Hi, is the heat transfer coefficient dependent on temperature difference? Explain why or why not, please thank you,
Answers
Heat Transfer coefficient for forced convection is different than natural convection.
Forced Convection:
1. Dimensionless heat transfer coefficient is represented by Nusselt number. Nusselt number is in general function of Reynolds and Prandtl numbers. For high temperature gradient, it may become function of thermophysical property of the fluid.
Nusselt number (Nu) is heat transfer coefficient (h) multiply by reference length (L) and divided by thermal conductivity of the working fluid (k), i.e., Nu=hL/k.
Reynolds number (Re) is average velocity of flow (V) multiply by length scale (L) divided by kinematic viscosity (nu), i.e. Re=VL/nu
Prandtl number (Pr) is the ratio between kinematic viscosity (nu) divided by thermal diffusivity (a), i.e., Pr=nu/a (which is thermophyical property of the fluid).
then heat transfer coefficient is function of (k/L, Re and Pr). The function itself depends on the flow conditions (laminar or turbulent) and geometry of flow direct and geometry of the solid surface.
Natural convection:
Just replace Re by Rayleigh number ( which gravity* delta T, L^3/(nu a), all above argument is valid.
In simple words, h is complex function of fluid property (not material property), flow conditions, geometry of the flow.