Genera Zized Variationa I Prhcz $ Zes for Convective Heat Transfer and Irreversz ’ b k Thermodynamics
Variational principles for convective heat transfer are extended to nonhomogeneous fluids and to fluids with thermal expansion. The results apply to nonlinear problems with temperature dependent properties. The context is restricted to heat transfer but the derivation and a discussion of the physics brings out the general validity of the variational principles for a large category of irreversible processes.
New Methods in Heat Flow Analysis With Application to Flight Structures
New methods are presented for the analysis of transient heat flow in complex structures, leading to drastic simplifications in the calculation and the possibility of including nonlinear and surface effects. These methods are in part a direct application of some general variational principles developed earlier for linear thermodynamics.1-3 They are further developed in the particular case of purely thermal problems to include surface and boundary-layer heat transfer, nonlinear systems with temperature-dependent parameters, and radiation. The concepts of thermal potential, dissipation function, and generalized thermal force are introduced, leading to ordinary differential equations of the Lagrangian type for the thermal flow field. Because of the particular nature of heat flow phenomena, compared with dynamics, suitable procedures must be developed in order to formulate each problem in the simplest way. This is done by treating a number of examples. The concepts of penetration depth and transit time are introduced and discussed in connection with onedimensional flow. Application of the general method to the heating of a slab, with temperature-dependent heat capacity, shows a substantial difference between the heating and cooling processes. An example of heat flow analysis of a supersonic wing structure by the present method is also given and requires only extremely simple calculations. The results are found to be in good agreement with those obtained by the classical and much more elaborate procedures.
Radiation Energy Transfer and Thermal Conductivity of Ceramic Oxides
The heat-transfer mechanisms in several ceramic oxides were investigated from room temperature to 1250/sup o/C. The conductivity of normal sintered porous polycrystalline oxides results from phonon conduction, and results agree with theoretical expectations for the effect of impurities, structure, and temperature dependence. Single crystals, glasses, and low-porosity aggregates show deviations from the expected phonon conduction which are attributed to a second mechanism of heat transfer, namely, photon conduction. Measurements of photon conduction are related to infrared transmission, structure, and conditions of the measurements. Photon conduction can be a significant portion of the total heat conduction at high temperatures; however, for normal polycrystalline ceramic oxides of porosities greater than 2%, this contribution is small. (auth)