Multi-zone 모델링을 통한 온도성층화와 농도성층화가 존재하는 DME HCCI 엔진의 운전영역에 관한 수치해석연구

정동원(Dongwon Jeong),임옥택(Ocktaeck Lim) 한국자동차공학회 2011 한국 자동차공학회논문집 Vol.19 No.2

This work investigates the potential of in-cylinder thermal stratification and fuel stratification for extending the operating ranges in HCCI engines, and the coupling between thermal stratification and fuel stratification. Computational results areemployed. The computations were conducted using both a custom multi-zone version and the standard single-zone version of the Senkin application of the CHEMKINII kinetics rate code, and kinetic mechanism for di-methyl ether (DME). This study shows that the potential of thermal stratification and fuels stratification for extending the high-load operating limit by a staged combustion event with reduced pressure-rise rates is very large. It was also found that those stratification offers good potential to extend low-load limit by a same mechanism in high-load. However, a combination of thermal stratification and fuel stratification is not more effective than above stratification techniques for extending the operating ranges showing similar results of fuel stratification. Sufficient condition for combustion (enough temperature for) turns misfire in low-load limit to operate engines, which also leads to knock in high-load limit abruptly due to the too high temperature with high. DME shows a potential for maximizing effect of stratification to lower pressure-rise rate due to the characteristics of low-temperature heat release.

Multi-Zone 모델링을 통한 온도성층화와 농도성층화가 존재하는 DME HCCI 엔진의 운전영역 관한 수치해석연구

정동원(Dongwon Jeong),임옥택(Ockteack Lim) 한국자동차공학회 2010 한국자동차공학회 부문종합 학술대회 Vol.2010 No.5

This work investigates the potential of in-cylinder thermal stratification and fuel stratification for extending the operating ranges in HCCI engines, and the coupling between thermal stratification and fuel stratification. Computational results are employed. The computations were conducted using both a custom multi-zone version and the standard single-zone version of the Senkin application of the CHEMKINII kinetics rate code, and kinetic mechanism for di-methyl ether (DME). This study shows that the potential of thermal stratification and fuels stratification for extending the high-load operating limit by a staged combustion event with reduced pressure-rise rates is very large. It was also found that those stratification offers good potential to extend low-load limit by a same mechanism in high-load. However, a combination of thermal stratification and fuel stratification is not more effective than above stratification techniques for extending the operating ranges showing similar results of fuel stratification. Sufficient condition for combustion (enough temperature for Φ) turns misfire in low-load limit to operate engines, which also leads to knock in high-load limit abruptly due to the too high temperature with high Φ. DME shows a potential for maximizing effect of stratification to lower pressure-rise rate due to the characteristics of low-temperature heat release.

Risk Stratification of Childhood Medulloblastoma Using Integrated Diagnosis: Discrepancies with Clinical Risk Stratification

Cho Hee Won,Lee Hyunwoo,Ju Hee Young,Yoo Keon Hee,Koo Hong Hoe,Lim Do Hoon,Sung Ki Woong,Shin Hyung Jin,Suh Yeon-Lim,Lee Ji Won 대한의학회 2022 Journal of Korean medical science Vol.37 No.7

Background: Recent genomic studies identified four discrete molecular subgroups of medulloblastoma (MB), and the risk stratification of childhood MB in the context of subgroups was refined in 2015. In this study, we investigated the effect of molecular subgroups on the risk stratification of childhood MB. Methods: The nCounter® system and a customized cancer panel were used for molecular subgrouping and risk stratification in archived tissues. Results: A total of 44 patients were included in this study. In clinical risk stratification, based on the presence of residual tumor/metastasis and histological findings, 24 and 20 patients were classified into the average-risk and high-risk groups, respectively. Molecular subgroups were successfully defined in 37 patients using limited gene expression analysis, and DNA panel sequencing additionally classified the molecular subgroups in three patients. Collectively, 40 patients were classified into molecular subgroups as follows: WNT (n = 7), SHH (n = 4), Group 3 (n = 8), and Group 4 (n = 21). Excluding the four patients whose molecular subgroups could not be determined, among the 17 average-risk group patients in clinical risk stratification, one patient in the SHH group with the TP53 variant was reclassified as very-high-risk using the new risk classification system. In addition, 5 of 23 patients who were initially classified as high-risk group in clinical risk stratification were reclassified into the low- or standard-risk groups in the new risk classification system. Conclusion: The new risk stratification incorporating integrated diagnosis showed some discrepancies with clinical risk stratification. Risk stratification based on precise molecular subgrouping is needed for the tailored treatment of MB patients.

An Investigation about EGR Stratification on DME HCCI Combustion using Numerical Analysis

Narankhuu Jamsran,Ocktaeck Lim 한국자동차공학회 2011 한국자동차공학회 학술대회 및 전시회 Vol.2011 No.11

This work investigates the potential of in-cylinder EGR stratification for extending the operating range engines, and the coupling both thermal stratification and fuel stratification. The numerical analysis were done using both single-zone and multi-zone version of CHEMKIN-II kinetics rate code, and kinetic mechanics for DME. CO₂ is used the presence of EGR in calculation. In order to explore the appropriate stratification of EGR, its four cases were tested by coupling on thermal stratification, fuel stratification and combination of both fuel and thermal stratification and evaluated as fueling was increased to knocking conditions with very high loads (ØDME=0.40-0.55) for fixed intake temperature at BDC (400K). The EGR stratification performed in this work showed that the ringing can be reduced to an acceptable level by the use of a charge with proper CO₂ distribution and charge stratification to lengthen the combustion event in a compression stroke.

태양열 축열조 성층화 파괴 방지를 위한 분배기의 효과

손효석(Hyo Seok Son),홍희기(Hiki Hong) 대한설비공학회 2013 대한설비공학회 학술발표대회논문집 Vol.2013 No.6

Stratification in thermal storage tank is necessary to gain available energy in the active solar heat system. When the stratification is maintained for a long time, it represents higher available energy than non-stratified tank. Especially, if solar heat system has a return pipe line, it needs appropriate stratification enhancement device to prevent destruction of stratification. In this study, experiments were carried out on the effect of maintaining stratification by using diffuser and distributor. The experiment was performed 60 minutes in the same condition. As a result of experiments, when we installed the distributor, stratification was well maintained comparing with the diffuser. Moreover many punched distributor was shown the good degree of stratification and it was identified that there exsit flow rate range about internal distributor diameter.

Stratification and Salt-wedge in the Seomjin River Estuary under the Idealized Tidal Influence

황진환,장동민,김용훈 한국해양과학기술원 2017 Ocean science journal Vol.52 No.4

Advection, straining, and vertical mixing play primary roles in the process of estuarine stratification. Estuaries can be classified as salt-wedge, partially-mixed or well-mixed depending on the vertical density structure determined by the balancing of advection, mixing and straining. In particular, straining plays a major role in the stratification of the estuarine water body along the estuarine channel. Also, the behavior of a salt wedge with a halocline shape in a stratified channel can be controlled by the competition between straining and mixing induced by buoyancy from the riverine source and tidal forcing. The present study uses Finite Volume Coastal Ocean Model (FVCOM) to show that straining and vertical mixing play major roles in controlling along-channel flow and stratification structures in the Seomjin river estuary (SRE) under idealized conditions. The Potential Energy Anomaly (PEA) dynamic equation quantifies the governing processes thereby enabling the determination of the stratification type. By comparing terms in the equation, we examined how the relative strengths of straining and mixing alter the stratification types in the SRE due to changes in river discharge and the depth resulting from dredging activities. SRE under idealized tidal forcing tends to be partially-mixed based on an analysis of the balance between terms and the vertical structure of salinity, and the morphological and hydrological change in SRE results in the shift of stratification type. While the depth affects the mixing, the freshwater discharge mainly controls the straining, and the balance between mixing and straining determines the final state of the stratification in an estuarine channel. As a result, the development and location of a salt wedge along the channel in a partially mixed and highly stratified condition is also determined by the ratio of straining to mixing. Finally, our findings confirm that the contributions of mixing and straining can be assessed by using the conventional non-dimensional parameters with respect to salt-wedge behavior.

Experimental Study on Axial Stratification Process and Its Effects (I) - Stratification in Engine -

Ohm, In-Yong,Park, Chan-Jun The Korean Society of Mechanical Engineers 2002 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.16 No.11

This paper is the first of several companion papers, which investigate axial stratification process and its effects in an Sl engine. The axial stratification is very sophisticate phenomenon, which results from combination of fuel injection, port and in-cylinder flow and mixing. Because of the inherent unsteady condition in the reciprocating engine, it Is impossible to understand the mechanism through the analytical method. In this paper, the ports were characterized by swir and tumble number in steady flow bench test. After this, lean misfire limit of the engines, which had different port characteristic, were investigated as a function of swirl ratio and injection timing for confirming the existence of stratification. In addition, gas fuel was used for verifying whether this phenomenon depends on bulk air motion of cylinder or on evaporation of fuel. High-speed gas sampling and analysis was also performed to estimate stratification charging effect. The results show that the AFR at the spark plug and LML are very closely related and the AFR is the results of bulk air motion.

The Effect of Turbulence Penetration on the Thermal Stratification Phenomenon Caused by Coolant Leaking in a T-Branch of Square Cross-Section

Choi, Young-Don,Hong, Seok-Woo,Park, Min-Soo The Society of Air-Conditioning and Refrigerating 2003 International Journal Of Air-Conditioning and Refr Vol.11 No.2

In the nuclear power plant, emergency core coolant system (ECCS) is furnished at reactor coolant system (RCS) in order to cool down high temperature water in case of emergency. However, in this coolant system, thermal stratification phenomenon can occur due to coolant leaking in the check valve. The thermal stratification produces excessive thermal stresses at the pipe wall so as to yield thermal fatigue crack (TFC) accident. In the present study, effects of turbulence penetration on the thermal stratification into T-branches with square cross-section in the modeled ECCS are analysed numerically. Standard k-$\varepsilon$ model is employed to calculate the Reynolds stresses in momentum equations. Results show that the length and strength of thermal stratification are primarily affected by the leak flow rate of coolant and the Reynolds number of duct. Turbulence penetration into the T-branch of ECCS shows two counteracting effects on the thermal stratification. Heat transport by turbulence penetration from main duct to leaking flow region may enhance thermal stratification while the turbulent diffusion may weaken it.

SIMULATION OF THERMAL STRATIFICATION IN INLET NOZZLE OF STEAM GENERATOR

JOON-SUK JI,BUM-SU YOUN,HYUN-CHUL JEONG,김상녕 한국원자력학회 2009 Nuclear Engineering and Technology Vol.41 No.3

Due to thermal hydraulics phenomena, such as thermal stratification, various events occur to the parts of a nuclear power plant during their lifetimes: e.g. cracked and dislocated pipes and thermally fatigued, bent, and damaged supports. Due to the operational characteristics of the parts of the steam generator feedwater inlet horizontal pipe, thermal stratification takes place particularly frequently. However, the thermal stress due to thermal stratification at the steam generator feedwater inlet horizontal pipe was not reflected in the design stage of old plants(Kori Unit No. 1, 2, 3 and 4, Yeonggwang Unit No. 1 and 2, and Uljin Unit No. 1 and 2; referred to as old-style power plants hereinafter). Accordingly, a verification experiment was performed for thermal stratification in the horizontal inlet nozzle steam generator of old-style plants. If thermal stratification occurred in the horizontal pipe of an old-style power plant, numerical analysis of the temperature distribution of the pipes and fluids was conducted. The temperature distributions were compared at the curved part of the pipe and the horizontal pipe before and after the installation of the improved thermal sleeves designed to alleviate thermal stress due to thermal stratification. The thermal stress reduction measure was proven effective at the steam generator inlet horizontal pipe and the curved part of the pipe.

SIMULATION OF THERMAL STRATIFICATION IN INLET NOZZLE OF STEAM GENERATOR

Ji, Joon-Suk,Youn, Bum-Su,Jeong, Hyun-Chul,Kim, Sang-Nyung Korean Nuclear Society 2009 Nuclear Engineering and Technology Vol.41 No.3

Due to thermal hydraulics phenomena, such as thermal stratification, various events occur to the parts of a nuclear power plant during their lifetimes: e.g. cracked and dislocated pipes and thermally fatigued, bent, and damaged supports. Due to the operational characteristics of the parts of the steam generator feedwater inlet horizontal pipe, thermal stratification takes place particularly frequently. However, the thermal stress due to thermal stratification at the steam generator feedwater inlet horizontal pipe was not reflected in the design stage of old plants(Kori Unit No.1, 2, 3 and 4, Yeonggwang Unit No. 1 and 2, and Uljin Unit No. 1 and 2; referred to as old-style power plants hereinafter). Accordingly, a verification experiment was performed for thermal stratification in the horizontal inlet nozzle steam generator of old-style plants. If thermal stratification occurred in the horizontal pipe of an old-style power plant, numerical analysis of the temperature distribution of the pipes and fluids was conducted. The temperature distributions were compared at the curved part of the pipe and the horizontal pipe before and after the installation of the improved thermal sleeves designed to alleviate thermal stress due to thermal stratification. The thermal stress reduction measure was proven effective at the steam generator inlet horizontal pipe and the curved part of the pipe.