Dittus boelter equation

How is Nusselt number calculated?

FormulaNusselt Number : Nu = hL/k.Convection Heat Transfer Coefficient : k = Nuk/L.Characteristic Length : L = Nuk/h.Thermal Conductivity of the Fluid : k = hL/Nu.Where, Nu = Nusselt Number, h = Convection Heat Transfer Coefficient, L = Characteristic Length, k = Thermal Conductivity of the Fluid.

What does the Nusselt number tell you?

In fluid dynamics, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid. A Nusselt number of value one represents heat transfer by pure conduction. A value between one and 10 is characteristic of slug flow or laminar flow.

What is difference between Biot No & nusselt no?

The real difference betweeh the two are; Nusselt number confirms whether conductive or convective heat transfer dominates across the fluid-solid interface while Biot number, whether significant thermal gradients will develop inside a solid by the ratio of heat transfer away from the surface of a solid to heat transfer

Can Nusselt number be less than 1?

The Nusselt number is the ratio of the heat transfer with convection to the original heat transfer with no fluid motion. Since the moving fluid always enhances heat transfer, Nusselt number can only be greater than or equal to 1.

Is Prandtl number dimensionless?

9.9. The Prandtl number is an example of a dimensionless number that is an intrinsic property of a fluid. Fluids with small Prandtl numbers are free-flowing liquids with high thermal conductivity and are therefore a good choice for heat conducting liquids.

What is h in heat transfer?

where: q: heat flux, W/m2; i.e., thermal power per unit area, q = d /dA h: heat transfer coefficient, W/(m2•K) ΔT: difference in temperature between the solid surface and surrounding fluid area, K. It is used in calculating the heat transfer, typically by convection or phase transition between a fluid and a solid.

How is Biot number calculated?

Bi = Lc * h / k ,Lc [m] is the characteristic length of a material,h [W/(m² * K)] is the heat transfer coefficient at the material’s surface,k [W/(m * K)] is the thermal conductivity of the material.Bi is the resulting Biot number.

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What is the dittus Boelter equation when does it apply?

When the difference between the surface and the fluid temperatures is large, it may be necessary to account for the variation of viscosity with temperature. Therefore a modified form of Dittus-Boelter equation was proposed by Sieder and Tate (1936).

What is Biot and Fourier number?

The Fourier number is the dimensionless time for a temperature change to occur. The Biot number represents the ratio of heat transfer resistance in the interior of the system (L/k) to the resistance between the surroundings and the system surface (1/h).

What is temperature in forced convection flow?

The film temperature is often used as the temperature at which fluid properties are calculated when using Prandtl number, Nusselt number, Reynolds number or Grashof number to calculate a heat transfer coefficient, because it is a reasonable first approximation to the temperature within the convection boundary layer.

How is Fourier number calculated?

FormulaFourier Number : Fo = αt/L2Thermal Diffusivity : α = Fo L2/t.Characteristic Time : t = Fo L2/ αCharacteristic Length : L = √αt/Fo.Where, Fo = Fourier Number, α = Thermal Diffusivity, t = Characteristic Time, L = Characteristic Length.

Can Nusselt numbers negative?

yes, Nusselt number is negative if h (convective heat transfer coeff.) is negative. h is not a thermophysical propertiy and only represents the ratio between quantities defined with respect to different geometric scales, that is the heat flux q and temperature difference.

Can the overall heat transfer coefficient be negative?

In addition, with traditional definition of convective heat transfer coefficient, which is based on the temperature difference of an interior surface and room air, the coefficient value can be negative.

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