the flow and temperature distribution for steady single-phase flow through a bundle of 19 heated rods spaced by helical wire wraps in a hexagonal duct. Any combination of flow subchannels can be blocked at the inlet. The section can have an unheated entrance length followed by a heated section and an unheated exit length. In the heated section, the linear heat rate of each of the rods can be individually specified. Turbulent interaction, sweeping crossflow due to the wire wrap, and transverse thermal conduction are considered. An approximate relationship for pressure-diversion crossflow in terms of local axial mass velocities is used to eliminate pressure as a variable. Hence, the computational procedure does not require iterative techniques.

Effect of Fiber Positioning on the Effective Physical Properties of Composite Materials

Bounds that incorporate both fiber shape information — i.e., 3 shape factors — and fiber positioning information — i.e., 2 packing parameters — are derived for the transverse effective thermal conductivity of a fiber reinforced composite. Comparisons are made with the previously presented bounds by Hashin [3] and by Beran and Silnutzer [2]. An illustration is presented to demonstrate the potential utility of controlling fiber posi tioning to improve the design of a composite material.

The influence of nuclear spin decoupling on the transverse thermal conductivity coefficient of HD

Transport properties of diatomic molecules in a magnetic field usually obey an H/p law. Deviations are to be expected for magnetic-field values in the order of the coupling strength between the nuclear spin and the rotational magnetic moment. Experimental evidence for this effect is presented for the transverse thermal conductivity of HD.

The wiedemann-franz ratio of aluminium at liquid helium temperatures

Abstract The Wiedemann-Franz ratio of pure Al single crystals has been measured at temperatures between 1·5 and 4·2 K and shown to extrapolate to the theoretically predicted value at zero temperature. The possibility of observing a transverse phonon lattice thermal conductivity in Al has been discussed and shown to be very small, in agreement with the experimental results.

Thermomagnetic Gas Torque in Apparatus with Varied Dimensions

An axial magnetic field causes a rarefied gas to exert a torque on a heated cylinder (r) suspended in a gas chamber (R). The torque is a peaked function of both H and of p and is related to the Senftleben effect in dilute gases. The direction of the torque is independent of pressure (0.001 &amp;lt; p &amp;lt; 4 torr) and of chamber dimensions. The shape θ(H) of torque peaks is the same for N2, CO, and CH4 and is independent of pressure and of chamber dimensions for R / r &amp;gt;̄ 1.3. The shape is identical to that of the transverse thermal conductivity and wider than that predicted by the volume effect theory of Levi and Beenakker (L–B). The O2 torque shape is pressure dependent and can be represented accurately as the sum of two component peaks related to the J = K and K ± 1 states of O2. The position H0 / p of the torque peak is almost independent of chamber dimensions for p &amp;gt; 0.05 torr. The size of the torque varies as (p lnR / r)−1 for p &amp;gt; 1.0 torr, in agreement with both volume and surface effect theories. The torque varies as p1.5 for p &amp;lt; 0.01 torr. The measured pθ0 for N2 above 1.0 torr agrees with the L–B volume effect theory for R / r = 18 but drops to 25% of the theory as R / r is decreased to 1.07 and (R − r) to 1.5 mm. Also for (R / r) = 1.5 mm, the entire HD torque curve and the O2 and NO torques for H &amp;gt; 10H0 reversed direction. These data support the validity of the second-order Chapman Enskog approximation used in the volume effect theory and suggest that a reverse surface creep flow becomes important for chambers with small wall spacings.