Nusselt number 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 is the function of Nusselt number?

Definition. The Nusselt number is the ratio of convective to conductive heat transfer across a boundary. 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.

What is meant by Nusselt number?

The Nusselt number is defined as the ratio of convection heat transfer to fluid conduction heat transfer under the same conditions.

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 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 does Prandtl number tell us?

In heat transfer problems, the Prandtl number controls the relative thickness of the momentum and thermal boundary layers. When Pr is small, it means that the heat diffuses quickly compared to the velocity (momentum). The mass transfer analog of the Prandtl number is the Schmidt number.

What is the difference between Nusselt number and Biot number?

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

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 do I find my Prandtl number?

The Prandtl number (Pr = ν / α) is defined as the dimensionless ratio between kinematic viscosity (ν) and thermal diffusivity, α = k / (ρ·cp); where k stands for thermal conductivity, ρ stands for density, and cp is the (isobaric) specific heat capacity.

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 Reynolds number is turbulent?

For flow in a pipe of diameter D, experimental observations show that for “fully developed” flow, laminar flow occurs when ReD < 2300 and turbulent flow occurs when ReD > 2900.

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.

What does Biot number mean?

The Biot number (Bi) is a dimensionless quantity used in heat transfer calculations. This ratio determines whether or not the temperatures inside a body will vary significantly in space, while the body heats or cools over time, from a thermal gradient applied to its surface.

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