3rd ASME/JSME Joint Fluids Engineering

July 18, 1999, San Francisco, California

FEDSM99-6774

CAVITATION STIMULATION TECHNIQUE USING WATER-REPELLENT COATING IN A PROPELLER MODEL TEST

ABSTRACT

It is well known that cavitation extent on a propeller model is often smaller than on a full-size propeller even with the same combination of advance coefficient and cavitation number. Some reasons can be thought of such as difference in Reynolds' number, air nucleus distribution in the water, surface tension, or molecular attraction between water and a propeller blade surface. In order to stimulate cavitation, several experimental techniques have been utilized such as a trip-wire, leading edge roughness, or a hydrogen bubble method.

In this paper the authors propose a new experimental technique of spreading water-repellent coating on the tested propeller blade to avoid smaller cavities. This paper describes the effectiveness of this new technique.

The authors compared the effect on the cavitation stimulation of three types of water-repellent coating. Silicone-type and fluorine-type coating showed sufficient effect, while Teflon-type coating showed less effect. The fluorine-type coating spread very thinly and was hard enough, which is highly recommended.

The authors also showed how this technique has influence on the propeller characteristics, and concluded that this new technique is very useful to simulate the full-scale cavitating propeller.

INTRODUCTION

With the high speed and shallow draft of ships in recent years, cavitation on ship propellers has become a problem that has a large influence upon propeller performance. In order to increase the ship speed, it is essential to predict the characteristics of a propeller under the cavitating condition. One major way to investigate the propeller characteristics is a model test in a cavitation tunnel, in which it is important to control the cavitation based on grasping the fluid dynamical situation around the propeller model. This is indispensable in order to develop propellers with high efficiency and safety for high speed vessels.

In a usual propeller cavitation test, in order to simulate a full-scale propeller, similarity laws of the flow geometry and the pressure will be satisfied by adjusting two non-dimensional parameters, the propeller advance ratio J and the cavitation number , between the model and the full-scale. Under some test conditions, however, the cavity extent on the propeller model is different from that on the full-scale propeller in spite of adjusting J and . A typical case is that sheet cavitation is observed on the full-scale propeller while less cavitation such as bubble cavitation or streak cavitation occurs on the model, or the model is fully wetted. In this case, the pressure on the non-cavitating part of the propeller model might become less than the vapor pressure so that the thrust coefficient and torque coefficient tend to be larger than those of the full-scale propeller.

Because a model test is just a simulation of the full-scale phenomena, there are several reasons for this difference, among which Reynolds number, air nucleus distribution, surface tension and molecular attraction between water and a propeller blade surface are considered to affect the test result, especially the cavitation. Due to these influences, cavity extent on the propeller model generally tends to be less than that expected to simulate the full-scale. In order to stimulate cavitation, the following experimental techniques have been adopted.

A. Trip-wire method (Yokoo, 1960)

This method makes cavitation occur behind a thin wire fixed on the propeller blade surface near the leading edge. This method tends to make a cavity thicker and the torque larger.

B. Leading edge roughness method (Wills, 1986)

This method makes cavitation occur behind fine carbon particles pasted on the propeller blade surface near the leading edge. Similar to the trip-wire method, this leads to a thicker cavity and larger torque.

C. Hydrogen bubble method (Ukon, 1982)

This method electrically supplies hydrogen nuclei to the propeller disk region. Although this method successfully simulates the full-scale cavitation in the propeller cavitation test behind a ship model set in a cavitation tunnel, few examples were reported in a uniform flow.

In the present paper, the authors propose a new method that is spreading water-repellent coating on a propeller blade surface. By this method, two effects are expected; one is hastening cavitation occurrence by reducing the molecular attraction between water and the blade surface and the other is prevention of streak cavitation by increasing the contact angle between the cavity surface and the blade surface.

NOMENCLATURE

D Diameter

J Propeller advance ratio

v Cavitation number based on velocity

MODELS AND INSTRUMENTATION

Propeller Models

One 3-bladed propeller (D=250mm) and one 4-bladed propeller (D=194.4mm) were selected for the present test. Both showed typical streak cavitation in past cavitation tests.

Instrumentation

The test was carried out in the First Working Section (D=750mm) of the Large Cavitation Tunnel of Ship Research Institute. The main dynamometer (K&R J26) was used for force measurement.

Water-Repellent Coating

Three types of water-repellent coatings, silicone-type, fluorine-type and Teflon-type coatings, were tested. These are usually sold for car maintenance and are easily obtained.

EXPERIMENTS AND RESULTS

Cavitation Observation

In order to test the effect of each water-repellent coating, the coatings were spread on individual blades of the 3-bladed propeller and tested twice as follows.

Blade A Blade B Blade C

Test 1: no coat silicone fluorine

Test 2: no coat fluorine Teflon

In Test 1, the fluorine-type coating came off and showed no effect after 10 minutes from the beginning of the test. Later however, it became clear that the preparative cleaning of the blade surface before spreading was not good enough, so the fluorine-type coating was tested again in Test 2. The durability of each coating was also confirmed in Test 2 by continuous running for 2.5 hours under the cavitating condition. In addition to these tests, cavitation was observed after r/R lines had been painted on the coating to confirm that these lines did not affect the cavity.

Fig. 1 Cavity on an uncoated blade (J=1.546, v=0.25	Fig. 2 Cavity on a blade coated with silicone-type water-repelent
Fig. 3 Cavity on a blade coated with fluorine-type water-repelent	Fig. 4 Cavity on a blade coated with Tefron-type water-repelent

Photographs of both tests are compared in Figs. 1 to 4. The measuring conditions were J=1.546 and v=0.25. Compared with the uncoated blade (Fig. 1), blades coated with water-repellent coatings had more and thinner streak cavities and a larger cavity extent. The effect of the Teflon-type coating seemed smaller than the other coatings.

Fig. 5 Cavity on an uncoated blade (J=1.546, v=0.50)	Fig. 6 Cavity on a blade coated with fluorine-type water-repelent

In Figures 5 and 6, photographs of cavitation on the uncoated and coated blades are shown, respectively. Several cavities were observed sparsely near the trailing edge on the uncoated blade, foaming cavitation occurred densely on the coated blade as observed often on a full-scale propeller. One of the reasons for this difference was considered to be the difference in the number of minute cavities at the leading edge.

Fig. 7 Cavity on an uncoated blade (J=0.916)	Fig. 8 Cavity on a blade coated with silicone-type water-repelent

In order to investigate the effect of the water-repellent coating upon propeller characteristics, the 4-bladed propeller was tested with all blades coated with the silicone-type water-repellent coating. Photographs of cavitation on an uncoated or coated propeller is shown in Figs. 7 and 8, respectively, and the measured thrust coefficient and efficiency in Fig. 9. When the propeller was uncoated, there was a hysteresis at J=0.916 as shown in Fig. 9. The thrust coefficient at J=0.916 varied depending on whether J=0.916 was approached from above or below; when approached from below the thrust coefficient was less than when approached from above. Figure 7 was taken when the thrust coefficient showed the higher value. Cavitation extent was intermediate between Fig. 7 and Fig. 8 when the thrust coefficient was the lower value.

Fig. 9 Propeller Characteristics

The hysteresis did not exist with the coated propeller.

A large number of steady cavities were observed and the thrust coefficient was lower than that of an uncoated propeller. This appearance is considered similar to the full-scale propeller.

Under non-cavitating conditions in the region of high J and supercavitating conditions in the region of low J, no effect of water-repellent coating was found.

CONCLUSIONS

It was confirmed through the present tests that three types of water-repellent coatings had the effect of cavitation stimulation in a cavitation test of a propeller model. The properties of each coating are:

(1) Silicone-type water-repellent coating is thicker than the others so that it affects the shape of a propeller blade and there is a little difficulty with marking r/R lines, while it is easy to spread and treat.

(2) Fluorine-type water-repellent coating needs careful cleaning, especially cleaning of oil, on a propeller blade surface before spreading the coating, while the coating is very thin and there is no problem with marking the r/R lines. This is suitable for a cavitation test.

(3) Teflon-type water-repellent coating is strong but has less effect than the others.

Among the three coatings, the fluorine-type coating is considered most adequate and is recommended for a cavitation test. Another method such as a water-repellent plate should be studied in the future.

From the present test results, it was concluded that the differences in cavitation extent sometimes affects the propeller characteristics and that the water-repellent method is effective for cavitation stimulation in order to simulate the full-scale propeller cavitation.

ACKNOWLEDGMENTS

This research was carried out with help of Mr. Matsuda and Mr. Fujisawa in the Ship Research Institute.

REFERENCES

Yokoo, Koichi and Kitagawa, Hiromitsu, Some Scale Effect Experiments on Propeller, Report of Transportation Technical Research Institute, No. 43, 1960

Wills, C. B. and Ball, W. E., A Study into the Effect of Artificial Stimulation of the Boundary Layer on Model Propeller Cavitation Performance, ARE TR86303, 1986

Ukon, Yoshitaka, et al., Pressure Fluctuation Induced by Cavity Volume on Highly Skewed Propeller for a Ro/Ro Ship, Report of Ship Research Institute, Vol. 19 No. 3, 1982