You've probably seen it before....an out-of-control fire hose, and someone trying to get a hold of it. Some parents may say that it's similar to what they have to deal with when their kids on a sugar high. But the incredible power seen in a loose fire hose is due to the high velocity and flow rate of the water. It's the velocity that allows the stream of water to travel a long distance so the firefighter can maintain a safe distance from the fire. And MOST of us know that using more water at higher flow rates would be faster at putting the fire out. There are two factors that affect the velocity and flow rate from the fire nozzle: the pressure of the water entering, and the nozzle design itself.
Akron Brass is a company that designs a variety of equipment used in firefighting, including nozzles. We spoke with Ajit Singh, a senior engineer there to understand the operation and design of the "Assault" nozzle. Ajit explained that some past nozzles were designed to ensure a constant pressure water flow, no matter how much water was being pumped through. That nozzle design is called "automatic". They have a spring loaded baffle mechanism at the exit to restrict the water flow. The spring allows the baffle to open more if there is an increase in water supply to the unit, maintaining the pressure while allowing more water to flow through. Their main advantage is that the adjustable opening provides a steady stream flow pattern with varying water supply conditions.
But fire crews encounter different water flow conditions at each fire scene. A traditional automatic nozzle may shoot a water spray that looks good but could severely restrict the amount of water flow in case of a pressure drop. In this case, the amount of water in the spray may actually be very different, leading to some risk for the firefighter. That's why Akron Brass designers came up with a fixed baffle design. This ensures that the controlling gap at the baffle remains constant and a pressure drop will not cause a severe decrease in the amount of water flow.
To allow a fire crew to use the nozzle for a variety of locations, the baffle is interchangeable so the fire crews can match the nozzle to their desired water flow to get an optimum stream quality. Ajit and his team used direct modeling to quickly develop different geometries for their baffle designs. They could then import the designs into a computational fluid dynamics program to simulate the flow and determine which ones provided the best stream quality. This saved them a lot of time and expense compared to creating many prototypes and testing each one. Modeling also helped in ensuring that the unit was robust enough to handle water pressure of up to 250 psi and flow rates of 350 gallons per minute. This, while keeping the unit light and easy to operate, since some fires can take hours to extinguish.