Why is the Tesla turbine not used?
June 08, 2022
How are Tesla turbines different from modern day turbines?
As we saw in the previous article(link), the tesla turbine works on a series of closely packed parallel disks attached to a shaft and arranged within a sealed chamber. This is a bladeless turbine patented by Nikola tesla. But nowadays, the Tesla turbine is not used. Instead of this turbine, conventional turbines are mostly used. Let's discuss this in this article.
Overview of the Tesla turbine
He applies the water on disk, this will produce the viscous force tangential to a disc which makes it spin. But viscous force was very minimal. He increased the fluid speed to produce sufficient viscous force. Disk’s spiral shape increases the contact area between fluid and disk surface, this will result in increasing the production of viscous force. He had implemented the boundary layer thickness concept in his design. I will explain it here in detail.
Boundary Layer thickness- Detailed explanation
In Fig:1, you can see in this system that the fluid particles which are in close contact with the disk adhere to it and form a stationary layer. The next layer of molecules tries to pull the stationary layer in the flow direction. However, in this process they lose some energy to the stationary-layer molecules. The same thing happens with subsequent layers. This tendency of fluid particles to resist the flow of the other particles is known as ‘viscosity.’
Fig:1 - The next layer of molecules tries to pull the stationary layer in the flow direction
In this way, velocity gets varied here as I have illustrated in(Fig: 2A). The region up to which this velocity variation exists is known as ‘boundary layer region.’ Clearly, inside the boundary layer, one fluid layer produces a drag force on the neighboring layer since a relative motion occurs between the layers(Fig:2B). However, outside the boundary layer no relative motion occurs between the layers(Fig:2C), or the force between the layers is zero.
Fig:2A - Velocity variation
Fig:2B - Drag force produce on the neighboring layer
Fig:2C - Free flow - outside the boundary layer no relative motion
How did Tesla use the boundary layer thickness concept in his turbine?
To make use of this boundary layer phenomenon Nikola Tesla added two more disks in parallel. He kept disks closer, keeping the gap approximately twice the boundary layer. Now the shear effects are dominant in between the disk space. Tesla also found that by increasing the effective area between disk and fluid, the turbine can produce more torque, so he added more disks.
Tesla turbine operation
However, this design failed horribly. Did you know the reason behind it? The issue was that this turbine would run at a very high speed - 35,000 RPM(Fig:3A). Nikola Tesla never thought that this turbine would produce such a high RPM, and the disk strength was not sufficient enough to withstand the huge centrifugal force produced in the material, resulting in material expansion and disk failure by warping(Fig:3B).
Nikola Tesla could not find any material to withstand such a high RPM at that time. Eventually, he had to reduce the RPM to less than 10,000 to save the disks from mechanical failure, that’s why TT’s efficiency had gone down.
Fig:3A - Tesla turbine running at very high speed 35000 RPM
Fig:3B - Disk wrapping: To save disk from mechanical failure,
he had reduced the RPM
Now, I have the big question here: despite the fact that Tesla turbines are so easy to construct, Then why aren’t they used in the power-generation industries?
We will see the reasons for it.
1)The first reason is that the modern day steam turbines are more than 90% efficient than this turbine. We know that the Tesla turbine becomes more efficient as the rotor’s speed has to increase. But for the Tesla turbine to achieve such a high efficiency level, the rotor has to spin at a very high RPM — maybe 50,000! And this is impossible because of material expansion and disk failure by warping.
Fig:4 - Comparison between Tesla turbine and Steam turbine
2)The next major challenge is that for industrial applications, usually we need a disk size of two or three meters. Consider these hypothetical Tesla turbine disks, with a diameter of 3 meters. It’s an engineering impossibility to operate such large diameter disks at a speed of 50000 RPM.
Fig:5 - Hypothetical Tesla turbine disks
3)The last main issue is that of the blade tip velocity. The most modern steam turbine blades are able to achieve a mach number of 1.8 at their tips, or 1.8 times the speed of sound. A rough calculation shows that these hypothetical disks will be having a mach number of 13 at the tips - definitely an engineering impossibility. The only option left is to reduce the RPM, and we know this act will lead to a huge drop in the turbine’s efficiency.
Fig:6 - Blade tip velocity
Therefore, Nikola Tesla’s claim of 97% efficiency for his six-inch model seems unrealistic. Remember, he was able to run this turbine only under less than 10000 RPM.
Advantages of Tesla turbine:-
1)Simpler design than others conventional turbines
2)The Tesla turbine can work reversibly into a pump.
3) Low cost for production and maintenance. Disk cost is lower than the blade's cost.
Disadvantages of Tesla turbine
1)High speed but low torque generation, because power generation needs high torque.
2)Energy loss due to friction at high speed.
Application of Tesla turbine
Despite these drawbacks, the Tesla turbine has found some niche applications.
1.The Tesla turbine is reversible. It can work as a pump if you supply energy to the rotor.
3.The petroleum industry, and ventricular assistance pumps.
I hope you understood it and enjoy this explanation. 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?