#### Convective Heat Transfer over a Flat Plate: Summary

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

##### Key points from this module:

- When a fluid flows over a flat plate, most of the heat transfer occurs at the leading edge. This is because the thermal boundary layer is thinnest at the leading edge. The thicker thermal boundary layer downstream acts like a “blanket” and reduces heat transfer.
- The thicknesses of the momentum and thermal boundary layers are governed by the Prandtl number:
- Air has a Prandtl number of about 0.7. Its momentum boundary layer is a bit thinner than its thermal boundary layer.
- Liquid mercury (a liquid metal with low viscosity and high thermal conductivity) has a very small Prandtl number, about 0.02. Its momentum boundary layer is much thinner than its thermal boundary layer.
- Engine oil (a liquid with high viscosity and low thermal conductivity) has a Prandtl number of about 10,000. Its momentum boundary layer is much thicker than its thermal boundary layer.

- Due to the complexity of turbulent fluid flow over a flat plate, correlations must be used to calculate the average heat transfer coefficient. To use these correlations you need to know the Reynolds number and the Prandtl number. They allow you to calculate the Nusselt number, with you then use to calculate the heat transfer coefficient.

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

- Describe the meaning of a thermal boundary layer and sketch its thickness over a heated flat plate.
- Name one or two fluids that have momentum boundary layers that are thicker than their thermal boundary layers.
- Name one or two fluids that have momentum boundary layers that are thinner than their thermal boundary layers.
- Calculate the relative thicknesses of the thermal and momentum boundary layers using the Prandtl number.
- Use an appropriate correlation to calculate the average heat transfer coefficient over a flat plate.
- Calculate the total heat loss from the plate (in Watts) using the average heat transfer coefficient.

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