How does the Tesla turbine work?
June 02, 2022
Introduction of tesla turbine
The maverick engineer Nikola Tesla also made his contribution in the mechanical engineering field too. I took a look at one of his favourite inventions — a bladeless turbine, or Tesla Turbine. I was amazed to know that it had a simple, unique design, and yet it was able to beat the efficiency levels of steam turbines at that time. Normal turbines are complex in design, with blades of complicated geometry and stator parts.
In this article I will take you on a design journey to understand this interesting piece of technology and in the next article (link), I will also verify Tesla’s 97% efficiency claim.
How turbines work?
Modern-day turbines work on the airfoil principle. However, to make Tesla’s turbine spin, He relied on a totally different phenomenon: the viscous effect of fluid on solid surfaces. In the next section I will discuss more about this effect, and also the construction details of the Tesla turbine.
Construction of Tesla Turbine
What is the viscous force?
To understand the effect of viscous force, let's imagine a stone in the river. The water is flowing over the rounded stone. It makes the stone move because of the viscous force between the water and stone surface(Fig:1).
Fig:1 - Stone move due to viscous force between the water and stone surface
With the help of viscous force, I will try to spin a disk. Here I have a disk and apply the water on it (Fig:2). This arrangement will produce the viscous force tangential to a disc which makes it spin.
I have produced the simplest form of Tesla Turbine. But, this is quite an inefficient turbine; most of the jet energy is lost here. Let’s make this design more efficient and practical.
Fig:2 - Disk will spin because of viscous force applied on the disc.
In this design, I will place this shaft-disk pair inside a casing as illustrated in Fig 3. Now, the fluid enters through the outer casing, tangential to it. A provision for the fluid to exit is at the centre of the turbine.
Fig:3 - Shaft disc pair inside a casing.
I assume that an inlet fluid with slightly higher pressure than the atmospheric pressure is entering into the inlet nozzle at low speed. What do you think about the path this fluid takes? Since the fluid has a low velocity, the viscous force between the disc and the fluid is very minimal. Because of this, the disc will not rotate.The exit hole is at atmospheric pressure, which means the fluid will have a slightly higher pressure than the atmosphere and naturally flows towards the centre, almost in a straight line. (Fig:4)
Fig:4 - Fluid is flowing toward the center almost in straight line
Now, If I increase the fluid speed, the interaction between the fluid and disc surface produces sufficient viscous force, to turn the disc.
Now I am moving to an interesting twist. The fluid particles need a certain amount of centripetal force to maintain the rotating motion(Fig:5A). A fluid particle of the same velocity requires more centripetal force near the center than away from it. For this reason, the rotating fluid particles have a tendency to move away from the center. However, the turbine exit is at the center, so the fluid particles have to reach it eventually. Due to these opposing effects in the rotating case, the particle motion is curved out as shown in the Fig:5B below. If I do a comparison here, the radii of particle A in these two cases, clearly the curved path particles have more radius.
Fig:5A Rotating fluid particles need centripetal force to maintain motion
Fig:5B - A, B, C particle motion has curved out
What is the spiral path in a Tesla Turbine?
Now, let’s gradually increase the fluid speed. You can see in fig 6A, the curvature of the fluid particles has increased and forms a kind of spiral. This concept is clearer when you track the same fluid particle for different disc speeds (Fig:6B). The greater the disc speed, the more the particle moves away from the center.
Fig:6A - Fluid particle’s curvature has increased here and it forms kind of spiral
Fig:6B - The disc speed is greater , that;s why the particles are moved away from the center.
The fluid flow’s spiral shape is in fact a blessing in disguise. The spiral shape increases the contact area between the fluid particles and the disc surface, thus increasing the viscous force production on the disc. This effect also means that the faster the turbine rotates, the more energy it will extract from the fluid. In other words, the Tesla turbine exhibits high efficiency during high-speed operations.
Boundary layer thickness
When viscous fluid flows over a surface it experiences a resistance due to its viscosity. This generates a velocity gradient near the surface. This is known as boundary layer effect. You can read more about boundary layers here(link). To improve this design further let's use the boundary layer thickness concept.
In fig 7A, we can see that a boundary layer is formed between two disks. As we know the particles in the boundary layer region will try to drag or rotate the respective disk. However, there is a region which is outside both the boundary layers where fluid particles are flowing freely without any velocity gradient(Fig:7B). This free flow does not impart any energy to the disk, producing little contribution for the torque generation.
Fig:7A- Boundary layer form on every surface
Fig:7B - Fluid particles moving freely without velocity gradient
To make his turbine more efficient lets bring the disks closer keeping the gap approximately twice the boundary layer. Here there is no free flow. The two boundary layer regions are touching each other. Here, the shear effects are now dominant in between the disk space. For steam this ideal distance was found to be 0.4 mm. This way Tesla has improved the torque output of his turbine to a greater level.
Tesla found that by increasing the effective area between disk and fluid the turbine can produce more torque, so he added more disks. This model had a diameter of 6 inches.
Fig:8 -For More torque generation, Nikola Tesla added more disk
That’s all about the construction and working of the Tesla Turbine. I hope you understood it and enjoy this explanation. In the next article I will explain why we don't use the Tesla turbine? Is there any future of the Tesla turbine? I will elaborate on many interesting points.
Thank you for reading the article.
Other more articles:
Working of Steam Turbine
Why is the top flow faster over an Airfoil?
How does an Airfoil generate Lift?