I have written blogs about laminar and turbulent flows as related to the Reynold’s number. Now, let’s demonstrate the difference between the two flows and the advantages of laminar flow from EXAIR’s engineered air nozzles; as demonstrated by our VariBlast Safety Air Gun.
Fluid mechanics is the field that studies the properties of fluids in various states. There are two areas, fluid statics and fluid dynamics. Fluid dynamics studies the forces on a fluid, either as a liquid or a gas, during motion. Osborne Reynolds, an Irish innovator, popularized this dynamic with a dimensionless number, Re. This number determines the state in which the fluid is moving; either laminar flow or turbulent flow. Equation 1 shows the relationship between the inertial forces of the fluid as compared to the viscous forces.
Equation 1: Re = V * Dh/u
Re – Reynolds Number (no dimensions)
V – Velocity (feet/sec or meters/sec)
Dh – hydraulic diameter (feet or meters)
u – Kinematic Viscosity (feet^2/sec or meter^2/sec)
The value of Re will mark the region in which the fluid (liquid or gas) is moving. If the Reynolds number, Re, is below 2300, then it is considered to be laminar (streamline and predictable). If Re is greater than 4000, then it is considered to be turbulent (chaotic and violent). The area between these two numbers is the transitional area where you can have eddy currents and some non-linear velocities. To better show the differences between each state, I have a picture below that shows water flowing from a drain pipe into a channel. The water is loud and disorderly; traveling in different directions, even upstream. With the high velocity of water coming out of the drain pipe, the inertial forces are greater than the viscosity of the water. This indicates turbulent flow with a Reynolds number larger than 4000. As the water flows into the mouth of the river after the channel, the waves transform from a disorderly mess into a more uniform stream. This is the transitional region. A bit further downstream, the stream becomes calm and quiet, flowing in the same direction. This is laminar flow. Air is also a fluid, and it will behave in a similar way depending on the Reynolds number.
Why is this important to know? In certain applications, one state may be better suited than the other. For mixing, suspension and heat transfer; turbulent flows are better. But, when it comes to effective blowing, lower pressure drops and reduced noise levels; laminar flows are better. In many compressed air applications, the laminar region is the best method to generate a strong force efficiently and quietly. EXAIR offers a large line of products, including the Super Air Knives and Super Air Nozzles that utilizes that laminar flow for compressed air applications. If you would like to discuss further how laminar flows could benefit your process, an EXAIR Application Engineer will be happy to help you.
Whenever there is a discussion about fluid dynamics, Bernoulli’s equation generally comes up. This equation is unique as it relates flow energy with kinetic energy and potential energy. The formula was mainly linked to non-compressible fluids, but under certain conditions, it can be significant for gas flows as well. My colleague, Tyler Daniel, wrote a blog about the life of Daniel Bernoulli (you can read it HERE). I would like to discuss how he developed the Bernoulli’s equation and how EXAIR uses it to maximize efficiency within your compressed air system.
In 1723, at the age of 23, Daniel moved to Venice, Italy to learn medicine. But, in his heart, he was devoted to mathematics. He started to do some experiments with fluid mechanics where he would measure water flow out of a tank. In his trials, he noticed that when the height of the water in the tank was higher, the water would flow out faster. This relationship between pressure as compared to flow and velocity came to be known as Bernoulli’s principle. “In fluid dynamics, Bernoulli’s principle states that an increase in the speed of fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluids potential energy”1. Thus, the beginning of Bernoulli’s equation.
Bernoulli realized that the sum of kinetic energy, potential energy, and flow energy is a constant during steady flow. He wrote the equation like this:
Not to get too technical, but you can see the relationship between the velocity squared and the pressure from the equation above. Being that this relationship is a constant along the streamline; when the velocity increases; the pressure has to come down. An example of this is an airplane wing. When the air velocity increases over the top of the wing, the pressure becomes less. Thus, lift is created and the airplane flies.
With equations, there may be limitations. For Bernoulli’s equation, we have to keep in mind that it was initially developed for liquids. And in fluid dynamics, gas like air is also considered to be a fluid. So, if compressed air is within these guidelines, we can relate to the Bernoulli’s principle.
- Steady Flow: Since the values are measured along a streamline, we have to make sure that the flow is steady. Reynold’s number is a value to decide laminar and turbulent flow. Laminar flows give smooth velocity lines to make measurements.
- Negligible viscous effects: As fluid moves through tubes and pipes, the walls will have friction or a resistance to flow. The surface finish has to be smooth enough; so that, the viscous effects is very small.
- No Shafts or blades: Things like fan blades, pumps, and turbines will add energy to the fluid. This will cause turbulent flows and disruptions along the velocity streamline. In order to measure energy points for Bernoulli’s equation, it has to be distant from the machine.
- Compressible Flows: With non-compressible fluids, the density is constant. With compressed air, the density changes with pressure and temperature. But, as long as the velocity is below Mach 0.3, the density difference is relatively low and can be used.
- Heat Transfer: The ideal gas law shows that temperature will affect the gas density. Since the temperature is measured in absolute conditions, a significant temperature change in heat or cold will be needed to affect the density.
- Flow along a streamline: Things like rotational flows or vortices as seen inside Vortex Tubes create an issue in finding an area of measurement within a particle stream of fluid.
Since we know the criteria to apply Bernoulli’s equation with compressed air, let’s look at an EXAIR Super Air Knife. Blowing compressed air to cool, clean, and dry, EXAIR can do it very efficiently as we use the Bernoulli’s principle to entrain the surrounding air. Following the guidelines above, the Super Air Knife has laminar flow, no viscous effects, no blades or shafts, velocities below Mach 0.3, and linear flow streams. Remember from the equation above, as the velocity increases, the pressure has to decrease. Since high-velocity air exits the opening of a Super Air Knife, a low-pressure area will be created at the exit. We engineer the Super Air Knife to maximize this phenomenon to give an amplification ratio of 40:1. So, for every 1 part of compressed air, the Super Air Knife will bring into the air streamline 40 parts of ambient “free” air. This makes the Super Air Knife one of the most efficient blowing devices on the market. What does that mean for you? It will save you much money by using less compressed air in your pneumatic application.
We use this same principle for other products like the Air Amplifiers, Air Nozzles, and Gen4 Static Eliminators. Daniel Bernoulli was able to find a relationship between velocities and pressures, and EXAIR was able to utilize this to create efficient, safe, and effective compressed air products. To find out how you can use this advantage to save compressed air in your processes, you can contact an Application Engineer at EXAIR. We will be happy to help you.
When I see turbulent flow vs. laminar flow I vaguely remember my fluid dynamics class at the University of Cincinnati. A lot of times when one thinks about the flow of a liquid or compressed gas within a pipe they want to believe that it is always going to be laminar flow. This, however, is not true and there is quite a bit of science that goes into this. Rather than me start with Reynolds number and go through flow within pipes I have found this amazing video from a Mechanical Engineering Professor in California. Luckily for us, they bookmarked some of the major sections. Watch from around the 12:00 mark until around the 20:00 mark. This is the good stuff.
The difference between entrance flow, turbulent flow and laminar flow is shown ideally at around the 20:00 mark. This length of piping that is required in order to achieve laminar flow is one of the main reasons our Digital Flowmeters are required to be installed within a rigid straight section of pipe that has no fittings or bends for 30 diameters in length of the pipe upstream with 5 diameters of pipe in length downstream.
This is so the meter is able to measure the flow of compressed air at the most accurate location due to the fully developed laminar flow. As long as the pipe is straight and does not change diameter, temperature, or have fittings within it then the mass, velocity, Q value all stay the same. The only variable that will change is the pressure over the length of the pipe when it is given a considerable length.
Another great visualization of laminar vs. turbulent flow, check out this great video.
If you would like to discuss the laminar and turbulent flow please contact an Application Engineer.
1 -Fluid Mechanics: Viscous Flow in Pipes, Laminar Pipe Flow Characteristics (16 of 34) – CPPMechEngTutorials – https://www.youtube.com/watch?v=rQcZIcEa960
2 – Why Laminar Flow is AWESOME – Smarter Every Day 208 – SmarterEveryDay – https://www.youtube.com/watch?v=y7Hyc3MRKno