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Water nozzles and efficiency

Written by Ben
Last updated on 2008-11-11


Improved water gun designs can be more efficient. Today, people don't build a water gun and hoping that it will performs well—they design knowing it will perform well. This page will outline what can be done to improve water gun efficiency.

Fluid mechanics basics

Read our page about how water guns work if you don't know how they work, because that information is important in this article.

Pressure is the main factor in water gun water power. There are many different types of pressure chambers, such as rubber CPS systems and air-pressure systems. Each system has its own advantages and disadvantages that I will not cover in this guide.

Viscosity is the tendency for a fluid to stick together. Certain fluids, such as glycerin, have a higher viscosity than water. Viscosity varies by temperature, so as a general rule viscosity improves slightly as a fluid gets colder.

The orifice size is the size of the part commonly called the nozzle. The orifice size determines much of how a nozzle will perform. For water to exit a smaller orifice (hole), it must accelerate. For that reason, smaller nozzles typically have a greater speed than larger nozzles at the same pressure. Smaller nozzle orifices will have a greater speed and lower output (flow), while larger nozzle orifices will have a lower speed and higher output.

Drag on the stream is caused by the stream hitting the air. Depending on the stream cohesion (a combination of the viscosity of the stream and how well the stream is formed), stream speed (determined by the nozzle orifice size and pressure) and the size of the stream, stream break-up will occur a distance from the nozzle. Stream break-up can be described best as the stream breaking into many smaller droplets of water by the drag on the stream. These droplets will fall to the ground early because the stream's speed was reduced by drag as well. Shoot any water gun and you will see that by the end of the stream, the stream breaks into smaller droplets that hit the ground. Almost never will a cylindrical stream shaped as it exited the nozzle hit the ground due to steam break-up.

Flow is said to be laminar when it has a tendency to follow a linear (i.e. straight) path. Laminar flow is affected less by drag because it does not flow randomly. The opposite of laminar flow is turbulent flow. Turbulent flow is less linear and more random.

The shot angle can greatly affect the distance of a water gun. Due to projectile physics, the range of any projectile including a water stream will be greatest at a 45 degree angle. An angle greater or less than 45 degrees will result in less distance. A higher angle (up to 90 degrees) will give the stream more height, while a lower angle (as low as -90 degrees or 270 degrees) will reduce the height of the stream.

Internal diameter is the smallest diameter the water must pass through as it travels from the pressure chamber to the nozzle. Internal diameters of all other portions of the water gun are not important because the water will not flow from the pressure chamber to the nozzle in those parts. A larger internal diameter allows for more flow but also allows for more turbulence. Typically the increase in flow offsets any range decrease due to turbulence.

The effects of flow restrictions such as the nozzle orifice and the internal diameter of a pipe can be mathematically measured as a value, commonly called the Cv value (the coefficient of velocity). That value can be found by solving the equation Q = Cv * sqrt(P) for it where Q is water flow and P is gauge pressure. The higher the Cv value, the more efficient the water gun is in making water flow. Note that this equation for flow is approximate at best and should be thought of as an educated guess to what the water gun's flow might be.

Factors that affect output

Many factors will affect the output of a water gun. Output is defined as how much water exits the nozzle orifice per second. Output is proportional to the area of the nozzle and the square root of the pressure. As explained earlier, the equation Q = Cv * sqrt(P) can approximate the flow from a Cv value and the gauge pressure. The Cv value is affected by restrictions in flow, such as the nozzle orifice. Therefore, as the area of the nozzle (pi*r^2) increases, so does the Cv value. Large nozzles combined with high pressure will create high flow. Similarly, large nozzles with moderate pressure will create high flow. However, small nozzles with high pressure still will have relatively low flow. The equation uses the square root of the pressure, so pressure obviously is less important than nozzle diameter.

Linear design

We described turbulent flow in our definitions section as seemingly random, non-linear flow. Likewise, laminar flow was described as straight flow. Turbulent flow is not good because the stream will be affected (i.e. break up) by the random tendencies in the stream. Thus, it would be in our best interests to keep flow as laminar as possible.

The easiest way to create laminar flow is to avoid creating turbulent flow. Turbulent flow that we can reduce is created by turns in pipe. Laminar flow can be created by allowing the water to travel in a straight path from the pressure chamber to the nozzle orifice.

This practice calls for several potential design changes, the largest of which is valve selection. Ball valves are the ideal firing valve to use by this practice. Ball valves are the simplest valves that allow for perfectly straight and unobstructed flow. In constant, the typically plug valves used in older Super Soaker water guns obstruct the flow, reducing velocity and creating turbulence. Today's Super Soaker's use Max-D valves which are essentially ball valves, illustrating that ball valves are more efficient.

Of course, a design isn't bad if the flow isn't linear. If a design can get a lot of flow, even with some curves, it is not an inefficient design. Parts such as tubing that allow for smoother changes in direction are superior to 90° bends, obviously. Avoid making a frivolous curves and bends.

Also, a length of straight pipe does reduce turbulence by making the flow travel straight.

Maximizing distance through ideal nozzle orifice size

It comes as no surprise that there is an optimal nozzle orifice size for specific water guns. Some nozzle orifices are too big for maximum distance, and some are too small for maximum distance.

Essentially, if the nozzle diameter is small, it will have high velocity, but be small. Small streams have less mass, and are affected by drag more because of the lack of mass and higher velocities. In contrast, large diameters are slow, but have a lot of mass, making them resistant to drag. The ideal nozzle is the perfect combination of small and large diameters.

You can find the perfect balance in a nozzle orifice. Optimizing the nozzle diameter can add many extra feet to your distance. Test a variety of nozzle diameters and see which one works best. If two perform about the same, one between them might work better. Plot out nozzle diameter vs. range. You should see something like a bell curve. All you have to do is find the peak of that curve where maximum range is achieved.

Additives

You can add certain chemicals to your water to change the properties of the water. The most major property you could change is the viscosity. Some fluids have higher viscosities than others, however, not all of those fluids are water soluble. Glycerin is one fluid that is water soluble and relatively cheap. Some water treatment chemicals reportedly have higher viscosity as well. At the moment, glycerin is the only tested and verified additive and thus will be the only one we will be discussing.

Glycerin is fairly easy to obtain. I originally bought a smaller bottle from a CVS pharmacy in the skin care section that was actually quite expensive per ounce. Later, I bought a full gallon from McMaster-Carr, an online catalog. The part number was 3190K245.

Conical nozzles

PVC endcaps are relatively inefficient. They are common and easy to use, but a small amount of energy is lost by using them. A nozzle that reduces in diameter smoothly is more efficient. These nozzles can be hard to find, but some 3/4 inch ones are sold at home improvement and hardware stores for garden hoses in the gardening section. Big fire hose nozzles (such as the one pictured below) are usually conical in shape, but they are very large in diameter. The nozzle below is McMaster-Carr part number 6424T62.

Making flow more laminar

While not always necessary, sometimes you will have less laminar flow going to your nozzle. This can be remedied to a certain extent with "straighteners.

The most common and recognizable form of "straighteners" is some sort of honeycomb or straw piece in the nozzle. These will make the flow more laminar by reducing the internal diameter for certain parts of the flow while still maintaining full flow.

Another possibility includes using small pieces of string to make water flow more laminar. The flow will follow along the strings and become more laminar.

You can read more information about these techniques in the various patents on these nozzles.

Misconceptions

"Straws" and water flow

Some people seem to believe that making the flow more laminar through straws will increase the water output/flow of the water gun. That is not true. Adding straws to a nozzle will not increase the flow any substantial amount and likely will decrease it slightly. They straws make the flow more laminar. That means that a range might increase (yes, might, not will), not that range and flow will increase. Flow should stay approximately the same but it might decrease due to reduced cross sectional area.

Barrels

One popular misconception is that a long barrel will improve range. After all, lengths of pipe make the flow more laminar, so why not make a big long one?

After testing barrels for myself, I concluded that they actually decrease performance, likely due to friction in the barrel. Barrels also add unnecessary length to your water gun.

Rifling water streams

To those who are not familiar with guns, rifling makes the bullet spin. The spin on the bullet breaks up the air before the bullet, allowing the bullet to be less affected by drag. Rifling essentially makes guns more accurate and shoot a farther distance.

Rifling would reduce performance rather than improve it. Those who claimed to have rifled streams claimed to use long screws to give the water a spin. Placing a long screw in the nozzle would not only reduce the velocity of the water stream, but increase the turbulence of the stream as well. Regardless of the internal problems, had the stream obtained a spin it would only break apart due to centrifugal force.

Summary

Basically, what was said about can be summarized into a few points

  • What matters for range is high output (flow) with the right diameter nozzle
  • High flow can be achieved through high pressure and the right design
  • The right design limits the number of turns water makes before the nozzle and has pipe diameters of at least larger than the nozzle
  • Certain nozzle diameters shoot farther than others
  • Conical shaped nozzles are the most efficient
  • Glycerin can be added to water to make water guns shoot further

I used a lot of the things I explained in this article when designing Supercannon II. Consequently, the water gun was extremely efficient and achieved a range of 73 feet! That's what efficient design can do for you.


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