Jo Sep 20, 2022
A shell and tube heat exchanger among a variety of heat exchangers is still widely used in many industrial applications as it has several advantages like a long working life, simple manufacture and wide operation range.
In multi-channel systems like the shell and tube heat exchanger, flow distributions significantly affect their performance and lifetime. Uniform distribution is commonly assumed in designing conventional heat exchangers, but flow maldistribution may be an inevitable occurrence in practice. In particular, flow distribution in the tube-side of the SSTHX with axial connections of bonnet nozzles is extremely non-uniform.
Therefore, numerical and experimental studies on flow distributions of the shell and tube heat exchangers have been widely carried out.
In particular, many attempts have been made for flow uniformity in multi-channel systems by means of different geometry of headers or manifolds. Most studies tended to be focused on shell-side flows due to their complexity in shell and tube heat exchangers. What is difficult in numerical studies on heat exchangers is that their geometries with lots of channels cannot be modeled as it is. One way to solve it is to model tube bundles as porous media. The most important problem is to determine porous medium parameters accurately. In most references, they were obtained by empirical correlations.
Various baffles have been proposed to improve velocity distributions and heat transfers of heat exchangers.
However, few researchers have addressed the problems concerning the tube-side flow of the shell and tube heat exchanger.
It should be noted that most studies have been focused on limited regime like laminar flow or turbulent flow.
Pak Sin Myong, a researcher at the Faculty of Heat Engineering, has clarified that introduction of new header baffles makes it possible to uniformalize dramatically flow distribution in the tube-side of SSTHX. He has also modeled tube bundles as porous media as they have lots of tubes, and he has obtained porous media parameters through CFD simulation for one isolated channel.
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Jo Sep 18, 2022
Recently, with the development of nanotechnology, it has become possible to change the size and the shape of nanoparticles, so a lot of investigations are being conducted into the optical properties of various shaped nanogold and semiconductor quantum dots. The properties are affected by their associate structure and a core-shell structure has been the main focus of most investigations. However, the optical property of a semiconductor quantum dot aqueous solution that contains nanogold has not been discussed.
While Discrete Dipole Approximation(DDA) and Finite Difference Time Domain(FDTD) methods are widely used in the optical property simulation of nanoparticles, Mie theory is employed in the simulation of the scattering property of various shaped nanoparticles.
Ri Myong Il, a researcher at the Faculty of Physical Engineering, has chemically prepared an aqueous solution of CdTe quantum dots with gold nanoparticles and measured its absorption and scattering spectra. On the basis of it, he has simulated the scattering property of this composite material by combining Mie theory with FDTD method.
The aqueous solution of CdTe quantum dots (3nm in diameter) with gold nanoparticles (30nm in diameter) was prepared by the chemical crystal growth method. The scattering property of the aqueous solution of CdTe quantum dots with gold nanoparticles was simulated by the Finite Difference Time Domain (FDTD) method and it was verified by an absorption spectrophotometer.The results showed that compared with a pure aqueous solution of CdTe quantum dots, it has stronger absorption and scattering spectra, that is, 1.5 times higher in scattering intensity. Therefore, it is concluded that aqueous solutions of CdTe quantum dots with gold nanoparticles can be widlely used in the fields of medicine and optics.
More information can be found in his paper “Scattering Property of Aqueous Solution of CdTe Quantum dots with Gold Nanoparticles” presented to the SCI Journal “Optics Communications”.
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Jo Sep 16, 2022
Most practical engineering problems are currently solved by the well-developed standard finite element method (FEM). However, the FEM has its inherent shortcomings as it is a numerical method that relies on meshes. While conventional mesh-based techniques such as FEM can accurately approximate governing equations, labor and computational costs associated with creating the conforming mesh undermine the efficiency of such methods. The Boundary Element Method (BEM) is another technique that can alleviate the difficulties associated with the implementation of the standard FEM, but it still relies on meshes.
Thus, an idea of getting rid of meshes in the process of numerical treatments has naturally evolved, and the concepts of meshfree or boundary meshfree methods have been shaped up.
Boundary meshfree methods can be roughly sorted into two: MFS (Method of Fundamental Solutions)-based type and the BIE (Boundary Integral Equation)-based type. The former is based on the concept of the MFS. The traditional MFS uses only a fundamental solution, which is a response due to a concentrated point source, in the construction of the solution of a problem without using any integrals. It is a natural boundary meshfree method.
Studies show that the MFS has been developed by improving the locations or types of sources. In the traditional MFS, a fictitious boundary is required to have the source points on it to avoid the singularity of fundamental solutions. The determination of the distance between the real boundary and the fictitious boundary is based on experience and therefore it is troublesome. In recent years, a number of efforts have been made aiming to remove this barrier in the MFS so that the source points can directly be placed on the real boundary.
And the MFS has been improved by replacing concentrated point sources of the traditional MFS with area-distributed sources covering the source points for 2D problems (volume-distributed sources for 3D problems).
Kim Un Ok, a lecturer at the Faculty of Applied Mathematics, has successfully improved the boundary meshfree method with area-distributed sources placed on the real boundary called boundary distributed source (BDS) method, by moving distributed sources outside the boundary.
To show the effectiveness of the new improved method, she has plotted the relative errors in the computed results using the BDS method and her new improved method respectively.
She has demonstrated the accuracy of the new method by solving 2D potential problems in a square domain covering 0≤x and y≤1 with Dirichlet BC,Ф(x, y)=x2-y2. She has moved sources 1.5 times d, average distance between the original boundary distributed sources, outside the boundary in the direction of outward normal vectors.
When BDS method is used, the solution is inaccurate near the boundary regions. However, the proposed method works well in improving the accuracy of the numerical solution. The results say that moving distributed sources outside the boundary can improve the BDS method.
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Jo Aug 29, 2022
Stability of earth and rockfill dams (ERD) usually includes sliding stabilities of steady seepage (at levels of normal water or flood water), earthquake and transient seepage (drawdown or heading-up).
In transient seepage such as rapid drawdown (RDD) unlike steady seepage, the phreatic surface, shear strength, boundary condition and effective stress of the slope soil, etc. dynamically change and they have coupled seepage-stress and nonlinear characteristics. These cause a number of problems with evaluating stability for transient seepage.
It can be a great expense to examine one by one the stability of earth and rockfill dams for rapid drawdown in civil engineering practice.
ERDs frequently met in civil engineering practice can be classified into homogeneous, sloping upstream core and central core according to whether they have an impervious core or not and where the impervious core exists.
Ri Yong Nam, a researcher at the Faculty of Earth Science and Technology, has proved that fully coupled seepage-stress analysis makes it possible to select the safest type for rapid drawdown among all common types of earth and rockfill dams and to approximately estimate the minimum safety factor for RDD from the steady safety factors.
The slope stability for RDD is very important in embankments such as a reservoir dam of relatively small storage capacity than its height, embankments under the effects of the high tide and the ebb tide, dams of agricultural reservoir, etc. In these cases, a central core dam (CCD) to improve the stability of ERD for RDD is recommended.
On the contrary, slope stability for RDD is not so important in embankments such as the reservoir dams of relatively large storage capacity compared to its height, embankments of gentle water level variation, etc. In these cases, an ERD which has larger steady stability for the same slope gradients is economically better and thus, a sloping upstream core dam (SUCD) is recommended.
The details about this is carried in his paper “Study on Stability Reduction Characteristics of Earth and Rockfill Dams under Rapid Drawdown Using Fully Coupled Seepage-Stress Analysis” presented to the SCI Journal “Advances in Civil Engineering”.
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Jo Aug 24, 2022
A research group led by Hong Sang Ryong, a researcher at the Faculty of Heat Engineering, has been working hard to make further achievements since he established an energy-saving recirculation system for dust collection and ventilation that can keep clean environment at the working places in a timber mill.
The system makes it possible to reduce power assumption to the minimum by adjusting the number of ventilators and dust collectors to the operation rate of wood working machinery.
They determined reasonable working parameters of the air control valve to prevent deposit of rough wood chips in the pneumatic conveying pipe.
They also selected optimal jet time and close-down time of the pulse system in the dust collector.
As a result, they reduced the number of working ventilators and dust collectors and ensured a long working life of the bag filter, which led to high working safety and low operational cost of the dust collection and ventilation system.
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Jo Aug 21, 2022
An inductive conductivity sensor is based on electromagnetic induction and it has some advantages such as robust structure and low manufacturing cost over conductive sensors. These sensors have no bare metal electrodes directly contacted with solutions, so they are free from damage by chemical corrosive solutions such as sulphuric acid and sodium hydroxide.
A transformer type sensor has robust structure and good linearity. It is easy to install and convenient to use in the industrial field. Using these characteristics, Kang Hwi Song, a section head at the Faculty of Electronics, has designed a new type transformer inductive conductivity sensor (TICS).
TICS has one or more magnetic cores. Its permeability values change with temperature and pressure. It is very important to compensate or eliminate the effects of magnetic core permeability change for improving the sensing performance.
TICS consists of a drive coil, a sense coil and a temperature sensor. Applying alternating voltage to the drive coil induces an ionic electric current in the solution around the sensor. This ionic electric current also induces an electric current in the sense coil which is proportional to the conductivity of the solution. Here, the two coils are encapsulated by chemical-resistant plastic material that protects them from corrosive electrolytes.
The equivalent loss resistances of the drive coil and the sense coil depend on both their turn number and frequency. From the investigation into the new model, he set the rational turn numbers as 10 each. The signal voltage was hardly affected when the drive frequency over 10kHz was used. As frequencies higher than 10kHz may cause magnetic loss, 10kHz was selected as a drive frequency.
In a word, it is important to use magnetic cores which have high permeability and small magnetic loss. He selected HS10 as the most suitable magnetic core for his purpose.
The drive coil and the sense coil are fixed on the PCB and welded to the lead wire. The main body encapsulates the two coils and PCB. The seal ring, fixing cap and gland are assembled each other. The seal ring and gland are for waterproof structure and the fixing cap is for the tank. Each part is fabricated of ABS plastic by a 3D printer. ABS plastic material has high chemical resistance and high water resistance and it is cheap. The thickness of the main body is 2 mm.
This sensing device is cheap and consumes less power. It also has simple circuitry, good linearity, high sensitivity and wide measuring range.
You can find more information in his paper “A new design of inductive conductivity sensor for measuring electrolyte concentration in industrial field” presented to the SCI Journal “Sensors and Actuators A: Physical”.
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