• Transactions of the Korean Society of Mechanical Engineers
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• v.6 no.2
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• pp.160-168
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• 1982

수평으로 놓여진 2개의 동심타원 사이의 환상공간에서의 자연대류현상이 수치계산 및 실험적으로 연구되었다. 수치적 연구는 정상 2차원 층류유동영역에서 수행되었으며 유선 및 등온선이 구하여 졌다. 또한 특수한 경우로서 수평동심원 사이의 환상공간에 관한 기존연구와 비교하여 좋은 일치 를 보았다. 실험적 연구에서는 3개의 실험모델을 제작하였으며 마하젠더 간섭계에 의한 간섭사진 으로부터 등온선 분포를 얻었으며 아울러 유동에 관한 정보도 얻을 수 있었다. 실험과 수치계산 결과는 상당히 좋은 일치를 보였으며 기존 동심원환상공간에 편심율이 추가된 이론전개 및 수치 계산방법의 적합성을 알 수 있었고 이로서 환상공간에서의 자연대류에 관한 연구를 확장시켰다.

• The Korean Journal of Rheology
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• v.4 no.1
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• pp.35-45
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• 1992

어미현상의 예측을 위하여 사각단면이 링을 환상압연하는 공정을 3차원 강소성 유 한요소법으로 해석하였다. 환상압연공정의 점진적이고 변형영역이 두롤 사이 근처로 국한되 므로 계산시간을 줄이기 위하여 두롤사이를 통과하는 부분만을 해석하였다. 변형도율의 분 포, 압연력, 구동롤의 토오크, 링과 롤 사이의 접촉면에서의 압력분포 및 변형된 링의 단면 모양을 계산하였다. 압연력, 구동롤의 토오크, 특히 변형된 링의 단면모양, 즉 어미현상은 계 산결과와 실험이 잘 일치하였다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.39 no.12
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• pp.953-963
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• 2015

There is an increasing need to understand the thermal-hydraulic phenomena, including the critical heat flux (CHF), in narrow rectangular channels and consider these in system design. The CHF mechanism under a saturated flow boiling condition involves the depletion of the liquid film of an annular flow. To predict this type of CHF, the previous representative liquid film dryout models (LFD models) were studied, and their shortcomings were reviewed, including the assumption that void fraction or quality is constant at the boundary condition for the onset of annular flow (OAF). A new LFD model was proposed based on the recent constitutive correlations for the droplet deposition rate and entrainment rate. In addition, this LFD model was applied to predict the CHF in vertical narrow rectangular channels that were uniformly heated. The predicted CHF showed good agreement with 284 pieces of experimental data, with a mean absolute error of 18. 1 % and root mean square error of 22.9 %.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.3
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• pp.213-219
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• 2013

This study examined the static characteristics of a spool valve with a hybrid lap between the spool land and the sleeve. The static equation for the pressure characteristics was derived from flow equations that depend on the spool displacement, and the final model was derived from $q_a=q_b=0$ because the pressure characteristics test needs to block the control port in the valve. The static equation for the flow characteristics was derived from the pressure characteristics when the control port is open ($q_a=q_b$, $p_a=p_b$). The characteristic equation in the shifted region was assumed from the proportional relationship between the pressure-flow characteristics and the spool displacement.

• Journal of Advanced Marine Engineering and Technology
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• v.39 no.7
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• pp.696-701
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• 2015

Two-phase flow boiling experiments were conducted in 15 micro-channels with a depth of 0.2 mm, width of 0.45 mm, and length of 60 mm. FC-72 was used as the working fluid, and the mass fluxes ranged from 200 to $400kg/m^2s$. Tests were performed over a heat flux range of $5-40kW/m^2$ and vapor quality range of 0.1-0.9. The heat transfer coefficient sharply decreased at a lower heat flux and then was kept approximately constant as the heat flux is increased. Based on the measured heat transfer data, the flow pattern was simply classified into bubbly, slug, churn, and wavy/annular flows using the existing method. In addition, these classified results were compared to the transition criterion to wavy/annular regime. However, it was found that the existing transition criterion did not satisfactorily predict the transition criterion to annular regime for the present data.

• Journal of Hydrogen and New Energy
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• v.18 no.3
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• pp.275-283
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• 2007

본 논문은 세관(ID<7 mm) 내 R-22와 R-134a의 증발 열전달과 유동양식에 대한 실험적 연구이다. R-22와 R-134a의 유동양식을 관찰하기 위해 내경 2와 8 mm의 파이렉스 튜브를 사용하였고, 열전달 계수는 내경 1.77, 3.35, 5.35 mm의 수평 평활동관에 대해서 측정하였다. 증발 유동양식에서 내경 2 mm의 환상류 영역이 내경 8 mm에 비해 저건도 영역에서 발생하는 것을 확인할 수 있었고, 내경 2 mm의 유동양식은 Mandhane의 선도와 많은 오차를 보였다. 세관(ID<7 mm) 내 증발 열전달 계수는 종래의 대구경관(ID>7 mm)에 비해 관직경에 대한 영향이 많이 나타나는 것을 알 수 있었다. 내경 1.77 mm의 열전달 계수는 내경 3.36 mm와 5.35 mm에 비해서 20내지 30% 정도 높은 것을 나타났다. 또한 종래의 열전달 상관식(Shah's, Jung's, Kandlikar's and Oh-Katsuda's correlation)과 비교한 결과, 실험 데이터는 상관식과 많은 이탈 정도를 보였다. 따라서 실험데이타를 기초로 세관내 R-22와 R-134a에 적용할 수 있는 증발 열전달 상관식을 새로이 제안하였다.

• Journal of Energy Engineering
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• v.9 no.4
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• pp.334-341
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• 2000

회전 히트파이프의 열 전달 특성은 내부 관벽에 형성되는 응축 액막 두께와 증발부로 귀한되는 응축액의 유동율에 의해 결정된다. 본 연구는 축 방향으로 그루브(groove)를 갖는 회전 히트파이프의 열 전달 성능에 대한 실험 연구로써, 그루브에 의한 효과를 파악하기 위해 2종류의 히트파이프를 제작하고 작동성은 시험을 수행하였다. 회전 히트파이프가 작동시, 원심력에 의해 그루브로 응축액의 유동을 촉진시키며, 따라서 응축부 벽면에 형성되는 액막 두께가 얇게 된다. 응축부에 그루브를 갖는 히트파이프의 열전달 계수는 풀 유동에서 2000~4000W/$m^2$$^{0}$ C, 환상 유동 영역에서 1500~2500W/$m^2$$^{0}$ C로써, 전체 원형단면을 갖는 히트파이프와 비교하여 약 1.5배 정도의 열저달 향상을 볼 수 있었으며, 열전달 한계는 약 40% 정도 향상되는 것으로 나타났다.

• Proceedings of the KSME Conference
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• 2001.11b
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• pp.330-336
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• 2001

Water heat transfer experiments were carried out in a uniformly heated annulus with a wide range of pressure conditions. The local heat transfer coefficients for saturated water flow boiling have been measured just before the occurrence of the critical heat flux (CHF) along the length of the heated section. The trends of the measured heat transfer coefficients were quite different from the conventional understanding for the heat transfer of saturated flow boiling. This discrepancy was explained from the nucleate boiling in the liquid film of annular flow under high heat flux conditions.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.10 no.6
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• pp.732-742
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• 1998

Prediction method for heat transfer coefficients in a horizontal smooth tube with forced convection condensation is proposed. In this paper, the analogy between momentum and heat transfer was applied to an annular flow regime and the logarithmic velocity distribution is applied to describe the velocity profile within the liquid film. Prediction results are compared with those of experimental ones. The test refrigerants are R113, R22, R134a, R407C(R33/R125/R134a, 23/25/52 wt%), R410A(R32/R125, 50/50 wt%) and R134a＋R123(R134a/R123, 85.5/14.5 wt%) which are used under operating conditions in a condenser of air-conditioner. The proposed prediction method shows good agreement with experimental data within$\pm 30%$ for pure refrigerants. For the mixture refrigerants including the ternary mixture refrigerant R407C, condensation heat transfer from this study are higher than those from experiments. By correcting the constant in two-phase frictional multiplier, the predicated heat transfer coefficients become similar to the experimental results.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.9
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• pp.1217-1228
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• 1998

In order to elucidate the operational characteristics of rotating heat pipes, the internal flow patterns and heat transfer performance are investigated. Flow patterns and its transition are studied with various rotational speeds by visualizing flows established inside a rotating tube. To verify those results of analysis, 2 heat pipes of the same geometries but fill charge rates of 7, 30% were manufactured and submitted to operating tests. Comparison of experimental results on heat transfer rate show a fairly good agreement with the analytical results. The analysis reveals that the optimum charge ratio is ranged in 4~7% depending on the quantity of thermal loads. but the heat pipe with 7% of fill charge ratio reached dry-out limitation at heat flux of $q^{{\prime}{\prime}}=6.2kW/m^2$ lower than that of analytic results. Transition of flow regime was well related to the correlation by Semena & Khmelev on transient centrifugal Froude Number Frc. But hysteresis phenomenon was observed in transition of flow regime, when the rotational speed was stepwisely changed in the way to undergo 1 cycle.