Cavitation - Easily explained!

welcome to e-learning in thermodynamics and fluid dynamics of our bachelor program in renewable energies and environmental technology in this series we are going over the phenomenon of cavitation in water and its consequences what is cavitation cavitation is a rapid formation and collapse of vapor bubbles within a liquid such cavitation bubbles are formed mainly in locations within the liquid where the static pressure becomes smaller than a liquid swiper pressure the phase diagram of water shows if water is in its gaseous liquid or solid state depending on temperature and pressure the horizontal axis of a diagram represents temperature when its vertical axis shows pressure the wiper pressure curve is part of a phase diagram it indicates at which pressures and temperatures water evaporates from liquid to vapor or condenses from way per to liquid at a constant pressure of 1 bar absolute water in its solid state which is ice melts to liquid water at a temperature of zero degrees Celsius keeping the pressure constant at 1 bar absolute liquid water evaporates when the temperature is increased above 100 degrees Celsius evaporation of water is not only possible when the temperature is increased at a constant pressure the other possibility is to keep the temperature constant and reduce the pressure below the way / pressure water can evaporate and condensate at temperatures below 100 degree Celsius if a static pressure is low enough this is exactly what happens when cavitation occurs bubbles of water vapor form when the pressure in a certain location decreases below the way / pressure the way / condensates back to liquid once the bubble has reached a location with a pressure above the way / pressure but how is it possible that the pressure becomes very low in certain areas of a fluid let's have a look at an example for illustration if objects such as blades of a ship propeller are moved through water there is a higher pressure on the side facing the flow and a lower pressure on the opposite side on other edges exposed to the flow low pressure can also lead to cavitation the bubbles collapse as soon as they reach an environment with a pressure above the way / pressure when pumps circulate water through piping systems cavitation can occur whether pressure is lowest at the inlet of pumps and in narrow pipe sections if the pressure at the inlet of a pompous load and approaches the Weipa pressure cavitation can occur at the pump impeller as we have seen it at the ship propeller the pump can deliver less water because the cavitation bubbles occupy a part of a cross sections where the water should flow damage from collapsing vapor bubbles will be shown later the same amount of water per time flows through the pipe with a large cross-section as through the narrow cross-section therefore the velocity is higher in the narrow part and the static pressure is lower the pressure is especially load at the exposed edge at the inflow side of a narrow part we show a detail of a phase diagram of water which allows to analyze a situation in a narrow pipe section the temperature of a flowing water can be twenty degrees Celsius which is a vertical line in the diagram it crosses the way / pressure line at an absolute pressure of 2300 Pascal as long as the pressure is higher water remains liquid if the absolute pressure decreases below 2,300 Pascal water evaporates the colors in the left visualization correspond to the colors indicated on the vertical axes of a phase diagram on the right upstream of a restriction the static pressure is well above a way / pressure a static pressure in the flow decreases as a flow reaches the narrow pipe section in our example the pressure at the exposed edge is below the way / pressure consequently water evaporates and cavitation bubbles are formed the low pressure at the exposed edge causes cavitation bubbles to form at a pressure of 2300 Pascal a certain mass of water vapor requires over 50,000 times more volume than in its liquid state the bubbles collapse as soon as they reach a region with a pressure above the Weipa pressure because water from the vapor bubble occupies fifty thousand times less volume after having condensed its liquid state the collapse of each bubble is a spectacular event and emits pressure shops Prussia shops for money collapses result in an audible is very characteristic to cavitation the sound used now in the video has been recorded in an experimental setup similar to the one in animation but how does it look like when cavitation bubbles are formed and collapse in detail we can see the formation and the collapse of cavitation bubbles with a diameter of approximately nine millimeters in slow motion in real time this process takes about four milliseconds and it is a spectacular physical event because liquid water occupies many thousand times less volume than weigh / the bubble implodes it first assumes a ring shape shock waves and the strong microjet are the result of a collapse let's go through the whole process again if the local pressure decreases below the vapor pressure water evaporates a cavitation bubble is forming is growing larger and is transported with a flow to a region with a higher pressure the bubble stops growing if a local pressure exceeds the way / pressure vapor condenses starting from the bubbles wall its surface starts to break down at its weakest spot a microjet forms and pieces into the bubbles opposite surface Wabble is now collapsing the bubble becomes a torus and disintegrates further the microchip continues to flow in the liquid and can hit a wall through its concentrated impact even high strength material can be damaged the implosion releases energy in a very short time concentrated in a very small spot if the micro chat hits a surface it damages the material by a localized short-term overstraining the material starts to evolve micro cracks and later material can break off we show a few examples of damage in pump impellers kindly provided by the Swiss pump manufacturer Haney a brand new Impala to the left in comparison with an impeller having been continuously exposed to cavitation during 6 months another impeller with damage from 12 months operation in cavitation and one from 24 months the new impeller shows a geometry as it should be under normal operation without cavitation after 6 months of operation under cavitation material has been eroded in the center of the wheel the respective areas are highlighted with grey color the impeller blades are undamaged after a year of operation under cavitation the damaged geometry has a different shape and therefore the flow has a different pattern the collapsing cavitation bubbles caused material break off at the top edge of a blades after two years the blades have been eroded completely which makes it impossible for the pump to function in this video we have shown the basic mechanism leading to the formation of cavitation bubbles which would use the flow of water we have also seen the spectacular collapse of cavitation bubbles leading to noise and damage even to high strength material systems have to be designed such that cavitation is avoided

Channel: IET Institute for Energy Technology

Views: 823,538

Keywords: physics, thermodynamics, fluid dynamics, cavitation, e-learning, fluids, pumps, pipings, thermofluiddynamics, implosion, Micro Jet, Microjet, Fluid Flow, Fluid Flow Machines, Triple point, critical point, isobaric, isothermal, pressure, temperature, vapour bubbles, ship propeller, cavitation bubbles, experimental, phase diagram, pump, pressure levels, collapsing bubbles, slow motion, Micro cracks, IET, HSR, Energy Technology, Institute, diagram, phase, Liquid, solid, vapour, gas, EPFL, simulation, vapor

Id: U-uUYCFDTrc

Length: 10min 11sec (611 seconds)

Published: Mon Feb 15 2016