The Basic Research for Pulverization of Rice Using Underwater Shock Wave by Electric Discharge

Makoto Ide1,*, Manabu Shibuta1, Naoyuki Wada1, Shigeru Tanaka2, Hideki Hamashima3, Shigeru Itoh4 1 Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto City, Kumamoto 860-8555, Japan 2 Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto City, Kumamoto 860-8555, Japan 3 Kumamoto Industrial Research Institute, Kumamoto, Japan 4 Okinawa National College of Technology, Kumamoto, Japan


INTRODUCTION
In recent years, the food self-support rate of Japan is 40%, and this value is the lowest level in major developed countries.This reason includes decreasing of diverting rice consumption in Japan and increasing abandonment of cultivation.Therefore, these problems are solved by using rice powder instead of expensive flour, and we manage to increase the food selfsupport rate.
Previously, the rice powder is manufactured by two methods.One is dry type, and the other is wet type.The former is the method getting rice powder by running dried rice to rotating metal, and has a problem which that starch is damaged by heat when processing was performed.The latter is performed same method against wet rice, and has a problem which a large quantity of water is used.As a method to solve these problems, an underwater shock wave is used.Shock wave is the pressure wave which is over speed of sound by discharging high energy in short time.Propagating shock wave in water is underwater shock wave.The characters of underwater shock wave are long duration of shock wave because water density is uniform, water is low price and easy to get and not heat processing.
Thinking of industrialization, the electric discharge is used as the generating source of underwater shock wave in the experiment.As the results, the efficiency of obtaining enough grain size, 100µm, of rice powder was too bad only using the simple processing using underwater shock wave.Therefore, in Okinawa National College of Technology collaborating with us, obtaining rice powder with higher efficiency by using converged underwater shock wave is the goal of this research.
In this research, the underwater shock wave with equal energy of the experimental device of underwater shock wave is measured by the optical observation.In addition, the appearance converging underwater shock wave is simulated by numerical analysis, and the pressure appreciation rate between the first wave and converged underwater shock wave is calculated by using the pressure history of 2nd focal point.

EXPERIMENTAL METHOD 2.1 THE OPTICAL OBSERVATION OF THE DISCHARGE PHENOMENON
The experimental outline is shown in Fig. 1.Cu-W which is difficult to melt by electric discharge is used for electrodes, and distance between electrodes (Gap) is able to be changed by fixing nut.Polyoxymethylene and polyethylene, strong against shock wave, are used to the other parts.By changing the Gap into 1.0mm, 2.0mm, 3.0mm, 4.0mm and performing the experiment until succeeding five times, the comparison about these velocity and pressure.
The photograph and spec of the capacitor used in Okinawa National College of Technology are shown in Fig. 2, Table 1.The photograph and spec of the capacitor used on this experiment are shown in Fig. 3, Table 2.In this experiment, charge voltage is configured 28kV for adjusting charge energy to the value (4.9kJ) of Okinawa National College of Technology experiment.This experimental condition is shown in Table 3.    Fig. 4 shows the optical observation using shadowgraph method.This is the observation method of shadow by density change of medium.By this, underwater shock wave velocity and pressure can be calculated.The specs of high speed camera, HPV-1, is shown in Table 4.

NUMERICAL ANALYSIS
The numerical analysis model of underwater shock wave converging using LS-DYNA3D is shown in Fig. 7.Because the purpose of numerical analysis is calculation of pressure appreciation rate on 2 nd focal point, 1/5 model is used.The numerical analysis condition is shown in Table 4.When the model of device having ellipse shape is made, Solid Works (manufactured by Solid Works Japan Co., Ltd.) is used for making IGES data.After that, Hyper Mesh (manufactured by Altair Co., Ltd.) is used for meshing of the IGES data.According to Fig. 9 and 10, the underwater shock wave generating could be confirmed because all velocities is over the sonic speed in water,1450m/s,.And, the highest velocity and pressure was obtained in case of experimental No.2.Herewith, it is thought that the best Gap is 2mm when the charge voltage is 28kV.There were a lot of energy losses to water on No.3 condition because the condition was boundary whether to discharge.In addition, uneven to the whole of the value, it is thought that the energy losses to water or not discharging of all charged voltage are caused.

ABOUT CONVERGING EXPERIMENT
The framing photographs of converging experiment are shown in Fig. 11.According to these photographs, it is found that the underwater shock wave generated from 1 st focal point manage to converge on 2 nd focal point.However, it was not possible to judge accurately because the detonation gas prevents the reflecting underwater shock wave from converging.

THE RESULT OF NUMERICAL ANALYSIS
The result of numerical analysis is shown in Fig. 12.According to this result, it is confirmed that the underwater shock wave generated from 1 st focal point by detonating SEP converged twice on 2 nd focal point.Additionally, the time history graph of 2 nd focal point is shown in Fig. 13.The pressure value of 1 st shock wave was 37.1MPa.The pressure value of 1 st converging shock wave was 96.3MPa whose value was 2.59 times bigger than that of 1 st wave.The pressure value of 2 nd converging shock wave was 170MPa whose value was 4.57 times bigger than that of 1 st wave.

Figure 1 Figure 2
Figure 1 Experimental device for electric discharge

Figure 3
Figure 3 The capacitor of this experiment

118Figure 5
Figure 5 Experimental device for the optical observation of underwater shock wave converging

Figure 7
Figure 6 The detail of experimental device

Figure 10 Figure 11 Figure 13
Figure 10 Pressures of underwater shock wave by electric discharge

Table 1
Specs of this capacitor

Table 2
Specs of this capacitor

Table 3
Experimental conditions

Table 4
4. THE RESULTS OF THE OPTICAL OBSERVATION 4.1 ABOUT ELECTRIC DISCHARGEThe framing photographs of No.1 at 5µm intervals are shown in Fig.8.Herewith, underwater shock wave generating was confirmed.