ABSTRACT
A smoke plume results from the release of hot combustion products by a fire. The temperature difference with the surrounding ambient air induces a buoyant force that drives the combustion products upwards. A smoke plume is therefore also called a buoyant plume. The study of buoyant plumes is of pinnacle importance in fire dynamics and fire safety because the release of toxic gases (such as carbon monoxide, the ‘chief killer’ in fires) is the first cause of fatalities, well before the exposure to radiant heat from flames. Furthermore, a good understanding of smoke plume dynamics is fundamental in the design of efficient smoke management systems (chapter 6). The fire source (thus the smoke plume source) can be of several idealized shapes, e.g., circular, square and rectangular (see Fig. 4.1). If the source is circular the plume is referred to as an axisymmetric plume. If the source is rectangular with an aspect ratio great enough (i.e., long and narrow source), the plume is called a line plume. In addition to the geometrical shape of the fire source, the plume dynamics depend on the interaction with nearby or bounding walls (see section 3.5 and Fig. 4.2). When a line or axisymmetric plume is remote from any walls, ambient air is entrained from all sides. In the presence of a wall, air entrainment is inhibited from its side, yielding a different mixing process (see chapter 3). This effect can be exemplified by spill plumes, which are a typical example of line plumes generally encountered in atria. The smoke layer moves first horizontally beneath the ceiling of a room or a balcony before it is released to the open space at the spill edge and rises upwards. If the air entrainment occurs at both sides, the plume is called free or double-sided plume. If the plume is ‘attached’ on one side to a wall, it is called an adhered plume. The plume may also interact with a cross flow. Typical examples would be a plume in open atmosphere conditions and subjected to a wind or when a smoke control system with longitudinal ventilation is activated in a tunnel (see Fig. 4.3) or a car park.
