Tesla Turbine | The interesting physics behind it

the maverick engineer nikola tesla made his contribution in the mechanical engineering field too look at one of his favorite inventions a bladeless turbine or tesla turbine the tesla turbine had a simple unique design yet it was able to beat the efficiency levels of steam turbines at the time normal turbines are complex in design with blades of complicated geometry and stator parts nikola tesla once said the tesla turbine is his favorite invention and he even claimed an efficiency level of 97 percent for this turbine let's start a design journey to understand this interesting piece of technology and towards the end we will also verify tesla's efficiency claim modern-day turbines work on the airfoil principle you can see the fluid gushing over the airfoil cross-section will generate lift force on it and make the blade turn however to make this turbine spin nikola tesla relied on a totally different phenomenon the viscous effect of fluid on solid surfaces you might have seen this effect before when water flows over a rounded stone it makes the stone move because of the viscous force between the water and the stone surface nikola tesla extrapolated this very force to run his turbine who knows tesla might have got inspiration for his turbine from this very example if you produce the viscous force tangential to a disc it will start to spin hooray you've produced the simplest form of tesla turbine however this is quite an inefficient turbine most of the jet's energy is lost here let's make this design more efficient and practical let's place this shaft disc pair inside a casing now the fluid enters through the outer casing tangential to it a provision for the fluid to exit is at the center of the turbine assume an inlet fluid with slightly higher pressure than the atmospheric pressure is entering 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 will be very minimal and the disc won't 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 center almost in a straight line now let's increase the fluid speed and see what happens here since the fluid has a greater speed the interaction between the fluid and disc surface will produce sufficient viscous force to turn the disc here comes an interesting twist when the fluid particles are rotating they need a certain amount of centripetal force to maintain that motion 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 will curve out as shown if you compare the radii of particle a in these two cases clearly the curved path particles have more radius now let's gradually increase the fluid speed you can see the curvature of the fluid particles will further increase and form a kind of spiral this concept is clearer when you track the same fluid particle for different disk speeds the greater the disk speed the more the particle moves 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 to improve this design further we need to understand a key concept called boundary layer thickness we can observe 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 in this way you can clearly observe a velocity variation 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 however outside the boundary layer no relative motion occurs between the layers or the force between the layers is zero to make use of this boundary layer phenomenon nikola tesla came up with a unique idea he added two more parallel disks now let's observe the flow a boundary layer is formed on every surface as we saw earlier the particles in the boundary layer region will try to drag or rotate the respective disk however you can see a region outside both the boundary layers where fluid particles are flowing freely without any velocity gradient this free flow does not impart any energy to the disk and contributes little to the torque generation to make his turbine more efficient nikola tesla brought the disks closer keeping the gap approximately twice the boundary layer here no free flow occurs the two boundary layer regions are touching each other and we can see the shear effects are now dominant in between the disk space for steam this ideal distance was found to be 0.4 millimeters using this method tesla improved the torque output of his turbine tesla found that by increasing the effective area between disc and fluid the turbine can produce more torque so he added more disks this model had a diameter of six inches however this design failed horribly the issue was that this turbine would run at a very high speed 35 000 rpm nikola tesla never thought that this turbine would produce such a high rpm and the disc strength was not sufficient enough to withstand the huge centrifugal force produced in the material resulting in 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 ten thousand to save the discs from mechanical failure now for the big question despite the fact that tesla turbines are so easy to construct why aren't they used in the power generation industries the reason is that the modern day steam turbines are more than 90 percent efficient we know that the tesla turbine becomes more efficient as the rotor speed increases but for the tesla turbine to achieve such a high efficiency level the rotor has to spin at a very high rpm maybe fifty thousand the major challenge is that for industrial applications we need a disk size of two or three meters consider these hypothetical tesla turbine disks with a diameter of three meters it's an engineering impossibility to operate such large diameter discs at a speed of 50 000 rpm the 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 therefore nikola tesla's claim of 97 efficiency for his 6-inch model seems unrealistic remember he was able to run this turbine only under less than 10 000 rpm despite these drawbacks the tesla turbine has found some niche applications interestingly for instance the tesla turbine is reversible it can work as a pump if you supply energy to the rotor also we know that tesla turbines work based on fluid's viscous effects so these kinds of pumps are used in high viscosity applications like wastewater plants the petroleum industry and ventricular assistance pumps before you leave don't forget to be a member of the lessex team thank you

Channel: Lesics

Views: 3,829,323

Rating: 4.9101276 out of 5

Keywords: Tesla turbine, boundary layer, spiral flow, tesla turbine efficiency

Id: AfCyzIbpLN4

Length: 9min 24sec (564 seconds)

Published: Thu Jun 03 2021