Jeff Doersch Krystle Ervin K evin Fennelly. Dr P.Y. J ulien CIVE

Transcription

1 Jeff Doersch Krystle Ervin K evin Fennelly Dr P.Y. J ulien CIVE

2 The formation and collapse of small air bubbles in a liquid. This vapor collapse occurs almost instantaneously after evaporation takes place. Can cause serious damage to pumps, propellers, and other components of a hydraulic system. Figure 1. Cavitation damage to a Francis turbine

3 Reduced pump capacity. Reduction in the head of the pump. Formation of bubbles in a low pressure area of the pump. A noise that can be heard when the pump is running. Damage that can be seen on the pump impeller. Figure 2. Process of cavitation in an impeller

4 Cavitation is experienced when a liquid s pressure (p) at a specific location falls to the vapor pressure (p v ) of the liquid. Result: Vapor bubble formation and collapse (a fraction of a second later) The Bernoulli Equation (right) suggests that total energy head at two points in a hydraulic system (i.e. 1 and 2) must be constant in order to maintain equilibrium. pressures at different points in a system fluctuate as velocities and elevation change, conditions suitable for cavitation can be created Bernoulli Equation: Eqn. 1 V 1 = velocity at point 1 p 1 =pressure at point 1 z 1 = elevation at point 1 V 2 = velocity at point 2 p 2 = pressure at point 2 z 2 = elevation at point 2 = density of fluid g = acceleration due to gravity

5 1. Pressure drop due to increased velocity Fundamental fluid mechanics principles tell us that the velocity (V) of a flowing fluid will increase as its cross-sectional area (A) decreases. Fluid travelling at a very high velocity through a small area will have very little pressure. Relationship evident from the Bernoulli Equation, previous slide Careful thought must be put into design of pumps, turbines, propellers, and other hydraulic elements such that cavitation conditions will not occur.

6 2. Pressure drop due to increase in elevation. Atmospheric pressure (p atm ) is inversely proportional to elevation gain (i.e. as elevation increases, p atm will decrease). Example: a pump in a hydraulic system in Denver, Colorado (elev ft) is more likely to experience cavitation than that same pump and hydraulic system in Miami, Florida (sea-level).

7 The phase change from liquid to vapor is significant. Why? Because water vapor molecules have much higher energy levels compared to liquid water. When boiling occurs, vapor bubble burst, causing a shock wave. Effects are minimal because: Energy is released in all directions above the water surface. Pressure in the bubble is only slightly higher than atmospheric pressure. Only one phase change occurs. Figure 3. Nondestructive release of energy from boiling water

8 Imploding vapor bubbles release much more energy. Collapsing bubbles change back into the liquid state upon implosion. Water molecules rush to fill the void, resulting in high energy molecular collisions and a high energy shockwave. Larger bubble means larger quantity of molecules to collide and more energy release. Figure 4. Process leading to the air bubble collapse

9 There are two phases of Cavitation Incipient cavitation and desinent cavitation Incipient Cavitation The state when cavitation first begins as the pressure approaches vapor pressure Desinent Cavitation Once the pressure is past the vapor pressure Supercavitation Supercavitation is a point in desinent cavitation where instead of lots of smaller bubbles there is one large bubble

10 The cavitation index is used to describe the magnitude of cavitation The smaller the cavitation index the more likely cavitation will occur Cavitation Index: Eqn. 2 σ = P o = P v = ρ = U o = Cavitation Index Pressure at a point Vapor Pressure Density Velocity

11 Maximum damage from from the release of energy always occurs downstream from the initial cavitation The distance downstream energy is released can be calculated by Cavitation Cloud Length: Eqn. 3 L k /H = 2(σ s / σ) 2.63 L k = H = σ = σ s = Length of Cavitation Cloud Characteristic Dimension Cavitation Index Cavitation Index where damage begins

12 Cavitation also occurs in natural systems, typically where the stream bed roughness or slope changes. Figure 5. Cavitation created naturally from hydraulic vortex

13 Flow depths greater than 10 meters Intake tunnels High capacity pumps Pumps set to deliver more water than received Figure 6. Cavitation resulting from a submarine torpedo (large flow depth). e582144dde7d jpg

14 Increase pressure of overall system Multi-stage control valves in locations of large pressure drop Routine inspection of pipe system Reducing temperature of water in pipe Increase velocity of the fluid Increasing upstream roughness to gradually drop pressure

15 Various universities across the world study cavitation; both its potential uses and its damaging effects. Cavitation tunnels (shown on the right), can be used to test propellers and control surfaces. Bubble Dynamics chambers are used to study the behavior of cavitation bubbles to better understand and model cavitation. Figure 7. Illustration of cavitation tunnel at University of Tasmania

16 In biomedical practices cavitation is used on kidney stones This is done using shock wave lithotripsy Ultrasounds can create cavitation results in side effects such a nausea, headache, dizziness, and fatigue Cracking of joints Expanding of the joint space causes a decrease in pressure causing cavitation

17 Cavitation is used to mix, and breakdown suspended solids in different mixtures This is done by either using a propeller or forcing the liquid through a narrow opening with a large exit therefore causing cavitation Cavitation can also be used to force chemicals together Protein Folding Figure 8. Illustration of of cavitation potential in mixing application b8d-ab2d7a4d06b8_00caa634-bf12-421c-83e4-dbc4323c7701.jpg

18 Cavitation can be used to breakdown toxic compounds. A study done on the benefits of cavitation on mature landfill leachate showed a 22% increase in Biodegradability Index. Hydrodynamic cavitation can be used on a larger, more energy efficient scale compared to acoustic cavitation for water treatment. Figure 9. Landfill leachate

19 Cavitation is a hydraulic phenomenon that can have detrimental effects to machinery and other elements, but has a variety of advantageous applications in chemical and biomedical practices. Different forms and consequences of cavitation can occur based on factors such as the pressure, velocity, and elevation. With proper engineering design and fabrication, the risk of cavitation can be reduced to an acceptably low level.

20 Cavitation - An Introduction. The Engineering Toolbox. N.p., n.d. Web. 20 Oct Bis, M, Montusiewicz, A, Ozonek, J, Pasieczna-Patkowska, S. Application of hydrodynamic cavitation to improve the biodegradability of mature landfill leachate. Ultrasonics Sonochemistry. Sept Web 21 Oct Evans, Joe. "When Bubbles Don't Burst: Why Cavitation Is Damaging."When Bubbles Don't Burst: Why Cavitation Is Damaging. N.p., n.d. Web. 19 Oct Fundamentals Cavitation and Bubbly Flows Group at St. Anthony Falls Laboratory. (n.d.). Retrieved October 21, "Madera6 - Boiling Point by Dana Grisham." Madera6. N.p., n.d. Web. 19 Oct The Effect of Cavitation. McNally Institute. N.p., Sep Web 20 Oct Unsworth, A., Dowson, D., & Wright, V. (n.d.). 'Cracking joints'. A bioengineering study of cavitation in the metacarpophalangeal joint. Retrieved October 22, 2015.