• Nuclear Engineering and Technology
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• 제34권2호
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• pp.165-175
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• 2002

Effects of gap sizes (3.9-44.3 mm) of vertical annuli on nucleate pool boiling heat transfer of water at atmospheric pressure have been obtained experimentally. Through the study, tubes of the closed bottom have been investigated and results are compared with those of a single unconfined tube. According to the results, the annular condition gives much increase in heat transfer coefficient at moderate heat fluxes. The increase is much enhanced 3s the gap size decreases. At the same tube wall superheat (about 3.1 K) the heat transfer coefficient for the least gap size (i.e., 3.9 mm) is more than three times greater than that of the unconfined tube. However, deterioration of heat transfer occurs at high heat flux for confined boiling.

• 설비공학논문집
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• 제3권1호
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• pp.51-60
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• 1991

The aim of this study is to investigate the heat transfer characteristics in the transient cooling process of a high temperature wall. The slow transient cooling experiment was carried out with a copper block of high thermal capacity. The results of these experiments are as follows. 1. Temperature histories measured by the thermocouple, which is 0.99, 2.00, 2.99mm from the heat transfer surface showed monotonous during the cooling process. These variation are the curves of typical temperature histories in film-boiling, transition-boiling, and nucleate-boiling regions. 2. The temperature histories were measured by thermocouple installed in the copper block. The variations of the surface heat fluxes and surface temperature were computed from the numerical solution method TDMA from the measured temperature histories for radial position one dimensional heat transfer inverse problem. The boiling curves were found by the computed temperature histories. 3. The rewetting point which starts to change from film boiling to nucleate boiling is not connected with the mass velocity and it were found that the temperature of rewetting point indicated about $100^{\circ}C$. 4. The heat flux of rewetting point was about $10^5Kcal/m^2h$, at that time, the heat transfer coeficient indicated about $1000Kcal/m^2h^{\circ}C$ irrelevent to mass velocity. 5. The wall superheat decreases as the pressure increases. But I found that rewetting point appeared under higher condition in the wall temperature.

• 대한기계학회논문집B
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• 제38권6호
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• pp.465-474
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• 2014

본 연구에서는 비등 열유속 분배 모델의 예측 정확성을 검증하기 위하여 수직평판 자연대류 과냉 비등에서 기화, 급랭, 및 단상대류 열전달 기구에 대한 열유속 분배 특성을 실험적으로 조사하였다. 비등 열유속의 분배를 위해 적외선 열화상 기법과 전반사 가시화 기법을 동기화하여 비등 표면의 열유속 분포와 액상-기상 분포를 동시에 측정하여 분석하는 실험을 수행하였다. 실험은 대기압 조건에서 과냉도 $10^{\circ}C$를 가지는 물을 이용하여 수행하였으며, 벽면과열도 $12^{\circ}C$ 및 평균 열유속 $283kW/m^2$ 조건에 대한 실험 결과를 분석에 활용하였다. 실험을 통해 획득된 열유속 분배 결과는 상관식을 이용한 예측 결과와 큰 차이를 보였으며, 기포이탈직경과 기포이탈 시 주변의 과열액체층이 함께 뜯겨져 나가는 효과를 고려한 기포영향인자가 차이를 만드는 주요 원인들로 파악되었다.

• 대한기계학회논문집
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• 제16권1호
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• pp.87-94
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• 1992

본 연구에서는 실험적 해석을 통하여 액체-액체계의 직접접촉 막비등열전달 특성을 최소막비등지점과 막비등열유속의 측면에서 해석하고자 한다. 이는 직접접촉 비등에 대한 효율적인 열전달의 온도영역이나 주어진 액체-액체 계의 조합에 대한 증 기폭발의 발생가능성의 예측에 활용될 수 있을 것이다.

• 대한기계학회논문집B
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• 제26권11호
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• pp.1513-1520
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• 2002

Effect of Convex Surface Curvature on the Onset of Nucleate Boiling of Subcooled Fluid Flow in Vertical Concentric Annuli An experimental study has been carried out to investigate the effect of the transverse convex surface curvature of core tubes on heat transfer in concentric annular tubes. Water is used as the working fluid. Three annuli having a different radius of the inner cores, Ri=3.18mm, 6.35mm, and 12.70mm with a fixed ratio of Ri/Ro=0.5 are used over a range of the Reynolds number between about 40,000 and 80,000. The inner cores are made of smooth stainless steel tubes and heated electrically to provide constant heat fluxes throughout the whole length of each test section. Experimental result shows that heat flux values on the onset of nucleate boiling of the smaller inner diameter model is much higher than that of the larger size test model.

• 대한기계학회논문집B
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• 제30권1호
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• pp.74-81
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• 2006

Experiments to measure the pool boiling heat transfer on the micro-fin surfaces were performed with PF5060. The effects of various orientation and subcooling of heat surface on pool boiling performance were investigated under various heat-flux conditions for plain and micro-fin surfaces. The comparison between the results of this study and those of previous work shows a similar trend at the same conditions. From the results, it is proved that nucleate boiling performance is strongly dependent on the orientation, the micro-fin structure and the subcooling of heat surface. The heat flux on the surface with orientation angles of $45^{\circ}$ and $90^{\circ}$ was larger than that on horizontal surface(${\theta}=0^{\circ}$) at same wall superheat because of the effect of bubble sweeping. The nucleate boiling performance of micro-fin surfaces is enhanced by decreasing the fin size(WxL) and the pitch, respectively. The subcooling makes nucleate boiling performance lower for both micro-fin and plain surfaces.

• 대한기계학회논문집B
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• 제21권4호
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• pp.525-540
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• 1997

The effect of mold rotation on the transport process and resultant macrosegregation pattern during solidification of an Al-Cu alloy contained in a vertical axisymmetric annular mold cooled from the inner wall is numerically investigated. The mold initially at rest starts to rotate at a prescribed angular velocity simultaneously with the beginning of cooling. Computed results for a representative case show that the mold rotation essentially suppresses the development of both thermal and solutal convections in the melt, creating distinct characteristics such as the liquidus front, flow pattern and temperature distribution from those for the stationary mold. Thermal convection which develops at the early stages of cooling is soon extinguished by the rotating flow induced during spin-up, and thus does not effectively remove the initial superheat from the melt. On the other hand, solutal convection, though it weakens considerably and is confined within the mushy zone, still predominates over the solute redistribution process. With increasing the angular velocity, the solute transport in the axial direction is enhanced, whereas that in the radial direction is reduced. The final macrosegregation formed in the mold rotating at moderate angular velocities appears to be favorable in comparison with the stationary casting, in that not only relatively homogenized composition is achieved, but also a severely positive-segregated channel is restrained.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2000년도 춘계학술대회논문집B
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• pp.373-378
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• 2000

An experimental investigation on the incipience of nucleate boiling in forced flow of water is performed as a verification and extension of previous analysis. The effects of the subcooling, Reynolds number and surface curvature on the onset of nucleate boiling(ONB) in a concentric annulus flow channel with smooth inner heating surface is investigated experimentaly. Through flow visualization, the boiling phenomenon was observed directly and the experimental results were examined to find ONB heat flux. The results show that the variation of heat flux at ONB is increased linearly as the Reynolds number and subcooling are increased. The effect of surface curvature is very great specially for a small radius when radius of the inner heating tube is increased, the heat flux at ONB is almost inversely increased for the range of this investigation. It is found that the effect of convex surface curvature on ONB heat flux is very significant for a small radius.

• International Journal of Air-Conditioning and Refrigeration
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• 제6권
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• pp.93-103
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• 1998

The nucleate boiling heat transfer experiments are performed using a ternary refrigerant R407C which is a candidate of alternatives of HCFC 22. The boiling phenomena of R-32, R-125 and R-134a which are the constituent refrigerants of R407C are also investigated. The nucleate boiling heat transfer coefficients of R407C are less than those of HCFC 22 which have the similar physical and transport properties. In our experimental pressure range, which is similar to the operational pressure of air conditioning system, the deterioration of boiling heat transfer coefficients of mixture refrigerant R407C does not appear for moderate wall superheat region. Since nucleate boiling heat transfer coefficients cannot be obtained from ideal mixing law of mixture, Thome's method was used to predict. To account for the heat flux effect and system pressure in Thome's method, the correcting factor, a(P.L1T), was introduced and obtained from experiments for ternary refrigerant R407C.

• 한국주조공학회지
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• 제22권3호
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• pp.109-113
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• 2002

A good fluidity of high strength Al-alloys is required to cast thin wall castings needed to reduce the weight of cast parts. The fluidity, measured as the length to which the metal flows in a standard channel, is affected by many factors, such as the pouring temperature, solidification type of the alloy, the channel thickness, melt head, mold materials and temperature, coating etc. Therefore the experimentally measured fluidity scatters very much and makes it difficult to estimate the fluidity of a melt with a few measurements. The effect of Ti content and grain refinement on the fluidity of high strength aluminum alloy was investigated with a test casting with 8 thin flow channels to reduce the scattering of the fluidity results. The fluidity of Al-4.8%Cu-0.6%Mn Al-6.2%Zn-1.6%Mg-1.0%Cu and well-known commercial aluminum alloy, A356 was tested. Initial content of Ti was varied from 0 to 0.2wt% and Al-5Ti-B master alloy was added for grain refinement. The flow length varied linearly with superheat. By adding Ti and Al-5Ti-B, the fluidity increased. The grain size decreased by adding grain refiner at the same time. The fluidity depended on the degree of grain refinement. The fluidity of the alloy solidifying in mushy type is improved by grain refinement, because grain refinement increases the solid fraction at the time of flow stoppage.