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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Nusselt number ： ウィキペディア英語版 Nusselt number In heat transfer at a boundary (surface) within a fluid, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer across (normal to) the boundary. In this context, convection includes both advection and diffusion. Named after Wilhelm Nusselt, it is a dimensionless number. The conductive component is measured under the same conditions as the heat convection but with a (hypothetically) stagnant (or motionless) fluid. A similar non-dimensional parameter is Biot Number, with the difference that the thermal conductivity is of the solid body and not the fluid. A Nusselt number close to one, namely convection and conduction of similar magnitude, is characteristic of "slug flow" or laminar flow. A larger Nusselt number corresponds to more active convection, with turbulent flow typically in the 100–1000 range. The convection and conduction heat flows are parallel to each other and to the surface normal of the boundary surface, and are all perpendicular to the mean fluid flow in the simple case. :$\backslash mathrm\_L\; =\; \backslash frac\}\; =\; \backslash frac$ where ''h'' is the convective heat transfer coefficient of the flow, ''L'' is the characteristic length, ''k'' is the thermal conductivity of the fluid. * Selection of the characteristic length should be in the direction of growth (or thickness) of the boundary layer; some examples of characteristic length are: the outer diameter of a cylinder in (external) cross flow (perpendicular to the cylinder axis), the length of a vertical plate undergoing natural convection, or the diameter of a sphere. For complex shapes, the length may be defined as the volume of the fluid body divided by the surface area. * The thermal conductivity of the fluid is typically (but not always) evaluated at the film temperature, which for engineering purposes may be calculated as the mean-average of the bulk fluid temperature and wall surface temperature. In contrast to the definition given above, known as ''average Nusselt number'', local Nusselt number is defined by taking the length to be the distance from the surface boundary to the local point of interest. :$Nu\_x\; =\; \backslash frac$ The ''mean'', or ''average'', number is obtained by integrating the expression over the range of interest, such as: :$\backslash overline=\backslash frac\; \backslash int\_^$ The mass transfer analog of the Nusselt number is the Sherwood number. ==Introduction== An understanding of convection boundary layers is necessary to understanding convective heat transfer between a surface and a fluid flowing past it. A thermal boundary layer develops if the fluid free stream temperature and the surface temperatures differ. A temperature profile exists due to the energy exchange resulting from this temperature difference. The heat transfer rate can then be written as, :$=hA\backslash left(\; -\; \backslash right)$ And because heat transfer at the surface is by conduction, :$=-kA\backslash frac$ These two terms are equal; thus :$-kA\backslash frac=hA\backslash left(\; -\; \backslash right)$ Rearranging, :$\backslash frac=\backslash frac\}$ Making it dimensionless by multiplying by representative length L, :$\backslash frac=\backslash frac\}\}$ The right hand side is now the ratio of the temperature gradient at the surface to the reference temperature gradient. While the left hand side is similar to the Biot modulus. This becomes the ratio of conductive thermal resistance to the convective thermal resistance of the fluid, otherwise known as the Nusselt number, Nu. :$\backslash mathrm\; =\; \backslash frac$ 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nusselt number」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.