Dittus-Boelter Equation
The Dittus-Boelter equation (for turbulent flow) is an explicit function for calculating the Nusselt number. It is easy to solve but is less accurate when there is a large temperature difference across the fluid. It is tailored to smooth tubes, so use for rough tubes (most commercial applications) is cautioned. The Dittus-Boelter equation is:
where:
- is the inside diameter of the circular duct
- is the Prandtl number
- for heating of the fluid, and for cooling of the fluid.
The Dittus-Boelter equation is valid for
Example The Dittus-Boelter equation is a good approximation where temperature differences between bulk fluid and heat transfer surface are minimal, avoiding equation complexity and iterative solving. Taking water with a bulk fluid average temperature of 20 °C, viscosity 10.07×10¯⁴ Pa·s and a heat transfer surface temperature of 40 °C (viscosity 6.96×10¯⁴, a viscosity correction factor for can be obtained as 1.45. This increases to 3.57 with a heat transfer surface temperature of 100 °C (viscosity 2.82×10¯⁴ Pa·s), making a significant difference to the Nusselt number and the heat transfer coefficient.
Read more about this topic: Nusselt Number, Empirical Correlations, Forced Convection in Turbulent Pipe Flow
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