تكهف: الفرق بين النسختين
| [نسخة منشورة] | [مراجعة غير مفحوصة] |
تم حذف المحتوى تمت إضافة المحتوى
ASammourBot (نقاش | مساهمات) ط روبوت: +تصنيف:ميكانيكا الموائع +تصنيف:هيدروليكا +تنسيقات تجميلية |
وسم: تعديل مصدر 2017 |
||
سطر 11:
== التكهف اللاعطالي ==
==
==Cavitation damage{{anchor|Cavitation erosion}}==
[[Image:Turbine Francis Worn.JPG|thumb|right|250px|Cavitation damage to a [[Francis turbine]].]]
Cavitation is, in many cases, an undesirable occurrence. In devices such as [[propeller]]s and [[pump]]s, cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency. Cavitation has also become a concern in the renewable energy sector as it may occur on the blade surface of [[Tidal stream generator|tidal stream turbines]].<ref>{{cite journal |last=Buckland H. C. |first2=Baker T. |last2=Orme J. A. C. |first=Masters I. |doi=10.1177/0957650913477093 |year=2013 |title=Cavitation inception and simulation in blade element momentum theory for modelling tidal stream turbines |journal=Institution of Mechanical Engineers, Part A: Journal of Power and Energy |volume=227 |issue=4 |pages=479–485 }}</ref>
When the cavitation bubbles collapse, they force energetic liquid into very small volumes, thereby creating spots of high temperature and emitting shock waves, the latter of which are a source of noise. The noise created by cavitation is a particular problem for military [[submarine]]s, as it increases the chances of being detected by [[passive sonar]].
Although the collapse of a small cavity is a relatively low-energy event, highly localized collapses can erode metals, such as steel, over time. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller or pump's lifetime.
After a surface is initially affected by cavitation, it tends to erode at an accelerating pace. The cavitation pits increase the turbulence of the [[fluid flow]] and create crevices that act as nucleation sites for additional cavitation bubbles. The pits also increase the components' surface area and leave behind residual stresses. This makes the surface more prone to [[stress corrosion cracking|stress corrosion]].<ref>{{Cite book |author1=Stachowiak, G.W. |author2=Batchelor, A.W. | title=Engineering tribology | year=2001 | publisher=Butterworth-Heinemann | location=Boston | isbn=0-7506-7304-4 | page=525}}</ref>
===Pumps and propellers===
Major places where cavitation occurs are in pumps, on propellers, or at restrictions in a flowing liquid.
As an impeller's (in a pump) or propeller's (as in the case of a ship or submarine) blades move through a fluid, low-pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blade moves, the lower the pressure around it can become. As it reaches [[vapor pressure]], the fluid [[vaporization|vaporizes]] and forms small [[liquid bubble|bubble]]s of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shock waves in the fluid, which may be audible and may even damage the blades.
Cavitation in pumps may occur in two different forms:
====Suction cavitation====
[[Suction]] cavitation occurs when the pump suction is under a low-pressure/high-vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump, where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition can have large chunks of material removed from its face or very small bits of material removed, causing the impeller to look spongelike. Both cases will cause premature failure of the pump, often due to bearing failure. Suction cavitation is often identified by a sound like gravel or marbles in the pump casing.
In automotive applications, a clogged filter in a hydraulic system (power steering, power brakes) can cause suction cavitation making a noise that rises and falls in synch with engine RPM. It is fairly often a high pitched whine, like set of nylon gears not quite meshing correctly.
====Discharge cavitation====
Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller, it must pass through the small clearance between the impeller and the pump housing at extremely high flow velocity. This flow velocity causes a vacuum to develop at the housing wall (similar to what occurs in a [[Venturi effect|venturi]]), which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump housing. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.
Discharge cavitation in joint fluid is thought to cause the popping sound produced by bone [[Cracking joints|joint cracking]], for example by deliberately cracking one's knuckles.
== التطبيقات في الهندسة الكيميائية ==
| |||