Jo Sep 19, 2024
Aluminum and its alloys are light and cheap with good thermal and electrical properties and excellent mechanical properties and machinability, so they are widely used in various fields of the national economy such as mechanical industry, aerospace industry, daily necessities industry, electronic industry, building materials industry, etc.
Aluminum and its alloys are easily oxidized in the atmosphere because of their strong chemical activity, resulting in a thin oxide film on the surface, which is protective because of its good corrosion resistance in dry air, but is easily damaged in contaminated atmosphere, water, seawater, soil and various corrosive media. Therefore, surface treatment is usually applied to enhance the surface performance of aluminum and its alloys such as corrosion resistance, wear resistance and decorativeness.
Surface treatment methods of aluminum and its alloys usually include mechanical surface treatment (mechanical grinding), chemical surface treatment (chemical coating), electrochemical surface treatment (anode oxidation, electroplating), and physical surface treatment (coating, deposition), etc.
Anode oxidation is the most commonly used electrochemical surface treatment method, which allows aluminum and its alloys to form protective coatings with a major component of amorphous aluminum oxide and a thickness of several tens of micrometers under the action of an external electric field in an acidic electrolyte (e.g. sulfuric acid, oxalic acid, phosphoric acid, etc.). The increase in the temperature of electrolyte due to the electrical resistance heat during anode oxidation promotes the dissolution of the surface oxide film and decreases its thickness, which leads to the reduction of its mechanical properties and corrosion resistance. Therefore, in the anode oxidation process of aluminum and its alloys, it is necessary to cool the electrolyte with cooling equipment to prevent excessive rise in the temperature, which means consumption of a large amount of energy.
Kim Chang Sok, a researcher at the Science Engineering Institute, has proposed a surface treatment method for excellent surface properties and low energy consumption by applying wide temperature anode oxidation technique to the surface treatment of aluminum alloy 6061 commonly used as dry bulb.
Addition of polycarboxylic acid and inorganic salts to the sulphuric acid electrolyte of aluminum alloys can inhibit or retard the electrochemical and chemical dissolution of aluminum, resulting in an anodic oxide film with excellent surface properties even at wide temperatures, which can significantly reduce energy consumption and cost during anode oxidation. Considering it, he selected inexpensive oxalic acid as polycarboxylic acid and nickel sulfate which is widely used in the coloration process of aluminum alloy as inorganic salt, and conducted a study to increase the allowable electrolytic temperature of anodic oxidation.
The results showed that the surface characteristics of the samples anodized at 30℃ are almost the same as those of the samples anodized by the existing method at 20℃, so that the acceptable temperature range of anode oxidation can be increased by 10℃ and the energy consumed for cooling of electrolyte during anodization can be significantly reduced.
...
Jo Sep 18, 2024
The real-world optimization problems which we encounter nowadays are becoming more and more complicated and difficult to solve by traditional heuristic methods. For those problems, metaheuristic algorithms have proven to be efficient and reliable techniques for finding near optimal solutions with a reasonable computational cost. Most of the today’s successful metaheuristic methods were inspired by swarm behavior of animals, biological systems and natural phenomena.
Recently, gravitational search algorithm (GSA) has been newly introduced, imitating the law of gravity in physics. The GSA is distinguished from other metaheuristic algorithms for its unique concepts and operators and it is regarded as one of the powerful metaheuristic algorithms. Actually, it has been widely utilized to solve various kinds of optimization problems arising in many fields, for example, image processing, effective solving of aircraft landing planning (ALP) problems in the air traffic control, predicting the structure of protein and estimating the minimum ignition energy.
Hybrid algorithms of GSA with other optimization methods have been presented in several works. Still, a large number of open problems exist for GSA despite these exertions.
Choe Thae Ryong, a researcher at the Faculty of Applied Mathematics, has investigated a method for improvement of GSA by mixing it with the invasive weed optimization, thus proposing a new hybrid algorithm called IWO-GSA. In IWO-GSA, the agents generate new seeds and scatter them at each iteration of GSA. Then, elite agents are selected according to their fitness.
He applied IWO-GSA to 23 benchmark functions and compared it with GSA and IWO. From the results of numerical experiments, he found that the proposed algorithm is superior to the competitors for most benchmark functions.
...
Jo Sep 15, 2024
Permanent magnet (PM) brushless DC (BLDC) motors are widely used in various applications demanding speed regulation and control. PMBLDC motors are a typical example with advantages of high efficiency, high power density and maintenance-free operation. However, in some applications with limited space, high temperature or severe vibrations, use of Hall-effect sensors for rotor position detection may encounter hazards or problems. For example, high temperature can affect the characteristics or cause failure of sensors while insufficient space would not be able to accommodate sensors. Other conditions such as severe vibration or in-water operation may involve a risk of sensor destruction. Therefore, sensorless control is considered as an appropriate technique for such applications.
Sensorless control can reduce not only the number of external wiring/connections between the motor and the driver but also the maintenance requirement caused by vibration. This will improve the system reliability and reduce cost.
There have been a lot of studies reporting sensorless control techniques for BLDC motors. Some of them are based on detection of zero crossing points (ZCPs) of back electromotive force (EMF) or terminal voltages. The ZCP techniques often require the voltage of the neutral point or virtual neutral point, which are used for comparison with terminal voltages for commutation. However, neutral or virtual neutral points often contain high-frequency noise and windings of common motors may have no lead-outs for neutral points.
To avoid this problem, some studies indirectly obtained back-EMF ZCPs with a series of signal subtractions and manipulation. However, a thirty electric degree phase shift is still required for correct commutation, and the entire circuitry is complex. Some others directly compared the filtered average terminal voltages to obtain the required signals for commutation. For the above methods, the inclusion of low pass filters (LPFs) may solve the noise problem due to pulse width modulation (PWM) switching but will limit the speed range. In addition, some literature discussed the speed range that can be achieved under heavy load conditions.
Ji Sang Guk, a researcher at the Faculty of Electrical Engineering, has proposed a sensorless technique with a wide speed range and high efficiency using an enhanced commutation method.
This technique uses a low-pass filter with two cut-off frequencies that can be regulated for various speed operations. The higher cut-off frequency is designed for high speed to reduce phase lag, and the lower one is used for start-up to reject the noise.
From the experiments, with loads applied, he proved that the developed technique is capable of increasing the speed beyond the rated speed. For the entire speed range, the proposed technique demonstrated high efficiency even at high torque, high speed conditions.
...
Jo Sep 13, 2024
Channel induction furnace (CIF) has been widely used for melting, holding and casting of metal owing to its advantages such as higher power factor, low power consumption and less stirring of molten metal because magnetic induction line follows the closed iron core.
In general, CIF is composed of induction units with iron core, inductor and channel, and a molten metal bath. The induction units are assembled in the furnace body and are separated when the furnace needs repairing. If an electric current flows through the inductor, very large induced current flows through the channel due to electromagnetic induction phenomenon. Then, the channel is heated by Joule heat from the induced current, and this heat makes the metal molten through continuous transfer into the molten metal bath. Therefore, the temperature of the channel gets higher than that of the molten metal bath during melting, and it shortens the repair cycle of the furnace due to severe erosion of lining of the channel.
To overcome such problems, Song Hak Myong, a researcher at the Department of Mechanical Engineering, analyzed the melting and flow of molten metal in the channel of CIF using COMSOL Multiphysics 5.4 and Taguchi method, and decided the design parameters of a single loop channel induction furnace (SLCIF).
The simulation result demonstrated that the proposed parameters (establishment angle of channel: 36° and cross section size of channel: 100×74mm2) can sufficiently increase the working life because of the lower maximum local superheating temperature of channel (52K) than other CIFs.
...
Jo Sep 12, 2024
Fluid flow and heat transfer problems in porous media are widely applied in drying, filtration, preservation of agricultural products, fluidized bed reactors, etc.
Today, a significant amount of work has been focused on the numerical simulation of the problems involving non-equilibrium heat and mass transfer. Especially, Computational Fluid Dynamics (CFD) is used to simulate such problems.
Recently, there have been several studies on the numerical modeling of diverse problems involving heat and mass transfer including drying. In this respect, heat and mass transport equations have been proposed to model different heat and mass transfer problems based on the thermal non-equilibrium assumption.
However, few studies have been proposed on non-equilibrium heat and mass transfer models for prediction of drying process using commercial software FLUENT.
Choe Song Gun, a section head at the Faculty of Heat Engineering, has developed a numerical formulation for modeling non-equilibrium heat and mass transfer inside porous media and solved the equations by using FLUENT. To achieve the objective, he reformed the moisture transport equation and energy transport equation for the solid matrix of porous media into a user-defined scalar equation type in FLUENT. In addition, he redefined the properties of solid and some terms of these equations in consideration of the coefficients and terms of UDS equation in FLUENT.
Then, he compared the calculation results and the experimental results in order to check the validity of the reformation of the model and equations. The deviations between them are less than 3% in heat transfer and less than 8% in mass transfer. The results show that his predictions follow the expected trends with respect to temperature and velocity at inlet.
...
Jo Sep 10, 2024
Wireless sensor network (WSN) is one of the most popular areas of research due to the constant improvement in the field of wireless technology and embedded system. Now, WSNs are extensively deployed in every area such as industry, civilian and military applications. These applications include monitoring, tracking, event detection and disaster management. The long term usefulness of WSN mainly relies on the lifetime of sensor nodes. The lifetime of SN mainly depends on its battery source. Since these batteries can be hardly replaceable, the improvement in the lifetime of WSNs can be achieved through the conservation of battery energy. In order to achieve this purpose, it is very important to design an energy-efficient routing protocol for efficient use of the limited energy of WSNs.
Many researchers applied the concept for clustering of sensor nodes and efficient routing to conserve the energy of sensor nodes and these techniques significantly improved the lifetime of WSN. LEACH (Low Energy Adaptive Clustering Hierarchy), the first adaptive hierarchical clustering routing protocol that considers energy saving, comprises two phases i.e. setup phase and steady phase.
But LEACH protocol has some critical drawbacks: It does not consider residual energy of a node, so a node with low energy can be selected as a cluster head. Also, due to randomized election of cluster heads, it is possible that elected cluster heads very close to each other might cause congestion in particular areas.
To solve the critical problem due to residual energy in LEACH, some protocols similar to LEACH, but considering residual energy of nodes were introduced. Nevertheless, LEACH based protocols considering residual energy of nodes can never deal with irregular election of cluster heads.
In response to the above issue, Kim Chol Man, a researcher at the Faculty of Information Science and Technology, has presented an improved cluster head election algorithm using residual energy and network address of nodes.
This algorithm is based on LEACH and considers both remaining energy and network address of nodes to revise the cluster head threshold equation. He assigned the network addresses of nodes by distributed address assignment mechanism (DAAM) of ZigBee. As he used the network address of a node in the threshold equation, the cluster type created in a new round is similar to the old cluster type. In addition, he dynamically changed the upper limit of the interval that generates random numbers to optimize the number of cluster heads.
He evaluated the performance of the proposed LEACH-NA using network Simulation platform NS-2. The simulation results demonstrate that the proposed algorithm shows better performance than the existing ones in terms of network lifetime and energy conservation, and the number of cluster heads in every round is more stable than other algorithms based on LEACH.
...
