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The Prandtl number (Pr) is a dimensionless ratio of momentum diffusivity to thermal diffusivity. It characterizes how a fluid transports momentum versus heat:
Pr = Cp × μ / k = ν / α
where Cp is specific heat capacity, μ is dynamic viscosity, k is thermal conductivity, ν is kinematic viscosity, and α is thermal diffusivity. A high Pr (thick boundary layer) means heat transfer is limited by conduction relative to momentum transfer.
Pr appears directly in the Nusselt number correlations used to calculate the convective heat transfer coefficient h. For example, the Dittus–Boelter equation for turbulent flow in a tube:
Nu = 0.023 × Re0.8 × Prn
where n = 0.4 for heating or 0.3 for cooling. Propylene glycol solutions have Pr values far above water — especially at low temperatures and high concentrations — meaning convective performance drops significantly compared to a water-only system.
Note: The Prandtl number of propylene glycol solutions is strongly temperature-dependent and increases sharply near the freezing point.