• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2005.11a
• /
• pp.437-444
• /
• 2005

Experiments were carried out in an atmospheric pressure, lab-scale gas turbine combustor to see the effect of partial premixing on unstable flame structure and Nox emission characteristics. The swirl angle is 45 deg., fuel-air mixing degrees were varied 0, 50 and 100% respectively at equivalence ratio ranging from 0.53 to 0.79. The evolution of phased-locked OH chemiluminescence images were acquired with an ICCD. NOx emission characteristics were also investigated at each experimental condition. The effect of the fuel-air mixing degree on the flame structure was obtained from phase-locked $OH^*$ images. And it was obtained from local heat release characteristics that the information about the region which the combustion instability was amplified or damped. It also could be confirmed that $\sigma$ has greatly influence on NOx emission characteristics at lean regimes. It would be expected that it could provide invaluable data for understanding the mechanism of combustion instability.

• Journal of Advanced Marine Engineering and Technology
• /
• v.37 no.8
• /
• pp.807-815
• /
• 2013

A numerical study on soot formation in a laminar ethylene diffusion flame at atmospheric pressure was conducted to obtain a better understanding of the effects of buoyancy on sooting flames under 0g and 1g using a gas-phase reaction mechanism and thermal and transport properties. A simple model was employed to predict soot formation, growth and oxidation with interactions between the gas phase chemistry and the soot chemistry taken into account. Results showed that the flames in 0g are much wider than that of 1g because of the thicker diffusion layer and reduction in axial velocity. The reduction in the axial velocity in 0g results in longer residence times, and resulting in greatly enhanced soot volume fraction. And, under zero-gravity, due to the lack of a buoyancy-induced instability, flame instability disappears.

• Atmosphere
• /
• v.18 no.4
• /
• pp.317-338
• /
• 2008

Analyses of observational data and numerical simulations were performed to understand the mechanism of MCSs (Mesoscale Convective Systems) occurred on 13-14 July 2004 over Jindo area of the Korean Peninsula. Observations indicated that synoptic environment was favorable for the occurrence of heavy rainfall. This heavy rainfall appeared to have been enhanced by convergence around the Changma front and synoptic scale lifting. From the analyses of storm environment using Haenam upper-air observation data, it was confirmed that strong convective instability was present around the Jindo area. Instability indices such as K-index, SSI-index showed favorable condition for strong convection. In addition, warm advection in the lower troposphere and cold advection in the middle troposphere were detected from wind profiler data. The size of storm, that produced heavy rainfall over Jindo area, was smaller than $50{\times}50km^2$ according to radar observation. The storm developed more than 10 km in height, but high reflectivity (rain rate 30 mm/hr) was limited under 6 km. It can be judged that convection cells, which form cloud clusters, occurred on the inflow area of the Changma front. In numerical simulation, high CAPE (Convective Available Potential Energy) was found in the southwest of the Korean Peninsula. However, heavy rainfall was restricted to the Jindo area with high CIN (Convective INhibition) and high CAPE. From the observations of vertical drop size distribution from MRR (Micro Rain Radar) and the analyses of numerically simulated hydrometeors such as graupel etc., it can be inferred that melted graupels enhanced collision and coalescence process of heavy precipitation systems.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.28 no.11
• /
• pp.1368-1375
• /
• 2004

Measurements of OH chemiluminescence in an atmospheric pressure, laboratory-scale dump combustor at equivalence ratios ranging from 0.63 to 0.89 were reported. The signal from the first electronically excited state of OH to ground state was detected through a band-pass filter with an ICCD. The objectives of this study are two: One is to see the effects of equivalence ratio on global heat release rate and local Rayleigh index distribution. To get the local Rayleigh index distribution, the line-of-sight images were inverted by tomographic method, such as Abel do-convolution. Another aim is to investigate the validity of using OH chemiluminescence acquired with an ICCD as a qualitative measure of local heat release. For constant inlet velocity and temperature, the overall intensities of OH emission acquired at different equivalence ratio showed periodic and higher value at high equivalence ratio. OH intensity averaged over one period of pressure increased exponentially with equivalence ratio. Local Rayleigh index distribution clearly showed the region of amplifying or damping the combustion instability as equivalence ratio increased. It could provide an information/insights on active control such as secondary fuel injection. Finally, local heat release rate derived from reconstructed OH images were presented fur typical locations.

• Korean Journal of Remote Sensing
• /
• v.34 no.3
• /
• pp.465-480
• /
• 2018

On 20 January 2017, the fresh snow cover which is more than 20 cm, accompaning with lightning occurred over Yeongdong coastal region for the first 3-hour of the heavy snowfall event. This study analyzed sounding observations in the heavy snow period which were including the measurements of wind profiler, radiometer and rawinsonde. The features examined from the vertical wind and temperature data at the two adjacent stations, Bukgangneung and Gangneung-Wonju National University, are summarized as follows: 1) The strong (30-40 kts) north-east winds were observed in the level from 2 to 6 km. The Strong atmospheric instability was found from 4 to 6 km, in which the lapse rate of temperature was about $-18^{\circ}C\;km^{-1}$. These features indicate that the deep convective cloud develops up to the height of 6 km in the heavy snowfall period, which is shown in the satellite infrared images. 2) The cooling was observed in the level below 1 km. At this time, the surface air temperature at Bukgangneung station decreased by $4^{\circ}C$. The narrow cooling zone estimated from AWS and buoy data was located in east-west direction. These are the features observed in the cold front of extratropical cyclone. The distributions of radar echo and lightning also show the same shape in east-west direction. Therefore, the results indicate that the Yeongdong thundersnow event was the combined precipitation system of deep convective cloud and cold frontal precipitation.

• Journal of the Korean earth science society
• /
• v.31 no.4
• /
• pp.322-334
• /
• 2010

Spatiotemporal characteristics of precipitation in Korea, associated with the weakening of Tropical Cyclones (TCs) around the Korean Peninsula ($32-36^{\circ}N$, $122-132^{\circ}E$) over the last 30 years (1979-2008), were investigated. Weakened TCs are classified as WEC (Weakened to Extratropical Cyclone) and WTD (Weakened to Tropical Depression). In WEC, precipitation was evenly distributed all over the Korean Peninsula and the greater precipitation was recorded in the southern coast. In WTD, the most precipitation was recorded in the southern coast but low precipitation was recorded in the central and inland areas of Korea. The difference of precipitation between WEC and WTD was not statistically significant in Region 2 (Jeollanam-do, Gyeongsangnam-do, southeastern part of Gyeongsangbuk-do, Jeju-do); however, the precipitation resulting from WEC was greater than that resulting from WTD in Region 1 (central area of Korea, Jeollabuk-do, inland of Gyeongsangbuk-do). In WEC, the developed upper-level potential vorticity (PV) and low-level temperature trough shifted to the northwest of TCs approaching Korea. In addition, an upper-level jet stream and strong divergence field were observed to the northeast of the TCs. It was assumed that these meteorological factors had induced baroclinic instability and diabatic process, which created a large precipitation area around the TCs. However, the intense PV, temperature trough, jet stream were not observed in WTD, which created a small precipitation area around the TCs.

• Atmosphere
• /
• v.29 no.3
• /
• pp.311-324
• /
• 2019

We employed a Multi-Angle Snowflake Camera (MASC) to quantitatively analyze snow particles at the ground level in the Yeongdong region of Korea. The MASC captures high-resolution photographs of hydrometeors from three angles and simultaneously measures fallspeed. Based on snowflake images of the several episodes in 2017 and 2018, we derived statistics of size, aspect ratio, orientation, complexity, and fallspeed of snow crystals, which generally showed similar characteristics to the previous studies in other regions of the world. Dominant snow crystal habits of January 22, 2018 generated by northerly were melted aggregates when 850 hPa temperature was about $-6{\sim}-8^{\circ}C$. Average fallspeed of snow crystals was $1.0m\;s^{-1}$ though its size gradually increased as temperature decreased. Another snowfall event (March 8, 2018) was driven by the baroclinic instability as accompanied with a deep trough. Snow crystal habits were largely rimed aggregates (complexity ~1.8) and melting particles of dark images. Meanwhile, in the extreme snowfall event whose snow rate was greater than $10cm\;hr^{-1}$ on January 20, 2017, main snow crystals appeared to be heavily rimed particles with relatively smaller size when convective clouds developed vertically up to 9 km in association with tropopause folding. MASC also could successfully measure a decrease in snow crystal size and an increase in riming degree after AgI seeding at Daegwallyeong on March 14, 2017.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.43 no.7
• /
• pp.585-592
• /
• 2015

Effects of opening distance on liquid-gas spray of pintle injector were experimentally investigated under atmospheric condition by using water and air as simulants of propellants. Discharge coefficients($C_d$) and mass flow rates were calculated with various injection conditions; 0.1 bar - 1.0 bar for water pressures and 0.2 mm - 1.0 mm for the pintle opening distances. Spray angles were measured from the spray images that were obtained by a shadowgraphy method. When opening distance is 0.2 mm, liquid sheet is not formed properly and it show non-uniform spray. than it can result in combustion instability. it has a weak correlation between the momentum flux ratio and the spray angle, while it has a strong correlation between the momentum ratio and spray angle. Finally the spray angles reduced exponentially when the momentum ratio increased and the spray angles converged to about 40 degrees.

• Atmosphere
• /
• v.27 no.4
• /
• pp.455-466
• /
• 2017

In Korea, property and human damages occur annually due to heavy precipitation during the summer. On August 8, 2015, heavy rainfall occurred in the Seoul metropolitan area due to an outflow boundary, and $77mmhr^{-1}$ rainfall was recorded in Gwangju, Gyeonggi Province. In this study, the simulation of the WRF numerical model is performed to understand the cause and characteristics of heavy rainfall using the Conditional Instability of the Second Kind (CISK), potential vorticity (PV), frontogenesis function, and convective available potential energy (CAPE) analyses, etc. Convective cells initiated over the Shandong Peninsula and located on the downwind side of an upper level trough. Large amounts of water vapor were supplied to the Shandong Peninsula along the southwestern edge of a high pressure system, and from the remnants of typhoon Soudelor. The mesoscale convective system (MCS) developed through CISK process and moved over to the Yellow Sea. The outflow boundary from the MCS progressed east and pushed cold pool eastward. The warm and humid air over the Korean Peninsula further enhanced convective development. As a result, a new MCS developed rapidly over land. Because of the latent heat release due to convection and precipitation, strong potential vorticity was generated in the lower atmosphere. The rapid development of MCS and the heavy rainfall occurred in an area where the CAPE value was greater than $1300Jkg^{-1}$ and the fronto-genesis function value of 1.5 or greater coincided. The analysis result shows that the MCS driven by an outflow boundary can be identified using CISK process.

• Atmosphere
• /
• v.26 no.1
• /
• pp.159-183
• /
• 2016

On 31 July 2014, there was a localized torrential rainfall ($58.5mm\;hr^{-1}$) caused by a strong convective cell with thunder showers over Daegwallyeong. In the surface synoptic chart, a typhoon was positioned in the East China Sea and the subtropical high was expanded to the Korean peninsula. A WRF (Weather Research and Forecasting) numerical simulation with a resolution of 1 km was performed for a detailed analysis. The simulation result showed a similar pattern in a reflectivity distribution particularly over the Gangwon-do region, compared with the radar reflectivity. According to the results of the WRF simulation, the process and mechanism of the localized heavy rainfall over Daegwallyeong are as follows: (1) a convective instability over the middle part of the Korean peninsula was enhanced due to the low level advection of warm and humid air from the North Pacific high. (2) There was easterly flow from the coast to the mountainous regions around Daegwallyeong, which was generated by the differential heating of the insolation among Daegwallyeong and the Yeongdong coastal plain, and nearby coastal waters. (3) In addition, westerly flow from the western part of Daegwallyeong caused a strong convergence in this region, generating a strong upward motion combined by an orographic effect. (4) This brought about a new convective cell over Daegwallyeong. And this cell was more developed by the outflow from another thunderstorm cell to the south, and finally these two cells were merged to develop as a strong convective cell with thunder showers, leading to the record breaking maximum rainfall per hour ($58.5mm\;hr^{-1}$) in July.