• 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.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 2001년도 춘계학술발표회요약집
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• pp.154-154
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• 2001

• Nuclear Engineering and Technology
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• 제52권9호
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• pp.1939-1944
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• 2020

Pool boiling heat transfer of a copper microporous coating was experimentally studied in borated water with a concentration of boric acid from 0.0 to 5.0 vol percent (vol%) to determine the effect of boric acid on boiling heat transfer in water. A high-temperature, thermally conductive microporous coating (HTCMC) was created by sintering copper powder with an average particle size of 67 ㎛ onto a 1 cm × 1 cm plain copper surface with a coating thickness of ~300 ㎛ within a furnace in a vacuum environment. The tests showed that the nucleate boiling heat transfer coefficient (NBHT) of HTCMC became slightly less enhanced as the concentration of boric acid increased but the NBHT coefficient values were still significantly higher than those of the plain surface. The critical heat flux (CHF) values from 0 to 1.0 vol% were maintained at ~2,000 kW/㎡, and then, they gradually decreased down to ~1,700 kW/㎡ as the concentration increased further to 5.0 vol%. It is believed that the micro-scale pores of the HTCMC were partially blocked by the high boric acid concentration during the nucleate boiling such that the small bubbles were not effectively created using the HTCMC reentrant cavities as the boric acid concentration increased.

• 대한기계학회논문집B
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• 제34권11호
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• pp.983-990
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• 2010

본 연구에서는 냉각면의 표면온도가 비교적 낮은 강제대류 및 핵비등영역에서 다양한 종류의 마이크로 휜이 가공된 냉각면의 분무냉각 열전달에 대해 실험적으로 연구하였다. 실험결과로부터 냉각면 표면에 가공된 마이크로 휜은 분무냉각 열전달을 촉진시키며, 냉각휜의 크기와 종류에 상관없이 분무유량이 증가할수록 분무냉각 열전달도 큰 폭으로 증가하고 있음을 알 수 있었다. 또한 희박한 분무영역에서는 냉각면에 가공된 휜의 크기와 형상이 분무냉각 열전달에 큰 영향을 미치고 있으나, 분무유량이 증가할수록 이러한 경향은 점차 약해져 본 실험에서 가장 높은 분무유량조건에서는 편평한 냉각면을 제외한 모든 냉각면의 열유속이 거의 동일하게 나타나고 있음을 알 수 있었다.

• 대한기계학회논문집B
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• 제29권5호
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• pp.633-642
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• 2005

Nucleate pool boiling experiments for R11 under a constant wall temperature condition were carried out. A microscale heater array was used for the heating and the measurement of high temporal and spatial resolution by the Wheatstone bridge circuit. Very sensitive heat flow rate data were obtained by the control for the surface condition with high time resolution. The measured heat flow rate shows a discernable peak at the initial growth stage and reaches an almost constant value. In the thermal growth region, bubble shows a growth proportional to $t^{\frac{1}{5}}$. The bubble growth behavior is analyzed with a dimensionless parameter to compare with the previous results in the same scale. As the wall superheat increases, the departure diameter and the departure time increase, and the waiting time decreases. But the asymptotic growth rate is not affected by the wall superheat change. The effect of the wall superheat is resolved into the suggested growth equation. Dimensionless parameters of time and bubble radius characterize the thermal growth behavior well, irrespective of wall condition. The comparison between the result of this study and the previous results shows a good agreement at the thermal growth region. The quantitative analysis for the heat transfer mechanism is conducted with the measured heat flow rate behavior and the bubble growth behavior. The required heat flow rate for the volume change of the observed bubble is about twice as much as the instantaneous heat flow rate supplied from the wall.

• 대한기계학회논문집B
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• 제29권2호
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• pp.169-179
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• 2005

Nucleate boiling experiments on heating surface of constant wall temperature were performed using R113 for almost saturated pool boiling conditions. A microscale heater array and Wheatstone bridge circuits were used to maintain a constant wall temperature condition of heating surface and to measure the heat flow rate with high temporal and spatial resolutions. Bubble images during the bubble growth were taken as 5000 frames per second using a high-speed CCD camera synchronized with the heat flow rate measurements. The bubble growth behavior was analyzed using the new dimensionless parameters for each growth regions to permit comparisons with previous experimental results at the same scale. We found that the new dimensionless parameters can describe the whole growth region as initial and later (thermal) respectively. The comparisons showed good agreement in the initial and thermal growth regions. In the initial growth region including surface tension controlled, transition and inertia controlled regions as divided by Robinson and Judd, the bubble growth rate showed that the bubble radius was proportional to $t^{2/3}$ regardless of working fluids and heating conditions. And in the thermal growth region as also called asymptotic region, the bubble showed a growth rate that was proportional to $t^{1/5}$, also. Those growth rates were slower than the growth rates proposed in previous analytical analyses. The required heat flow rate for the volume change of the observed bubble was estimated to be larger than the heat flow rate measured at the wall. Heat, which is different from the instantaneous heat supplied through the heating wall, can be estimated as being transferred through the interface between bubble and liquid even with saturated pool condition. This phenomenon under a saturated pool condition needs to be analyzed and the data from this study can supply the good experimental data with the precise boundary condition (constant wall temperature).

• 한국태양에너지학회 논문집
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• 제29권5호
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• pp.35-44
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• 2009

In this study, boiling heat transfer coefficients(HTCs) and critical heat flux(CHF) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nano tubes(CNTs) dispersed at $60^{\circ}C$. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001, 0.001, 0.01, and 0.05%. For dispersion of CNTs, polyvinyl pyrrolidone(PVP) is used in distilled water. Pool boiling HTCs are taken from $10kW/m^2$ to critical heat flux for all nanofluids. Test results show that the pool boiling HTCs of the nanofluids are lower than those of pure water in entire nucleate boiling regime. On the other hand, critical heat flux is enhanced greatly showing up to 200% increase at volume concentration of 0.001% CNTs as compared to that of pure water. This is related to the change of surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of heat transfer surface are decreased due to this layer. The thin layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, maintains the nucleate boiling even at very high heat fluxes and reduces the formation of large vapor canopy at near CHF resulting in a significant increase in CHF.

• Journal of Advanced Marine Engineering and Technology
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• 제39권7호
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• pp.702-708
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• 2015

본 연구에서는 FC-72를 작동 유체로 사용하여 이상 유동 비등에 관한 실험을 수행하였다. 마이크로 채널은 깊이 0.2mm, 폭 0.45mm, 길이 60mm 그리고 채널의 개수는 15개로 구성되었다. 실험은 질량유속과 열유속 각각 $200-400kg/m^2s$, $5.6-49.0kW/m^2$ 범위와 증기 건도 0.02-0.93 범위에서 수행되었다. 실험 결과에 따르면 낮은 건도(x<0.2)영역에서는 핵비등에 의한 열전달이 지배적으로 작용하며, 그 이상의 증기 건도 영역에서는 핵비등과 강제 대류 비등의 영향이 복합적으로 작용하는 것으로 나타났다. 핵비등과 강제 대류 비등은 각각 비등수와 대류수에 관한 함수로 표현할 수 있으며, 실험에서 얻어진 열전달 계수는 기존의 상관식에 의한 열전달 계수와 비교하였다.

• 한국에너지공학회:학술대회논문집
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• 한국에너지공학회 1995년도 춘계학술발표회 초록집
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• pp.185-198
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• 1995

This is an experimental study conducted to visualize the nucleate boiling phenomena and flow regimes occurring inside the liquid pool in a closed two-phase thermosyphon. To meet this purpose, an annular-type thermosyphon was designed and manufactured using a glass tube and a stainless steel tube, being assembled axisymmetrically. The heat to be supplied to the working fluid is generated within a very thin layer of stainless steel tube wall by applying a high frequency electromagnetic field through the induction coil, axisymmetrically set around the evaporator zone. Some important results were as follows ; 1) Considering the structural complexity of the tested thermosyphon, it showed good performance for the range of heat flux 2＜ q" ＜25kW/$m^2$ and saturation vapor pressure, 0.1＜Pv＜1.1bar 2) different type of nucleating boiling regimes were observed as described below, -Pulse boiling regime : Flow pattern changed cyclically with time during 1 cycle of pulse boiling process. The onset of Nucleation was followed by expulsive growing of vapor bubble, resulting in the so called blow-up phenomenon, massive expulsion of large amount of liquid around the bubble. -Transient : Some spherical vapor bobbles were observed growing out from 2~3 nucleating sites, that was dispersed at the lower part of the heated tube wall in the liquid pool. But the rest upper region above the nucleating sites were filled with churns or bubbles of vapor. -Continuous nucleate boiling regime : The whole zone of evaporator was filled with lots of spherical vapor bubbles, and the bubbles showed tendency to decrease in diameter as the heat flux increased.ased.