Abstract
In this chapter, a brief explanation about the basics of cavities has been given. Natural convection has been investigated as one of the important problems in cavities. The low heat transfer rate between the fluid and the walls is one of the basic problems of natural convection. This issue leads to the development of strategies to improve the heat transfer rate, such as increasing the thermophysical properties of the working fluids, making cavities with complex geometries, using cavities filled with porous media, and creating a magnetic field. Also, the review of formulas related to cavities in the presence of nanofluids has been fully reviewed in this chapter.