Understanding the fundamentals of hose stream mechanics is vital to the success of fire suppression operations on the fireground. This includes understanding: when and how to utilize air entrainment to your advantage; when air entrainment can be detrimental; and how to best apply the water through various application patterns, stream angles and deflection methods to successfully map the compartments that are inside of a structure.
UL’s Fire Safety Research Institute (FSRI) fire attack study that was funded by an Assistance to Firefighter Grant (AFG) was designed to provide both context for and substantiation of fire suppression methods, tools and tactics that have been utilized for decades. The AFG-funded coordinated fire attack study built up knowledge that was gained during fire attack by applying the results to fires in acquired structures. The results from the studies provide the fire service with scientific knowledge on the effect of both interior and exterior fire suppression on victim survivability and firefighter safety.
FSRI engineers and the project technical panel members from the fire attack study agreed that a priority of the study was to quantify hose stream mechanics intendent of fire. To accomplish this, the first two parts of the study were designed to examine the principles of air entrainment in hose streams and water distribution, or mapping, in structures that lack the presence of fire. The results from these experiments can be used to increase knowledge into how nozzles distribute water via different hose stream types, nozzle movements and suppression locations, in addition to how nozzles entrain and move air throughout a structure.
Entrainment
Before discussing the effect of air entrainment on suppression operations, we must first understand how a water droplet that moves through space entrains air.
As the bale of the nozzle opens and water leaves the end of the nozzle, air resistance breaks up the stream into droplets. The momentum of the hose stream propels the droplets forward. As the droplets impact the air that’s ahead of them, an area of high pressure is created. In the wake of these droplets, an area of low pressure is formed. The quiescent air that’s around the stream is at a higher pressure than that of the wake and flows toward that lower pressure area. The air that’s drawn in by these moving droplets is known as entrainment.
Consider how the number of droplets might change as a smooth bore or straight stream is employed or if a fog pattern is utilized.
Stream types/applications
To see how different stream types and application patterns affect entrainment consider the scenarios that were examined during the study.
Suppose that you are positioned at the start of a hallway holding a straight or a solid stream that’s aimed down range in a fixed position, with no manipulation. In this case, you are entraining about 1,000–2,000 cubic feet per minute (cfm) of air into the stream. As soon as you start to manipulate the stream into an O-pattern, you increase the air that’s entrained to approximately 4,000–6,000 cfm. The increase will occur regardless of the type of pattern (O-pattern, Z-pattern, T-pattern or wall-ceiling-wall) that’s selected and is more dependent on the speed of the application.
If a straight or solid stream is changed to a fog stream, air entrainment increases. For example, a stationary narrow fog entrains approximately 10,000 cfm of air. If that fog stream is rotated in an O-pattern, the air entrainment can increase to values of more than 15,000 cfm.
When put together, the data showed that the key factors that affected entrainment were the stream type (straight stream or smooth bore versus narrow fog) and the speed at which the nozzle was manipulated.
Stream manipulation
How we manipulate the hose stream on the fireground can change the amount of air that’s entrained and has the potential to affect fire dynamics and victim survivability during suppression, both interior and exterior.
For interior suppression, a flow-and-move approach with a straight or solid stream that’s manipulated into an O-pattern, for example, will take advantage of a manageable amount of air entrainment to essentially seal off the space that’s ahead of the advancing suppression crew and entrain fresh air into the structure behind the crew.
For exterior suppression, it’s critical that you ensure that you get the most amount of water into the fire compartment with the least amount of air that’s entrained. As such, straight or solid streams should be employed with care, to limit the manipulation of the stream and to ensure that the ventilation opening isn’t occluded.
Water mapping
The second phase of the fire attack study was to look at water mapping, or how water is distributed within a compartment.
Surface cooling is vital to the success of extinguishment, stopping the heat absorption and subsequent re-radiation of the hot surfaces to fuels or potential occupants within the compartment.
If a hose stream is applied perpendicular to a surface, water will radiate 360 degrees. The more shallow the angle into the compartment, the more the water will be propelled forward (Figure 1). This won’t provide surface cooling to the ceiling or walls that are between the nozzle and the point of impact of the stream. The steeper the angle into the compartment, the more the water will radiate outward in all directions, which would provide better surface coverage and better cooling.
Once the water coats the ceiling surface and reaches the side walls of the compartment, it falls down these surfaces in sheets. As the water reaches the floor, it accumulates and pools in different areas, depending on the obstructions that might or might not be present.
Coupling together the concepts of air entrainment in hose streams and the principles that surround effective water mapping of structures enables firefighters to most effectively apply water on the fireground to maximize the potential survivability of any trapped occupants and to quickly suppress the fire.
Hose stream prop
To bring the results from the study to the training ground, FSRI engineers, along with the help of trusted fire service partners and live training demonstrations throughout the country, designed the hose stream prop to study the suppression principles that were developed as a part of the fire attack study.
A long hallway and an attached room, with overall size and ventilation openings that are similar to those that are used in the fire attack study, are the primary components to the prop. The walls and roof are lined with plexiglass for easy visualization of the tactics that are employed (Figure 2).
The hallway-and-room setup allows for visualization of the principles regarding successful extinguishment tactics:
· Interior suppression down a hallway that can be used to show air entrainment behind an advancing suppression crew as well as utilizing alternate hits, such as deflecting the stream off of the king stud of the doorway to the bedroom (Figure 3)
· Exterior suppression that utilizes streams that are angled off of the ceiling at various angles as well as utilizing alternate hits, such as deflecting the stream off of the window header (Figure 3)
· A roof structure atop the bedroom allows for visualization of the principles that are needed to understand successful attic fire suppression methods, such as an eave attack, or to examine exterior suppression methods from below a simulated floor above
· A small, simulated cockloft atop the hallway allows for a visualization of different specialty nozzles, such as a cellar distributor, a cockloft nozzle and void-space nozzles
FSRI is proud to freely share fully dimensioned construction plans of its water mapping prop along with instructional videos and lesson plans for fire departments to incorporate into their training curriculum. Everything that’s needed to build the prop and to implement the accompanying training is provided in a comprehensive hands-on-training toolkit via the FSRI website: https://fsri.org/hose-stream-mechanics.
