• 한국전산유체공학회지
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• 제19권3호
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• pp.37-43
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• 2014

In this paper, hydraulic & thermal developing and fully developed laminar forced convection flow of a water-$Al_2O_3$ nanofluid in a circular horizontal tube with uniform heat flux at the wall, are investigated numerically. A single phase model employed with temperature independent properties. The thermal entrance length is presented in this paper. The variations of the convective heat transfer coefficient and shear stress are shown in the entrance region and fully developed region along different nanoparticles concentration and Reynolds numbers. Convective heat transfer coefficient for nanofluids is larger than that of the base fluid. It is shown that heat transfer is enhanced and shear stress is increased as the particle volume concentration increases. The heat transfer improves, as Reynolds number increases.

• International Journal of Air-Conditioning and Refrigeration
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• 제9권2호
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• pp.36-43
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• 2001

In order to understand the laminar heat transfer enhancement by swirl flow, the effects of heat transfer in a circular pipe with twisted inserts are investigated experimentally. In the present study, a uniform heat flux condition is considered. Laminar heat transfer correlations are developed using least square fit method from surface temperature distributions of an electrically-heated pipe and flow properties. Average Nusselt number correlations with twisted inserts are expressed as a function of swirl parameter, Reynolds number and prand시 number. When the twisted ratio is 6.50, mean Nusselt number and friction factor increase by approximately 500% and 300%, respectively, compared with the values for a pipe without inserts.

• 설비공학논문집
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• 제3권1호
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• pp.78-85
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• 1991

An experimental result for heat transfer of pulsating turbulent pipe flow was presented under the condition of fully developed dynamic regime and uniform wall heat flux. Experiments were performed at following conditions ; Inlet time-averaged Reynolds number varied from 5000 to 11000; The peak pressure fluctuation were 1.3, 2.3 and 3.5 percent of the mean pressure; Pulsating frequency ranged from 53 Hz to 320 Hz The measurements showed that the effect of pulsation on local heat transfer is greater at downstream, in which pulsating source exists, than upstream and the heat transfer rate, averaged over the pipe length, was higher or lower than in an equivalent non-pulsating flow according to the pulsating conditions. In addition, the significant change of heat transfer rate was observed in acoustically resonant conditions, when the pulsating frequency of the flow corresponded to the pipe natural frequency.

• ETRI Journal
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• 제20권4호
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• pp.380-394
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• 1998

A numerical study has been performed to predict the characteristics on the transient operation of the heat pipe with multiple heaters for electronic cooling. The model of the heat pipe was composed of the evaporator section with four heaters, insulated transport section, and the condensor section with a conductor which is cooled with uniform heat flux condition to surrounding. The governing equations and the boundary conditions were solved by the generalized PHOENICS computational code employing the finite volume method. Two test cases are investigated in present study; Case 1 indicates that the 1st and 2nd heaters among four heating sources are heated off, while the 3rd and 4th heaters are heated on. Case 2 is the inverse situation switched from heating location of Case 1. The results show that the transient time to reach the steady state is shorter for Case 1 than for Case 2. Especially, the temperature difference of the heater during switching operation is relatively small compared to the maximum allowable operating temperature difference in electronic system. Hence, it is predicted that the heat pipe in present study operates with thermally good reliability even for switching the heaters.

• Korean Journal of Hydrosciences
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• 제6권
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• pp.67-79
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• 1995

The reattachment of buoyant efflluent to a shore in a crossflow is investigated experimentally. The effluent is prodeced by discharging heated water through a projected side channel into a confined crossflow of the same depth. In the projecting effluent, the size of recirculating region, which is formed by deflected thermal plume on the lee of the effluent, tends to increase, but the maximum temperature decreases in the direction of the crossflow and it has more uniform transverse spreading compared to non-projected type. The heat flux across the crossflow is found to be independent of the projceted length of the side channel under relatively high buoyancy flux on the contrary to low buoyancy flux. The reattachment of th effluent can be specified by both velocity ratio and densimetric Froude number, whereas only the velocity ratio is governing factor to the reattachment of the effluent in the case of non-projecting type.

• 대한기계학회논문집B
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• 제20권10호
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• pp.3371-3380
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• 1996

각종 노(furnace)를 포함하여 보일러, 가스터빈, 우주선 추진기구, 원자로 및 연료전지 등 고온이 열전달 문제를 다루는데 있어서 복사 열전달 연구는 매우 중요하다. 이러한 복사열전달에 대한 연구동향은 최근 복사 물성치(특히 가스에서의)를 예측하기 위한 이론 모델의 개발 및 측정분야와 복사전달 방정식의 해를 비교적 간단하게 구하기 위한 근사해법에 대한 연구로 크게 대별되고 있다. 이러한 두가지 연구방햐은 완전히 분리, 독립되어 있는 것은 아니며 서로 많은 연관성을 가지고 있다. 특히, 비회체가스(nongray gas)의 복사 성질에 대한 모델링은 근사 해법의 계산결과에 큰 영향을 미치게 되므로 가스의 복사 성질의 예측은 매우 중요한 연구 과제가 된고 있다. a Low resolution spectral modeling of water vapor is carried out by applying the weighted-sum-of-gray-gases model (WSGGM) to a narrow band. For a given narrow band, focus is placed on proper modeling of gray gas absorption coefficients vs. temeprature relation used for any solution methods for the Radiative Transfer Equation(RTE). Comparison between the modeled emissivity and the "true" emissivity obtained from a high temperatue statistical narrow band parameters is made ofr the total spectrum as well as for a few typical narrow bands. Application of the model to nonuniform gas layers is also made. Low resolution spectral intensities at the boundary are obtained for uniform, parabolic and boundary layer type temeprature profiles using the obtained for uniform, parabolic and boundary layer type temperature profiles using the obtained WSGGM's with 9 gray gases. The results are compared with the narrow band spectral intensities as obtained by a narrow band model-based code with the Curtis-Godson approximation. Good agreement is found between them. Local heat source strength and total wall heat flux are also compared for the cases of Kim et al, which again gives promising agreement.

• 설비공학논문집
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• 제18권12호
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• pp.1007-1016
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• 2006

Although the advance in electronic technology enables a large number of circuity to be packed in a small volume, it is simultaneously required to remove the high heat load produced by them. In this study, the heat transfer and pressure drop characteristics of a mini-channel heat exchanger, which is designed for liquid cooling of electronic components, are investigated by varying operating conditions. Water and FC-72 were used as working fluids. The mini-channel heat exchanger was made with circular shape channels having din-meters of 2, 3, and 4 mm in regular intervals, and the channel length was 100 mm. The header and inlet guide pathway to provide uniform inflow were attached at the inlet of the test section. Copper block including the heaters was attached at the sidewall of the test section as a heat source, which provided the heat flux from 5 to $15W/cm^2$. The entrance effects enhanced the heat transfer coefficient in the mini-channel significantly. In addition, the single-phase pressure drop in the mini-channel was very similar to that predicted by the laminar flow correlation except that the transition Re decreased due to flow instability in the entrance region.

• 한국염색가공학회지
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• 제25권2호
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• pp.134-139
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• 2013

A tenter is very important to dry and heat-set fabrics in textile dyeing and finishing industry. However, the tenter machine typically utilizes more than 80% of all the power in dyeing system, and yet is one of the primary machine which affects quality of fabrics. Therefore, performance optimization of the tenter machine is required to reduce energy consumption and enhance quality of fabrics. To optimize the tenter machine, it is important to maintain the uniform flow rate, which can be obtained by optimizing a nozzle geometry. In this study, emboss hole angle was investigated as main parameter in flow rate uniformity and heat flux efficiency. The analysis results were compared with those acquired from bench-scale dryer test in the laboratory. The tenter machine performance simulated by Computational Fluid Dynamics(CFD) was optimized by controlling emboss hole angle.

• 한국가시화정보학회:학술대회논문집
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• 한국가시화정보학회 2006년도 추계학술대회 논문집
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• pp.125-130
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• 2006

The characteristics of boiling heat transfer and critical heat flux (CHF) behavior of nano-fluids were studied by using various sized silver and alumina nanoparticles. The diameter of nanoparticles was from 2 nm to 250 nm for silver and from 20nm to 40nm for alumina. Pool boiling characteristics and CHF enhancement of nano-fluids with different sized nanoparticles were compared with those of pure water and each nano-fluids. The experiment was performed at atmospheric pressure and the temperature of the pool was maintained constantly by using a flat immersed heater. The concentration of nano-fluids was uniform in all experiments as 0.01g/liter. The results showed that the measured boiling curves were shifted to the right. It demonstrated that the occurrence of nucleate boiling regime in nano-fluids retarded, compared with that of pure water. Also, in nano-fluids, the boiling curves showed that CHF of nano-fluids is significantly enhanced and represented the effect of particle size on boiling characteristics.

• 태양에너지
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• 제6권2호
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• pp.62-69
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• 1986

An analysis is made of the fully developed laminar flow and heat transfer in a parallel flat plate with heated rectangular block arrays to investigated the influence of bouyancy force. The shrouds is considered as adiabatic, while the heated block surface transmit a uniform rate of heat flux per unit axial length. The governing equations for velocity and temperature are solved by SIMPLE(Semi-Implicit Method Pressure Linked Equation) algorithm. Detailed velocity and temperature fields and overall heat transfer on wide range of Rayleigh number and various aspect ratios of heated rectangular blocks are computed. The result show that bouyancy leads to a significient enhancement in heat transfer along with a smaller increase in pressure drop, with the great enhancement found when the aspect ratio is 3.0.