Qiaojue Liu

Generation of Electrohydraulic Shock Waves by Plasma-Ignited Energetic Materials: I. Fundamental Mechanisms and Processes

Shock waves in water have many applications, such as lithotripsy, sterilization, weapons, and so on. Underwater electrical wire explosion can be utilized to produce shock waves with fast front and short duration time efficiently. In order to amplify the energy of shock waves, energetic materials (EMs) were considered. In this paper, Al, Cu, Mo, and W wires with EM covers were designed and tested. The results showed that wire explosion ignited the EMs, which in turn affected the process of wire explosion. As a result, the duration time of shock waves was elongated.

Signal Analysis and Waveform Reconstruction of Shock Waves Generated by Underwater Electrical Wire Explosions with Piezoelectric Pressure Probes

Underwater shock waves (SWs) generated by underwater electrical wire explosions (UEWEs) have been widely studied and applied. Precise measurement of this kind of SWs is important, but very difficult to accomplish due to their high peak pressure, steep rising edge and very short pulse width (on the order of tens of μs). This paper aims to analyze the signals obtained by two kinds of commercial piezoelectric pressure probes, and reconstruct the correct pressure waveform from the distorted one measured by the pressure probes. It is found that both PCB138 and Müller-plate probes can be used to measure the relative SW pressure value because of their good uniformities and linearities, but none of them can obtain precise SW waveforms. In order to approach to the real SW signal better, we propose a new multi-exponential pressure waveform model, which has considered the faster pressure decay at the early stage and the slower pressure decay in longer times. Based on this model and the energy conservation law, the pressure waveform obtained by the PCB138 probe has been reconstructed, and the reconstruction accuracy has been verified by the signals obtained by the Müller-plate probe. Reconstruction results show that the measured SW peak pressures are smaller than the real signal. The waveform reconstruction method is both reasonable and reliable.

Generation of Electrohydraulic Shock Waves by Plasma-Ignited Energetic Materials: III. Shock Wave Characteristics With Three Discharge Loads

As an important plasma-assisted technology to generate shock waves (SWs), underwater pulsed discharge has drawn much attention in recent years for its complex physical process. Based on three discharge loads, the water gap (WG) load, the electrical wire (EW) load, and the energetic material (EM) load, the discharge processes are briefly introduced and the characteristics of the associated SWs are analyzed. First, the experimental setups were built and typical structures of the three loads were presented. Second, the inherent characteristics of SWs under the three loads, such as their peak pressure, impulse, and time duration of positive pressure and power spectral density (PSD), were studied and compared. Finally, a cracking effect experiment is carried out to study the SW fracturing characteristics. The results show that SWs generated with the WG load have the lowest peak pressure, impulse, and power density, SWs generated with the EW load have a better energy conversion efficiency and the largest peak pressure, and SWs generated with the EM load have the maximum impulse and power density. Furthermore, SW fracturing characteristics are mainly affected by its inherent characteristics. The peak pressure and impulse determine the shock number of fracturing, and the fracture pattern is significantly affected by the PSD.

Generation of Electrohydraulic Shock Waves by Plasma-Ignited Energetic Materials: II. Influence of Wire Configuration and Stored Energy

Electrohydraulic shock waves (SWs) have been used for shale gas exploitation in recent years. The traditional method to generate SWs is underwater pulsed discharge with a water gap load or an electrical wire (EW) load, but the SW energy is limited heavily by the stored energy. To obtain stronger SW, the EW explosion ignited energetic material (EM) load is proposed here. To obtain a good ignition performance, the influence of capacitor charging voltage, stored energy, wire diameter, and wire material (Cu and Mo) were studied. The results show that the higher the charging voltage is, the more effective the ignition is; liquidation and vaporization will affect the discharge but will not ignite the EM explosion effectively; the EMs given in this paper are more sensitive to the arc plasma. Besides, the nonrefractory copper is more suitable for the EM ignition than the refractory molybdenum.

A novel design of Rogowski coil for measurement of nanosecond-risetime high-level pulsed current

In pulsed power systems, pulsed currents with risetimes from nanosecond to microsecond can be effectively measured by self-integrating Rogowski coils. Appropriate design of the structure and the integrating resistor is crucial to the high-frequency response of a coil. In this paper, several novel designs of Rogowski coils integrating resistors were proposed and tested. Experimental results showed that the optimized coil could response square waves with fronts of ∼1.5 ns and had a sensitivity of ∼0.75 V/kA. The maximal peak current was designed as 100 kA.

Electrode Erosion Characteristics of Repetitive Long-Life Gas Spark Switch Under Airtight Conditions

With the rapid developments in pulse power technology, the gas spark switch has broadened its application in both industrial fields and research fields, such as the extraction of unconventional gas resources and underwater electrical wire explosion. Thus, the repetitive switch needs high current capability, small breakdown voltage variance, and long lifetime in an airtight cavity. The performances of the switch directly influence the output characteristics of the system, but electrode erosion is inevitable, which may decrease the self-breakdown voltage and the lifetime. The purpose of this paper was to investigate the electrode erosion characteristics under adverse conditions. The cathode showed a worse erosion surface. Compared with the central zone of the electrodes, the outer zone showed a worse surface roughness for both the cathode and the anode.

Hybrid PCB Rogowski Coil for Measurement of Nanosecond-Risetime Pulsed Current

Self-integrating Rogowski coil has been widely used for measurements of pulsed currents from nanosecond timescale to microsecond timescale in pulsed power systems. Appropriate designs of coils structure are vital to high-frequency response. This paper proposed a new structure of Rogowski coil so as to provide a better response to fast pulsed current. Four Rogowski coils with different structures were tested in three different platforms. Experimental results showed that the hybrid Printed Circuit Board Rogowski coil had the best performance in this paper, which could respond to square waves with fronts of

Fracturing Effect of Electrohydraulic Shock Waves Generated by Plasma-Ignited Energetic Materials Explosion

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A Novel Strain Measurement System in Strong Electromagnetic Field

ns and had a sensitivity of 0.585 V/kA. The designed maximal peak current was 100 kA.

Research of high-power repetitive spark-gap switch under adverse conditions

Electrohydraulic shock waves (ESWs) are widely applied to many fields such as sterilization, lithotripsy, food processing, and so on. Based on high-pulsed power technology, electrical explosion is increasingly utilized to generate shock waves with steep fronts and short duration. In order to further magnify the shock waves, we have proposed a new technique by using energetic materials (EMs) loads. This paper investigated the fracturing effect of the ESWs generated by plasma-ignited EMs explosion. During the ESWs fracturing process, a large-scale triaxial stress pressurizing equipment and a dynamic strain measurement system were applied to the shale samples. The most evident experimental results showed that a large number of cell-shaped multiple cracks developed after the ESWs fracturing process. In addition, interior crack morphology and fluorescent tracing proved the evidence of penetration cracks. These cracks contributed to a great reduction of fracture pressure in fracturing test, which laid a scientific foundation for ESWs technique to be a promising method of well stimulation in those low permeability reservoir and further optimized into a new plugging relief and injection gain technology.