POLYFLOW - What value could be assigned to a heat transfer coefficient along a boundary?
Non-isothermal simulations require imposing boundary conditions for the energy equation. Imposed temperatures as well as vanishing fluxes are easy to set. Sometimes however, a heat transfer occurs, which depends on a temperature difference. When no data is available, what value could typically be assigned to such a heat transfer coefficient (governing heat transfer through convection)?
This is a difficult question, as it depends on both materials in contact, as well as on the possible velocity of the surrounding fluid (the ones that is used for cooling or heating). Generally, these heat transfer coefficients are reported in tables, whose usage is not too easy. Indeed, such tables involve Reynolds, Prandtl and Nüsselt numbers. Such a table can be found e.g. on pages 407 and ss. in the book "Transport phenomena" by Bird, Stewart and Lightfoot.
In the mean time, a few useful and approximative numbers:
- holding a cup of hot coffee in a hand: heat transfer coefficient is of about 10-20 W/m2/°C
- contact between polymer and mould in blow moulding: heat transfer coefficient of about 100 W/m2/°C
- a solid object in warm water: heat transfer coefficient of about 100-500 W/m2/°C
- a solid object in boiling oil: heat transfer coefficient of up to 1000 W/m2/°C (one of the best heat transfer coefficients)
- forced cooling with a water jet, the heat transfer coefficient may apparently reach values as high as 100000 W/m2/°C
Those numbers are indicative, and should not be taken for granted. Whenever possible, the above information will be expanded.