<Li> 10,000 for polymer melts </Li> <Li> around 1 × 10 for Earth's mantle . </Li> <P> Small values of the Prandtl number, Pr <<1, means the thermal diffusivity dominates . Whereas with large values, Pr>> 1, the momentum diffusivity dominates the behavior . For example, the listed value for liquid mercury indicates that the heat conduction is more significant compared to convection, so thermal diffusivity is dominant . However, for engine oil, convection is very effective in transferring energy from an area in comparison to pure conduction, so momentum diffusivity is dominant . </P> <P> In heat transfer problems, the Prandtl number controls the relative thickness of the momentum and thermal boundary layers . When Pr is small, it means that the heat diffuses quickly compared to the velocity (momentum). This means that for liquid metals the thickness of the thermal boundary layer is much bigger than the velocity boundary layer . </P>

When is a system dominated by conductive heat transfer (compared to momentum diffusivity)