Submerged Washing - Technical Advices

In water jet washing, the method of washing a target in the atmosphere is ordinarily used. Depending on the purpose of washing, submerged washing is employed.
We call our developed method of submerged washing "U-Jet" and have had it on the market since 1985. The following describes the effects of submerged washing.

Injecting a water jet underwater greatly drops the jet power with the resistance of the surrounding water.

Figure 1 shows the relationship between the distance from the nozzle outlet and the jet injection pressure for submerged and atmospheric injection. In the atmosphere, jetting is not attenuated as long as it is kept within about 100 mm from the nozzle outlet.

At the submerged injection the jetting force begins to be attenuated when the distance from the nozzle outlet attains about 20 mm. In a distance of about 90 mm, the jetting force is dropped by 1/2. Like this, the jetting force drops greatly with the water resistance. But cavitation is generated between the jet and surrounding water.
Ultrasonic washing has broad uses for washing semiconductors and precision machine parts utilizing cavitation.

Figure1. Comparison of injection force

Cavitation means an event where the liquid internal pressure drops below the saturated vapor pressure and produces fine air bubbles. In submerged jetting, a vortex is produced between the jet and surrounding water. At the center of the vortex, the pressure drops like a hurricane and produce cavitation. Minute air bubbles generated by cavitation in large amounts. Minute air bubbles are forced to flow downstream. With the increase of the surrounding pressure, they are momentarily squeezed.

The impact pressure at that time attains several GPa, and produces noise attacks the surface of an object located downstream of the jet. In submerged washing, multiplies the washing force of cavitation and the injection force of the jet.

Submerged jet injection produces cavitation. Our originally-developed "horn type nozzle" efficiently produces cavitation in spite of its simple configuration.
This nozzle has been tapered in a shape that is widened toward the end. Generation of a vortex around the jet is accelerated by the water jetting into the taper to produce cavitation efficiently.(See Figure 2.)

In addition, it has been reported that the cavitation clouds where minute air bubbles are collected at the downstream has a motive force greater than one air bubble.

Figure2. Horn type nozzle

Figure 3 shows the amounts of brick erosion when submerged injection using the hone type nozzle and the conventional nozzle with a flat nozzle outlet. The conditions are 1mm in nozzle diameter, 30 MPa in pressure, and 10 seconds in the injection time.

It is presumable that the horn type nozzle is 10 times greater amount of erosion than the conventional nozzle to produce cavitation efficiently. In addition, the maximum erosion amount is obtained when the distance between the nozzle and sample is 40 to 50 times longer than the nozzle diameter. Our submerged washing machine uses this hone type nozzle.

Figure3. Comparison of erosion amounts

In addition, Photo 1 shows the efficiency of submerged injection using the horn type nozzle. When submerged and atmospheric injection are generated against the brick, submerged injection penetrates the brick and the hole diameter is 2 times greater than the atmospheric injection's. It is shown that submerged injection is superior to atmospheric injection. The conditions of test are 1.5mm nozzle diameter, 30 Mpa in pressure, 10 seconds in the injection time and the distance between the nozzle and sample is 30mm.

Like this, submerged injection using cavitation with the horn type nozzle demonstrates excellent washing force that is more than atmospheric injection.

Submerged washing		Atmospheric washing

A crack is generated with generation of a through hole.
Photo1. Brick erosion test


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