What are Nusselt number correlations?
In fluid dynamics, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid. It is a dimensionless number, closely related to the fluid’s Rayleigh number. A Nusselt number of value one represents heat transfer by pure conduction.
What is natural convection cooling?
Natural convection cooling describes a situation where there is no forced air flow from a fan, blower, or any other source. Under natural convection cooling, the temperature of the air within the heat sink’s fins increases due to heat from the heat source.
What is the most important parameter in natural convection?
The driving force for natural convection is gravity. For example if there is a layer of cold dense air on top of hotter less dense air, gravity pulls more strongly on the denser layer on top, so it falls while the hotter less dense air rises to take its place.
Why is the Nusselt number important?
Nusselt number is an important parameter that can contribute to a better rate of heat exchange. It is basically a function of Reynolds and Prandtl number. The correlation is provided in Eq. This is the so-called Dittus Boelter-type correlation.
What is the physical significance of the Nusselt number how is it defined?
Nusselt number is required to find ‘h’ which is convective heat transfer coefficient. The physical interpretation of Nusselt number is the enhancement of heat transfer due to convection over conduction alone. If Nu=1, then, than your fluid is stationary and all heat transfer is by conduction.
How is Nusselt number calculated?
- Nusselt 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 correlation for natural convection?
The magnitude of the natural convection heat transfer between a surface and a fluid is directly related to the flow rate of the fluid induced by natural convection. The higher the flow rate, the higher the heat transfer rate.
What are the physical significance of nusselt no and prandtl no?
Reynolds determines if the flow is laminar or turbulent. Nusselt calculates the transference of heat relative to pure conduction. Prandtl represents how big is the viscous boundary layer relative to thermal boundary layer.
How is the Nusselt number of a fully developed flow changing along the position?
In developing flow inside pipes the Nu number changes along the axial position. After the flow is fully developed the Nu number remains constant along the axial position (Nu=4.36 for laminar flow, constant heat flux at wall, circular pipe). It is a coefficient indicating the rate at which a geometry transfer heat.
What does a large Nusselt number mean?
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 the Nusselt number in natural convection?
The simple empirical correlations for the average Nusselt number, Nu, in natural convection are of the form: Nux = C. Raxn The values of the constants C and n depend on the geometry of the surface and the flow regime, which is characterized by the range of the Rayleigh number.
What is the relationship between Nusselt number and Rayleigh number?
The simple empirical correlations for the average Nusselt number, Nu, in natural convection are of the form: The values of the constants C and n depend on the geometry of the surface and the flow regime, which is characterized by the range of the Rayleigh number.
How do you calculate Nusselt number correlation?
Nusselt number correlations. In case of free-convection, the Nusselt number is represented as the function of Rayleigh number (Ra) and Prandtl Number (Pr), In simple representation Nu = f (Ra, Pr). In case of forced-convection, the Nusselt number is represented as the function of Reynold’s number (Re) and Prandtl Number (Pr), in simple way
What is the Nusselt number of a boundary?
“The Nusselt number is the ratio of convective to conductive heat transfer across a boundary.” The convection and conduction heat flows in-parallel to each other. The surface will be normal of the boundary surface, and vertical to the mean fluid-flow. k = the thermal conductivity of the fluid.