A nozzle works by forcing fluid (liquid or gas) through a narrowing channel, which speeds it up. The core principle is simple: when the same amount of fluid has to pass through a smaller opening, it must move faster to get through. This tradeoff between pressure and velocity is what makes every nozzle function, from a garden hose attachment to a rocket engine.
The Pressure-Velocity Tradeoff
The physics behind a nozzle comes down to energy conservation. A fluid flowing through a pipe carries two forms of energy: pressure energy (the force pushing it forward) and kinetic energy (its actual speed). The total energy stays constant, so when one goes up, the other goes down. This relationship is described by Bernoulli’s principle, and it explains why squeezing fluid through a smaller space makes it go faster.
When fluid enters the wide end of a nozzle, it’s moving relatively slowly under higher pressure. As the channel narrows, that pressure energy converts into velocity. The fluid accelerates. By the time it exits the narrow end, it’s moving much faster but at lower pressure. You can feel this yourself by partially covering a garden hose with your thumb: you’re creating a smaller opening, and the water shoots out faster as a result.
The relationship between pressure and flow rate isn’t one-to-one, though. To double the flow rate through a hydraulic spray nozzle, you need to increase the pressure by four times. This square-root relationship means that small pressure changes produce modest speed gains, while large pressure jumps are needed for dramatic increases in output.
Why Shape Matters
Not all nozzles are simple funnels. The internal geometry has a major effect on how efficiently the nozzle converts pressure into velocity and how much energy gets wasted as friction and turbulence along the walls.
Conical nozzles, which taper smoothly from wide to narrow, lose relatively little energy. Their energy loss coefficients range from about 0.08 to 0.50, meaning they can convert the majority of pressure energy into useful velocity. Cylindrical nozzles, which are essentially straight tubes with a constant diameter, perform worse. Their energy loss coefficients range from 0.21 to 0.78, making them the least efficient common design. The smoother and more gradual the transition from wide to narrow, the less energy gets eaten up by turbulence inside the nozzle.
Other geometric details matter too. The ratio of the nozzle’s length to its diameter, the sharpness of the edges at the opening, and even the shape of the pipe section upstream all influence how much fluid actually makes it through compared to what the math would predict in a perfect system. Engineers measure this with something called a discharge coefficient: the ratio of actual flow to theoretical ideal flow. A perfect nozzle would score 1.0, but real nozzles always fall short due to friction and flow disruption.
Convergent vs. Convergent-Divergent Nozzles
For everyday applications like hoses, sprinklers, and pressure washers, a simple convergent nozzle (one that just gets narrower) works fine. The fluid speeds up as the channel shrinks, and it exits at a higher velocity than it entered. This design works well for any flow that stays below the speed of sound.
Rocket engines and jet propulsion systems need something more specialized: a convergent-divergent nozzle, sometimes called a CD nozzle or de Laval nozzle. This design narrows down to a tight point called the throat, then widens back out again. It seems counterintuitive that a widening section would speed things up further, since in everyday experience, a wider pipe slows fluid down. But something unusual happens when the flow reaches the speed of sound at the throat.
At that critical point, the rules flip. In subsonic flow, a wider channel slows fluid down. In supersonic flow, a wider channel speeds it up. This happens because at supersonic speeds, the fluid’s density drops faster than its velocity increases. To keep the same mass flowing through per second, the fluid has to accelerate as the channel expands. The result: exhaust gases enter the converging section at subsonic speed, hit the speed of sound at the throat, then accelerate to supersonic speeds in the diverging section. NASA notes that the final speed depends on the ratio between the exit area and the throat area. A larger exit relative to the throat produces faster exhaust.
Controlling Droplet Size in Spray Nozzles
Many nozzles aren’t just about speed. Spray nozzles in agriculture, manufacturing, and medicine need to break liquid into droplets of a specific size. The size of those droplets determines whether a pesticide coats a leaf evenly, whether paint applies smoothly, or whether medication reaches the lungs.
Droplet size is measured by something called Volume Median Diameter, or VMD: the size where half the spray volume is in larger droplets and half is in smaller ones. Agricultural sprayers classify droplets into categories. Fine droplets (145 to 225 microns) provide thorough coverage for fungicides and contact insecticides. Coarse droplets (326 to 400 microns) work better for systemic herbicides that only need to land on the leaf surface. Very coarse and extremely coarse droplets (400 microns and above) are used for soil applications where drift would waste product.
The nozzle’s orifice diameter is the primary control. In medical inhalers, for example, orifice diameters typically range from about 0.3 to 0.5 millimeters. Shrinking the orifice produces finer particles that penetrate deeper into the lungs. One study found that reducing the orifice from 0.42 mm to less than 0.14 mm increased the fraction of drug particles fine enough for deep lung delivery from 19% to over 70%. The tradeoff: smaller openings take longer to release the dose, stretching spray time from about 200 milliseconds to over a full second.
How Nozzles Wear Out
Nozzles don’t last forever. The high-speed flow that makes them useful also gradually destroys them. Erosion from particles in the fluid, corrosion from chemical exposure, and the sheer turbulence of high-velocity flow all degrade the orifice over time. As the opening changes shape, spray patterns become uneven and flow rates drift from their design specifications.
The damage tends to concentrate at specific points. In industrial nozzle pipes carrying water and sand, failure analysis shows that the inlet section takes the worst beating, where incoming particles slam into the wall at high speed and turbulence is greatest. In one case study of mild steel nozzle pipes, leak perforations appeared within just 300 hours of operation, with the combined effects of erosive wear and corrosion eating through the pipe wall. Severe wall thinning was also found near the exit section. Regular inspection, particularly ultrasonic thickness measurement, can catch thinning before it becomes a failure.
For spray nozzles in agriculture or industrial coating, even minor erosion changes the orifice geometry enough to shift droplet sizes and spray patterns. A worn nozzle that was once producing medium-sized droplets may start producing finer ones, increasing chemical drift and waste. Most manufacturers recommend replacing spray tips on a set schedule rather than waiting for visible damage.