#### Free Convection: Summary

The answers to the ConcepTests are given below and will open in a separate window.

##### Key points from this module:

- Hot surfaces generally cause fluids to expand and rise. Cold surfaces generally cause fluids to contract and sink. These buoyancy-induced flows result in convective heat transfer.
- It is complicated to mathematically model these flows, so experimental correlations must be used to calculate the average heat transfer coefficients. To use these correlations you need to know the Rayleigh number and the Prandtl number. They allow you to calculate the Nusselt number, which you then use to calculate the heat transfer coefficient.
- The geometry and orientation of the surface relative to gravity makes a difference. For example, if the hot surface on a horizontal plate is oriented upward, the heat transfer away from the plate will typically be higher than the heat transfer away from the same plate with the hot surface oriented downward.

##### From studying this module, you should now be able to:

- Provide a conceptual description of natural convection around various geometries, such as a vertical and horizontal flat plates, cylinders, and spheres.
- Use appropriate correlations to calculate the average heat transfer coefficient due to free convection over these surfaces.
- Calculate the total heat loss (in Watts) from a hot object suspended in a cold, quiescent fluid due to free convection.
- Perform a similar calculation for a cold object suspended in a hot, quiescent fluid.

*Prepared by Jeffrey Knutsen, Department of Mechanical Engineering, University of Colorado Boulder*