PLUMES Natural Convection Flows

When an object is warmer than the surrounding air, the air is heated, and the warm air moves upward due to buoyancy. The air current created in this way is called a natural convection flow or plume. Also, if the object is colder than the surroundings, the descending cool air current is called a natural convection flow or plume. Generally, heat transfer involving mo­tion of air or some other fluid caused by a difference in density is called natural or free convection. As a result of free convection, a flow of air or other fluid is produced in the form of a boundary layer moving along a surface or as a thermal plume above a surface. In a building, natural con­vection flows can be formed along the cold or warm vertical surfaces of the external walls and windows, along vertical hot surfaces of process equip­ment, etc., as shown in Fig. 7.58. Convection flows or thermal plumes are created above people, lights, hot horizontal surfaces of process equipment, and other objects with a surface temperature greater than the room air temperature (Fig. 7.59).

As can be seen from Figs. 7.58 and 7.59, the amount of air in the con­vection flows increases with height, due to entrainment of the surrounding air. The amount of air transported in a natural convection flow depends on the temperature and the geometry of the surface or source and the temper­ature of the surrounding air. Because the driving force in convection flows

Ft < ft

FIGURE 7.58 Convection flows along vertical surfaces.

Is the buoyancy force caused by the density difference (i. e., the temperature difference), a temperature gradient in the room influences the plume rise height. Information on thermal plume characteristics is essential for de­signing ventilation systems with displacement air supply and for dimen­sioning overhead hoods above heat sources. Empirical, analytical, and computational fluid dynamics are the commonly used approaches to evalu­ate air temperatures, velocities, and airflow rates in thermal plumes above different heat sources and in convection flows at vertical surfaces.

This section treats

• Natural convection flows in nonconfined and nonstratified environments (Section 7.5.2)

• Plume interaction (Section 7.5.3)

• The influence of a confined space on convection flows (Section 7.5.4)

• The influence of a temperature stratification (Section 7.5.5)

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