캐비테이션의 이해 캐비테이션의 형성, 붕괴 및 결과

유체에서 캐비테이션 기포가 형성되고 붕괴되어 펌프 임펠러와 같은 재료에 소음과 심각한 손상을 일으킵니다.

### 캐비테이션의 이해 유체 역학의 원인, 효과, 결과: 캐비테이션의 이해
* 💧 캐비테이션은 액체 내에서 증기 기포가 빠르게 형성되고 붕괴되는 현상으로, **정압이 액체의 증기압 아래로 떨어질 때** 발생합니다.
* 📊 물의 상도는 온도와 압력에 따른 물의 상태(고체, 액체 또는 기체)를 보여주며 **증기압 곡선은 증발 및 응축 조건**을 나타냅니다.
* 🌡 절대 1bar의 일정한 압력에서 물은 **0°C에서 얼음에서 액체로 녹고 100°C 이상의 온도에서 증발**하지만, 압력이 충분히 낮으면 **100°C 이하에서도 증발**할 수 있습니다.
* 🚢 선박 프로펠러 블레이드와 같은 물체가 물속을 이동할 때 고압 및 저압 영역이 생성되어 **저압 영역에서 기포가 형성**되고 **고압 영역에서 붕괴**되면서 캐비테이션이 발생합니다.
* 🔍 펌프에서 캐비테이션의 예를 볼 수 있는데, **입구와 좁은 파이프 섹션의 낮은 압력**으로 인해 증기 기포가 형성되어 이러한 기포가 흐름 공간을 차지하면서 펌프의 효율이 저하되는 현상입니다.
* 📉 좁은 파이프 섹션의 압력 강하로 인해 물이 증발하여 기포가 형성될 수 있으며, **기포는 증기압 상태에서 액체 물보다 50,000배 이상의 부피**를 차지할 수 있습니다.
* 💥 이러한 기포의 붕괴는 **충격파와 마이크로제트**를 발생시켜 캐비테이션 노출로 인해 시간이 지남에 따라 펌프 임펠러에서 관찰되는 침식 현상에서 알 수 있듯이 재료에 심각한 손상을 일으킬 수 있습니다.
* 🛠 캐비테이션 손상을 방지하려면 유체 흐름 중에 증기 기포가 형성되지 않도록 **압력 조건을 유지**하도록 시스템을 신중하게 설계해야 합니다.

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 vapor 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, while its vertical axis shows pressure. The vapor pressure curve is part of a phase diagram. It indicates at which pressures and temperatures water evaporates from liquid to vapor or condenses from vapor 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 degrees 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-section 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 low 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 2300 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. Static pressure in the flow decreases as 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 50000 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 impeller 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 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.

Cavitation - Easily explained!

IET Institute for Energy Technology

캐비테이션은 액체에서 증기 기포가 빠르게 형성되고 붕괴되는 현상으로, 정압이 액체의 증기압 아래로 떨어질 때 발생합니다.

물의 상 다이어그램은 온도와 압력의 관계를 보여주며, 다양한 조건에서 물이 어떻게 증발하거나 응축될 수 있는지 보여줍니다.

캐비테이션은 종종 유체를 통과하는 물체가 저압 영역을 만들어 증기 기포를 형성하기 때문에 발생합니다.

펌핑 시스템에서 캐비테이션은 입구와 좁은 파이프 구간에서 압력이 가장 낮을 때 발생합니다.

좁은 영역에서 물의 속도가 빠르면 정압이 낮아져 증발과 캐비테이션 기포가 형성됩니다.

이러한 기포는 압력이 높은 영역에서 붕괴되어 잠재적으로 펌프를 손상시키고 물의 흐름을 감소시킵니다.

직경 약 9밀리미터의 캐비테이션 버블은 4밀리초 만에 형성되고 붕괴되는 장관을 연출하며 압력 충격과 특유의 소리를 내뿜습니다.

기포는 저압 영역에서 성장한 후 고압 영역에서 격렬하게 붕괴하면서 충격파와 마이크로제트를 발생시켜 고강도 소재도 손상시킬 수 있습니다.

캐비테이션에 장시간 노출되면 펌프 임펠러에 심각한 손상이 발생할 수 있으며 시간이 지남에 따라 눈에 보이는 미세 균열과 재료 파손이 발생할 수 있습니다.

캐비테이션이 펌프 작동에 미치는 영향

캐비테이션은 휠 중앙의 재료 침식으로 이어지며, 처음에는 임펠러 블레이드가 손상되지 않은 상태로 유지됩니다.

시간이 지남에 따라 손상된 형상과 흐름 패턴이 변경되어 블레이드의 상단 가장자리에서 재료가 끊어짐니다.

결국 2년이 지나면 블레이드가 완전히 침식되어 펌프가 작동하지 않게 되고, 캐비테이션을 방지하도록 시스템을 설계해야 할 필요성이 강조됩니다.