What is the dittus-Boelter equation?
The Dittus–Boelter equation (for turbulent flow) is an explicit function for calculating the Nusselt number. The Nusselt number is the ratio of convective to conductive heat transfer across (normal to) the boundary.
What is the famous dittus-Boelter correlation for?
The Dittus-Boelter correlation may be used for small to moderate temperature differences, Twall – Tavg, with all properties evaluated at an averaged temperature Tavg.
What is Nusselt number and its significance?
In fluid dynamics, the Nusselt number (Nu) is the ratio of convective to conductive heat transfer at a boundary in a fluid. A value between one and 10 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.
How do you derive a Nusselt number?
the Nusselt number, which is NuDh = 890. the hydraulic diameter of the fuel channel is: Dh = 13,85 mm. the thermal conductivity of reactor coolant (300°C) is: kH2O = 0.545 W/m.K.
What is L in Nusselt number?
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.
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.
What is the overall heat transfer coefficient?
The overall heat transfer coefficient, or U-value, refers to how well heat is conducted through over a series of resistant mediums. Its units are the W/(m2°C) [Btu/(hr-ft2°F)].
What is meant by fouling factor?
The fouling factor represents the theoretical resistance to heat flow due to a build-up of a layer of dirt or other fouling substance on the tube surfaces of the heat exchanger, but they are often overstated by the end user in an attempt to minimise the frequency of cleaning.
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.
What does Prandtl number tell us?
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.
Why is Nusselt number negative?
Popular Answers (1) 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.
What is the difference between Nusselt 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.
When is the Dittus Boelter equation less accurate?
The DittusBoelter equation is easy to solve but is less accurate when there is a large temperature difference across the fluid and is less accurate for rough tubes (many commercial applications), since it is tailored to smooth tubes.
What is the temperature of the core of A Dittus Boelter?
At higher elevations of the core the bulk temperature may reach up to 330°C. The temperature difference of 29°C causes the subcooled boiling may occur (330°C + 29°C > 350°C).
Where can I find the Nusselt number for a 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.
Why does the Sieder-Tate correlation change as the Nusselt number changes?
) due to temperature change between the bulk fluid average temperature and the heat transfer surface temperature, respectively. The Sieder-Tate correlation is normally solved by an iterative process, as the viscosity factor will change as the Nusselt number changes.