# Natural Convection in Enclosures

### From Thermal-FluidsPedia

Enclosures are referred to as finite spaces bounded by walls and filled with fluid (liquid or gas). Natural convection in enclosures, also known as internal convection, takes place in rooms and buildings, furnaces, cooling towers, fire safety, as well as electronic cooling systems. Internal natural convection is different from the cases of external convection, where a heated or cooled wall is in contact with the quiescent fluid and the boundary layer can be developed without any restriction. As will become evident later, internal convection usually cannot be treated using simple boundary layer theory because the entire fluid in the enclosure engages to the convection.

Figure 1 illustrates several examples of both two- and three dimensional internal natural convections (Yang, 1987). Figures 1(a) and (b) depicts natural convection in a rectangular enclosure with left and right sides heated and cooled while the top and bottom walls are insulated. For a shallow enclosure (*H* / *L* < 1), a boundary layer may exist near the heated and cooled wall and the buoyancy flow in the enclosure is of the recirculating type. For the case that *H/L* is greater than unity, natural convection in the enclosure can no longer be described using the boundary layer theory. Natural convection in an inclined rectangular enclosure can be considered as a generalized case for natural convection in a rectangular enclosure since Fig. 1(a) and (b) are special cases with . When , Fig. 1(c) becomes the case of natural convection in a rectangular enclosure heated from below. Natural convection in an enclosure can be suppressed by vertical partitions such as that shown in Fig. 1(d). Natural convection in an annular enclosure formed by two differentially heated concentric cylinders is illustrated in Fig. 1(e). The flow will be of the recirculating type and axisymmetric. Figure 1 (f) and (g) depict natural convection in a box enclosure and truncated annular enclosure, respectively. Both of them can be tilted relative to gravity and various heating and cooling boundary conditions can be imposed at different walls. It should be pointed out that the examples shown in Fig. 1 are illustrative, rather than inclusive. Natural convection can occur in different and more complex enclosures.

In this section, scale analysis, analytical solutions, numerical solutions and empirical correlations pertaining to natural convection in enclosures will be presented.

## References

Yang, K.T., 1987, “Natural Convection in Enclosures,” *Handbook of Single Phase Convective Heat Transfer*, Eds. Kakac, S., Shah, R., and Aung, W., Wiley & Sons, New York, NY.