Jo Oct 29, 2024
Subcooled flow boiling is widely used in many cooling systems due to its high cooling effect. In such systems, it is very important to accurately analyze the boiling heat transfer behavior of working fluid. Therefore, there have been several researches on accurately analyzing the behavior of subcooled flow boiling heat transfer by using commercial Computational Fluid Dynamics (CFD) codes. Most of the researchers have used the RPI (Rensseler Polytechnic Institute) model within Eulerian multi-phase framework in their CFD simulations of subcooled flow boiling heat transfer.
In a word, there have been many studies on the CFD analysis of subcooled flow boiling and a lot of progress has been made in terms of accuracy. However, most of the previous works for CFD simulations considered water as the working fluid, and the works for the CFD simulations with CO2 as the working fluid are rare.
Ma Kuk Chol, a researcher at the Faculty of Heat Engineering, has developed a mathematical model to simulate the subcooled flow boiling of CO2 by utilizing the Eulerian multi-phase model combined with the RPI and IAC models, and solved the model by means of the CFD solver, ANSYS FLUENT 19.3.
Thus, he proved that the coefficient of heat transfer is greatly affected by bubble waiting time coefficient (CWT) and critical Weber number (Wecri) and determined reasonable CWT and Wecri.
The details are found in his paper “Determining of bubble waiting time coefficient, critical Webber number and turbulence model for modeling of subcooled flow boiling heat transfer of CO2 at low temperature” in “Applied Physics A” (SCI).
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Jo Oct 26, 2024
Dye solar cells (DSCs), which are based on photosensitization of dyes adsorbed on wide-bandgap oxide semiconductors such as TiO2, have attracted a great deal of attention on account of their semitransparency, color harmony function and simple fabrication processes.
Transparent glass or polymer films with a conductive oxide layer (TCO electrode), such as FTO/GLASS and ITO/PET(Polyethyleneterephthalate), are often used as the substrates in DSCs because of their good conductivity and outstanding transmittance. However, these TCO electrodes account for a lot in the price of inorganic–organic hybrid solar cells such as dye and perovskite solar cells due to their dependence on exclusive deposition techniques.
Ti metallic substrates, such as Ti wire, Ti mesh, Ti foil and the like, are deemed to be a good choice to replace TCO electrodes. However, the Ti metallic substrates suffer from certain shortages. For example, metal foil has no transmittance, metal mesh owns a small effective area for accepting sunlight and metal wire is hard to support the manufacture of large-scale DSCs. Moreover, the high price of titanium substrates limits its wide application.
Jon Sang Mo, a researcher at the Faculty of Chemical Engineering, has proposed a new method of manufacturing a porous Ti back contact layer by using a facile printing method in normal ambient conditions.
In addition, conjugating the new back contact electrode technology with the traditional monolithic structure using the carbon counter electrode, he fabricated low-cost TCO-free DSCs. These four-layer structured DSCs consist of a dye-adsorbed nanocrystalline TiO2 film on a glass substrate, a porous Ti back contact layer, a ZrO2 spacer layer, and a carbon counter electrode in a layered structure, and the interior of the layers is filled with iodine/triiodide redox electrolyte.
For more information, please refer to his paper “Journal of Saudi Chemical Society” in “TCO-free dye solar cells based on Ti back contact electrode by facile printing method” (SCI).
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Jo Oct 25, 2024
Industrial robots can perform repetitious, difficult and hazardous tasks with precision. They can improve quality and productivity, and reduce delivery time and production cost. There are many industrial robots with different specifications and applications, so selecting a suitable industrial robot for particular application and manufacturing environment from lots of robots available is one of the important and decisive problems in practice.
Industrial robot selection in consideration of multiple conflicting performance attributes is a very difficult multi-attribute decision making (MADM) problem. For industrial robot selection, many MADM methods (MADMs) are applicable.
Many researchers have applied some different MADMs to practical industrial robot selection problems. But they only compared the results from MADMS, and failed to consider which MADM is the most suitable and reasonable to the given robot selection problem. Moreover, few of them determined reasonable final results by combining the results from different MADMs.
Choe Myong Song, a researcher at the Faculty of Mechanical Science and Engineering, proposed a reasonable method for industrial robot selection combined with multiple MADMs based on final comprehensive performance (CP) values using the weighted average of the CP values from individual MADMs. Then, he applied the proposed method to the selection of the best industrial robot for some pick-n-place operations avoiding some obstacles.
The results showed that the proposed method is a reasonable industrial robot selection method for determining the FCP values and FCRs of candidate robots in consideration of the preference weights of each MADM, and that it could be widely used in many practical problems for industrial robot selection and other MADM applications.
For further information, please refer to his paper “A reasonable method for industrial robot selection combined with several multi-attribute decision making methods” in “International Journal on Interactive Design and Manufacturing” (EI).
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Jo Oct 24, 2024
There are many kinds of nitrogen oxide exhausted from vehicles using fuel, but most of them are changed into NO2 after a certain period of time in the natural world and a very small amount of them fatally affect the human body. The very good level of NO2 gas concentration for human body in the atmosphere is below 100μg/m3 and the bad level is over 400μg/m3.
Thus, accurate estimation of NO2 concentration is very important to protect human lives.
Kim Yong Hyok, a researcher at the Faculty of Electronics, designed the structure of a three electrodes system (3ES)-electrochemical NO2 gas sensor and analyzed the action principle based on Nafion membrane material by using qualitative analysis method. In addition, he established the manufacturing process of the sensor and experimentally analyzed the NO2 sensing property depending on the bias voltage properties.
He proved that the optimal bias voltage range is 0.9~1.2V and the sensitivity is 0.31μA/ppm, and that it can sense NO2 gas up to 100ppm linearly.
You can find more information in his paper “Design of Three Electrodes - Electrochemical Nitrogen Oxide Gas Sensors and its Bias Characteristics” in “Journal of Analog and Digital Devices” (EI).
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Jo Oct 22, 2024
IH technology finds its wide application to melting, welding, hardening, cooking and other fields. IH applications use high current passing through an induction coil to produce a sufficient magnetic field and Eddy current within work pieces. Operating frequency is selected by heating application. Resonant circuits are commonly used in IH application to produce powerful current and high voltage to provide maximum output power to the heating load. Recently, the L-LC resonant topology has been widely introduced for its many advantages.
It is widely known that switching devices can be damaged from NON-ZVS operation and spike current generated during the phase shift angle adjustment with fixed frequency to vary the output power and that when the load parameter is changed due to the workpiece, the temperature is close to the Curie temperature. Therefore, the switching frequency must be controlled when the phase shift angle and load parameters are varied to maintain the ZVS operation and prevent the switching device from spike current.
Ri Nam Jin, a researcher at the Faculty of Automation Engineering, proposed a variable frequency phase shift control (VFPSC) to guarantee the ZVS operation and protect the switch from spike current during the heating process.
The proposed technique achieved more efficient performances than conventional fixed frequency control strategies by eliminating NON-ZVS operation and spike current caused by the change of output power and load and guaranteeing zero voltage switching (ZVS) during output power regulation. The theoretical results were verified experimentally on L-LC resonant inverter.
The experiments to validate the proposed control method were conducted on an induction melting of a 3kg Nickel work piece from room temperature to the melting point of 1 600℃.
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Jo Oct 20, 2024
Electrohydraulic servovalves are compact, accurate, broad-bandwidth modulating valves, which are widely used in several industrial applications that need high power and rapid response. Electrohydraulic servovalves can proportionally transform electric analog or digital input signals into stepless hydraulic output signals and they can be single-stage or two-stage servovalves. The first stage (pre-stage) of such servovalves may assume a variety of forms, such as a sliding spool, a nozzle-flapper, a jet-pipe and a deflector-jet.
The present designs of pilot operated servovalves are either of a nozzle-flapper type or of a jet pipe type, and the design aspects and various configurations of servovalves, particularly of a nozzle-flapper type, are available in many references.
However, there have been few studies on the deflector-jet type servovalves. Furthermore, no detailed design approaches and modeling of a deflector-jet type servovalve have been reported so far. Conventional design methods for structural parameters of the fluidic amplifier in the deflector-jet servovalve (DJSV) are confined in their applications due to the lower resolution and narrower working bandwidth of the deflector-jet type servovalve.
Hence, it is very important to develop a new type of electro hydraulic servovalve that has more ideal characteristics. To improve the characteristics of resolution and working bandwidth of DJSV, the most reasonable flow amplification coefficient and pressure amplification coefficient of a fluidic amplifier should be obtained, by properly determining the structural parameters of fluidic amplifiers in the pre-stage.
In order to obtain the most ideal coefficients (pressure amplification coefficient and flow rate amplification coefficient) of fluidic amplifier in the DJSV, Pang In Ho, a researcher at the Robotics Institute, conducted a computer simulation of the effects of the geometric parameters of a fluidic amplifier on the two coefficients.
First, he decided that the important geometric parameters which give the greatest effect on the characteristics of fluidic amplifier are the area and shape of the returning channel of receive ports. Then, he studied the characteristics of the fluidic amplifier, while changing the area of the discharging channel by changing its height.
He built 3D and fluid models for the fluidic amplifier in the DJSV using SolidWorks. He used ANSYS CFD 15.0 to solve the 3D continuity and momentum equations for incompressible flows. The results showed that the pressure amplification coefficient was Kp≈25MPa/mm and the flow rate amplification coefficient was KQ≈0.127L/s·mm-1, which means better sensitivity and better linearity.
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