词条 | Nusselt number |
释义 |
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. where h is the convective heat transfer coefficient of the flow, L is the characteristic length, k is the thermal conductivity of the fluid.
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[1] to the local point of interest. The mean, or average, number is obtained by integrating the expression over the range of interest, such as:[2] The mass transfer analog of the Nusselt number is the Sherwood number. IntroductionAn 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, And because heat transfer at the surface is by conduction, These two terms are equal; thus Rearranging, Making it dimensionless by multiplying by representative length L, 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. DerivationThe Nusselt number may be obtained by a non-dimensional analysis of Fourier's law since it is equal to the dimensionless temperature gradient at the surface: , where q is the heat transfer rate, k is the constant thermal conductivity and T the fluid temperature. Indeed, if: , and we arrive at then we define so the equation becomes By integrating over the surface of the body: , where Empirical CorrelationsTypically, for free convection, the average Nusselt number is expressed as a function of the Rayleigh number and the Prandtl number, written as: Otherwise, for forced convection, the Nusselt number is generally a function of the Reynolds number and the Prandtl number, or Empirical correlations for a wide variety of geometries are available that express the Nusselt number in the aforementioned forms. Free convectionFree convection at a vertical wallCited[3] as coming from Churchill and Chu: Free convection from horizontal platesIf the characteristic length is defined where is the surface area of the plate and is its perimeter. Then for the top surface of a hot object in a colder environment or bottom surface of a cold object in a hotter environment[3] And for the bottom surface of a hot object in a colder environment or top surface of a cold object in a hotter environment[3] Forced convection on flat plateFlat plate in laminar flowThe local Nusselt number for laminar flow over a flat plate, at a distance downstream from the edge of the plate, is given by[6] The average Nusselt number for laminar flow over a flat plate, from the edge of the plate to a downstream distance , is given by[6] [4]Forced convection in turbulent pipe flowGnielinski correlationGnielinski's correlation for turbulent flow in tubes:[5][6] where f is the Darcy friction factor that can either be obtained from the Moody chart or for smooth tubes from correlation developed by Petukhov:[5] The Gnielinski Correlation is valid for:[5] Dittus-Boelter equationThe Dittus-Boelter equation (for turbulent flow) is an explicit function for calculating the Nusselt number. It is easy to solve but is less accurate when there is a large temperature difference across the fluid. It is tailored to smooth tubes, so use for rough tubes (most commercial applications) is cautioned. The Dittus-Boelter equation is: where: is the inside diameter of the circular duct is the Prandtl number for the fluid being heated, and for the fluid being cooled.[3] The Dittus-Boelter equation is valid for [7]Example The Dittus-Boelter equation is a good approximation where temperature differences between bulk fluid and heat transfer surface are minimal, avoiding equation complexity and iterative solving. Taking water with a bulk fluid average temperature of 20 °C, viscosity 10.07×10−4 Pa·s and a heat transfer surface temperature of 40 °C (viscosity 6.96×10−4, a viscosity correction factor for can be obtained as 1.45. This increases to 3.57 with a heat transfer surface temperature of 100 °C (viscosity 2.82×10−4 Pa·s), making a significant difference to the Nusselt number and the heat transfer coefficient. Sieder-Tate correlationThe Sieder-Tate correlation for turbulent flow is an implicit function, as it analyzes the system as a nonlinear boundary value problem. The Sieder-Tate result can be more accurate as it takes into account the change in viscosity ( and ) due to temperature change between the bulk fluid average temperature and the heat transfer surface temperature, respectively. The Sieder-Tate correlation is normally solved by an iterative process, as the viscosity factor will change as the Nusselt number changes.[8] [3] where: is the fluid viscosity at the bulk fluid temperature is the fluid viscosity at the heat-transfer boundary surface temperature The Sieder-Tate correlation is valid for[3] Forced convection in fully developed laminar pipe flowFor fully developed internal laminar flow, the Nusselt numbers tend towards a constant value for long pipes. For internal Flow: where: Dh = Hydraulic diameter kf = thermal conductivity of the fluid h = convective heat transfer coefficient Convection with uniform surface heat flux for circular tubesFrom Incropera & DeWitt,[9] Convection with uniform surface temperature for circular tubesFor the case of constant surface temperature,[9] OEIS sequence {{OEIS link|A282581}} gives this value as . See also
External links
References1. ^{{cite book|title=Heat Transfer: a Practical Approach|author=Yunus A. Çengel|year=2003|publisher=McGraw-Hill|edition=2nd}} {{NonDimFluMech}}{{Use dmy dates|date=September 2010}}{{DEFAULTSORT:Nusselt Number}}2. ^{{cite journal|title=Transitional natural convection flow and heat transfer in an open channel|year=2012|author=E. Sanvicente|doi=10.1016/j.ijthermalsci.2012.07.004|volume=63|pages=87–104|journal=International Journal of Thermal Sciences|display-authors=etal}} 3. ^1 2 3 4 5 {{Cite book |first=Frank P. |last=Incropera |authorlink=Frank P. Incropera |last2=DeWitt |first2=David P. |title=Fundamentals of Heat and Mass Transfer |edition=4th |page=493 |location=New York |publisher=Wiley |year=2000 |isbn=978-0-471-30460-9 }} 4. ^Incropera, Frank P. Fundamentals of heat and mass transfer. John Wiley & Sons, 2011. 5. ^1 2 3 4 {{cite book |authorlink=Frank P. Incropera |last=Incropera |first=Frank P. |last2=DeWitt |first2=David P. |title=Fundamentals of Heat and Mass Transfer |edition=6th |location=Hoboken |publisher=Wiley |pages=490, 515 |year=2007 |isbn=978-0-471-45728-2 }} 6. ^{{cite journal |last=Gnielinski |first=Volker |title=Neue Gleichungen für den Wärme- und den Stoffübergang in turbulent durchströmten Rohren und Kanälen |pages=8–16 |year=1975 |journal=Forsch. Ing.-Wes. |volume=41 |issue=1}} 7. ^Incropera, Frank P.; DeWitt, David P. (2007). Fundamentals of Heat and Mass Transfer (6th ed.). New York: Wiley. p. 514. {{ISBN|978-0-471-45728-2}}. 8. ^{{cite web |url=http://www.profjrwhite.com/math_methods/pdf_files_hw/sgtm3.pdf |title=Temperature Profile in Steam Generator Tube Metal |website= |accessdate=23 September 2009 }} 9. ^1 {{Cite book |first=Frank P. |last=Incropera |last2=DeWitt |first2=David P. |title=Fundamentals of Heat and Mass Transfer |edition=5th |pages=486, 487 |location=Hoboken |publisher=Wiley |year=2002 |isbn=978-0-471-38650-6 }} 5 : Convection|Dimensionless numbers of fluid mechanics|Dimensionless numbers of thermodynamics|Fluid dynamics|Heat transfer |
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