Fire Dynamics is the study of how chemistry, fire science, material science and the mechanical engineering disciplines of fluid mechanics and heat transfer interact to influence fire behavior. In other words, Fire Dynamics is the study of how fires start, spread and develop. But what exactly is a fire?
Fire can be described in many ways - here are a few:
- NFPA 921: "A rapid oxidation process, which is a chemical reaction resulting in the evolution of light and heat in varying intensities."
- Webster's Dictionary: "A fire is an exothermic chemical reaction that emits heat and light"
Fire can also be explained in terms of the Fire Tetrahedron - a geometric representation of what is required for fire to exist, namely, fuel, an oxidizing agent, heat, and an uninhibited chemical reaction.
Heat Energy is a form of energy characterized by vibration of molecules and capable of initiating and supporting chemical changes and changes of state (NFPA 921). In other words, it is the energy needed to change the temperature of an object - add heat, temperature increases; remove heat, temperature decreases. Heat energy is measured in units of Joules (J), however it can also be measured in Calories (1 Calorie = 4.184 J) and BTU's (1 BTU = 1055 J).
Temperature is a measure of the degree of molecular activity of a material compared to a reference point. Temperature is measured in degrees Farenheit (melting point of ice = 32 º F, boiling point of water = 212 º F) or degrees Celsius (melting point of ice = 0 º C, boiling point of water = 100 º C).
Normal human oral/body temperature
Human skin begins to feel pain
Human skin receives a first degree burn injury
Human skin receives a second degree burn injury
A phase where burned human tissue becomes numb
Human skin is instantly destroyed
Water boils and produces steam
Glass transition temperature of polycarbonate
Melting temperature of polycarbonate
Charring of natural cotton begins
Charring of modern protective clothing fabrics begins
Temperatures inside a post-flashover room fire
Heat Release Rate (HRR) is the rate at which fire releases energy - this is also known as power. HRR is measured in units of Watts (W), which is an International System unit equal to one Joule per second. Depending on the size of the fire, HRR is also measured in Kilowatts (equal to 1,000 Watts) or Megawatts (equal 1,000,000 Watts).
Heat Flux is the rate of heat energy transferred per surface unit area - kW/m2.
Heat Flux (kW/m2)
Typical firefighter exposure
Pain to skin within seconds
Threshold flux to floor at flashover
Thermal Protective Performance Test (NFPA 1971)
60 - 200
Flames over surface
Temperature vs. Heat Release Rate
One candle vs. ten candles - same flame temperature but 10 times the heat release rate!
HRR: ~ 80 W Temperature:
HRR: ~ 800 W
Heat transfer is a major factor in the ignition, growth, spread, decay and extinction of a fire. It is important to note that heat is always transferred from the hotter object to the cooler object - heat energy transferred to and object increases the object's temperature, and heat energy transferred from and object decreases the object's temperature.
Conduction is heat transfer within solids or between contacting solids.
The governing equation for heat transfer by conduction is:
Where T is temperature (in Kelvin), A is the exposure area (meters squared), L is the depth of the solid (meters), and k is a constant that unique for different materials know as the thermal conductivity and has units of (Watts/meters*Kelvin).
Thermal Conductivity of Common Materials
Copper = 387
Gypsum = 0.48
Steel = 45.8
Oak = 0.17
Glass = 0.76
Pine = 0.14
Brick = 0.69
PPE = 0.034 - 0.136
Water = 0.58
Air = 0.026
Convection is heat transfer by the movement of liquids or gasses.
The governing equation for heat transfer by convection is:
Where T is temperature (in Kelvin), A is the area of exposure (in meters squared), and h is a constant that is unique for different materials known as the convective heat transfer coefficient, with units of W/m2*K. These values are found empirically, or, by experiment. For free convection, values usually range between 5 and 25. But for forced convection, values can range anywhere from 10 to 500.
Radiation is heat transfer by electromagnetic waves.
The governing equation for heat transfer by radiation is:
Where T is temperature (in Kelvin), A is the area of exposure (in meters squared), α is the thermal diffusivity (a measure of how quickly a material will adjust it's temperature to the surroundings, in meters squared per second) and ε is the emissivity (a measure of the ability of a materials surface to emit energy by radiation).
Fire Development is a function of many factors including: fuel properties, fuel quantity, ventilation (natural or mechanical), compartment geometry (volume and ceiling height), location of fire, and ambient conditions (temperature, wind, etc).
Traditional Fire Development
WatchWindows: Traditional Fire Development in a Compartment Fire
Fire Behavior in a Structure
Flashover is the transition phase in the development of a contained fire in which surfaces exposed to the thermal radiation, from fire gases in excess of 600° C, reach ignition temperature more or less simultaneously and fire spreads rapidly through the space. This is the most dangerous stage of fire development.