• 대한기계학회논문집
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• 제19권9호
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• pp.2353-2364
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• 1995

The ignition phenomena of a solid fuel plate of polymethyl-methacrylate(PMMA), which is vertically positioned and exposed to a thermal radiation source, is numerically studied here. A two-dimensional transient model includes such various aspects as thermal decomposition of PMMA, gas phase radiation absorption, gas phase chemical reaction and air entrainment by natural convection. Whereas the previous studies considers the problem approximately in a one-dimensional form by neglecting the natural convection, the present model takes account of the two-dimensional effect of radiation and air entrainment. The inert heating of the solid fuel is also taken into consideration. Radiative heat transfer is incorporated by th Discrete Ordinates Method(DOM) with the absorption coefficient evaluated using gas species concentration. The thermal history of the solid fuel plate shows a good agreement compared with experimental results. Despite of induced natural convective flow that induces heat loss from the fuel surface, the locally absorbed radiant energy, which is converted to the internal energy, is found to play an important role in the onset of gas phase ignition. The ignition is considered to occur when the rate of variation of gas phase reaction rate reaches its maximum value. Once the ignition takes place, the flame propagates downward.

• 생물환경조절학회지
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• 제28권3호
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• pp.204-211
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• 2019

본 연구는 국내 온실의 환경설계 기준 설정에 필요한 기초자료 제공을 목적으로 온수난방 방식을 채택하고 있는 상업용 온실 2곳에서 난방 중 열환경 계측 실험을 실시하고, 온수난방 배관의 열전달 특성을 분석하여 난방배관의 단위길이 당 방열량 자료를 제시하였다. 실험 기간동안 두 온실의 평균기온은 각각 $16.3^{\circ}C$와 $14.6^{\circ}C$로 조절되었으며, 난방배관의 온수 온도는 평균 $52.3^{\circ}C$와 $45.0^{\circ}C$로 관측되었다. 실험결과 난방배관 표면의 자연대류열전달계수는 $5.71{\sim}7.49W/m^2^{\circ}C$의 범위로 분석되었다. 난방배관 내의 유속이 0.5m/s 이상일 때에는 관내의 수온과 관 외부의 표면온도 차이가 크지 않은 것으로 나타났다. 이를 바탕으로 난방배관의 관류열전달계수를 수평원통에서의 층류 자연대류열전달계수의 형태로 유도하였다. 유도된 관류열전달계수 식을 변형하여 관의 규격과 온수-실내공기의 온도차를 입력 변수로 하는 난방배관의 단위길이 당 방열량 산정식을 개발하였다. 본 연구 결과를 기존에 제공되고 있는 국내외 여러 자료와 비교한 결과 JGHA 자료와 가장 유사한 것으로 나타났다. 국내 온실의 설계에서 적용하고 있는 NAAS 자료와 국외의 BALLS 및 ASHRAE 자료는 값이 너무 큰 것으로 판단된다. 따라서 국내 온실의 환경설계기준을 제정하고, 고시하기 위해서는 추가적인 실험을 통해 이 부분에 대한 충분한 검토가 필요할 것으로 판단된다.

• 한국해양공학회지
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• 제24권5호
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• pp.22-30
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• 2010

The fuel tank of a ship is filled with heavy fuel oil (HFO) that has a very high viscosity. In order to inject the HFO into the engine easily, heating coils are usually installed inside the fuel tank to heat the HFO and lower its viscosity. Currently, several different types of heating coils are used, e.g., fin-type, bare-type, drum-type, and shell-and-tube-type. It is well known that the shell-and-tube-type heating coil has good performance and high efficiency. In this study, experiments were conducted to determine the heat transfer efficiencies of three different shell-and-tube-type heating coils. Heat transfer efficiency was evaluated by using FLUENT 6.3.26 software. Also, structural safety was assessed by using ANSYS.simulation software.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 1998년도 춘계학술발표회논문집(1)
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• pp.812-817
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• 1998

Experimental studies on heat transfer and solidification of the molten metal pool with overlying coolant with boiling were performed The simulant molten pool material is tin (Sn) with the melting temperature of 232$^{\circ}C$. Demineralized water and R113 are used as the working coolant. This work examines the crust formation and the heat transfer characteristics of the molten metal pool immersed in the boiling coolant. The Nusselt number and the Rayleigh number in the molten metal Pool region of this study are compared between the water coolant case and the R113 coolant case. The experimental results or the water coolant are higher than those for R113. Also, the empirical relationship of the Nusselt number and the Rayleigh number is compared with the literature correlations measure from mercury. The present experimental results are higher than the literature correlations. It is believed that this discrepancy is caused by the effect of heat loss to the environment on the natural convection heat transfer in the molten pool.

• 한국결정성장학회지
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• 제28권2호
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• pp.63-68
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• 2018

Natural convection of a two dimensional laminar steady-state incompressible fluid flow in a rectangular enclosure has been investigated numerically for low aspect ratios with the physical vapor transport crystal growth. Results show that for aspect ratio (Ar = L/H) range of $0.1{\leq}Ar{\leq}1.5$, with the increase in Grashof number by one order of magnitude, the total mass flux is much augmented, and is exponentially decayed with the aspect ratio. Velocity and temperature profiles are presented at the mid-width of the rectangular enclosure. It is found that the effect of Grashof number on mass transfer is less significant when the enclosure is shallow (Ar = 0.1) and the influence of aspect ratio is stranger when the enclosure is tall and the Grashof number is high. Therefore, the convective phenomena are greatly affected by the variation of aspect ratios.

• 태양에너지
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• 제10권1호
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• pp.5-13
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• 1990

온도가 일정한 외벽을 열원으로 하는 수직원통형 축열조 내에 채워진 상변화물질의 내향용융 과정에서 자연대류에 의한 열전달현상을 유한차분법을 이용하여 수치해석하였고, 그 결과를 실험을 통하여 검증하였다. 상변화물질로는 용융점온포가 $42.5%^{\circ}C$ n-docosane paraffin($C_{22}H_{46}$)을 사용하였다. 본 연구 결과에서 상변화물질의 용융속도는 액상영역의 자연대류로 인하여 축열조 상부로 갈수록 빠른 것으로 나타났다. 그리고 상변화물질의 초기온도는 액상영역 속도분포에 큰 영향을 주지 않으며, 상변화물질의 초기온도가 낮을수록 용융속도는 늦어지고 축열조의 형상비($H/r_w$)가 커질수록 자연대류는 활발해지는 것으로 나타났다. 이러한 수치해석의 결과는 실험 결과와 잘 일치하였으나, 수치해석시 상변화물질의 용융에 따르는 체적팽창을 고려하지 않았기 때문에 용융이 진행 됨에 따라 오차가 발생하였다.

• 설비공학논문집
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• 제9권4호
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• pp.484-496
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• 1997

The fluid flow and heat transfer characteristics of conjugate forced and natural convection in the triple-glazed airflow window, where the outer air passes through a space contrived between the interior and exterior window panes, were studied numerically by a finite volume method for the elliptic form of the Navier-Stokes equations. The investigation focused on the influence of window geometry, ventilastion rate and solar energy on the temperature, velocity distribution and thermal performance of the airflow window. The comparison between the triple-glazed airflow window and the enclosed triple-glazed window was also made to evaluate the effect of buoyancy upon which the thermal performance of the window depended.

• 대한기계학회논문집
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• 제17권10호
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• pp.2601-2610
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• 1993

A numerical study on the melting process inside an isothermal spherical capsule is made. It is assumed that the phase change medium of its solid phase is heavier than the liquid phase and therefore the unmelted solid core is continuously moving downward on account of gravity forces. Such a gravity-assisted melting is commonly characterized by the existence of a thin liquid film below the solid core. The present study is motivated to present a full-equation-based analysis of the influences of the initial subcooling and the natural convection on the fluid flow and heat transfer characteristics associated with the gravity-assisted melting. In the light of the solution strategy, the present study is substantially distinguished from the existing works in that the complete set of governing equations in both the melted and unmelted regions are resolved without subdivision of the solution domains. For example, the liquid film region and the upper melted region are treated here as one domain and thus obviating laborious efforts to couple them. Numerical results are obtained by varying the Rayleigh numbers and the degree of subcooling. For the range of parameters examined, the presence of subcooling was found to impede the melting rate. The dropping velocity of the unmelted solid core was observed to affect the natural convection in the liquid significantly. When compared with the available experimental data, much improved prediction was achieved.

• Nuclear Engineering and Technology
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• 제48권6호
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• pp.1321-1329
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• 2016

To get an insight into the operating characteristics of the passive residual heat removal system of molten salt reactors, a two-phase natural circulation test facility was constructed. The system consists of a boiling loop absorbing the heat from the drain tank, a condensing loop consuming the heat, and a steam drum. A steady-state experiment was carried out, in which the thimble temperature ranged from $450^{\circ}C$ to $700^{\circ}C$ and the system pressure was controlled at levels below 150 kPa. When reaching a steady state, the system was operated under saturated conditions. Some important parameters, including heat power, system resistance, and water level in the steam drum and water tank were investigated. The experimental results showed that the natural circulation system is feasible in removing the decay heat, even though some fluctuations may occur in the operation. The uneven temperature distribution in the water tank may be inevitable because convection occurs on the outside of the condensing tube besides boiling with decreasing the decay power. The instabilities in the natural circulation loop are sensitive to heat flux and system resistance rather than the water level in the steam drum and water tank. RELAP5 code shows reasonable results compared with experimental data.

• Nuclear Engineering and Technology
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• 제44권6호
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• pp.597-610
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• 2012

The passive auxiliary feedwater system (PAFS) is one of the advanced safety features adopted in the APR+, which is intended to completely replace the conventional active auxiliary feedwater system. With an aim of validating the cooling and operational performance of PAFS, an experimental program is in progress at KAERI, which is composed of two kinds of tests; the separate effect test and the integral effect test. The separate effect test, PASCAL ($\underline{P}$AF$\underline{S}$ $\underline{C}$ondensing Heat Removal $\underline{A}$ssessment $\underline{L}$oop), is being performed to experimentally investigate the condensation heat transfer and natural convection phenomena in PAFS. A single, nearly-horizontal U-tube, whose dimensions are the same as the prototypic U-tube of the APR+ PAFS, is simulated in the PASCAL test. The PASCAL experimental result showed that the present design of PAFS satisfied the heat removal requirement for cooling down the reactor core during the anticipated accident transients. The integral effect test is in progress to confirm the operational performance of PAFS, coupled with the reactor coolant systems using the ATLAS facility. As the first integral effect test, an FLB (feedwater line break) accident was simulated for the APR+. From the integral effect test result, it could be concluded that the APR+ has the capability of coping with the hypothetical FLB accident by adopting PAFS and proper set-points of its operation.