Calculation of the individual heat transfer coefficient
Dimensionless numbers and equations
| Calculation of the individual heat transfer coefficients involves the following dimensionless numbers: | ||
| The Nusselt number, Nu |
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where
α =
individual heat transfer coefficient [W/m2·K] d = characteristic length [m] (e.g. tube diameter) λ = thermal conductivity [W/m·K] cp = specific heat [J/kg·K] μ = dynamic viscosity [Pas] v = velocity [m/s] ρ = density [kg/m3] |
| The Prandtl number, Pr |
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| The Reynolds number, Re |
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For natural convection one dimensionless number is added: | ||
| The Grashof number, Gr |
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where
g = gravitational acceleration [m/s2] β = volumetric expansion coefficient [K-1] ΔT = temperature difference [K] |
At natural convection the following generalized equation applies: | ||
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At forced convection the following generalized equation applies: | ||
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With dimensionless numbers and equations any consistent system of units can be applied. | ||
The physical meaning of the different dimensionless numbers are as follows: | ||
| Nu = (characteristic length)/(theoretical film thickness) | ||
| Pr = (momentum diffusivity)/(thermal diffusivity) | ||
| Re = (momentum by eddy diffusion)/(momentum by molecular transport) | ||
| Gr = (inertia forces)/(viscous shear forces)·(buoyancy forces)/ (viscous shear forces) | ||
| Back to heat transfer main page |
| Forced convection, laminar flow |
| Forced convection, turbulent flow |
| Typical Nu - Pr - Re relationship |
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