• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 추계학술대회논문집B
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• pp.124-130
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• 2001

The heat transfer and flow measurements were made on a cylindrical pedestal mounted on a flat plate with a turbulent impinging air jet. The heat transfer coefficient distributions on the flat plate were measured using the shroud-transient technique and liquid crystal was used to measure the surface temperature. The jet Reynolds number (Re) is 23,000, the dimensionless nozzle-to-surface distance (L/d) from 2 to 10, the dimensionless pedestal diameter-to-height (H/D) from 0 to 1.5, the dimensionless 2nd pedestal diameter-to-height ($H/D_2$) from 0 to 0.4 and the distance from the stagnation point to 2nd pedestal (p/D). The results show that for H/D = 0.5 to 1.5, the Nusselt number distributions on the plate surface exhibit a maximum between $r/d\;{\cong}\;1.0$ and 1.5. The presence of the pedestal appears to cause the flow separation and reattachment on the plate surface, which results in the maximum heal transfer coefficient. Also, for p/D = 2.5 and $H/D_2$ = 0.3, the local Nusselt number in the region corresponding to $r/d\;{\cong}\;1.1$ was increased up to 50% compared to that for $H/D_2=0$.

• 상하수도학회지
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• 제32권5호
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• pp.381-388
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• 2018

Microbubbles oxygen transfer to water was simulated based on experimental results obtained from the bubbles generation operated under varying liquid supply velocity to the multi-step orifices of the generator. It had been known that liquid supply velocity and bubble size are inversely related. In the oxygen transfer, a non-steady state was assumed and the pseudo stagnation caused the slow movement of bubbles from the bottom to the water surface. Two parameters were considered for the simulation: They represent a factor to correct the pseudo stagnation state and a scale which represented the amount of bubbles in supply versus time. The sum of absolute error determined by fitting regression to the experimental results was comparable to that of the American Society of Civil Engineers (ASCE) model, which is based on concentration differential as the driving force. Hence, considering the bubbles formation factors, the simulation process has the potential to be easily used for applications by introducing two parameters in the assumptions. Compared with the ASCE model, the simulation method reproduced the experimental results well by detailed conditions.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2009년 추계학술발표대회
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• pp.44-44
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• 2009

While the pinch instability theory (PIT) has been widely employed to analyze the spray transfer mode in the gas metal arc welding (GMAW), it cannot predict the detaching drop size accurately. The PIT is modified in this work to increase the accuracy of prediction and to simulate the molten tip geometry to be more physically acceptable. Since the molten tip becomes a cone shape in the spray mode, the effective wire diameter is formulated that the effective diameter is inversely proportional to current square. Modifications are also made to consider the finite length of the liquid column and current leakage through the arc. While the effective diameter influences drop transfer significantly, the current leakage has negligible effects. The effects of modifications on drop transfer are analyzed, and the predicted drop diameters show good agreements with the experimental data of the steel wire.

• 설비공학논문집
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• 제8권1호
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• pp.151-165
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• 1996

Relation between condensate retention and heat transfer performance is studied for condensation of CFC-11 on horizontal integral-fin tubes. Eight tubes with trapezoidally shaped integral fin density from 738fpm to 1654fpm and 10, 30 grooves are tested. The liquid retention angles are measured by the height gauge, and each tube is tested under static(non-condensing) condition (CFC-11, water) and under dynamic(condensing) condition (CFC-11). The analytical model predicts the amount of liquid retention on a horizontal integral-fin tubes within＋10 percent over most of the data. Average retention angle increases as both surface tension-to-density ratio($\sigma/\rho$) and fin density(fpm) increase, The tube having a fin density of 1299~1654fpm has the best heat transfer performance. The amount of surface flooding must keep below of 40 percent for best heat transfer performance at condensation. The tube having low number of fin density must be used for fluids having high values of $\sigma/\rho$(water, (TEX)$NH_3$, ect.) and the tube having high number of fin density must be used for the fluid having low values of $\sigma/\rho$(R-11, R-22, etc.)

• 대한설비공학회지:설비저널
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• 제5권3호
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• pp.169-177
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• 1976

The importance of surface conditions of nucleate boiling is well recognized and it has been known that the heat transfer to boiling liquid is closely related to the bubble population density. The bubble population density should depend on various factors such as heat flux, surface roughness, surface contamination, properties of liquid, etc. In this paper the effect of surface conditions on heat transfer in nucleate boiling is treated. The experiments were carried out with distilled water boiler, on the horizontal heating surfaces, sintered with various bronze particle, under atmospheric pressure. In addition, experimental investigation for the polished bronze surface was performed. By studing a coefficient Xb defined by eq. (9), which represents the bubble foaming ability of heating surface, generalized fomula on the heat transfer in the nucleate toiling were expressed. The coefficient $X_b$, determined empirically, is not constant and indicates a major influence of the sintered metal surfaces on the $\Delta$, necessary to sustain nucleate boiling at any given heat flux. In this study, the main results are obtained as follows; (1) At low temperature difference, the coefficient $X_b$ of sintered metal surface was found to he higher than the polished surface throughout the full range of experiments. (2) The optimum sintered structure showing the maximum coefficient $X_b$ has been confirmed to exist and it is encountered when particle diameter is $256{\mu}$.

• 한국분무공학회지
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• 제20권3호
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• pp.135-140
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• 2015

Flame transfer function is used to determine the relationship between flow fluctuations and heat release perturbations in a lean premixed gas turbine combustor. The characteristics of flame transfer function are known to depend greatly on flame geometries in addition to other various flow conditions. However, it is not easy to experimentally measure the flame transfer function under various actual combustor operating conditions in terms of time and cost. The current research tries to model the flame transfer function using CFD(Computational Fluid Dynamics). From the results, it is shown that the calculated steady flame geometry can be exactly captured with consideration of the wall heat transfer and radiations. Also, unsteady analysis results show the close characteristics of the flame transfer function to the measured one in both gain and phase.

• 설비공학논문집
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• 제11권6호
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• pp.912-920
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• 1999

The forced convective boiling heat transfer coefficients of R-407C were measured inside a horizontal tube 6.0mm I.D. and 4.0m long. The heat transfer coefficients increased according to an increase in heat flux at constant mass flux. Because nucleation was completely suppressed in the two-phase flow region with high quality, heat transfer coefficients in forced convective evaporation were higher than those in nucleate boiling region. Average heat transfer coefficients of R-407C were about 30 percent lower than the pure refrigerant correlation, due to mass transfer resistance at the gas-liquid interface. However, the total experimental data shows an agreement with the predicted data for ternary refrigerant mixtures with a mean deviation of 30%.

• Journal of Advanced Marine Engineering and Technology
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• 제7권1호
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• pp.64-78
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• 1983

In nucleate boiling, bubbles are created by the expansion of entrapped gas or vapor at small cavities in the surface of heat transfer. Namely, surface roughness is the important factor of heat transfer. This paper deals with the characteristics of boiling curve according to surface roughness. Freon-113 is used as the experimental fluid. The results are as follows; 1. In the case of the same as "q=C$\Delta$T$^{n}$ ", the lower numberical index "n", the larger heat transfer coefficient and the lower wall superheat "$\Delta$T" is obtained for the rougher surface. 2. In the working of every kind of heat transfer sruface with boiling, improvement of capabilities of heat transfer can be devised by adding suitable roughness on the heat transfer surface. 3. When the metal nets of moderate mesh number are established, the capabilities of heat transfer can be improved in evaporation of liquid in vessels. But in the case that the sucession of bubbles in checked by using the nets which are too tight, the generation of bubbles union decreases critical heat flux. decreases critical heat flux.

• 공업화학
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• 제8권3호
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• pp.416-424
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• 1997

본 연구에서는 고밀집 섬유체에 의한 정적 액-액 접촉기에 대한 이론적 모델링을 통하여 섬유체 접촉기 내부에서의 물질 이동과정과 각 상에서의 축 방향 농도분포 해석이 수행되었고, 여러 운전 조건하에서 출구 농도를 예측하여 실험치와 비교하였다. 본 섬유체 액-액 추출기는 그 구조적 특성 때문에 축 방향 분산이 없는 플러그 흐름 모델로 완벽하게 묘사될 수 있었으며 data fitting을 통해 결정되어 모델식에 적용된 물질전달 계수와 섬유체 추출기 축 방향 반위 길이 당 물질전달 면적의 곱의 항인 $k_a{\sigma}$은 그 섬유체 추출기의 고유 특성 값으로 사용될 수 있었고, 그 값은 약 0.0327cm2/sec이었다.

• Korean Chemical Engineering Research
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• 제46권3호
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• pp.521-528
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• 2008

중공사막 접촉기에 적절한 PVDF 중공사막을 상전이 공정에 의하여 제조하였으며, SEM 과 기체투과도에 의하여 그 특성을 분석하였다. 벤치규모 중공사막 접촉기에서 $SO_2$ 제거를 위한 흡수제는 수산화나트륨 수용액을 사용하였다. 기체는 shell side, 액체는 lumen side로 흐르는 counter-current로 실험하였으며, 흡수제의 농도, 기체의 유속, 액가스비, 유입 $SO_2$ 농도에 따른 영향을 알아보았으며, 수학적 모델링에 의해 막 물질전달계수($k_m$)를 측정하였다. 흡수제의 농도와 액기비가 증가함에 따라 기-액 계면에서 충분한 알칼리도가 제공되므로, 액체막 저항이 감소하며, 총괄물질전달 용량계수는 증가하였다. 기체유속이 증가함에 따라 기체막저항은 감소하게 되므로 총괄물질전달 용량계수는 증가하였다.