Nusselt number, Nu, is the dimensionless parameter characterizing convective heat transfer. It is defined as. where α is convective heat transfer coefficient, L is representative dimension (e.g., diameter for pipes) , and λ is the thermal conductivity of the fluid.
What is H in Nusselt number?
h = convective heat transfer coefficient .
How do I use a Nusselt number?
A Nusselt number of order unity would indicate a sluggish motion little more effective than pure fluid conduction : for example, laminar flow in a long pipe. A large Nusselt number means very efficient convection: For example, turbulent pipe flow yields Nu of order 100 to 1000.
What is K in heat transfer?
k: The factor k is called the thermal conductivity constant . The thermal conductivity constant k is larger for materials that transfer heat well (like metal and stone), and k is small for materials that transfer heat poorly (like air and wood).
What is Nusselt number and Prandtl number?
Nusselt number is the ratio of convection heat transfer to pure conduction heat transfer, (K- Thermal conductivity of fluid) Reynolds number is the ratio of the inertia to the viscous forces avting on a fluid. Prandtl number is the ratio of the momentum and thermal diffusivities .
Who found Nusselt number?
nondimensional group which is named the Nusselt number after Wilhelm Nusselt (1882–1957). For the formal relation between h and Nu see Equation (2).
Can Nusselt number be less than 1?
Nu can not be less than one since it gives relation between conduction and convection of the liquid.
How is Nusselt number calculated?
For fully developed (hydrodynamically and thermally) turbulent flow in a smooth circular tube, the local Nusselt number may be obtained from the well-known Dittus-Boelter equation. To calculate the Nusselt number, we have to know: the Reynolds number, which is Re Dh = 575600 . the Prandtl number, which is Pr = 0.89 .
What does Nusselt number depend on?
The Nusselt number in the fully developed laminar flow is constant and depends on the channel geometry and the wall heat transfer boundary condition .
What is the difference between Nusselt number and Biot number?
Biot number indicates how much solid temperature can change quickly based on the convection heat transfer in fluid. Biot number uses thermal conductivity of the body (not fluid), whereas Nusselt number uses thermal conductivity of the fluid.
Is a higher K value better?
The thermal conductivity (k-value) is the ability of a material to conduct heat; hence, the lower the k-value, the better the material is for insulation .
What is K value and U-value?
To indicate to what extent a material is thermally insulating, the term thermal transmittance or U-value (formerly known as K-value) is used in the construction industry. The lower the U-value, the higher the heat resistance of a material, meaning the better the insulation.
Is W mk the same as W MC?
The units W/m-K and W/m-C are the same .
Where is Prandtl number used?
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).
What is meant by Prandtl number?
The Prandtl number is a dimensionless quantity that puts the viscosity of a fluid in correlation with the thermal conductivity . It therefore assesses the relation between momentum transport and thermal transport capacity of a fluid.
What does Reynolds number represent?
The Reynolds number is the ratio of inertial forces to viscous forces . The Reynolds number is a dimensionless number used to categorize the fluids systems in which the effect of viscosity is important in controlling the velocities or the flow pattern of a fluid.
