Analysis:

steam

What about water? A small amount of air is usually dissolved in water, and further, the molecular groups of water can be converted into gas-water vapor. At this point, it is no different from beer. The difference is that water molecules will turn into gas at low pressure, producing bubbles, which are called vacuoles, also known as holes. The diameter of air cavity is sometimes as small as 10-5 cm. Don't underestimate this humble hole. It was and still is a terrible obstacle to navigation.

1894, when Brave, a small destroyer with a British tonnage of 240 tons, made its first sea trial, the propeller speed could only reach 384 revolutions, which was 1.54% lower than the rated design speed. After several debugging, it was not until 1897 that chief engineer Barnaby published a paper at the meeting of shipbuilding engineers, indicating that the initial effect was not good.

Fortunately, the propeller has cavitation. Twenty years later, 19 15, the new British torpedo boat "Linde" sailed into the Atlantic Ocean for trial. Its design speed is twice that of the previous model, but when the ship works at the maximum speed, the stern shakes and the seawater foam at the tail churns.

For example, when pouring beer, the speed is the same as the previous one. When the torpedo boat returned to the base, the propeller was in tatters. This is another bubble that creates trouble. It was not until 197 1 that thousands of ships were investigated, and it was found that after one year of use, 30% of propellers were damaged to varying degrees due to cavitation.

In order to study the mechanism and function of cavitation, people have started theoretical and experimental research since the last century. 1895, Britain built a small water tunnel dedicated to cavitation research, and then in the 1920s and 1930s, Britain, Germany, France, the Soviet Union, the United States and other countries successively built larger cavitation water tunnels. At the same time, theoretical research has also made corresponding progress.

Why does high-speed water bubble? It turns out that water boils at standard atmospheric pressure (one atmospheric pressure is equivalent to 10 1325Pa) and the temperature reaches 100℃. "Boiling" is a phenomenon that bubbles can appear in water. The boiling pressure of water is different at different temperatures, which is called saturated vapor pressure, also called vapor pressure. The saturated vapor pressure of water at different temperatures is:

Temperature/℃

Steam pressure/pa 600.662338.17381.2149344737101325198490.

Thus, when the pressure is 2338. 1Pa, water will boil at 20℃, and this phenomenon of boiling at room temperature can be called "cold boiling". On the plateau above 4000 meters above sea level, it is not easy to cook there because of the low air pressure and the boiling point of only 86℃. When the pressure reaches 198490 Pa, that is, less than two atmospheres, water does not boil until 120℃, which is almost the pressure of an ordinary pressure cooker.

As mentioned above, the high-speed movement of fluid will reduce the local pressure, especially when high-speed ships, propellers and torpedoes move in the water, which will cause the local water pressure to be very high and reach the steam pressure at room temperature. This is the reason why bubbles are produced in high-speed sailing water.

Once cavitation occurs, the resistance will increase and the generation of bubbles will consume a lot of energy. So the speed of the ship can't go up any more. If special measures are not taken to solve the cavitation problem, the speed of most large ships will not exceed 26 knots (about 14 m/s).

However, the harm of cavitation to navigation does not stop there. The problem is that after cavitation is formed in the low pressure area, it flows to the high pressure area with the water flow, where the pressure increases and bubbles cannot exist and close. The closure of bubbles will cause high pressure similar to explosion, even reaching 100000 atmospheric pressure. Under this atmospheric pressure, any metal material will be destroyed, so the propeller will soon be bitten by cavitation. Similar problems occur in large hydropower stations and dams. The flow velocity in the spillway tunnel is very high, and bubbles will erode the tunnel wall. The turbine blades of the hydropower station will be used in a few days.

Within dozens of millimeters, it was eaten by blisters.

Drops of water wear through the stone, and uninterrupted drops of water can wear through the stone. At first, people thought it was caused by the long-term erosion of water, but it was actually due to cavitation. With the development of high-speed cameras, some people "land" water droplets with 1500 cameras per second.

Location. The water drops flatten from a circle and then disperse. At this time, some local velocities near the center of the droplet are quite large enough to reach the low pressure of cavitation. So the cavitation gradually eroded the hard stone. In the flowing river, the running water beats against the rock bank. "The rock passes through the air,

The stormy waves broke through the coast. I am afraid that this effect of water is also caused by cavitation.