As we saw in the previous article, the Tesla turbine works on a series of closely packed parallel disks attached to a shaft and arranged within a sealed chamber(Fig:1). This is a bladeless turbine patented by Nikola tesla. We have learned about the Tesla turbine’s design journey. In this article I will explain why is the Tesla turbine is not used.
To get the answer of this question first you should understand why the conventional turbines are mostly used instead of the Tesla turbines.
Modern day turbines are more than 90% efficient than the Tesla turbines. Because they work on airfoil principles(link). You can see in Fig:2 below these turbines have blades(not smooth discs). When the high energy fluid passes through the turbine, it will create a pressure difference and this will create a lift force. And subsequently lift force will rotate the turbine. Due to this simple airfoil shape makes the turbine blade turn easily. You can say the airfoil shape is the heart of these turbines. These turbines can create high torque with less RPM, there is no need of high RPM like Tesla turbines.
Tesla turbines work on boundary layer phenomenon. Did you know? How did Tesla use the boundary layer thickness concept in his turbine? I will explain here. Boundary layer is a fluid mechanics, 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. In Fig below, you can see that a boundary layer is formed between two disks.(Fig:3B) As we know the particles in the boundary layer region will try to drag or rotate the respective disc. However, there is a region which is outside both the boundary layers where fluid particles are flowing freely without any velocity gradient(Fig:3B). This free flow does not impart any energy to the disk, producing little contribution for the torque generation.
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. The Tesla also 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:4B). This design is complete(Fig:4C). Now it's time to run this turbine.
But this design has failed horribly. The issue was that, this turbine would run at a very high speed - 35,000 RPM(Fig 4C). 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:5).
If the Tesla turbine is to achieve such a high efficiency level like other turbines, 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. 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.
2)Another major challenge is that for industrial applications, usually we need a disc size of two or three meters. This is impossible to operate such a large diameter(6 Inches) disc at a speed of 50000 RPM.
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.
These are the reasons why the Tesla turbines are not used. That’s all. I hope you have a good understanding of this article. See you in the next article.
Thank you for reading the article!