Of the three methods of heat transfer, conduction (the transfer of heat via a solid object), convection (the transfer of heat via a fluid medium, in this case, heated combustion gases), and radiation, radiation is probably the most interesting.
Radiation is the transfer of heat through electromagnetic energy in the form of waves that are created when a particle is transformed from a high energy state to a lower energy state.
Thermal radiation is actually just part of the total electromagnetic spectrum. The electromagnetic spectrum is composed of radio waves, x-rays, infrared light rays, ultraviolet light rays, and visible light. The radiation portion of the spectrum is actually made up of portions of the infrared and ultraviolet sections and all of the visible light section.
Most liquids and solids burn with luminous flames, radiating heat. The yellow/orange glow one sees in a flame is the emission coming from minute carbonaceous particles suspended within the flame. You may have noticed that alcohol flames burn less brightly than their gasoline counterpart. This is because gasoline radiates more carbonaceous particles than does alcohol. Generally, as a rule, the “sootier” a flame, the more radiation emissive it is.
Not only do flames radiate heat, but so do hot gases and heated solids. Although the process of flashover is still not well understood, the radiation coming from the heated layer of gas at ceiling level is generally accepted as being the primary factor in bringing a room to flashover. As this gas layer, which is made up of water vapor, carbon dioxide, and carbon monoxide, begins to develop, it will start to radiate heat to the objects below it. This heated gas layer starts the pyrolysis process at floor level, and flashover usually occurs when the radiative flux at floor level reaches two watts/cm2 or when the temperature of the gas layer reaches 600°C (approximately 1,100°F).
Heated substances also play an important role in fire development. Hot objects will radiate heat to their surroundings, possibly causing them to ignite. Walls and other surfaces near a burning object will radiate heat back to that burning object. Such an effect is called radiative feedback. This is what often causes rapid fire spread through hallways; a long, narrow corridor will enhance flame movement.
Tests where objects are burned in a corner where two walls and the floor meet simulate the effect of radiative feedback on a fire. Such tests indicate burning rates four times that of burning the same object in the open.
While calculations of the actual amount of radiation given off by a material can be made, such calculations are lengthy and often require the use of advanced mathematics. However, a few generalizations about radiation and firefighting may be made:
- The larger a burning object is, the more total radiation it will give off.
- The more closely a receiving surface comes parallel to the burning surface, the more radiation it will receive.
- A water curtain placed between a burning building and an exposed one will not affect radiation to any great extent. Water must be placed directly on the exposed surface to cool it and keep it below its ignition temperature.
- Don’t underestimate the power of radiation from a fire—It can travel over considerable distances.
- While making inspections, take a look at the building and imagine it as a source and a receiver of radiation to other nearby buildings. Does it have a large combustible exterior surface that may pose an exposure problem? Exterior sheathing such as wood or “gasoline siding” (old, untreated, asphalt shingles) pose a particularly difficult problem.
- The necessity of proper ventilation can never be overstated. Removing the hot, toxic gases will lessen the likelihood of flashover.
- Determine the presence of openings in the exterior wall of the radiation source and the receiver building. Windows are particularly vulnerable to radiant heat and may allow fire to enter into the exposed building if compromised. Wired glass helps, but these openings must still be protected.
Research into the phenomena of radiation and fire dynamics in general is constantly taking place. Hopefully this research will be relayed to the fire service and used in firefighting efforts. Thermal radiation is a force to be contended with and considered in every fire incident.
Waidwick, NJ, Volunteer Fire Department