### 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 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 The Reynolds number, Re For natural convection one dimensionless number is added: The Grashof number, Gr where g    = gravitational acceleration [m/s2] β    = volumetric expansion coefficient [K-1] ΔT  = temperature difference [K] At natural convection the following generalized equation applies: At forced convection the following generalized equation applies: 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)