표면 열전달율의 복사ㆍ대류성분 분리가 비정상 열부하 계산에 미치는 영향에 관한 연구

김영탁,최창호 대한건축학회 2005 대한건축학회논문집 Vol.21 No.11

The purpose of this paper was to analyze the influence of radiation and convection component separated from surface combined heat transfer coefficient on dynamic heat load calculation. In General, it was not considered the mutual radiation of walls that Heat load simulation calculated by surface combined heat transfer coefficient. In order to solve this problem, we had developed new simulation program to calculate radiation heat transfer and convection heat transfer respectively, and verified the influence of radiation component with this new program, in indoor heat transfer process.

수평미세관내 R-290의 비등열전달 특성

최광일(Kwang-Il Choi),A. S. Pamitran,오종택(Jong-Taek Oh) 대한설비공학회 2006 대한설비공학회 학술발표대회논문집 Vol.2006 No.6

The present paper deals with an experimental study of boiling heat transfer characteristics of R-290, and is focused on pressure gradient and heat transfer coefficient of the refrigerant flow inside horizontal smooth minichannel with inner diameter of 3.0 ㎜ and length of 2 000 ㎜. The direct heating method applied for supplying heat to the refrigerant where the test tube was uniformly heated by electric current which was applied to the tube wall. The experiments were conducted with R-290 with purity of 99.99% at saturation temperature of 0 to 10℃ . The range of mass flux is 50~250 ㎏/㎡s and heat flux is 5~20 ㎾/㎡. The heat transfer coefficients of R-290 increases with increasing mass flux and ㎡ saturation temperature, wherein the effect of mass flux is higher than that of the saturation temperature, whereas the heat flux has a low effect on increasing heat transfer coefficient. The significant effect of mass flux on heat transfer coefficient is shown at high quality. the effect of heat flux on heat transfer coefficient at low quality shows a domination of nucleate boiling contribution. The heat transfer coefficient of the experimental result was compared with six existing heat transfer coefficient correlation. Zang et al.'s correlation(2004) gave the best prediction of heat transfer coefficient.

수평미세관내 R-290의 비등열전달 특성

최광일,A.S. Pamitran,오종택 대한설비공학회 2006 설비공학 논문집 Vol.18 No.11

The present paper dealt with an experimental study of boiling heat transfer characteristics of R-290. Pressure gradient and heat transfer coefficient of the refrigerant flow inside horizontal smooth minichannel were obtained with inner tube diameter of 3.0mm and length of 2,000mm. The direct electric heating method was applied for supplying a heat to the refrigerant uniformly. The experiments were conducted with R-290 purity of 99.99%, at saturation temperature of 0 to 10℃, a mass flux range of 50∼250kg/m2s, and a heat flux range of 5∼20kW/m2. The heat transfer coefficients of R-290 increased with increasing mass flux and saturation temperature, wherein the effect of mass flux was higher than that of the saturation temperature. Heat flux has a low effect on the increasing of heat transfer coefficient. The heat transfer coefficient was compared with six existing heat transfer coefficient correlations. The Zhang et al.'s correlation(2004) gave the best prediction of heat transfer coefficient. A new correlation to predict the two-phase flow heat transfer coefficient was developed based on the Chen correlation. The new correlation predicted the experimental data well with a mean deviation of 11.78% and average deviation of -0.07%.

전열응고해석법을 이용한 마그네슘합금의 열전달계수 및 롤의 온도변화 측정

한창석,이찬우,Han, Chang-Suk,Lee, Chan-Woo 한국재료학회 2022 한국재료학회지 Vol.32 No.9

Research is being actively conducted on the continuous thin plate casting method, which is used to manufacture magnesium alloy plate for plastic processing. This study applied a heat transfer solidification analysis method to the melt drag process. The heat transfer coefficient between the molten magnesium alloy metal and the roll in the thin plate manufacturing process using the melt drag method has not been clearly established until now, and the results were used to determine the temperature change. The estimated heat transfer coefficient for a roll speed of 30 m/min was 1.33 × 10⁵ W/m²·K, which was very large compared to the heat transfer coefficient used in the solidification analysis of general aluminum castings. The heat transfer coefficient between the molten metal and the roll estimated in the range of the roll speed of 5 to 90 m/min was 1.42 × 10⁵ to 8.95 × 10⁴ W/m²·K. The cooling rate was calculated using a method based on the results of deriving the temperature change of the molten metal and the roll, using the estimated heat transfer coefficient. The DAS was estimated from the relationship between the cooling rate and DAS, and compared with the experimental value. When the magnesium alloy is manufactured by the melt drag method, the cooling rate of the thin plate is in the range of about 1.4 × 10³ to 1.0 × 10⁴ K/s.

Two-phase flow heat transfer of R-134a in microtubes

Yun Wook Hwang,김민수 대한기계학회 2009 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.23 No.11

The characteristics of the two-phase flow heat transfer of R-134a in microtubes with inner diameters of 430 μm and 792 μm were experimentally investigated. The effect of the heat flux on the heat transfer coefficient for microtubes was significant before the transition quality. The boiling number expressed the interrelation between the heat flux and the mass about the heat transfer coefficients. The smaller microtube had greater heat transfer coefficients; the average heat transfer coefficient for the tube A (Di = 430 μm) was 47.0% greater than that for the tube B (Di = 792 μm) at G = 370 kg/m2⋅s and q" = 20 kW⋅m2. A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following factors: the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer, and the nucleate boiling correction factor. The correlation developed in present study predicted the experimental heat transfer coefficients within an absolute average deviation of 8.4%.

일중 피복온실의 관류열전달계수 산정

황영윤,이종원,이현우 (사) 한국생물환경조절학회 2013 생물환경조절학회지 Vol.22 No.2

This study was conducted to suggest a model to calculate the overall heat transfer coefficient of singlelayer covering for various greenhouse conditions. There was a strong correlation between cover surface temperatureand inside air temperature of greenhouse. The equations to calculate the convective and radiative heat transfer coefficientsproposed by Kittas were best fitted for calculation of the overall heat transfer coefficient. Because the coefficientof linear regression between the calculated and measured cover surface temperature was founded to 0.98, theslope of the straight line is 1.009 and the intercept is 0.001, the calculation model of overall heat transfer coefficientproposed by this study is acceptable. The convective heat transfer between the inner cover surface and the inside airwas greater than the radiative heat transfer, and the difference increased as the wind speed rose. The convective heattransfer between the outer cover surface and the outside air was less than the radiative heat transfer for the low windspeed, but greater than for the high wind speed. The outer cover convective heat flux increased proportion to theinner cover convective heat flux linearly. The overall heat transfer coefficient increased but the cover surface temperaturedecreased as the wind speed increased, and the regression function was founded to be logarithmic and powerfunction, respectively. 본 연구의 목적은 일중피복온실의 피복재에 대하여 우리나라 환경에 적합한 관류열전달계수를 산정하는 방법을 찾아내고 검증하여 다양한 온실조건 및 환경조건에서관류열전달계수를 산정할 수 있는 모델을 제시하는 것이다. 온실내부 및 외부온도와 피복재 표면온도와의 상관관계를 분석한 결과 주간 및 야간 온도를 모두 고려하였을 때보다 야간온도만을 고려하였을 경우가 상관성이훨씬 더 높은 것으로 나타났다. 피복재의 표면온도가 온실의 외부온도보다는 내부온도와 상관성이 더 높은 것으로 나타났다. 관류열전달계수를 산정하는데 사용된 5가지 종류의 대류 및 복사열전달계수 산정식을 비교한 결과 Kittas가 제안한 대류 및 복사열전달계수 산정식이가장 적합한 것으로 나타났다. 피복재 표면온도의 측정값과 계산값의 상관성을 분석한 결과 직선의 기울기는1.009이고 절편은 0.001이며 결정계수가 0.98로 나타나본 연구에서 제시된 관류열전달계수 산정모델이 신뢰성이 있음을 확인할 수 있었다. 온실내부로부터 피복재 내부표면으로 전달되는 열흐름량의 경우 모든 풍속구간에대해 대류열전달량이 복사열전달량보다 더 컸으며 풍속이 증가할수록 그 차이가 증가하였다. 외부표면에서 손실되는 열흐름량의 경우 풍속이 낮을 때에는 대류열전달량에 비해 복사열전달량이 더 컸으나 풍속이 증가함에따라 그 차이는 점점 줄어들어 풍속이 높을 때에는 대류열전달량이 더 커지는 것으로 나타났다. 피복재 외부표면의 대류열전달량은 내부표면의 대류열전달량에 직선적으로 비례하여 증가하는 것으로 나타났다. 풍속이 증가함에 따라 관류열전달계수는 증가하고 피복재의 표면온도는 감소하는 것을 확인할 수 있었고, 변화추세를 보면 관류열전달계수는 거듭제곱함수와 그리고 표면온도는로그함수와 잘 일치하였다.

X-ray imaging analysis on behaviors of boiling bubbles in nanofluids

Park, Hanwook,Lee, Sang Joon,Jung, Sung Yong Elsevier 2019 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.128 No.-

<P><B>Abstract</B></P> <P>Nanofluid, a liquid suspension containing nanoparticles, has been widely used to enhance heat transfer. However, the heat transfer enhancement mechanism of nanofluids has not been clearly revealed yet. Therefore, understanding the boiling heat transfer of nanofluids is a challenging research issue in the field of heat transfer. When nanoparticles are added into a base fluid, the thermo-physical properties of the fluid and surface characteristics are modified. Those modifications induce changes in the behavior of boiling bubbles. In addition to the size and number of bubbles, the generating rate of boiling bubbles was newly defined to estimate the heat transfer coefficient directly from bubble behaviors according to nanofluid concentration. As the nanofluid concentration increases, both the generating rate of boiling bubbles and the heat transfer coefficient decrease. Wettability and hydrodynamic size were also employed to reason those degradations. Wettability increase leads to the reduction of activated nucleation sites, which reduces the bubble generating rate and heat transfer coefficient with an increase in nanofluid concentration. In this study, the feasibility and usefulness of synchrotron X-ray imaging and a newly defined boiling generating rate for examining boiling heat transfer were verified by observing boiling-bubble behaviors. Moreover, it was also shown that wettability plays an important role in changes of the bubble behaviors and the heat transfer coefficient as surface roughness modification.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The behavior of boiling bubbles were investigated using a synchrotron X-ray imaging. </LI> <LI> The bubble generating rate was newly defined to examine the nanofluid heat transfer. </LI> <LI> The bubble generating rate decreases with increase of nanofluid concentration. </LI> <LI> Wettability increase reduces the bubble generating rate and heat transfer. </LI> </UL> </P>

Numerical analysis for the conjugate heat transfer of skin under contrast therapy

Jeon, Byoung jin,Choi, Hyoung Gwon Elsevier 2015 INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER - Vol.86 No.-

<P><B>Abstract</B></P> <P>The effect of natural convection on the temperature distribution of skin was numerically investigated by solving the conjugate heat transfer of the skin under contrast therapy, where hot and cold stimulations are periodically induced on the skin. A finite volume method based on the SIMPLE algorithm was adopted to solve the axisymmetric incompressible Navier–Stokes equations coupled with an energy equation. Those equations were strongly coupled with the Pennes bio-heat equation of skin for the analysis of the conjugate heat transfer. The amplitudes of the sinusoidal temperature profiles of the skin at selected depths were found to become smaller when natural convection was considered, and the temperature evolution obtained by analyzing the conjugate heat transfer differed from that obtained with constant heat transfer coefficients or with a conduction effect only, especially when blood perfusion was included. The spatially-averaged heat transfer coefficient predicted from the present simulation of conjugate heat transfer of contrast therapy was also confirmed to be close to the typical values employed in previous numerical studies. More importantly, the present conjugate heat transfer simulation for contrast therapy revealed that the heat transfer coefficient on skin varies both spatially and temporally. Therefore, the conjugate heat transfer analysis, in which the heat transfer coefficient is calculated as a function of time and space, must be employed in order to accurately predict the temperature evolution inside the skin when subjected to contrast therapy.</P>

Heat Transfer Characteristics and Fouling Control in a Fluidized Bed Heat Exchanger with Circulating Solid Particles

Ho-Keun Kang,Soo-Whan Ahn,Jong-Woong Choi,Byung-Chang Lee 한국마린엔지니어링학회 2009 한국마린엔지니어링학회 학술대회 논문집 Vol.2009 No.-

Fluidized bed type in heat exchangers are known to increase the heat transfer and eliminate the common problem of heat transfer surface scaling. For resonable design of circulating fluidized bed heat exchanger it is important to know the effect of design and operating parameters on the bed to the wall heat transfer coefficient. The present experimental and numerical study was conducted to investigate the effects of circulating solid particles on the characteristics of flow and heat transfer in the fluidized bed vertical shell and tube type heat exchanger with counterflow, at which a variety of solid particles such as glass(3㎜Ф), aluminum(2~3㎜Ф), steel(2~2.5㎜Ф), copper(2.5㎜Ф) and sand(2~4㎜Ф) were used in the fluidized bed with a smooth tube. Seven different particles have the same volume of 14㎣, and the effects of water flow rates, particle diameter, materials and geometry were investigated. Our results showed that the flow velocity range for collision of particles to the tube wall is higher with heavier density solid particles, and the increase in heat transfer was in the order of sand < copper < steel < aluminum < glass. This behaviour might be attributed to the parameters such as surface roughness or particle heat capacity. Fouling examination using 25,500 ppm of ferric oxide (Fe₂O₃) revealed that the tube inside wall is cleaned by a mild and continuous scouring action of fluidized solid particles. The fluidized solid particles not only keep the surface clean, but they also break up the boundary layer improving the heat transfer coefficient even at low fluid velocities.

Heat transfer and fluid flow of MgO/ethylene glycol in a corrugated heat exchanger

H. Arya,M. M. Sarafraz,M. Arjomandi 대한기계학회 2018 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.32 No.8

The present work aims to investigate the thermo-hydraulic performance of a counter-current corrugated plate heat exchanger working with MgO/ethylene glycol nanofluid. MgO nanoparticles were dispersed in ethylene glycol at different weight (mass) concentrations of 0.1 %, 0.2 % and 0.3 % and nanofluids were introduced to a heat exchanger in form of a counter-current flow to exchange heat with water. The test rig provided conditions to measure the influence of different operating parameters such as fluid flow, mass concentration and inlet temperature of the nanofluid on heat transfer coefficient, pressure drop, and thermal performance index of the heat exchanger. Results showed that flow rate and mass concentration can intensify the convective heat transfer coefficient. However, they both increase the pressure drop of the system. The heat transfer coefficient, pressure drop was found to be enhanced by 35 % and 85 %, respectively at wt.% = 0.3. Interestingly, inlet temperature was found to only increase the heat transfer coefficient slightly (up to 9.8 % at wt.% = 0.3) and had no influence on the values of pressure drop. The presence of MgO nanoparticles was found to increase the thermo-hydraulic performance index of the heat exchanger by 34 %.