Sonic Cavitation in Water

Source: 20170911-infinity-supercritical-cavitation-water-review

Source: Darner, C. L. (1970). Sonic cavitation in water (No. NRL-7131). NAVAL RESEARCH LAB WASHINGTON DC.

An introduction is given to talk a little bit about water. The point being, water is quite complex and if we didn’t interact with it everyday, it’s properties compared to other liquids would be rather outstanding.

Moving past that, the point of the report is to document a way to suppress sonic cavitation in water, a problem that is of interest of many naval communities.

Originally, back in the 1700s, Euler anticipated difficulties due to the areas of low pressure caused by turbines. He saw that they could cause bubbles which lower resistance and thus thrust. Later on, we found that there was also damage to the metal turbine as well. When the steam turbine was created, these two issues arose. There was a decrease in the increase of thrust with increasing propeller shaft speed. Also, the propellers were being eaten away rather quickly. It was found that this was due to the collapsing bubbles formed due to the low- pressure areas. Indeed, pressures over 400,000 psi can be found due to cavitation.

When high-power sonar is being used, similar problems arise. Cavitation happens and efficiency drops rapidly as acoustic intensity increases. Also, there is rapid mechanical destruction of the transducer.

Pure water in this report is considered distilled water. Even after distillation, contaminates still remain. If any water is even exposed to atmosphere with no mixing, CO2 in introduced and lowers the pH to ~5.5.

There are some laboratory methods to inhibit cavitation. These include maintaining a low pressure (44 to 58 psi) or after the application and then release of pressure around 15,000 psi. Another method to inhibit cavitation is to filter micro-sized particulate or to degas the water. To investigate the degassing of the water, a large tank was built and steel was introduced to allow it to rust. A piece of wood floated at the top to stop the reintroduction of oxygen. As the iron rusted and consumed the oxygen, a sound source was put at one end and the power required to induce cavitation was recorded. As the oxygen saturation lowered (down to 35 percentage from 100), the power had to be increased.

After the oxygen saturation reached 2 percentage and the total gas content down to 67 percentage, the power required to induce cavitation still increased. The increase from just deoxygenation was around 2 fold, while after letting the water sit past this, it allowed for up to 8 fold increases in power.

It seems that resting dexoygenated water seems to lead to an increasing “strength” against cavitation, while it was not seen with “regular” water. This “resting” was found to increase the power required by up to 10 fold.

This was found to be due to hydrogen being introduced into the system through the rusting process of the steel. When the process was repeated while introducing hydrogen into the water as well, the water withstood cavitation up to 36 fold the original amount.

When investigating the cohesiveness of the “strong” water, it was found that it took 40 drops of the treated water to make 1 cubic centimeter, but only 20 drops of untreated water.

It was also found that some dilute high- molecular-weight polymer solutions help to reduce cavitation.

One such polymer, polyethylene oxide, was able to increase the cavitation resistance by up to 8 fold. This however was reversed over time, due to the fact that it continued to fall out of solution and precipitate on the glass and sound producer.

Some other ways to cause these solutions (or seen by ships navigating at sea) is through algae that exude a polysaccharide that when in the water, increases resistance to cavitation.

When these algae were added to the tank, a resistance to cavitation by up to 10 fold was found. After removing the algae and boiling off the water, a solute of 2.5 grams/liter remained. When just the compound was re- introduced, the cavitation resistance returned. Adding more solute did not change the resistance however.

Different algae seem to produce different polysaccharides, some that produce this increase in resistance of 10 fold at concentrations of 0.25 grams/liter.

The paper concludes that the deoxygenation method (which also produces hydrogen) seems to be the best method for use inside a sonar dome. The issue being that cavitation starts to occur outside the dome past the increase in resistance. Thus, the polymer produced by the algae could be used if cheap enough, since it could be slowly leaked outside the sonar dome continuously, allowing for a higher original power due to a resistance to cavitation also outside the dome.

The paper has an appendix that seems to be of interest. It goes over the physics and chemistry of water.

Surprisingly, when looking at the other hydrides of the 6th main group of elements, (H2Te, H2Se, and H2S), they produce a rather good trend for reducing boiling and freezing points. They are also all colorless, pungent, and poisonous gases. However, the extrapolated value for the boiling-freezing point for water is not only wrong, the correct values are nearly twice these values.

Another surprising quality of water is the presence of “heavy” water (deuterium oxide) which is oxygen bonded to two hydrogens, both with an extra neutron bound to the proton. This type of water is not biologically active, and has numerous different qualities. It’s in concentrations of around 150-200ppm in natural water. There is also tritium (hydrogen with two neutrons) which can also form an oxide with oxygen.

This, along with the other isotopes of oxygen, allow for a total of 18 different molecular compounds. However, most are quite low in concentrations. On top of this, there are H+ and OH- ions found in pure water.

Water can also be superheated and supercooled. These events can be seen by slowly heating or slowly cooling water free of impurities and gases. People have been able to reduce water’s temperature to -4 degrees Fahrenheit before it froze. Small mechanical bumps can shock the system into the next physical state.

Another interesting behavior is that at low temperatures, the viscosity of water decreases with increasing hydrostatic pressure, while most other fluid’s viscosity’s increase with pressure.

Possibly the most striking quality of water is it’s hydrogen bond. Due to the bend in the

molecule, the positive charges of the protons (hydrogen), and the pull from the oxygen of the electrons, a strong dipole moment is achieved. The side of the oxygen has a strong negative charge that with form a hydrogen bond with other hydrogens who have their electrons pulled.

This could be from water, or from many other molecules. This bond is what allows for the increase in the extrapolated boiling-freezing point, as each molecule of H2O is strongly attached to one another.

Ice also has some interesting qualities. Different forms of ice form under different temperatures, pressures, and other conditions. Some of these have densities that don’t float in liquid water.

End of review.

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