#### Interactive Self-Study Module: Free Convection

##### Overview:

This module uses screencasts to explain how to characterize and quantity heat transfer toward or away from an object that is suspended in a quiescent (essentially motionless) fluid. Your retention of material in this module will increase if you write down reasons for your answers to ConcepTests, and you try to solve the example problems before watching the screencast solutions. We suggest using the learning resources in the following order:

- Attempt to answer the multiple choice ConcepTest and solve the example problem before watching the screencasts or working with the simulations.
- Watch the screencast that explains the physics of free convection and introduces pertinent correlations.
- Review the important equations for free convection.
- Try to solve the example problems before watching the solutions in the screencasts.
- Answer the ConcepTests.
- Look at the list of key points, but only after you try to list the key points yourself.

##### Motivation:

- When a relatively warm or cold object is placed in an essentially motionless fluid (sometimes called a “quiescent” fluid), it causes the fluid to rise or sink around the object due to buoyancy. For example, if the object is warmer than the surrounding fluid, it causes the nearby fluid to warm up, expand, and rise, much like a hot air balloon. Conversely, if the object is cooler than the surrounding fluid, it causes the fluid to contract and sink. In these cases the bulk fluid velocity, the variable we called \(U_∞\) in forced convection, is zero. However, buoyant effects still result in convective heat transfer, which we call “free” or “natural” convection. The goal is to calculate the heat transfer coefficient, h, under these conditions.
- This module is primarily intended for a Heat Transfer course.

##### Before studying this module, you should be able to:

- Explain and work with dimensionless parameters, such as the Reynolds number.
- Select, justify, and use correlations for forced convection to calculate the Nusselt number.
- Use a Nusselt number to calculate the heat transfer coefficient.

##### After studying this module, you should 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.