Hak-Joon Kim

Generation of the basis sets for multi-Gaussian ultrasonic beam models—An overview

By using a small number of Gaussian basis functions, one can synthesize the wave fields radiated from planar and focused piston transducers in the form of a superposition of Gaussian beams. Since Gaussian beams can be transmitted through complex geometries and media, such multi-Gaussian beam models have become powerful simulation tools. In previous studies the basis function expansion coefficients of multi-Gaussian beam models have been obtained by both spatial domain and k-space domain methods. Here, we will give an overview of these two methods and relate their expansion coefficients. We will demonstrate that the expansion coefficients that have been optimized for circular piston transducers can also be used to generate improved field simulations for rectangular probes. It will also be shown that because Gaussian beams are only approximate (paraxial) solutions to the wave equation, a multi-Gaussian beam model is ultimately limited in the accuracy it can obtain in the very near field.

Fine Particle Removal Performance of a Two-Stage Wet Electrostatic Precipitator Using a Nonmetallic Pre-Charger

ABSTRACT A novel two-stage wet electrostatic precipitator (ESP) has been developed using a carbon brush pre-charger and collection plates with a thin water film. The electrical and particle collection performance was evaluated for submicrometer particles smaller than 0.01∼0.5 μm in diameter by varying the voltages applied to the pre-charger and collection plates as well as the polarity of the voltage. The collection efficiency was compared with that calculated by the theoretical models. The long-term performances of the ESP with and without water films were also compared in tests using Japanese Industrial Standards dust. The experimental results show that the carbon brush pre-charger of the two-stage wet ESP had approximately 10% particle capture, while producing ozone concentrations of less than 30 ppb. The produced amounts of ozone are significantly lower than the current limits set by international agencies. The ESP also achieved a high collection rate performance, averaging 90% for ultrafine particles, as based on the particle number concentration at an average velocity of 1 m/sec corresponding to a residence time of 0.17 sec. Higher particle collection efficiency for the ESP can be achieved by increasing the voltages applied to the pre-charger and the collection plates. The decreased collection efficiency that occurred during dust loading without water films was completely avoided by forming a thin water film on the collection plates at a water flow rate of 6.5 L/min/m2. IMPLICATIONS Current two-stage electrostatic precipitators (ESPs) have several technical problems such as a drop in collection efficiencies by small-particle re-entrainment during rapping and corrosion of metallic electrodes of the ESPs by corrosive gases. This paper evaluates a novel two-stage ESP that uses a nonmetallic pre-charger and water film collection plates to avoid the above mentioned problems of other ESPs. This ESP can be used not only for industrial applications but also for residential purposes because it has a high removal performance for fine particles with low ozone generation and maintains its efficiency due to the continuous cleaning of the collection plates with water film.

Development of an intelligent system for ultrasonic flaw classification in weldments

Abstract Even though ultrasonic pattern recognition is considered as the most effective and promising approach to flaw classification in weldments, its application to the realistic field inspection is still very limited due to the crucial barriers in cost, time and reliability. To reduce such barriers, previously we have proposed an intelligent system approach that consisted of the following four ingredients: (1) a PC-based ultrasonic testing (PC-UT) system; (2) an effective invariant ultrasonic flaw classification algorithm; (3) an intelligent flaw classification software; and (4) a database with abundant experimental flaw signals. In the present work, for performing the ultrasonic flaw classification in weldments in a real-time fashion in many real word situations, we develop an intelligent system, which is called the ‘Intelligent Ultrasonic Evaluation System (IUES)’ by the integration of the above four ingredients into a single, unified system. In addition, for the improvement of classification accuracy of flaws, especially slag inclusions, we expand the feature set by adding new informative features, and demonstrate the enhanced performance of the IUES with flaw signals in the database constructed previously. And then, to take care of the increased redundancy in the feature set due to the addition of features, we also propose two efficient schemes for feature selection: the forward selection with trial and error, and the forward selection with criteria of the error probability and the linear correlation coefficients of individual features.

Integration of a nonmetallic electrostatic precipitator and a wet scrubber for improved removal of particles and corrosive gas cleaning in semiconductor manufacturing industries

To remove particles in corrosive gases generated by semiconductor industries, we have developed a novel non-metallic, two-stage electrostatic precipitator (ESP). Carbon brush electrodes and grounded carbon fiber-reinforced polymer (CFRP) form the ionization stage, and polyvinyl chloride collection plates are used in the collection stage of the ESP. The collection performance of the ESP downstream of a wet scrubber was evaluated with KCl, silica, and mist particles (0.01–10 μm), changing design and operation parameters such as the ESP length, voltage, and flow rate. A long-term and regeneration performance (12-hr) test was conducted at the maximum operation conditions of the scrubber and ESP, and the performance was then demonstrated for 1 month with exhaust gases from wet scrubbers at the rooftop of a semiconductor manufacturing plant in Korea. The results showed that the electrical and collection performance of the ESP (16 channels, 400 × 400 mm2) was maintained with different grounded plate materials (stainless steel and CFRP) and different lengths of the ionization stage. The collection efficiency of the ESP at high air velocity was enhanced with increases in applied voltages and collection plate lengths. The ESP (16 channels with 100 mm length, 400 × 400 mm2 × 540 mm with a 10-mm gap) removed more than 90% of silica and mist particles with 10 and 12 kV applied to the ESP at the air velocity of 2 m/s and liquid-to-gas ratio of 3.6 L/m3. Decreased performance after 13 hours of continuous operation was recovered to the initial performance level by 5 min of water washing. Moreover, during the 1-month operation at the demonstration site, the ESP showed average collection efficiencies of 97% based on particle number and 92% based on total particle mass, which were achieved with a much smaller specific corona power of 0.28 W/m3/hr compared with conventional ESPs. Implications: Current electrostatic precipitators (ESPs) for the semiconductor manufacturing industry have economic drawbacks. For example, ESPs with metal components require the use of expensive anticorrosive metallic materials to protect against the corrosive gases produced in the semiconductor industry. This paper evaluates a nonmetallic, two-stage ESP that uses a carbon brush charger, carbon forced resin plate ground channels, and polyvinyl chloride collection plates into which aluminum sheets are inserted. This nonmetallic ESP can be used to remove particles and mists from highly corrosive gases in the semiconductor industry as well as in other industries.

Fine particle collection of an electrostatic precipitator in CO2-rich gas conditions for oxy-fuel combustion

The collection of particles in CO(2)-enriched environments has long been important for the capture of CO(2) in order to clean gases via oxy-fuel combustion. We here report on the collection characteristics of fine and ultrafine particles using an electrostatic precipitator (ESP) in a CO(2)-enriched atmosphere. In order to understand the characteristics of particle collection in CO(2)-rich gas mixtures, the ionic properties of a CO(2)-enriched atmosphere was also investigated. The electrical mobility of the ions in a CO(2)-enriched atmosphere was found to be about 0.56 times that found in a conventional air atmosphere, due to the higher mass of CO(2) gas compared to that of air. The low electrical mobility of ions resulted in a low corona current under CO(2)-enriched conditions. The collection efficiency of particles in a CO(2)-rich atmosphere for a given power consumption was thus somewhat lower than that found in air, due to the low quantity of particle charging in CO(2)-enriched air. At the same time, higher temperatures led to the higher electrical mobility of ions, which resulted in a greater collection efficiency for a given power. The presence of a negative corona also led to a greater collection efficiency of particles in an ESP than that achieved for a positive corona.

Generation of Model Diesel Particles by Spark Discharge and Hydrocarbon Condensation

This study was conducted in order to generate model particles which were similar to particles in diesel emission. Spark discharge was used for carbon agglomerates and hydrocarbon condensation for particles that consist of carbon agglomerates and hydrocarbon. The size of the carbon agglomerates, whose mean size were 30 and 70 nm, ranged between 15 and 200 nm, and the total number concentration of the particles ranged from 3 to 5 × 107#/cm3 as the controllable variables in spark discharge generator changed. The result of the hydrocarbon condensation experiment showed that the final sizes of the particles enlarged by condensation did not depend on the initial sizes, but the maximum condensational growth of carbon agglomerates by dodecane (C12 H26) condensation was 112 times the initial size of 40 nm, while the size of the agglomerates by benzene (C6H6) was 3.25 times its initial size.

A Systematic Approach to Ultrasonic Pattern Recognition for Real-Time Intelligent Flaw Classification in Weldments

Flaw classification is one of the essential issues in quantitative ultrasonic nondestructive evaluation of weldments. Ultrasonic flaw classification can be divided into three approaches [1]; 1) conventional approaches which use heuristic experience-based echo-dynamic pattern identification techniques, 2) model-based approaches which use model-based strong features in ultrasonic flaw signals, and 3) ultrasonic pattern recognition approaches which use features and decision making algorithms and adopt various signal processing techniques and artificial intelligent tools. Among these approaches, ultrasonic pattern recognition approaches which are considered as the most promising tool have been investigated extensively in the ultrasonic nondestructive evaluation (NDE) community [2–6].

Feasibility Study on the Utilization of EMAT Technology for In-line Inspection of Gas Pipeline

If gas is leaking out of gas pipelines, it could cause a huge explosion. Accordingly, it is important to ensure the integrity of gas pipelines. Traditionally, over the years, gas-operating companies have used the ILI system, which is based on axial magnetic flux leakage (MFL), to inspect the gas pipelines. Relatively, there is a low probability of detection (POD) for the axial defects with the axial MFL-based ILI. To prevent the buried pipeline from corrosion, it requires a protective coating. In addition to the potential damage to the coating by environmental factors and external forces, there could be defects on the damaged coating area. Thus, it is essential that nondestructive evaluation methods for detecting axial defects (axial cracks, axial groove) and damaged coating be developed. In this study, an electromagnetic acoustic transducer (EMAT) sensor was designed and fabricated for detecting axial defects and coating disbondment. In order to validate the performances of the developed EMAT sensor, experiments were performed with specimens from axial cracks, axial grooves, and coating disbondment. The experimental results showed that the developed EMAT sensor could detect not only the axial cracks (minimum 5% depth of wall thickness) and axial grooves (minimum 10% depth of wall thickness), but also the coating disbondment.

Prediction of Insonifying Velocity Fields and Flaw Signals of the 2002 Ultrasonic Benchmark Problems

In the present study, the radiation issues related to the ultrasonic benchmark problems of year 2002 are explored by adopting two ultrasonic measurement models based on 1) the multi‐Gaussian beams and 2) the Rayleigh‐Sommerfeld integral with high frequency approximation, while keeping the choice of scattering model (the plane wave far‐field scattering amplitude estimated by the Kirchhoff approximation) unchanged. The insonifying beam fields and flaw signals calculated by two models showed very good agreement in most of the cases. However, they showed significant difference at the near‐critical angle (which is corresponding to the cases of refracted S‐waves with the refracted angle of 30°) due to the rapid variation in the transmission coefficient in that region.

Prediction of flaw signals of the ultrasonic benchmark problems by Sungkyunkwan University

This paper describes the approaches and results of Sungkyunkwan University (SKKU), Suwon, Korea to solve the ultrasonic benchmark problems proposed by the World Federation of Nondestructive Evaluation Centers. SKKU implemented a computationally efficient ultrasonic measurement model based on the multi-Gaussian beams with paraxial approximation, and validated the accuracy of the proposed model by use of 1) the generalized Rayleigh-Sommerfeld integral that can provide more exact solutions, and 2) the comparison to the experimental signal obtained from the flat-bottom hole in an ASTM standard block immersed in water.