Thermal diffiisivity

In equations 5-8, the variables and symbols are defined as follows p0 is reference mass density, v is dimensional velocity field vector, p is dimensional pressure field vector, x is Newtonian viscosity of the melt, g is acceleration due to gravity, T is dimensional temperature, tT is the reference temperature, c is dimensional concentration, c0 is far-field level of concentration, e, is a unit vector in the direction of the z axis, Fb is a dimensional applied body force field, V is the gradient operator, v(x, t) is the velocity vector field, p(x, t) is the pressure field, jl is the fluid viscosity, am is the thermal diffiisivity of the melt, and D is the solute diffiisivity in the melt. The vector Fb is a body force imposed on the melt in addition to gravity. The body force caused by an imposed magnetic field B(x, t) is the Lorentz force, Fb = ac(v X v X B). The effect of this field on convection and segregation is discussed in a later section. 

The considerable mass difference between reactants and carrier gas combined with often large thermal gradients means that thermal diffiision may contribute to the overall mass transfer. Thermal diffiision drives reactants... 

Figures 5 (a) and (b) show the thermal conductivities (X) of U-ZrHi 60 and U-ZrHi 9o, respectively, at the temperatures up to 773 K which were calculated from the relation of X=aCvd, where a, Cp and d represented the thermal diffiisivity, the specific heat and the density, respectively. It has been already demonstrated that the values of Cp and d can be expressed as the simple functions of temperature and composition [11], The high thermal conductivity at irradiation temperature of 700 K induces to the safety at high linear power in reactors.

The title compound has been synthesized by means of degradation. Its composition has been determined by chemieal analysis, TG and ICP. The studies of IR, UV, XRD, W-NMR indicate that the compound has the Keggin structure, and the results of TG-DTA show that the stable temperature of this compound is up to 400-450°C. We also studied the rare earth chemical thermal diffiision of the eomplex and found that the rare earth element can permeate into the surface, interface and the body cavity of the prepared material. The conductivity of the permeated sample is 1.935X 10 Q cm, which is 10 times higher than that of the parent K,oH3[Y(SiW, 039)2]. 

Separation of mixtures based on differences in thermal diffiisivity at present are feasible only for analytical purposes or for production on a very small scale of substances not otherwise recovered easily. Nevertheless, the topic is of some interest to the process engineer as a technique of last resort. 

Powdery chemical of the TDtype Coefffcieits of Eti. (+4) a b Radius of the cylinder f cm Bfifieetive thermal diffiisivity The Fraitk- Kamenetskii number The value of calculated herein r The temp, values measuied by other researchers X... 

In addition to acoustic properties, the transient grating measurements also provide information on the thermal diffusivity of the ILs. In this study, various ILs with different anions and cations were used in order to elucidate the effect of the anon and the cation on their thermal diffiisivities and... 

Electrical resistivity measurement adopted conventional four probes method. Seebeck coefficient was measured by the standard DC method. Thermal conductivity k was calculated from density, specific heat, and thermal diffiisivity. Specific heat measurement was carried out by use of a differential scanning calorimeter (DSC model 8230, Rigaku, Japan) compared with a standard material of a -AI2O3. The values of thermal diffiisivity obtained from a differential phase analysis of photo-pyroelectric signal (AL- A 0 analysis) [9]. All measiu ements were done at room temperature. 

U being the total heat transfer coefficient, Ri the inside convective resistance, Rk the conductive resistance of pipe walls, and R0 the outside convective resistance. This result may be rearranged in terms of the characteristic length 1/m = JkAjVP for fins [recall Eq. (2.113)], the thermal diffiisivity,... 

F. Takahashi, T. Mori, Y. Hamada, 1. Hatta, AC calorimetric thermal diffiisivity measurement in relatively thick samples by a distance-variation method, Jpn. J. Appl. Phys.,3S, 4741-4744, (2001). 

Molybdenum was chosen by AIST as a test material for this technique and the first results, recently presented, for thermal diffiisivity data in the range fix>m 2000 K to 2600 K indicate an improvement to the scarce and scattered literature data. 

In Eq. (20.23) for the temperature conductivity (thermal diffiisivity) a = as/ip -c),c indicates the specific or entropy capacity per mass and is the corresponding volumic... 

Various material properties (e. g. thermal diffiisivity, air permeability) have been determined for WHIPOX CMCs. Reliable data are important for potential applications such as thermal insulators, filters or burners. Thermal conductivity perpendicular to fiber orientation is about IW/mK. Closer inspection reveals lower conductivity if a mullite matrix is employed instead of alumina. Thermal conductivity in fiber direction, on the other hand, is about three times higher as perpendicular to the fiber direction, reflecting the non-isotropic structure of the composite (Figure 11). 

In optothermal transient emission radiometry a sample material is irradiated (excited) with optical wavelength radiation in the form of short-duration pulses and the thermal emission transient observed by means of a wide-band infrared detector (Figure 6.32). An optothermal decay curve plots the intensity of the thermal emission versus time. The shape of the decay curve depends upon the extent to which the incident radiation penetrates into the sample, as well as on the thermal diffiisivity and transparency to the emitted thermal infrared of the material. 

In these equations, Tj is the temperature at coordinates (r, t) in the macrograin, pcp is the average value of the heat capacity per unit volume of the macroparticle, is the effective thermal diffiisivity in the macrograin and (—Affr) the enthalpy of polymerization. In Equation 2.141, the parameter hp is the average convective heat transfer coefficient, usually calculated from a Nusselt number correlation. Early works tended to use the well-known Ranz-Marshall correlation for evaporation from a droplet however, it has been... 

Numerical results reported (2) on a typical TGDDM-DDS matrix laminate, assuming that the prepregs are suddenly expose to die cure temperature, are diown in Fig. 24 (a,b,c) as me variation of die tenqierature, decree of reaction and viscosity as a function of the processing time, bodi on the dam and on the core of the laminate. Input data of die full model are givmi in Table 9 (2). Due to the contribution of die thermal conductivity of the fibm the tenqierature at the center of die laminate nqiidly reaches the external inqiosed temperature and increases as a con uence of die imbalance between the rate of heat generation and the thermal diffiisivity of the composite (Fig. 24a). When these two quantities are comparable, the temperature profile reaches a maximum. 

Fourier number n (Np). A dimensionless group important in analysis of unsteady heat transfer in solids, such as sheets being heated or cooled in thermoforming, or cooling of extrudates and moldings, ATp = a f/x , where a is the material s thermal diffiisivity, t the heating or cooling time, and x is the a thickness or half-thickness in the direction of heat flow. 

The thermal conductivity A of pure fluids diverges at the critical point about halt as strongly as the isobaric heat capacity (Figs. 5a and 5b), and therefore the thermal diffiisivity, A/pCf>, goes to zero. 

Figure 10.17. Remarkably, low values of thermal conductivity have been obtained with these materials (e.g., as low as 0.014 W/m K as calculated from the product of density, thermal diffiisivity and specific heat), suggesting them as very promising candidates for themial superinsulation at atmospheric pressure.

Polymer processing is influenced by the thermal characteristics of polymer. They are melt temperature, glass transition temperature, thermal conductivity, thermal diffiisivity, heat capacity, coefficient of linear thermal expansion, and decomposition temperature. The effect of processing (shear and heat) is expected to have an effect on the polymer at the molecular level, reducing molar mass and altering its distribution [3]. 

Recent developments have been in the area of microthermal analysis using thermal conductivity with thermal diffiisivity signals or AFM to visualize specific areas or domains in the material and perform localized thermal analysis studies (183,184). Relaxational behavior over time and temperature is related to changes in free volume of the material. Positron annihilation lifetime spectroscopy (PALS) measurements of positron lifetimes and intensities are used to estimate both hole sizes and free volume within primarily amorphous phases of polymers. These data are used in measurement of thermal transitions (185,186) structural relaxation including molecular motions (187-189), and effects of additives (190), molecular weight variation (191), and degree of crystallinity (192). It has been used in combination with DSC to analyze the range of miscibility of polymethyl methacrylate poly(ethylene oxide) blends (193). 

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