• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2004년도 추계학술대회
• /
• pp.1172-1177
• /
• 2004

The evolution of incipient soot particles has been examined by high resolution electron microscopy (HRTEM) and elemental analyzer in ethylene-air inverse diffusion flames. Laser Induced Incandescence(LII) and laser scattering methods were introduced for examining the change of soot volume fraction and morphological properties in combustion generated soot qualitatively. Soot particles, collected by thermophoretic sampling were analyzed by using HRTEM to examine the nano structure of precursor particles. HRTEM micrographs apparently reveal a transformation of condensed phase of semitransparent tar-like material into precursor particles with relatively distinct boundary and crystalline which looks like regular layer structures. During this evolution histories C/H analysis was also performed to estimate the chemical evolution of precursor particles. The changes of C/H ratio of soot particles with respect to residence time can be divided into two parts: one is a very slowly increasing regime where tar-like materials are transformed into precursor particles (inception process) the other is an increasing region with constant rate where surface growth affects the increase of C/H ratio dominantly (surface growth region). These results provide a clear picture of a transition to mature soot from precursor materials.

• 대한기계학회논문집B
• /
• 제24권11호
• /
• pp.1540-1548
• /
• 2000

Near-wake flow field downstream of a circular cylinder in the wake-transition regime where fine-scale secondary vortices have a spanwise wavelength of around one diameter has been studied by means of phase-averaging from cinematic PIV data. A cross-correlation algorithm in conjunction with the FFT(Fast Fourier Transform)analysis and an offset correlation technique is used for obtaining the velocity vectors. Which the help of very high sampling rate compared to the shedding frequency, it is possible to obtain phase-averaged flow fields although the shedding is not forced but natural. Phase -locked three-dimensional vortical structures are reconstructed form the phase-averaged data in one x-y(cross-sectional) and several z-x(spanwise-streamwise)planes. In this process of phase-averaging in a z-x plane, a technique to freeze the secondary vortices relative to the centerline is applied. The formation process of the secondary vortices is shown by considering spatial relations between the primary Karman and the secondary vortices and their temporal evolutions.

• 대한의용생체공학회:의공학회지
• /
• 제33권4호
• /
• pp.177-183
• /
• 2012

The inspiratory flow rate of a human is changed with the amount of the workload. The flow characteristic is affected by the inspiratory flow rate. In the flow field of airway, the both of turbulence intensity and secondary flow affect the deposition pattern of particles which is important for the drug-aerosol targeting. Thus the analysis of the flow characteristic in a human airway is important. The purpose of this study is to investigate the effects of the inspiratory flow rate on the flow characteristics in a human airway. The tubular airway is consistent with the oral cavity, pharynx, larynx and trachea. The relatively inspiratory flow rate is used at each case of human states regarding the workload. By the effect of geometric airway changes, transition to turbulent airflow after the larynx can occur with relaminarization further downstream. The low Reynolds number k-${\omega}$ turbulence model is used for analysis with flow regime. As the inspiratory flow rate is larger, the turbulence kinetic energy and secondary flow intensity increase in airway. On the other hand, the area of recirculation zone is smaller.

• Nuclear Engineering and Technology
• /
• 제52권7호
• /
• pp.1417-1428
• /
• 2020

The design of a nuclear reactor core requires basic thermal-hydraulic information concerning the heat transfer regime at which onset of nucleate boiling (ONB) will occur, the pressure drop and flow rate through the reactor core, the temperature and power distributions in the reactor core, the departure from nucleate boiling (DNB), the condition for onset of flow instability (OFI), in addition to, the critical velocity beyond which the fuel elements will collapse. These values depend on coolant velocity, fuel element geometry, inlet temperature, flow direction and water column above the top of the reactor core. Enough safety margins to ONB, DNB and OFI must-emphasized. A heat transfer package is used for calculating convection heat transfer coefficient in single phase turbulent, transition and laminar regimes. The main objective of this paper is to study the possibility of power upgrading of WWR-S research reactor from 2 to 10 MW_th. This study presents a one-dimensional mathematical model (axial direction) for steady-state thermal-hydraulic design and analysis of the upgraded WWR-S reactor in which two types of plate fuel elements are employed. FOR-CONV computer program is developed for the needs of the power upgrading of WWR-S reactor up to 10 MW_th.

• 한국양식학회지
• /
• 제11권3호
• /
• pp.345-352
• /
• 1998

조피볼락 Sebastes schlegeli 자어와 점농어 Lateolabrax sp. 자어를 대상으로 기아시 성장과 간세포 핵 크기 변화를 조사하였다. 전장과 습중량 성장에 있어 조피볼락과 점농어의 간세포 핵 크기는 첫 먹이 섭취 자어의 영양상태와 연관성을 보였다. 본 연구의 조피볼락과 점농어 자어에서의 기아시 간세포 핵 크기 조사 결과는 조피볼락과 점농어에서 초기 영양상태 판명의 지표가 될 수 있으며 초기 먹이 공급 공정시 내생적 영양상태에서 외생적 영양상태로의 효율적 전환을 가능케 할 것으로 사료된다.

• 대한기계학회논문집
• /
• 제11권5호
• /
• pp.810-818
• /
• 1987

본 연구에서는 이러한 개념을 바탕으로 오리피스 내의 국소 압력강하치의 통 계적 해석을 통한 유동양식의 판별을 시도하였다. 수평관내 물과 공기를 매질로 2상 유동을 형성하여 각 유동양식에 따른 압력강하치의 확률밀도함수를 구하였으며 순간압 력강하의 통계치를 분석하였다.본 연구에서 다룬 유동양식은 기포유동, 플러그(pl- ug)유동, 슬러그(slug)유동, 성층(stratified)유동 등이며 천이영역에 대한 실험자료 도 포함하였다. 끝으로 이러한 유동양식 판별법을 사용하여 유동선도를 완성하였으 며 다른 선도와 비교분석하였다.

• 대한기계학회:학술대회논문집
• /
• 대한기계학회 2002년도 학술대회지
• /
• pp.245-248
• /
• 2002

Vortex lock-on or resonance in the flow behind a circular cylinder is visualized by a time-resolved PIV when a single frequency oscillation is superimposed on the mean incident velocity. Measurements are made of the $K{\'{a}}rm{\'{a}}n$ vortices in the wake-transition regime at the Reynolds number 360. Basically, natural shedding state is observed to compare with lock-on state. Wake motion by the change of the shedding frequency of lock-on state is investigated. When lock-on occurs, the vortex shedding frequency is found to be half the oscillation frequency as expected from previous experiments. The physical flow phenomena of natural shedding and lock-on states are analyzed with physical parameters of recirculation and vortex formation region. Consequently, it is found that the change of wake bubble plays an important role in the flow at the lock-on state. Vortex formation region is also actively changed like recirculation region as the lock-on occurs. Therefore, it is deduced that the recirculation region is closely related with the vortex formation region.

• 한국자기학회:학술대회 개요집
• /
• 한국자기학회 2003년도 하계학술연구발표회 및 한.일 공동심포지엄
• /
• pp.21-21
• /
• 2003

Microstructure and magnetic transport phenomina in rf sputtered AlN/Co type ten-layered discontinuous films of nanoscaled [AlN (3 nm)/Co (t nm)]...$\sub$10/ with t$\sub$Co/=1.0∼2.0 nm have been investigated. The microstructure and tunneling magnetic resistance of the samples are strongly dependent on the thickness of Co layer. Negative tunneling magneto-resistance due to the spin-dependent transport has been observed along the current-in-plane configuration in the samples having the Co layers below 1.6 nm thick. When the thickness of Co layer was less than 1,2 nm, randomly oriented granular Co particles were completely isolated and embedded in amorphous AlN matrix, and the films showed the superparamagnetic behavior with a high MR value of Δ$\rho$/$\rho$$\sub$0/=1.8 %. As t$\sub$Co/ increases, a transition from the regime of co-existence of superparamagnetic and ferromagnetic behaviors to ferromagnetic behavior was observed. Tunneling barrier called "decay length for tunneling" for the films having the thickness of Co layer from 1.4 to 1.6 nm was measured to be ranged from 0.004 to 0.021 ${\AA}$$\^$-1/.

• Progress in Superconductivity
• /
• 제3권2호
• /
• pp.140-145
• /
• 2002

An enhancement of the c-axis differential conductance around the zero-bias voltage near the superconducting transition temperature $T_{c}$ has been observed in $Au/Bi_2$$Sr_2$$CaCu_2$$O_{8+x}$ junctions. We attribute such an enhancement to the Andreev reflection (AR) between the surface Cu-O bilayer with suppressed superconductivity and the next superconducting Cu-O bilayer. The continuous evolution of the differential conductance, from gap like depression to an AR-like peak structure, around the zero-bias voltage points to weakening of the barrier strength of the nonsuperconducting layer between adjacent Cu-O bilayers as temperature approaches $T_{c}$ from below. The peak structure disappeared just below the bulk $T_{c}$ value of underdoped Bi2212 single crystals, whereas it survived up to ~1 K above $T_{c}$ in Junctions prepared on slightly overdosed crystals. According to a recently proposed theoretical consideration, a wider temperature range of the AR a bone $T_{c}$ is expected in the underdoped regime when phase-incoherent preformed pairs emerge in the pseudogap state. Our result is in contradiction to the preformed pair scenario. scenario.o.

• 대한기계학회논문집B
• /
• 제25권10호
• /
• pp.1355-1364
• /
• 2001

Characteristics of secondary vortices is topologically investigated in the near-wake region of a circular cylinder, where the Taylor's hypothesis does nut hold. The three-dimensional flow fields in the wake-transition regime were measured by a time-resolved PIV for various planes of view. The convection velocities of the Karman and secondary vortices are evaluated from the trajectory of the vortex center. Then, saddle points are determined by applying the critical point theory. It is shown that the inclination angle of the secondary vortices agrees well with the previous experimental data. The flow fields in a moving frame of reference have several critical points and the mushroom-like structure appears in the streamline patterns of the secondary vortices. Since the distributions of fluctuating Reynolds stresses defined by triple decomposition are closely related with the existence of secondary vortices, the physical meaning of them is explained in conjunction with the vortex center and saddle point trajectories.