Assignment 5 - Essay
|What is Cavitation|
|How to Detect Cavitation Damage|
|Hydro Turbine Damage|
|Aquatic Organism Cavitation|
Cavitation has been a recurring problem in many hydrodynamic devices, such as hydro turbines for electric generation, (1) pumps in industrial applications (2) and propellers and rudders for ships and submarines (3) . Other, less obvious places where cavitation occurs is diesel engine cylinder liners, (4) on the fins of tuna and dolphins, (5) and in human knuckles when they crack. (6) The theoretical possibility of cavitation was suggested by Euler in 1794, and Reynolds proved the physical existence via experiments in 1874. By 1900, evidence of cavitation as a real world problem occurred when Sir Charles Parson invented the first steam turbine ship, the Turbinia. (7) Today, the phenomena still limits performance of hydrodynamic devices, where the collapse of cavitation bubbles causes undesired noise and vibrations, a reduction in efficiency, and damage to mechanical devices.
Cavitation is similar to boiling water, but can occur at relatively low temperatures, as low as room temperature. (8) This happens when the pressure of the liquid decreases to such a point that the water transforms from a liquid to a vapor. These low pressure zones happen behind fast moving objects in the water, such as tuna fins, propeller blades, and even sections of piping. The damage occurs when the water vapor then transforms back into a liquid. During the collapse, the bubble collapses on itself, forming a very concentrate and powerful water-jet. (9) These micro-jets are capable of producing a tremendous amount of force, so much that small pieces of metal can be ejected from the surface. This leaves a volume for more bubbles to form, causing even more cavitation bubbles to form. This exponential rise in cavitation damage makes this form of damage very important to shipbuilder and pump designers. If improperly designed, a device can quickly fail.
To detect cavitation damage, there are two simple and easy ways to determine if a device has been damaged by cavitation. First, there is a noticeable drop in performance of a device. For pumps and propellers, a couple of minutes of intensive cavitation is more than capable in reducing performance of the device. A second way to determine if cavitation is occurring is to listen to the liquid flowing within the device. When the cavitation bubbles collapse, they produce a very loud, audible noise. The noise is usually between 8-15 kHz, well within the range of human hearing. The noise itself can best be described as "a can of marbles being shaken. " (10)
Turbinia was the very first steam turbine powered steam ship. It was designed by Charles Parsons and constructed just before the turn of the century. Parsons believed that his new ship would be capable of incredible speeds, far faster than any ship of the day, but when the newly built ship went out for sea trials, it was found Turbinia was not capable of even 20 knots. Parsons suspected that cavitation was limiting his performance, so he went about designing new propellers to mitigate the problem. He finally found that using 3 separate shafts, each with 3 propellers was enough to reduce the cavitation problem. The maximum top speed of the Turbinia would end up being a remarkable 34 knots. By overcoming the cavitation problem that limited the speed of the ship, Parsons was able to sell his turbine powered ship concept to the Royal Navy, giving them a speed advantage in the upcoming Great War. (11)
Hydro turbines are becoming increasingly important in today's world as a renewable and consistent source of energy, unlike wind and solar power. However, controlling cavitation is crucial to avoid performance losses and downtime caused by cavitation damage and subsequent repair. The most graphic and deadly example of cavitation damage happened on at the Sayano-Shushenskaya Hydroelectric Power Plant. One of the ten turbines experience catastrophic failure, resulting in the turbine cover being lifted off and flooding the nearby power plant. 75 people died, and it is estimated that over 100 million US dollars of damage had been done to the plant, much less the cost of downtime that happened to nearby aluminum producers. The cause was found to be excessive vibrations of the turbine. One hypothesis that caused the accident was that cavitation damage had unbalanced the turbine rotor, causing it to have severe vibrations, leading to the ultimate failure of the device.(12)
One interesting and non-obvious situation where cavitation is problematic is for dolphins' maximum speed. Scientists were confused why large dolphins with much stronger muscles were only slightly faster than much smaller dolphins, who were limited in speed by their muscles. Some scientists hypothesized that these larger dolphins couldn't swim any faster because it would become too painful. The pain was caused by cavitation bubbles collapsing on the tails of dolphins. Essentially dolphins were able to sense the stinging pain of cavitation, and would have to slow down the beating of the tail fin to avoid cavitating the water they were swimming in. Similarly, tuna would also experience cavitation on their fins, but because they had no nerve ending in their tail fins, they would not feel the pain of the cavitation bubbles collapsing and were able to swim at much faster speeds. Scientists discovered when examining tuna fins that some would have tiny holes located on the edges of their tail fin, similar to the size left by cavitation implosions on metal objects. (13)
Updated March 11, 2011