How do you find the Nusselt number for turbulent flow?
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 ReDh = 575600. the Prandtl number, which is Pr = 0.89.
Why is the Nusselt number over flat plate for the constant heat flux is larger than the constant temperature condition?
When Biot number Bi=0, it indicates the poor thermal contact or no external heat transfer from reservoir to pipe. That means the temperature difference between the wall and inside fluid (Tw-Tm) does not vary axially which resembles the thermally fully developed flow with constant wall heat flux.
How is Nusselt number calculated?
Formula
- 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.
How will you determine that the flow is laminar or turbulent for a flat plate?
As the fluid flows past the long flat plate, the flow will become turbulent at a critical distance xcr downstream from the leading edge. For flow past a flat plate, the transition from laminar to turbulent begins when the critical Reynolds number (Rexcr) reaches 5×105.
How to calculate the Nusselt number for a turbulent flow?
If the flow around the isothermally heated or cooled plate is a turbulent flow, the following formula can be used to calculate the average Nusselt number at constant wall temperature. This formula is based on the studies of Petukov ( High Temperature I, 1963) and Schlichting ( Grenzschicht-Theorie, 1958).
Which is a characteristic of a large Nusselt number?
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.
When to use Reynolds number or Nusselt number?
Note that when using the Reynolds number, the type of flow plays a decisive role, i.e. whether the flow is laminar or turbulent. In turbulent flows, the Nusselt numbers generally assume very large values, since the mixing caused by the turbulences leads to an increased transport of momentum and energy transverse to the flow direction.
Which is a larger Nusselt number convection or conduction?
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.