Thermal conductivity, definition

Diamond nanoparticles Nanofluids Thermal conductivity Definition... 

This definition is in terms of a pool of liquid of depth h, where z is distance normal to the surface and ti and k are the liquid viscosity and thermal diffusivity, respectively [58]. (Thermal diffusivity is defined as the coefficient of thermal conductivity divided by density and by heat capacity per unit mass.) The critical Ma value for a system to show Marangoni instability is around 50-100. 

Thickness. The traditional definition of thermal conductivity as an intrinsic property of a material where conduction is the only mode of heat transmission is not appHcable to low density materials. Although radiation between parallel surfaces is independent of distance, the measurement of X where radiation is significant requires the introduction of an additional variable, thickness. The thickness effect is observed in materials of low density at ambient temperatures and in materials of higher density at elevated temperatures. It depends on the radiation permeance of the materials, which in turn is influenced by the absorption coefficient and the density. For a cellular plastic material having a density on the order of 10 kg/m, the difference between a 25 and 100 mm thick specimen ranges from 12—15%. This reduces to less than 4% for a density of 48 kg/m. References 23—27 discuss the issue of thickness in more detail. 

From the definition of thermal conductivity, the heat transferred per unit time through unit area at a distance v from the surface is given by ... 

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... 

In the methods reported above, the temperature change AT used to measure the heat capacity C(T) was supposed to be so small that the time constant r = R C could considered constant in the AT interval. Let us consider, for example, the thermal discharge of a system with heat capacity C(T) a T and thermal conductance to the bath G(T) a T3 (e.g. a metal sample and a contact resistance to the bath at rB). A AT/TB = 10% gives a At/t = 20% over the interval AT, that is a time constant definitely not constant. 

Unlike Fourier s Law, Eq. (4.62) is purely empirical—it is simply the definition for the heat transfer coefficient. Note that the units of he (W/m -K) are different from those for thermal conductivity. Under steady-state conditions and assuming that the heat transfer area is constant and h is not a function of temperature, the following form of Eq. (4.62) is often employed ... 

The influence of small quantities of arsenic on copper has already been described (p. 55). The thermal conductivity of Cu-As alloys has been investigated,5 as also has the electrical behaviour at temperatures as low as 1-26° Abs., obtained by means of liquid helium 6 whether or not the alloys are supraconduetive at these temperatures has not been definitely determined. The structure of various Cu-As alloys has been investigated by means of the X-rays.7... 

The direct-effect a and T) must be positive definite because of Fourier s law of heat conduction (where the thermal conductivity is always positive, according to... 

This definition of thermal diffusivity gives the impression that it is simply a mathematical factor but, as Hands points out10, it is the parameter that relates heat flow to the energy gradient, analogous to thermal conductivity relating heat flow to the temperature gradient. 

Coefficient of Thermal Conductivity or Specif, ic Heat Conductivity ( is the quantity of heat transmitted per second thru a plate of material 1cm thick and 1cm2 in area, when the temp difference between the two sides of the plate is one degree centigrare. Some values are given in Ref and under individual compds described in this Encyclopedia Ref Clift Fedoroff, Vol 2(1943), Table of Physical Constants of Compounds Used in Explosives Industry and Definition of Terms Used in Table of Physical Constants [See also S. Nagayama Y.Mizushima KKK 21, 8-11 (I960) CA 55, 9877 (1961) Explosivst 1964, 2l]... 

We neglect the heat spent on heating the mixture in the reaction zone itself (cdT/ dt) and further consider the thermal conductivity in this zone to be constant and equal, for the sake of definiteness, to the thermal conductivity K1 of the reaction products at Tv Turning to the one-dimensional problem, we obtain the equation... 

When analyzing thermal processes, the thermal conductivity, k, is the most commonly used property that helps quantify the transport of heat through a material. By definition, energy is transported proportionally to the speed of sound. Accordingly, thermal conductivity... 

The most common detectors for GC are the non-selective flame ionisation detector and thermal conductivity detector. For element speciation, selectivity is definitely advantageous, allowing less sample preparation and less demanding separation. Of the conventional GC detectors, the electron capture detector is very sensitive for electrophilic compounds and therefore has some selectivity for polar compounds containing halogens and metal ions. It has been used widely... 

Using the experimental values for the width of the traveling wave front (portion be, Fig. 8), let us estimate the propagation velocity for the case of a thermal mechanism based on the Arrhenius law of heat evolution from the known relationship U = a/d, where a 10"2 cm2/s is the thermal conductivity determined by the conventional technique. We obtain 5 x 10"2 and 3 x 10-2cm/s for 77 and 4.2 K, respectively, which are below the experimental values by about 1.5-2 orders of magnitude. This result is further definite evidence for the nonthermal nature of the propagation mechanism of a low-temperature reaction initiated by brittle fracture of the irradiated reactant sample. 

As seen from Figure 10, the autowave solution of equation (2) exists only at G < G0 0.64. Physically this implies that in a system described by this model the autowave mode of the reaction propagation over a solid reactant mixture becomes impossible at a definite increase in the strength of the sample, decrease in the thermal effect and reaction velocity, and increase in the thermal conductivity. 

The hole model for molecular liquids was elaborated by Furth [12], who supposed that the free volume of a liquid is not distributed uniformly between its molecules like in crystals, but is concentrated like some holes which can disappear in one place and appear in another place. These holes are in permanent motion, so that the situation is different from the jumps of the holes in a crystal. The appearance and disappearance of the holes in a liquid are a result of the fluctuations connected with thermal movements. These holes in liquids have no definite shape and size they can increase or decrease spontaneously. Furth [12] tried to calculate a large number of properties of the liquids viscosity, compressibility, thermal expansion, thermal conductivity, but the results were not successful. However, Furth obtained a precise result of the calculation of the volume change by melting and the entropy of melting. 

Equation (1-1) is the defining equation for thermal conductivity. On the basis of this definition, experimental measurements may be made to determine the thermal conductivity of different materials. For gases at moderately low temperatures, analytical treatments in the kinetic theory of gases may be used to predict accurately the experimentally observed values. In some cases, theories are available for the prediction of thermal conductivities in liquids and solids, but in general, many open questions and concepts still need clarification where liquids and solids are concerned. 

The following symbols are used in the definitions of the dimensionless quantities mass (m), time (t), volume (V area (A density (p), speed (u), length (/), viscosity (rj), pressure (p), acceleration of free fall (p), cubic expansion coefficient (a), temperature (T surface tension (y), speed of sound (c), mean free path (X), frequency (/), thermal diffusivity (a), coefficient of heat transfer (/i), thermal conductivity (/c), specific heat capacity at constant pressure (cp), diffusion coefficient (D), mole fraction (x), mass transfer coefficient (fcd), permeability (p), electric conductivity (k and magnetic flux density ( B) ... 

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