Results 21 - 30 of 49386
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[en] A simplifying approach for calculating the radiant energy is achieved by introducing the concept of net transmittance, resulting in a novel variation of the net radiation method that provides an easy way for solving a variety of situations. In particular, a closed form for the net radiation between two grey plates through a radiation shield formed by a series of partially transparent partially reflecting partially absorbing plates is found. In addition, the method is generalized to cylindrical and spherical geometries
[en] This paper briefly reexamines the basic concepts used to describes radiative properties of opaque surfaces. It first recalls the connection between the specific intensity and the randomly fields produced by a thermal source. Then the paper discusses the concept of reflectivity and shows that there is a fundamental limit related to the uncertainty principle that prohibits defining a local reflectivity in space and in angle as it is usually done. As a consequence, it is shown that it is meaningless to define a reflectivity for an area smaller than the wavelength. On the other hand, the emissivity can be defined for a length scale on the order of the skin depth which can be much smaller than the wavelength
[en] This paper is a review of the main literature studies about the finned heat exchanger modelling and simulation. Finned heat exchangers are used in cryogenic industry, nuclear industry, food industry, medicine, etc.. The aspects concerning the air circulation through the heat exchanger component tubes are studied, as well as those involved in the thermal transfer from technological fluid to wall or from fin to air. A particular attention is devoted to the physical and mathematical complete models of finned heat exchangers, as well as to the criterial relationship for the calculation of the partial heat transfer to air (α2), fin efficiency (ηa) and friction coefficient (f). Details are also given about the influence of the thermal contact resistance on the finned heat exchanger performance. This study is a useful tool for a correct technological sizing of this type of heat exchangers. (authors)
[en] In the development of reactor systems, if cost penalties are not too great, direct testing of reactor components is usually undertaken. When the financial burden of such direct testing becomes too severe, ensuring the adequate performance of reactor components entails the use of proven design methods. The establishment of such methods involves: a) the carrying out of basic generic work to determine the physical phenomena and physical principles involved in component performance. b) the use of this generic data in the generation of mathematical models by means of which reactor situations can be assessed. c) the validation of these models against experimental data (produced at a reasonable cost) in fairly complex situations representative of the reactor configurations. This paper describes the present position, in the UK, in respect of the development of such design methods for the CDFR above sodium environment
[en] The conductive radiative transfer problem in homogeneous plane parallel medium of anisotropic scattering with specularly reflecting boundaries has been considered. The angular dependent reflectivity of the boundary is considered as Fresnel reflection probability function. A Galerkin-iterative technique is used to solve the coupled conductive radiative heat equations in integral forms. Numerical results are obtained for the temperature, the conductive, radiative and the total heat fluxes for homogeneous upon the reflective index of the medium with isotropic or anisotropic scattering. Results obtained for homogeneous medium with isotropic or anisotropic scattering are compared with the published calculations.
[en] A possible severe accident scenario is a general meltdown and relocation of the reactor core during which molten core material accumulates in the lower plenum of the reactor vessel. The decay heat generated in a radioactive material would have to be removed through the walls of the lower plenum in order to ensure the integrity of the reactor pressure vessel. Numerical simulations of turbulent natural convection in a geometry representing the lower plenum cavity of a reactor pressure vessel were conducted. A two-dimensional numerical code based on a finite-volume method was developed to simulate turbulent natural convection in a fluid with heat generation using large-eddy simulation. Simulations were performed at Rayleigh numbers 1010 and 2.1011 and Prandtl numbers 1.2, 7 and 8 which corresponds to conditions in the numerical investigations made by Nourgaliev et al. (1997) and in the experimental work done by Asfia and Dhir (1996). The results are shown to be in satisfactory agreement. (author)
[en] Elementary considerations are used to define and analyze the discrete spectrum for a general radiative transfer model that includes polarization effects. (author)
[pt]Consideracoes elementares sao usadas para definir e analisar o espectro discreto para um modelo geral de transferencia radiativa que inclui efeitos de polarizacao. (E.G.)
[en] Two phase heat transfer correlation for the slug to annular flow regimes. Successful design of heat exchanger such as evaporator requires accurate prediction of the local heat transfer coefficient for boiling regime. Investigations of two phase heat transfer in horizontal pipe flow have led to a new correlation for the heat transfer coefficient. The proposed correlation is: NuTP=2,90 [Re]0,82 [Pr]0,78 [Bo]0,62. This correlation was tested and experimental data obtained on covering the slug to annular flow regimes. The correlation produces satisfactory results. (author). 7 figs.; 14 refs
[en] Highlights: • A stochastic optimization approach is proposed to design organic heat transfer fluids. • Risk metrics are used to design fluids that withstand strong variability in system conditions. • Non-intuitive mixture compositions are identified. - Abstract: Over 50% of the heat generated in industry is in the form of low-grade heat (with operating temperatures below 370 °C). Recovering heat from these sources with standard Rankine cycles (using water as working fluid) is inefficient and expensive. Organic working fluids have become an attractive alternative to mitigate these inefficiencies. In this work, we address the problem of designing flexible multi-component organic fluids capable of withstanding variability in heat source temperatures and efficiencies of individual cycle equipment units. The design problem is cast as a nonlinear stochastic optimization problem and we incorporate risk metrics to handle extreme variability. We show that a stochastic optimization framework allows us to systematically trade-off performance of the working fluid under a variety of scenarios (e.g., inlet source temperatures and equipment efficiencies). With this, it is possible to design working fluids that remain robust in a wide range of operational conditions. We also find that significant flexibility of the working fluid can be obtained by using optimal concentrations as opposed to using single component mixtures. We also find that state-of-the-art nonlinear optimization solvers can handle highly complex stochastic optimization problems that incorporate detailed physical representations of the system.