• 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 %.

• Proceedings of the KSME Conference
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• 2001.06e
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• pp.131-135
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• 2001

The atomization characteristics of an annular liquid (water) sheet of small radius with a core gas (air) flow were studied. Different sizes of annular gaps (0.2, 0.4 and 0.8 mm) were tested to find the effect of liquid sheet thickness on SMD. The inner diameter of the gas port for the core gas flow was 4 mm. Cross-section averaged SMD was measured for various liquid and gas velocities. Regions of the SMD decrease with the increase of the liquid velocity always existed regardless of the liquid sheet thickness. This attributes to the transition of the flow patterns of spray and also to the aerodynamic interaction between the atomizing gas and the ripples on the liquid sheet surface.

• Proceedings of the KSME Conference
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• 2001.06d
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• pp.201-207
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• 2001

We modeled the liquid film dryout(LFD) process for both tube and annulus which have uniformly heated vertical channels. We set phenomenological initial conditions in the model. The initial void fraction on the onset of the annular flow location is derived from the physical chum-to-annular flow criterion with the help of the drift-flux-model. The initial thermodynamic-equilibrium-quality is calculated by iteration with the flow quality to find the onset of the annular-flow location. Present model tends to predict very well at the lower exit quality but under-estimates at the higher exit quality. We found that the prediction error of the present model is gradually bigger as the inlet subcooling approaches near the saturation. We obtained excellent results for both tube and annulus channels as the mean of 0.97 and root-mean-square error of 11% for the number of 3883 experimental data on tubes, and of 0.96 and of 12% for 593 on annuli. The present model extended the applicable range to the relatively low exit quality region than previous LFD models.

• 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 Korea Society for Energy Engineering kosee Conference
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• 1996.10b
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• pp.181-189
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• 1996

회전열파이크는 전기모터, 고온가스터빈 등의 냉각에 이용함으로써 성능개선을 얻을 수 있다는 점에서 지난 20년 동안 많은 연구가 이루어져왔다. 그동안 내부벽면형상개발은 Taper를 설치하거나, 계단식 단면변화 또는 Impeller설치와 같은 기술들이 연구되었다. 본 연구는 회전 열파이프 용기내벽에 Coil Insert를 설치하여 응축액막 유동효과에 대한 가시화실험 및 열전달 성능실험을 통하여 그 특성을 검토하였고 아래와 같은 결과를 얻을 수 있었다. 1) Pool 영역에서는 Coil Insert에 의한 축방향 펌프효과가 매우 크며, 그 효과는 나선각이 클수록 크게 나타난다. 2) Coil Insert는 Pool에서 환상류로 천이를 촉진시키며, 나선각이 클수록 그 효과가 크게 나타난다. 3) Coil insert 열파이프의 열저항에 미치는 효과는 크지 않은 것으로 보인다. 4) Coil Insert에 의해 Dry-out한계의 증가가 예상되지만, 실험장치의 열부하한계에 의해 확인할 수 없었다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.6
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• pp.714-725
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• 2003

The theory of the current-type electromagnetic flowmeter for a high temporal resolution was developed for two-phase flow measurements. To predict the output of the current-type flowmeter, the three-dimensional virtual potential distribution C and the newly introduced flow pattern coefficient f were derived and computed. The output of flowmeter depends on the liquid conductivity (sensitive to temperature) and flow configurations of the two-phase flow with the sinusoidal excitation over 100 Hz. The flow pattern coefficient was specially devised to separate the dependency on the flow configuration of the two-phase flow from that on the liquid conductivity which can be expressed with the calibration of single-phase flow. Using the finite difference method, the three-dimensional virtual potential distributions were computed for the electrode of finite size. By taking derivative of the virtual potential, the weight functions were evaluated and compared with existing analytic series solution for the point-electrode. There was a reasonable correspondence between the present and existing results. In addition, the flow pattern coefficients were evaluated for annular flows with various film thicknesses, and compared with the experimental results by the impedance spectroscopy. The numerical results agreed well with the experimental data.