Define and use basic terms and concepts associated with the chemistry and dynamics

science

Description

Course Learning Outcomes for Unit IV

Upon completion of this unit, students should be able to:

1. Explain the physical and chemical properties of fire.

1.1 Define the categorization of flames.

1.2 Describe laminar and turbulent flames

1.3 Categorize the flash point, fire point, and autoignition temperature of a flammable liquid.

4. Describe and apply the process of burning.

4.1 Compare the flammability limits of burning velocity.

4.2 Evaluate the three zones of the plume of a fire burning in the relationship of air entrainment into

the flame.

5. Define and use basic terms and concepts associated with the chemistry and dynamics

5.1 Analyze the minimum rate of heat release that leads to a flashover.

5.2 Compare the smoke flow through different types of buildings.

Reading Assignment

Chapter 7:

Fire Characteristics: Gaseous Combustibles

Chapter 8:

Fire Characteristics: Liquid Combustibles

Chapter 12:

Movement of Fire Gases

Unit Lesson

Understanding the flame phenomena is critical for the safety of firefighters. However, is it important to

understand diffusion of flame spread across a combustible item? How does that relate to firefighters making

entry into a structure? Why is it important to understand the categorization of flames from Bunsen burners,

Porous-plate flat-flame burners, or any other type of burner using natural gas or liquefied petroleum gas? Why

is it important to understand fuel being diffused with air? Shackelford (2009) stated, “The diffusive flaming

process is characterized by flames, generally yellow in nature as the burning process is not complete” (p. 58).

According to Shackelford the appearance is as though the fuel to air ratio is incorrect. He claimed, “It is the

most common type of flaming that will be encountered by firefighters” (p. 58) and the yellow flame suggests

carbon monoxide being formed. Corbett and Pharr (2011) support Shackelford, suggesting the color of the

flame is characterized by the percentage of oxygen available for combustion and fuel composition. Will the

entrainment of more air into the flame improve combustion? Is there another element that needs to be

considered?

Gann and Friedman (2015) state that, for combustion to occur, the air mixture and the fuel mixture must be

within the flammable limits range of the material involved, which is indicated by the change in flame color. The

authors continue by saying, “Flame types fall into the following categories: Premixed flames or diffusion

flames; laminar flames or turbulent flames; stationary flames or propagating flames; subsonic flames

(deflagrations) or supersonic flames (detonations)” (p. 96). The authors suggest there are other possible

combinations of these categories. McCaffery (1979) claimed buoyancy diffusion flame as another type. He

stated that buoyancy diffusion flame propagation is viewed as intermittent velocity of the plume or fluctuating

UNIT IV STUDY GUIDE

Physical and Chemical

Properties of Fire


FIR 3301, Fire Behavior and Combustion 2

UNIT x STUDY GUIDE

Title


of the flame. The graphic illustration below shows intermittent velocity of the plume from a fire in a chair and

the three zones of the fire plume structure. Is this all there is to combustion?

Click here to access an interactive media file.

According to Corbett and Pharr (2011), flames are affected by “the heat release rate, the diameter of the

burning fuel, and the mixing of the entrained air into the reaction zone [which] drives the pulsing structure

along its perimeter and at the top of the flame plume” (p. 76). Gann and Friedman (2015) suggested that, in

addition, there is the movement of fire gases as the result of gasification of the solid or liquid that is burning.

The authors state, “When the molecular fragments leave the fuel surface, they have almost no momentum.

They rise into the air above strictly due to buoyancy” (p. 213). In other words, looking at the graphic

illustration, as the air is being entrained into the perimeter of the flame the air picks up molecular fragments

that are being released from the chair through the pyrolysis process. In addition, there are many other factors

that affect the fire development: the number of openings allowing air entrainment, the size or volume of the

room or compartment the chair is in, the fire plume under the ceiling releasing thermal radiation, and the

smoke flow from the burning chair. In fact, if there were no openings in the graphic illustration one of two

things would begin to happen. According to Gann and Friedman (2015) the first thing to occur would be the

release of heat causing:

An increase in the pressure and temperature of the gases in the compartment, according to

the ideal gas law. Ordinary construction materials can withstand a substantial increase in

pressure if the pressure is applied evenly and gradually. However, windows can break in a

fire because of stresses created when the viewable area of the glass is heated and expands

more than the area shaded by the frame. (p. 217)

In this instance the room would be entrained with air and the fire will increase, building thermal layers of

heated gases evenly throughout the fire compartment. The superheated gas layered through the structure

begins to increase in temperature and the radiant heat from the fire heats all the combustible material in the

room at, or near, its ignition temperature (IFSTA, 1998). Corbett and Pharr (2011) suggest this would increase

the temperature and, as the temperature increased, the volume pressure would increase. The authors

referred to this as the expansion of matter. If all the combustible material does not reach ignition temperature

in the room this could result in a flameover with flames traveling across the ceiling. If all the combustible

material reached ignition temperature then the entire room could ignite at once in a flashover.

The second thing that could happen, “if no rupture occurs, the oxygen in the compartment becomes depleted

to the point that the combustion ceases” (Gann & Friedman, 2015, p. 217). As the material burns, the volume

of smoke increases and flows across the ceiling in thermal layers. As layers build they bank down the walls

being entrained back in to the flame of the chair burning. As this occurs the entrained air is depleted and the

fire decays. The fire remains at this state until either air is entrained into the flame and heated gases or the

heat is removed over a period of time and the fire becomes self-extinguished. However, if the compartment is

still very hot and the decaying fire is diffused with air a backdraft will occur. Corbett and Pharr (2011) describe

this as, “an example of fuels diffused in air and burning with rapid flame spread rates, which result in a

pressure rise of explosive force” (p. 38) or, in other words, a backdraft.

Instruction Files

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