• Journal of the Korean Vacuum Society
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• v.10 no.3
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• pp.341-349
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• 2001

Numerical calculation has been performed to investigate the fluid flow, heat transfer and local mass fraction of chemical species in the MOCVD(metalorganic chemical vapor deposition) manufacturing process. The mixing of reactants (trimethylgallium with hydrogen gas and ammonia) was presented by the concentration of each reactant to predict the uniformity of film growth. Effects of inlet size, location, mass flow rate and susceptor/cold wall tilt angle on the concentration were reported. From the numerical calculation, the concentration of reactants could be qualitatively predicted by the Nusselt number(heat transfer) and the optimum mass flow rate, wall tilt angle and inlet condition were considered.

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.5
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• pp.1032-1043
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• 1989

The heat transfer characteristics of the drag reducing polymer solutions are investigated experimentally in the thermal entrance region of circular tube flows. Fluids used in experiments are the aqueous solutions of high molecular polymer, polyacrylamide Separan AP-273 and the range of polymer concentrations is from 20 to 1000 wppm. Two stainless steel tubes with inside diameter 8.5mm(L/D=712) and 10.3mm(L/D=1160) are used for the heat transfer flow loops. The flow loop is set up to measure friction factors and heat transfer coefficients of test sections in two different modes; the recirculating flow system and once-through flow system. The test tubes are heated directly by electricity to apply the constant heat flux boundary conditions to the wall. Three different types of adaptors are used to observe the effects of the upstream flow conditions of the heat transfer test sections. The viscosity and characteristic relaxation time of the test fluids circulating in the flow system are measured by the capillary tube viscometer and falling ball viscometer at regular time intervals. The installed adaptors exhibit slight effect on the entrance heat transfer of Newtonian fluid. However, no noticeable effects are observed for the entrance heat transfer of the drag reducing fluids. The order of magnitude of the thermal entrance lengths of the drag reducing fluids which follow the minimum friction asymptote is much longer than that of Newtonian fluids in turbulent flows. A new dimensionless parameter, the viscoelastic Graetz number, is defined and all the experimental data are recasted in terms of the viscoelastic Graetz number. The local Nusselt number of the viscoelastic fluids is represented as a function of flow behavior index n and the viscoelastic Graetz number. As degradation continues the viscosity and the characteristic relaxation time of the testing fluids decrease. Weissenberg number defined by the relaxation time and D/V appears to be a proper dimensionless parameter in describing degradation effects on heat transfer of the viscoelastic fluids.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.11 no.6
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• pp.846-854
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• 1999

Measurements of the local heat transfer coefficients were made on a hemispherically convex surface with a round oblique impinging jet. The liquid crystal transient method was used for these measurements. This method, which is a variation on the transient method, suddenly exposes a preheated wall to an impinging jet while video recording the response of liquid crystal for the surface temperature measurements. The Reynolds number used was 23000 and the nozzle-to-surface distance was L/d=2, 4, 6, 8, and 10 and the jet angle was $\alpha$=$0^{\circ}\; 15^{\circ}\;30^{\circ}C\; and \;40^{\circ}C$. In the experiment, the Nusselt number at the stagnation point decreases as the jet angle increases and has the maximum value for L/d=6. The X-axis Nusselt number distributions exhibit Secondary maxima at $0^{\circ}C\re $\alpha$\re 15^{\circ}C, L/d\le6$ for X/d＜0(upstream) and at $0^{\circ}C\re $\alpha$40^{\circ}C,\;L/d\le4\;and\; at\; 30^{\circ}C\re $\alpha$$\leq$40^{\circ}C,\;L/d\le 6 $for X/d＞0(downstream). The secondary maxima occurs at long distance from the stagnation point as the jet angle increases or the nozzle-to-surface distance decreases. The Y-axis Nusselt number distributions exhibit secondary maxima at Y/d=$\pm$2 for $0^{\circ}C\le a\le30^{\circ}C\; and\; L/d\le4, and \;for\;$\alpha$=40^{\circ}C$and L/d=2. The displacement of the maximum Nusselt number from the stagnation point increases as the jet angle increases or the nozzle-to-surface distance decreases and the maximum distance is about 0.67 times of the nozzle diameter. The ratio of the maximum Nusselt number to the stagnation Nusselt number increases as the jet angle increases.

• Journal of Advanced Marine Engineering and Technology
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• v.36 no.8
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• pp.1016-1023
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• 2012

Because heat exchanger consists of many circular tubes, the analysis of local heat transfer and pressure drop at the surrounding of circular tubes, performance and calculation of size, economics play important roles in design. In this study, This study conducted experiment and analysis in order to observe convective heat transfer coefficient LMTD (logarithm mean temperature difference) and pressure losses according to water temperature and air flow rate using a cross flow heat exchanger of staggered arrangement. This heat exchanger was composed of staggered arrangement for five rows and seven columns of tube banks, and the condition of experiment and analysis are $40{\sim}65^{\circ}C$ of water temperature and $5.0{\sim}12.3m^3/s$ of air flow rate. As a result of it, since air density decreases as water temperature and flow rate increases, Reynolds number decreases with characteristics of low flow velocity but mean heat transfer coefficient increases with air flow rate increase, heat transfer performance has been improved and pressure losses decreased. And since heat transfer rate shows about 8~12% and pressure drop around 0.01~7.5% error as the analysis result, the feasibility of this study could be evaluated.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.6 s.249
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• pp.587-593
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• 2006

A ventilated disc brake having semi-cylindrical grooves has been proposed to improve the thermal judder by way of heat transfer enhancement. The local heat transfer coefficients were measured in the flow passage of disc brake. These measured local heat transfer data were utilized to do the finite element numerical analysis which predicts the maximum temperatures on the disc brake. The results show that the maximum temperatures on the disc surface with semi-cylindrical grooves are approximately 35.2% lower than those without them.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 1999.10a
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• pp.30-30
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• 1999

추진기관의 연소실과 같이 큰 열부하를 받는 경우 막냉각은 연소실을 보호하기 위해서 사용되는 대표적인 냉각기법이다. 막냉각의 성능을 극대화하기 위해서는 냉각유체를 2차원 슬롯을 통해서 분사시키는 것이 가장 효율적이지만 여러 가지 이유 때문에 실제 적용에 있어서는 슬롯입구 부근에 작은 구멍을 만들어 이로부터 분사되는 유체를 통해서 냉각시키게 된다. 본 연구에서는 주어진 덕트에서 분사율, 분사방법, 슬롯의 형상에 따른 슬롯 출구에서의 유동장 및 온도장 측정과 함께 물질전달 방법을 이용하여 슬롯 립(slot lip) 내벽에서 자세한 국소 열/물질전달계수를 구하였다. 분사율 0.5와 1.0에 대해서 주유동과 같은 방향으로 분사시켜줄 경우와, 유동을 제한시켜 주고 이차유동의 방향을 각각 주유동과 같은 방향과 반대방향으로 분사시키는 방법에 대하여 실험하였으며, 슬롯 입구에 경사슬롯을 설치하여 냉각유체를 분사시키는 방법에 대해서 실험하였다.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.35 no.12
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• pp.1335-1341
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• 2011

The purpose of this research is to investigate how separated and reattached flow affects mass transfer, by comparing the local mass transfer characteristics on an inclined flat plate with those on a parallel flat plate. The local mass transfer coefficients for the flat plate were measured using the naphthalene sublimation technique; the inclined angle of the flat plate was varied from $-10^{\circ}$ to $10^{\circ}$ at $5^{\circ}$ intervals, and the free-stream velocity was varied from 2m/s to 15m/s. At positive inclined angles, the local Sherwood numbers decreased gradually because the boundary-layer thickness increased. On the other hand, for negative inclined angles, the local Sherwood numbers assumed the minimum value at the separation point of the recirculation flow and the maximum value at the reattachment point. The average Sherwood numbers for both positive and negative inclined angles were lower than those in the case of the parallel plate.

• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.1
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• pp.197-206
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• 1990

Numerical results of heat transfer from a horizontal isothermal surface are presented for wall temperature T$_{w}$ = 0 .deg. C and ambient water temperature, T$_{\infty}$, from 1 .deg. C to 15 .deg. C. They include streamlines, temperature profiles, local heat transfer coefficients and average Nusselt numbers for the entire flow fields. For a upward-facing horizontal isothermal surface, the results show steady two dimensional flow regimes for T$_{\infty}$ .leg. 4.4 .deg. C, but no solution was obtained above T$_{\infty}$ = 4.4 .deg. C. For a downward-facing horizontal isothermal surface, the flow regimes are steady two dimensional flow for T$_{\infty}$ .geq. 4.9 .deg. C, and the numerical calculation was failed below this ambient water temperature. The mean Nusselt number has its maximum value at about T$_{\infty}$ = 3.4 .deg. C for upward-facing horizontal isothermal surface. For the case of downward-facing horizontal isothermal surface, the mean Nusselt number increases as the ambient water temperature increases.es.s.s.

• Transactions of the Korean hydrogen and new energy society
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• v.29 no.1
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• pp.105-110
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• 2018

The flow boiling heat transfer of water was experimentally investigated on plain and sintered microporous surfaces in a mini-channel. The effects of microporous coating on flow boiling heat transfer of subcooled water were investigated in a 300 mm long mini-channel with a cross section of $20{\times}10mm^2$. The test section has sufficiently long entrance length of 300 mm which provides a fully-developed flow before the channel inlet. The bottom side of the channel was heated by a copper block assembled with a high-density cartridge heater and other sides of the channel were insulated. The microporous surface was fabricated by sintering copper particles with the average particle size of $50{\mu}m$ on the top side of the copper block. Heat transfer measurement was conducted at the mass flux of $208kg/m^2s$ and the heat flux up to $500kW/m^2$. Microporous coated surface showed an earlier boiling incipience compared with plain surface regardless of the mass flux. Microporous coating were significantly attributed to local wall temperature and local heat transfer coefficient for flow boiling.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.2
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• pp.345-353
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• 1987

An analysis is performed to study flow and heat transfer characteristics of mixed free and forced convection about a sphere. Nonsimilar boundary layer equations which are valid over the entire regime of mixed convection are derived in terms of the mixed convection parameter, Gr/Re$^{2}$, through a dimensional analysis. The transformed conservation equations are solved by a finite difference method for the whole range of mixed convection regime. Numerical results for fluids having the Prandtl number 0.7 and 7 are presented. As the mixed convection parameter increases, the local friction coefficient and local heat transfer coefficient increases as well. For small Prandtl number, the friction coefficient is larger, while for large Prandtl number, the heat transfer coefficient is larger. Natural convection effect on the forced flow is more sensitive for small Prandtl number fluid. Flow separation migrates rearward as an increase in the mixed convection parameter. For small Prandtl number, the buoyancy effect is relatively small so that the flow separation occurs earlier.