• Design & Manufacturing
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• v.16 no.2
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• pp.33-38
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• 2022

Currently, Korea is an aging society, and it is expected to enter a super-aging society in about 4 years. Accordingly, many X-ray technologies are being developed. In X-rays, 99% of X-rays are converted into heat energy and 1% into light energy (X-rays). 99% of the thermal energy raises the temperature of the anode and its surroundings, and the cooling system is an important factor as overheating can affect the deterioration of X-ray quality and shortened lifespan. There is a method of forced air cooling using natural convection. Therefore, in this study, when X-rays were taken 5 times, Flow analysis was performed on heat removal according to temperature rise and cooling time for the heat generated at the anode of the X-ray tube (input power 60kW, 75kW, 90kW). Based on one-shot, the most rapid temperature rise section increased by more than 57% to 0.03 seconds, A constant temperature rises from 0.03 seconds to 0.1 seconds, It is judged that the temperature rises by about 8.2% or more at one time. After one-shot cooling, the cooling drops sharply from about 60% to 0.03 seconds, It is judged that the temperature has cooled by more than 86% compared to the temperature before shooting. One-shot is cooled by more than 86% with cooling time after 0.1 seconds, As the input power of the anode increases, the cooling temperature gradually increases. Since the tungsten of the anode target inside the X-ray tube may be damaged by thermal shock caused by a rapid temperature rise, an improvement method for removing thermal energy is required when using a high-input power supply.

• Journal of Aerospace System Engineering
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• v.18 no.4
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• pp.109-116
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• 2024

The validity of the analysis results was confirmed based on the insulation test results, and the vaporization mass generated in the common bulkhead was calculated to evaluate the common bulkhead propellant tank's insulation performance. The analysis results were validated by comparing the transient heat transfer analysis with the insulation test results. A transient heat transfer analysis was subsequently conducted on the common bulkhead propellant tank, considering the internal heat conduction in the propellant tank and natural convection heat transfer due to the outside air. This analysis extracted the heat flux generated in the common bulkhead and quantified the vaporization mass, a key indicator of insulation performance. Consequently, the vaporization mass was calculated at 0.09 kg, below the insulation design standard of 0.12 kg for the common bulkhead propellant tank, confirming it meets the insulation performance standard.

• Culinary science and hospitality research
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• v.20 no.6
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• pp.235-244
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• 2014

The purpose of this study was to investigate the effect of freezing and thawing rate on the physical properties of soybean sprouts to improve the quality of processed soybean sprouts during distribution and storage. Cooked soybean sprouts were frozen by air-blast freezing (ABF) system at $-45^{\circ}C$ or natural air convection freezing (NCF) system at $-24^{\circ}C$, then thawed using microwave oven by varying output power (0, 400, 800 and 1,000 W) until $75^{\circ}C$. The quality of soybean sprouts was measured by the water content, hardness and springiness. In addition, the internal microstructure of soybean sprouts was observed by optical microscope. For results, water content of soybean sprouts thawed by 1,000 W in a microwave showed the lowest value after natural air convection freezing. Springiness of soybean sprouts thawed by all amounts of output power was decreased in comparison with control. Hardness was increased only in soybean sprouts thawed by 1,000 W after air-blast freezing. However the gaps between springiness and hardness were relatively small with control at 1,000 W thawing, after air-blast freezing. Internal microstructure of the soybean sprouts was more damaged as freezing and thawing time were increased. In conclusion, high freezing and thawing rate might improves the quality of soy bean sprout, and IQF freezing and 1,000 W of microwave thawing appears to be the optimum condition for frozen HMR production. From the results freezing and thawing process parameters might can be use as quality control parameters as various type of sprout products processing.

• Journal of the Korean Society of Food Science and Nutrition
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• v.44 no.10
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• pp.1492-1503
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• 2015

This study evaluated the combination effect of various freezing and thawing techniques on the quality and nutritional aspects of onions. Onions were frozen by natural air convection freezing (NCF), air blast freezing (ABF), and liquid nitrogen freezing (LNF). Onions were frozen for 76 min by NCF, 9 min by ABF, and 9 min by LNF. The freezing treatment was stopped when the core temperature reached $-12^{\circ}C$ for NCF and ABF, and $-120^{\circ}C$ for LNF. Frozen samples were thawed through natural air convection thawing, running water thawing, sonication thawing (ST), or microwave thawing. The quality and nutritional aspects of frozen-thawed onions were evaluated by measuring thawing loss, pH, texture, water content, color, and SEM image. ST was found to cause the least loss in onion sample among the tested thawing methods, whereas the freezing methods did not cause any significant loss. In our experiment, thawing is found to be a more critical technique when compared to that of freezing. There were no clear quantifications or trends of pH and water content among different freezing and thawing techniques. The highest total color difference (${\Delta}E$) was observed in the NCF sample. For morphological observation, ABF gave the smallest ice crystal size, as well as minimum cell collapse. Loss of vitamin C, free sugar, and organic acid content was lower in the ABF and ST sample, when compared to other trials. In our study, we found that combination of ABF and ST could preserve the quality and nutritional aspects of frozen-thawed onions better than other methods.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.9
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• pp.605-612
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• 2016

In this study, numerical simulations were conducted for conjugate heat transfer around ice balls in an encapsulated ice thermal storage system. Four shapes of ice balls were modeled; the default one was a sphere, and the other three shapes were designed to enhance convective heat transfer through the ball surface. The flow around the ball was laminar, for which the Reynolds number was 300, and both forced and natural convections inside and outside the balls were considered. The simulations revealed that the magnitude of convective heat transfer for the different shapes decreased in the following order: bone, dimple, hole, and sphere. For the entire simulation, the maximum difference in the average temperatures of water inside the capsules was found to be $0.9^{\circ}C$. Therefore, it can be said that the effect of ice-ball shape on the performance of the ice thermal storage system is significant, considering that more than 0.3 million balls are used in this system.

• Journal of Biosystems Engineering
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• v.10 no.1
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• pp.24-38
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• 1985

경사진 밀폐 공간에서 마주 보는 두 벽면의 온도 차로 인하여 발생되는 자연 대류 현상은 여러 공학 분야에서 볼 수 있는 중요한 열전달 현상으로서, 최근 들어 평판형 태양열 집열기를 설계하려는 사람들에게 많은 관심의 대상이 되고 있다. 평판형 태양열 집열기의 경우 덮개판으로 부터의 대류 열손실을 감소시킴으로서 집열 효율을 높일 수 있을 뿐만 아니라 사용목적에 따라 소형 집열기를 제작할 수 있어 경제적으로 유리하게 될 것이다. 밀폐된 공간에서 최초에 정지 상태에 있는 얇은 유체층을 하부에서 가열시켜 주면 열팽창 현상이 일어나고, 이것에 의한 부력이 점도나 열전도도 등의 안정화 요인을 극복할 수 있을 정도로 커지면 System이 불안정하게 되어 자연 대류 현상이 수반되며 이 때문에 열전달율이 급격히 증가하게 된다. 이러한 현상의 지배 방정식은 연립 비선형 편미분 방정식으로 특수한 경계 조건외에는 일반적으로 해석적 해를 구하기가 어렵기 때문에 실험적 연구가 많이 실시되어 왔고 이들 결과의 대부분은 전반적인 열전달 특성치만을 구하는데 집중되어 왔다. 본 연구에서는 수치 해석법의 하나인 유한 차분법을 도입하여 이차원으로 가정한 경사진 평판형 밀폐 공간에서의 자연 대류 현상의 지배 방정식을 유한 차분화시켜, $$2.74{\times}10^3\leq_-Gr\leq_-2.0{\times}10^6$$, Pr=0.73, $$15^{\circ}\leq_-a\leq_-150^{\circ}$$, 종횡비는 1, 2, 3, 5, 9에 대하여 정상 상태에서의 해를 구하면서 수치적으로 실험하였다. 본 연구에서 얻어진 결론을 요약하면 다음과 같다. (1) 해석적으로 구하기 어려운 경사진 밀폐 공간에서 자연대류현상의 지배 방정식을 유한 차분법으로 해결할 수 있으며, 대류항과 확산항을 따로 고려한 유한차분법이 효과적임을 확인하였다. (2) 저온과 고온 벽면에서의 온도를 각각 균일하게 놓고 단변으로 이루어진 벽면은 완전히 절연되어 있는 경우에 대하여 수치해를 구한결과, 이전의 해석적 및 실험적 결과와 일치하였으며, 시간의 경과에 따른 온도 및 유선의 변화를 현상학적으로 관찰할 수 있었다. (3) 평균 열전달 계수에 미치는 경사각의 효과를 살펴본 결과 종횡비가 1 인 경우 경사각이 $45^{\circ}$에서, 종횡비가 2, 3, 5, 9인 경우 경사각이 $60^{\circ}$에서 각각 평균 열전달 계수 최대치가 나타났다. (4) Ra수(Rayleigh number) 가 증가될수록, 경사각에 상관없이 평균 열전달 계수도 증가되었다. 한편 Ra수 및 경사각의 변화에 따라 종횡비가 증가될수록 평균 열전달 계수는 경사각이 $90^{\circ}$인 경우를 제외하고는 감소됨을 볼 수 있었다. 경사각이 $90^{\circ}$인 경우, 평균 열전달 계수는 종횡비가 2 인 곳에서 최대치를 얻을 수 있었으며, 종횡비가 계속 증가될수록 평균 열전달 계수는 점차 감소되어짐을 볼 수 있었다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.37 no.8
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• pp.717-724
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• 2013

Urea-SCR technology is known as one of the powerful NOx reduction systems for vehicles as well as stationary applications. For its consistent and reliable operation in vehicle applications, however, the freezing and melting of the urea solution in cold environments have to be resolved. In this study, therefore, a numerical study of three-dimensional unsteady problems was analyzed to understand the urea freezing and heating phenomena and heat transfer characteristics in terms of urea liquid volume fraction, temperature profiles, and phase change behavior in urea solutions with time by using the commercial software Fluent 6.3. As a result, it was found that the freezing phenomenon proceeds with a phase change from the tank wall to the center, whereas the melting phenomenon occurs faster in the upper part of the storage tank by natural convection and in the adjacent part of the coolant pipe than in other parts. Furthermore, approximately 190 s were required to obtain 1L of urea solution using a 4-coiled coolant heater under conditions of $70^{\circ}C$ and 200 L/h.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.13 no.1
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• pp.5-14
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• 2003

Principles of a novel pulse growing method are described. The method realized in the crystal growing on a seed from melts under raw melt feeding provided a more reliable control of the crystallization process when producing large alkali halide crystals. The slow natural convection of the melt in the crucible at a constant melt level is intensified by rotating the crucible, while the crystal rotation favors a more symmetrical distribution of thermal stresses over the crystal cross-section. Optimum rotation parameters for the crucible and crystal have been determined. The spatial position oi the solid/liquid phase interface relatively to the melt surface, heaters and the crucible elements are considered. Basing on that consideration, a novel criterion is stated, that is, the immersion extent of the crystallization front (CF) convex toward the melt. When the crystal grows at a <> CF immersion, the raised CF may tear off from the melt partially or completely due to its weight. This results in avoid formation in the crystal. Experimental data on the radial crystal growth speed are discussed. This speed defines the formation of a gas phase layer at the crystal surface. The layer thickness il a function of time a temperature at specific values of pressure in the furnace and the free melt surface dimensions in the gap between the crystal and crucible wall. Analytical expressions have been derived for the impurity component mass transfer at the steady-state growth stage describing two independent processes, the impurity mass transfer along the <> path and its transit along the <> one. The heater (and thus the melt) temperature variation is inherent in any control system. It has been shown that when random temperature changes occur causing its lowering at a rate exceeding $0.5^{\circ}C/min$, a kind of the CF decoration by foreign impurities or by gas bubbles takes place. Short-term temperature changes at one heater or both result in local (i.e., at the front) redistribution of the preset axial growth speed.

• Journal of Energy Engineering
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• v.8 no.2
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• pp.224-232
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• 1999

Latent heat storage system using micro-encapsuled phase change material is effective method for floor heating of house and building. The temperature profile in capsule block and flow rate of hot water are important parameters for the development of heat storage system. In the present study, a mathematical model based on 3-D, non-steady state, Navier-Stokes equations, scalar conservation equations and turbulence model ($\kappa$-$\varepsilon$), is used to predict the temperature profiles in capsule and the velocity vectors in hot water pipe. The multi-block grids and fine grids embedding are used to join the circle in hot water pipe and square in capsule block. The phase change process of the capsule is quite complex not only because the size of phase change material is very small, but also because phase change material is mixed with the cement to form thermal storage block. In calculation, it's assumed that the phenomena of phase change is limited only the thermal properties of phase change material and the change of boundary is not happened in capsule. The purpose of this study is to calculate the temperature profiles in capsule block and velocity vectors in hot water pipe using the numerical calculation. Two kinds of thermal boundary condition were considered, the first (case 1) is the adiabatic condition for the both outside surfaces of the wall, the second (case 2) is the case in which one surface is natural convection with atmosphere and another surface is adaibatic. Calculation results are shown that the temperature profile in capsule block for case 1 is higher than that for case 2 due to less heat loss in adaibatic surface. Specially, in the domain of near Y=0, the difference of temperature is greater in case 1 than in case 2. The detailed experimental data of capsule block on the temperature profile and the thermal properties such as specific heat and coefficient of heat transfer with the various temperature are required to predict more exact phenomena of heat transfer.

• Fire Science and Engineering
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• v.29 no.3
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• pp.1-5
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• 2015

The productivity and demand of porous material has been increasing by development of industry and increasing income. In particular, the fire caused by using wood flour risks wood processing industry and stock farm. The heat transfer of wood flour is carried into the depth direction by effect of oxidizer around flame, flame sometimes is progressed as smoldering. In the case of progressing as combustion fire, identifying the location of fire is difficult, and it leads to failing fire aid fire fighting. Therefore potential cause is acted as raising additional damage. This paper conducts experiments of downward deep seated fire of natural convection conditions. The samples is New Zealand wood flour that is demanded much in the domestic and oversea market. In this experiment, temperature of deep seated side is measure by changing wood flour density in holder The densities used in experiment are 3%, 5%, 10%, 15%. As a result, the tendency of temperature inside decreases as wood flour density increases. But, in the case of density which is above $0.2140g/cm^3$, the phenomenon, decreasing temperature, is not shown. The result of measurement show that average flame spread speed of wood flour is 0.249 mm/min.