I am assuming that your case is purely natural convection. In that case the Rayleigh Number would probably be your answer. However, the Rayleigh Number would inform you whether the boundary layer that forms on the heater would be laminar or turbulent. Typically transition happens when Ra = 10^9.
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Kremella
Admin
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The Grashof number is the ratio of the buoyancy forces to the viscous forces:
Gr = [g*beta*(T_s - T_infinity)*L^3]/nu^2
where
g is the gravitational acceleration
beta is the thermal expansion coefficient
T_s is the surface temperature
T_infinity is the free-stream temperature
L is the characteristic length
nu is the kinematic viscosity
and the Prandtl number is the ratio of the momentum diffusivity to the thermal diffusivity:
Pr = nu/ alpha
where
nu is the kinematic viscosity
alpha is the thermal diffusivity