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Unit 1: The Fire Environment Part 2

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Heat Transfer

To better understand when and how ignition and combustion occur in a wildfire, we need to discuss the physical processes involved. Note that heat transfer refers to the physical processes by which heat energy moves to and through unburned fuel.

Heat transfer refers to the physical process by which heat energy moves from one area to another.

Heat transfer

The Heat Transfer Methods figure above illustrates various heat transfer methods. Branches above the fire are receiving heat by convection and radiation. Tree trunks and shrubs are receiving heat by radiation from the fire. Fuels on the ground are being preheated by conduction and radiation. Preheating of fuels may be occurring by all of these methods at the same time, depending on the arrangement or loading of the fuels.
We've stressed the importance of radiant heat transfer in the preheating of fuels and spread of the fire. How much heat will be received by fuels ahead of the fire? Well, this depends on the fire intensity and the distance, but how much?

Common Methods of Heat Transfer

Conduction

The first common method of heat transfer is conduction. Conduction is the transfer of heat from one molecule of matter to another. An example of this is fire smoldering through a solid piece of fuel. Since wood is generally a poor conductor of heat, conduction is the least important method of the three.

Convection

This is the transfer of heat resulting from the motion of air (or fluid). It is the natural buoyant rise of warm air over a heat source that induces an automatic circulation within an airmass. Examples of forced convection are fire spreading from surface fuels to aerial fuels, and columns of smoke rising high into the atmosphere. Convection also includes direct flame contact, a powerful heat transfer process, especially in a head fire.

Radiation

more

Radiation is the transmission of heat energy by rays passing from a heat source to an absorbing material. Examples are the heat received from the sun, and the preheating of fuels ahead of a flaming front. Radiation from glowing char or flames is very strong. This is why firefighters often must shield exposed skin. Radiation is the chief source of heat transfer in a backing fire.
Do the examples given for the three heat transfer methods suggest a relationship among ignition, fire intensity, and rate of spread? Well, they should, because fire behavior is the result of, and is affected by, the method and the amount of heat energy transfer within the fire environment.

Mass Transport of Firebrands

There is a fourth method by which fire spreads that is of great concern to firefighters. This is the mass transport of firebrands which can occur as a result of convection, wind, or gravity. We call this spotting. Small embers of burning material can be lifted in a convection column and be carried some distance ahead of a fire. Wind, in addition to strong convective currents, can carry embers or firebrands considerable distances downwind from the fire. Wind without convective lifting will result in shorter range spotting of firebrands.
Gravity also is responsible for spotting of firebrands, but always down slope. Usually, the steeper the slope, the greater the spotting problems from burning materials of various sizes rolling down slope. In each of these cases, we are dealing with new ignitions outside the fire perimeter, and not the normal growth of the fire.

Determing a Fire’s Potential Behavior

Why do some fires remain small while others get large very rapidly? What happens when a fire gets large in size and intensity? How does fire interact with its environment?
Let's first consider the extent of the fire's environment. For a very small fire, the fire environment is limited to a few feet horizontally and vertically. As a fire grows in size, so does the extent of the environment affected. In a large fire, the fire environment may extend many miles horizontally and thousands of feet vertically. High intensity wildfires, whether large or small in size, usually have considerable effect on the atmosphere vertically. This is evidenced by their convection columns. There are generally three factors that determine the extent of vertical development of a fire's convection or smoke column. These are the heat energy generated from the fire, the instability of the lower atmosphere, and the winds aloft. Stable air and/or strong winds tend to discourage vertical development of convection columns.

High Intensity Fire

A high intensity fire will create much stronger indrafts that can help feed convection columns to many thousands of feet into the atmosphere. This is sometimes called a three-dimensional fire.

  • Fire can control the environment
  • Sphere of influence becomes greater
  • Can significantly modify weather elements near or adjacent to the fire

Low Intensity Fires

Low intensity fires will create weak indrafts at the fire's edge that will feed a low, weak smoke or convection column over the fire. This we sometimes refer to as a two-dimensional fire.

  • Environment controls the fire
  • Sphere of influence is very small
  • Only slight modification of weather elements in immediate proximity of fire

Open vs. Closed Environment

These images illustrate open and closed fire environments. On the left, we see a fire burning through all levels of the vegetation and exposed to various winds and other weather elements. It will be readily affected by any atmospheric changes, and fire behavior can change drastically as a result of wind shifts, etc.
On the right, the fire is burning under a forest canopy. This is somewhat similar to a structural fire burning inside a building. Conditions outside the building have relatively little effect on the fire inside. Such fires usually remain low in intensity. However, once the fire breaks out of the building or out through the forest canopy, fire intensity and spread can increase drastically as outside atmospheric conditions then influence the fire.

Remember that any wildfire is a heat source that can and will interact with its natural environment. The size of that sphere of influence will depend on the size and intensity or heat energy output of the fire. The physical location of the fire, and the sheltering effect from surrounding terrain and vegetation is often a contributing factor to the potential behavior of that fire.
Let's once again compare low intensity fires to high intensity fires. We can generalize by saying that with low intensity fires, the environment mostly controls the fire. The sphere of influence is very small, and the fire causes only slight modification of weather elements in the immediate proximity of the fire.
On the other hand, high intensity fires can control the environment to a marked extent. The sphere of influence becomes much greater, and high intensity fires can significantly modify weather elements near and adjacent to the fire.

Low Intensity Fire

  • Environment controls the fire
  • Sphere of influence is very small
  • Only slight modification of weather elements in immediate proximity of fire

High Intensity Fire

  • Fire can control the environment
  • Sphere of influence becomes greater
  • Can significantly modify weather elements near or adjacent to the fire

Firefighter Predictions

There are four primary areas of concern to firefighters in predicting fire behavior: Forward rate of spread of fires, the future perimeter of the fires, the fireline intensities or flame lengths, and any unusual or extreme fire behavior such as crowning and spotting.

Firefighters are primarily concerned about predicting:

  • The forward rate of spread of fires
  • The future perimeter of the fires
  • The fireline intensities or flame lengths
  • Unusual fire behavior such as crowning and spotting

Note that we are now using the term "fireline intensity," rather than fire intensity. It's important that you understand that this is not the same as fire intensity. Fire intensity is a somewhat general term, referring to the entire heat release of a fire at a given time. It is very difficult to measure or to relate overall fire intensity to fire control activities.
In contrast, fireline intensity is a measurable and useful term that is related to flame length. In turn, flame length can be related to fire control jobs. We will explain fireline intensity and its relationship to flame length a little later.
The job of predicting fire behavior is indeed a complex one. How does one even start to analyze the many variables? Well, this course is intended to help you gain the basic knowledge required to assess fire behavior. It will also brief you about systems available to make fire behavior calculations or estimations

Assumptions of Fire Models

Let us take a closer look at the mathematical fire behavior prediction model. The model processes fuel and environmental conditions to give expected fire behavior. Methods of estimating the input values and interpreting the output values will be covered throughout the course.
In the final unit of this course, Unit XI--Predicting Fire Behavior, you will go through the entire process of assigning input values, using the model in the form of tables and graphs to obtain output values, and interpreting these output values.
An entire unit will be devoted to each of the following basic input values: Fuel bed description, fuel moisture, slope, and wind speed. These units will cover both the general information required for a firefighter to assess the fire situation and also the specific information that is required as input to the model.
In order to express the many interactions that occur during a forest fire in mathematical terms, certain simplifying assumptions have been made. Among these are the following: The model describes fire behavior only at the leading edge of a 'free burning fire; the fuels are assumed to be continuous, uniform and in a single layer contiguous to the ground; wind, slope, and fuel moisture content all are constant for the time period of the calculation; the fire is not spreading by spotting or crowning; and firewhirls and other fire-induced atmospheric disturbances are not occurring.

Assumptions made in the mathematical fire behavior prediction models:

  • The leading edge of a free burning fire is being considered
  • The fuels are continuous, uniform and in a single layer contiguous to the ground
  • Wind, slope and fuel moisture contents are constant for the calculation period
  • The fire is not spreading by spotting or crowning
  • Firewhirls and other fire-induced atmospheric disturbances are not occurring

It is important that you understand the implications of these assumptions. You know, of course, that fire does not occur in a continuous, uniform and constant environment. But predictions from the model can be used successfully in many situations. The closer actual conditions are to the model assumptions, the better the predictions will be. This is why human judgment is always used along with the model. Even though the model does not describe extreme fire behavior, you will see in later units that it can predict the potential for spotting and crowning.

Fireline Intensity

Fireline intensity is the heat released by a foot wide slice of the flaming combustion zone in one second.

The figure above presents a diagram of fireline intensity to further clarify this term. Fireline intensity is the heat released in 1 second by a foot-wide slice of the flaming front. This represents the heat that would impact a firefighter just ahead of the front. Since it has a direct relationship to flame length, it can be related to the kind of control actions and size of fireline that must be planned for on the fire.
The relationships between fireline intensity and flame length will be explained in more detail in later units. Flame length is also an output of the mathematical fire model. It should not be confused with flame height. The figure below illustrates how each measurement is taken. Flames usually bend forward at the head of a fire, depending on wind slope factors. Researchers have determined that flame length is a better parameter for describing fire behavior than flame height.
As mentioned earlier, fire behavior predictions are useful for planning fire control actions. Such planning includes the location of firelines, use of direct or indirect attack methods, the type of control forces which will be effective, and the standards for fireline construction. Certainly, good planning in each of these areas will make the suppression effort more safe and effective.

Uses of Fire Behavior Predictions

Fire behavior predictions are useful for planning fire control actions, such as:

  • Location of fireline
  • Direct or indirect attack methods
  • Type of control forces which will be effective
  • Standards for fireline construction

Summary

Fire environment is the surrounding conditions, influences, and modifying forces that determine the behavior of fire. Prediction of fire behavior for safe and effective control and use of fire requires understanding of the interactions of fire with its environment.
The fire environment consists of three major components—topography, fuel, and air mass. From a fire standpoint, topography does not vary significantly with time, but does vary greatly in horizontal space. The fuel component varies in space and also in time; however, fuel characteristics, except for the moisture content of dead fuel, change slowly enough to be considered static for any one fire. The air mass is usually the most variable component, changing rapidly in both space and time.
The thermal energy responsible for most environmental interaction comes from the sun. Because the earth’s surface is not heated uniformly, temperature and air circulation patterns are set up that create large scale, local, and microscale climatic and weather patterns. The interaction of these patterns with other conditions determines the fire environment for a particular area.
Fire can be considered as a local heat source. As such, it influences and modifies the fire environment. Because a fire creates high temperatures, it can dominate sun-caused heat sources.
The extent of the environment of concern to fire behavior depends primarily on the size and characteristics of the fire. It ranges from a few feet to many miles horizontally and thousands of feet vertically. The vertical extent of the environment varies with fire intensity.
Most changes in fire behavior occur as the fire moves over the terrain and as time passes. But abrupt changes can occur when a fire moves vertically from one kind of environment to another, as when a surface fire in timber crowns.
Fire behavior is the interactions of the environmental components with each other and with the fire. The current state of each of these influences and their interactions determine the behavior of a fire at any moment.
Fire behavior is the result of complex interrelationships of aerodynamics, chemistry, thermodynamics, and combustion physics. Nevertheless, it is possible for firefighters to acquire sufficient skill in predicting fire behavior to allow safe and efficient control and use of fire. Development of this skill must come from experience, and from training in the fundamentals of fire behavior and the fire environment.

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . admin. (2005, September 27). Unit 1: The Fire Environment Part 2. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: http://ocw.usu.edu/Forest__Range__and_Wildlife_Sciences/Wildland_Fire_Management_and_Planning/Unit_1__The_Fire_Environment_Part_2.html. This work is licensed under a Creative Commons License Creative Commons License