You might wonder how the energy of a fluid can cause a metal disk to spin. After all, if a disk is perfectly smooth and has no blades, vanes or buckets to "catch" the fluid, logic suggests that the fluid will simply flow over the disk, leaving the disk motionless. This, of course, is not what happens. Not only does the rotor of a Tesla turbine spin - it spins rapidly.
The reason why can be found in two fundamental properties of all fluids: adhesion and viscosity. Adhesion is the tendency of dissimilar molecules to cling together due to attractive forces. Viscosity is the resistance of a substance to flow. These two properties work together in the Tesla turbine to transfer energy from the fluid to the rotor or vice versa. Here's how:
As the fluid moves past each disk, adhesive forces cause the fluid molecules just above the metal surface to slow down and stick.
The molecules just above those at the surface slow down when they collide with the molecules sticking to the surface.
These molecules in turn slow down the flow just above them.
The farther one moves away from the surface, the fewer the collisions affected by the object surface.
At the same time, viscous forces cause the molecules of the fluid to resist separation.
This generates a pulling force that is transmitted to the disk, causing the disk to move in the direction of the fluid.
The thin layer of fluid that interacts with the disk surface in this way is called the boundary layer, and the interaction of the fluid with the solid surface is called the boundary layer effect. As a result of this effect, the propelling fluid follows a rapidly accelerated spiral path along the disk faces until it reaches a suitable exit. Because the fluid moves in natural paths of least resistance, free from the constraints and disruptive forces caused by vanes or blades, it experiences gradual changes in velocity and direction. This means more energy is delivered to the turbine. Indeed, Tesla claimed a turbine efficiency of 95 percent, far higher than other turbines of the time.
But as we'll see in the next section, the theoretical efficiency of the Tesla turbine has not been so easily realized in production models.
The Boundary Layer: It's a Real Drag
The boundary layer effect also explains how drag is created on an airplane wing. Air moving over the wing behaves as a fluid, which means air molecules possess both adhesive and viscous forces. As air sticks to the wing surface, it produces a force that resists the forward motion of the aircraft.
IV Barriers to Tesla Turbine Commercialization
Tesla, as well as many contemporary scientists and industrialists, believed his new turbine to be revolutionary based on a number of attributes. It was small and easy to manufacture. It only had one moving part. And it was reversible.
To demonstrate these benefits, Tesla had several machines built. Juilus C. Czito, the son of Tesla's long-time machinist, built several versions. The first, built in 1906, featured eight disks, each six inches (15.2 centimeters) in diameter. The machine weighed less than 10 pounds (4.5 kilograms) and developed 30 horsepower. It also revealed a deficiency that would make ongoing development of the machine difficult. The rotor attained such high speeds - 35,000 revolutions per minute (rpm) - that the metal disks stretched considerably, hampering efficiency.
In 1910, Czito and Tesla built a larger model with disks 12 inches (30.5 centimeters) in diameter. It rotated at 10,000 rpm and developed 100 horsepower. Then, in 1911, the pair built a model with disks 9.75 inches (24.8 centimeters) in diameter. This reduced the speed to 9,000 rpm but increased the power output to 110 horsepower.
Bolstered by these successes on a small scale, Tesla built a larger double unit, which he planned to test with steam in the main powerhouse of the New York Edison Company. Each turbine had a rotor bearing disks 18 inches (45.7 centimeters) in diameter. The two turbines were placed in a line on a single base. During the test, Tesla was able to achieve 9,000 rpm and generate 200 horsepower. However, some engineers present at the test, loyal to Edison, claimed that the turbine was a failure based on a misunderstanding of how to measure torque in the new machine. This bad press, combined with the fact that the major electric companies had already invested heavily in bladed turbines, made it difficult for Tesla to attract investors.
In Tesla's final attempt to commercialize his invention, he persuaded the Allis-Chalmers Manufacturing Company in Milwaukee to build three turbines. Two had 20 disks 18 inches in diameter and developed speeds of 12,000 and 10,000 rpm respectively. The third had 15 disks 60 inches (1.5 meters) in diameter and was designed to operate at 3,600 rpm, generating 675 horsepower. During the tests, engineers from Allis-Chalmers grew concerned about both the mechanical efficiency of the turbines, as well as their ability to endure prolonged use. They found that the disks had distorted to a great extent and concluded that the turbine would have eventually failed.
Even as late as the 1970s, researchers had difficulty replicating the results reported by Tesla. Warren Rice, a professor of engineering at Arizona State University, created a version of the Tesla turbine that operated at 41 percent efficiency. Some argued that Rice's model deviated from Tesla's exact specifications. But Rice, an expert in fluid dynamics and the Tesla turbine, conducted a literature review of research as late as the 1990s and found that no modern version of Tesla's invention exceeded 30 to 40 percent efficiency.
This, more than anything, prevented the Tesla turbine from becoming more widely used.
As the Office of Naval Research in Washington, D.C., plainly stated: "The Parsons turbine has been around a long time with entire industries built around it and supporting it. If the Tesla turbine isn't an order of magnitude superior, then it would be pouring money down the rat hole because the industry isn't going to be overturned that easily …".
Nikola Tesla's Electric Car
Although Tesla never tested his turbine in a car, he did, by some accounts, develop an electric car in 1931. The car was a Pierce-Arrow, which had been configured with an 80-horsepower, 1,800-rpm electric motor instead of a gas-powered engine. According to the story, Tesla assembled a mysterious black box containing vacuum tubes, wires and resistors. Two rods stuck out of the box. When the rods were pushed into the box, the car received power. Tesla drove the car for a week - up to speeds of 90 miles per hour (145 kilometers per hour). Unfortunately, many believed he had tapped into some unknown and dangerous force of nature. Others called him crazy. In a rage, he removed the box from the car, took it back to his lab, and it was never seen again. To this day, the fundamental working principles of Tesla's electric car remain a mystery.
The Future of the Tesla Turbine
Tesla always was a visionary. He did not see his bladeless turbine as an end itself, but as a means to an end. His ultimate goal was to replace the piston combustion engine with a much more efficient, more reliable engine based on his technology. The most efficient piston combustion engines did not get above 27 to 28 percent efficiency in their conversion of fuel to work. Even at efficiency rates of 40 percent, Tesla saw his turbine as an improvement. He even designed, on paper, a turbine motorcar, which he claimed would be so efficient that it could drive across the United States on a single tank of gasoline.
Tesla never saw the car produced, but he might be gratified today to see that his revolutionary turbine is finally being incorporated into a new generation of cleaner, more efficient vehicles. One company making serious progress is Phoenix Navigation and Guidance Inc. (PNGinc), located in Munising, Michigan. PNGinc has combined disk turbine technology with a pulse detonation combustor in an engine the company says delivers unprecedented efficiencies. There are 29 active disks, each 10 inches (25.4 centimeters) in diameter, sandwiched between two tapered end disks. The engine generates 18,000 rpm and 130 horsepower. To overcome the extreme centrifugal forces inherent to the turbine, PNGinc uses a variety of advanced materials, such as carbon-fiber, titanium-impregnated plastic and Kevlar-reinforced disks.
Clearly, these stronger, more durable materials are critical if the Tesla turbine is going to enjoy any commercial success. Had materials such as Kevlar been available in Tesla's lifetime, it's quite likely that the turbine would have seen greater use. But as was often the case with the inventor's work, the Tesla turbine was a machine far ahead of its time.