• Journal of Advanced Marine Engineering and Technology
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• v.31 no.2
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• pp.145-151
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• 2007

The evaporation heat transfer coefficient of $CO_2$(R-744) in a horizontal tube was investigated experimentally. The main components or the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and evaporator(test section). The test section consists of a smooth, horizontal stainless steel tube of inner diameter of 4.57 mm. The experiments were conducted at mass flux of 200 to $500\;kg/m^2s$, saturation temperature of -5 to $5^{\circ}C$, and heat flux of 10 to $40\;kW/m^2$. The test results showed the heat transfer of $CO_2$ has a greater effect on nucleate boiling more than convective boiling. Mass flux of $CO_2$ does not effect nucleate boiling too much. In comparison with test results and existing correlations, the best fit of the present experimental data is obtained with the correlation of Jung et al. But existing correlations failed to predict the evaporation heat transfer coefficient of $CO_2$, therefore, it is necessary to develope reliable and accurate predictions determining the evaporation heat transfer coefficient of $CO_2$ in a horizontal tube.

• Journal of Energy Engineering
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• v.7 no.1
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• pp.122-130
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• 1998

A simplified heat transfer model for the cooling capability of the AP 600 PCCS is proposed I this paper. As the PCCS domain is covered with very thin and long water film, it is phenomenologically divided into 3 regions; water entrance effect region, asymptotic region, and air entrance effect region. As the length of the asymptotic region is estimated to be over 90% of the whole domain, the phenomena in the asymptotic region is focused. Using the analogy between heat and mass transfer phenomena in a turbulent situation, a new dependent variable combining temperature and vapor mass fraction was defined. The similarity between the PCCs phenomena in the asymptotic region and the buoyant air flow phenomena on a vertical heated plate is derived. Using the similarity, the simplified heat transfer correlations for the interfacial heat fluxes and the ratios of latent heat transfer to sensible heat transfer were established. To verify the accuracy of the correlation, the results of this study were compared with those of other numerical analyses performed for the same configuration and they are well within the range of 15% difference.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.15 no.8
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• pp.690-696
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• 2003

This paper presents the heat recovery performance of water fluidized-bed heat exchanger. Temperature and humidity ratio of waste gas are considered as important parameters in this study. Therefore, the heat recovery rate through water fluidized-bed heat exchanger for exhaust gases with various temperatures and humidity ratios can be estimated from the results of this study. Mass flow ratio (the ratio of mass flow rate of water to that of gas) and temperature of inlet water are also considered as important operating variables. Increase of heat recovery rate can be obtained through either high mass flow ratio or low temperature of inlet water with resultant low recovered temperature. The heat recovery performance with the mass flow ratio of about up to 10 has been investigated. The effect of number of stages of water fluidized-bed on the heat recovery performance has been also examined in this study.

• Transactions of the Korean hydrogen and new energy society
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• v.18 no.2
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• pp.189-196
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• 2007

The evaporation heat transfer coefficient and pressure drop of $CO_2$(R-744) in a horizontal tube was investigated experimentally. The main components of the experimental apparatus are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and an evaporator(test section). The test section consists of a horizontal stainless steel tube of 4.57 mm inner diameter. The experiments were conducted at mass flux of $200{\sim}1000\;kg/m^2s$ saturation temperature of $0{\sim}20^{\circ}C$, and heat flux of $10{\sim}40\;kW/m^2$. The test results showed that the heat transfer coefficient of $CO_2$ has a greater effect on nucleate boiling more than convective boiling. Mass flux of $CO_2$ does not affect nucleate boiling too much. In comparison with test data and existing correlations, All of the existing correlations for the heat transfer coefficient underestimated the experimental data. However lung et al.'s correlation showed a good agreement with the experimental data. The evaporation pressure drop of $CO_2$ increases with increasing mass flux and decreasing saturation temperature. When comparison between the experimental pressure drop and existing correlations. Existing correlations failed to predict the evaporation pressure drop of $CO_2$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.11 s.254
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• pp.1043-1050
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• 2006

An experimental investigation was performed to study flow boiling heat transfer of deionized water in a microchannel. Measurement and evaluation of boiling heat transfer coefficients were carried out using a single horizontal rectangular microchannel having a hydraulic diameter of $100{\mu}m$. Tests were performed for mass fluxes of 90, 169 and 267 $kg/m^2$s and heat fluxes of 200-700 $kW/m^2$. Test results showed that the measured boiling heat transfer coefficients had no dependence on mass flux and vapor quality. Most macro-channel correlations of boiling heat transfer coefficient did not provide reliable predictions.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.8
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• pp.1051-1060
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• 2003

The heat and mass transfer on two kinds of tube surfaces (bare stainless steel tube and Teflon coated tube) in steam-air mixture flow are experimentally studied to obtain design data for the heat exchanger of the latent heat recovery from flue gas. In the test section, 3-tubes are horizontally installed, and steam-air mixture is vertically flowed from the top to the bottom. The pitch between tubes is 67mm, the out-diameter of tube is 25.4mm, and the thickness is 1.2mm ; blockage factor (cross sectional tube area over the cross sectional area of the test section) is about 0.38. All of sensors and measurement systems (RTD, pressure sensor, flow-meter, relative humidity sensor, etc.) are calibrated with certificated standard sensors and the uncertainty for the heat transfer measurement is surveyed to have the uncertainty within 7%. As experimental results, overall heat transfer coefficient of the Teflon (FEP) coated tube is degraded about 20% compared to bare stainless tube. The degradation of overall heat transfer coefficient of Teflon coated tube comes from the additional heat transfer resistance due to Teflon coating. Its magnitude of heat transfer resistance is comparable to the in-tube heat transfer resistance. Nusselt and Sherwood numbers on Teflon (FEP) coated surface and bare stainless steel surface are discussed in detail with the contact angles of the condensate.

• Proceedings of the KSME Conference
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• 2007.05b
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• pp.2097-2101
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• 2007

This paper presents an optimal design of a micro evaporator which maximizes the heat transfer coefficient. Number of gaps, spanwise distance and streamwise distance are selected as the geometric design parameters. Mass flow rate of the refrigerant is selected as the non-geometric design parameter. Temperature at the surface of the heater is measured to valuate the heat transfer coefficient. Nine experiments are conducted using $L_9(3^4)$ orthogonal array. Maximum heat transfer coefficient is 640 W/$m^2K$ at the parameters of 2 gaps, 0.2 mm spanwise distance, 1.0 mm streamwise distance and 0.72 g/s mass flow rate. Among the 3 geometric parameters, the spanwise distance is the most sensitive parameter influencing the heat transfer coefficient. We conduct a second stage of experiment to increase the heat transfer coefficient by reselecting the mass flow rate. We concluded that 0.87 g/s is the optimized flow rate for an active micro cooler resulting in a heat transfer coefficient of 651 W/$m^2K$.

• 유체기계공업학회:학술대회논문집
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• 2001.11a
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• pp.339-344
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• 2001

The present study investigates convective heat/mass transfer and flow characteristics in wavy ducts of primary surface heat exchanger. Experiments using a naphthalene technique are carried out to determine the local transfer characteristics for flow in the corrugated wall duct. The aspect ratios of the rectangular duct cross-section are 7.3, 4.7 and 1.8 with a corrugation angle of $145^{\circ}$. The Reynolds numbers, based on the duct hydraulic diameter, are ranged from 1000 to 5000. The local heat/mass transfer measurement is conducted in the spanwise directions. The results show that Tayler-Gortler vortices exist on the pressure surface. Flow separation on the suction surface appears at a high Reynolds number resulting in a sharp decrease in the local transfer rates, but relatively high transfer rates are obtained in the reattachment region.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.29 no.8 s.239
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• pp.911-920
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• 2005

The present study investigates heat/mass transfer and flow characteristics in a ribbed rotating passage with turning region. The duct has an aspect ratio (W/H) of 0.5 and a hydraulic diameter ($D_h$) of 26.67 mm. Rib turbulators are attached in the parallel arrangement on the leading and trailing surfaces of the passage. The ribs have a rectangular cross section of 2 m (e) $\times$ 3 m (w) and an attack angle of $70^{\circ}$. The pitch-to-rib height ratio (p/e) is 7.5, and the rib height-to-hydraulic diameter ratio (e/$D_h$) is 0.075. The rotation number ranges from 0.0 to 0.20 while the Reynolds number is constant at 10,000. To verify the heat/mass transfer augmentation, internal flow structures are calculated for the same conditions using a commercial code FLUENT 6.1. The results show that a pair of vortex cells are generated due to the symmetric geometry of the rib arrangement, and heat/mass transfer is augmented up to $Sh/Sh_0=2.9$ averagely, which is higher than that of the cross-ribbed case presented in the previous study for the stationary case. With the passage rotation, the main flow in the first-pass deflects toward the trailing surface and the heat transfer is enhanced on the trailing surface. In the second-pass, the flow enlarges the vortex cell close to the leading surface, and the small vortex cell on the trailing surface side contracts to disappear as the passage rotates faster. At the highest rotation number ($R_O=0.20$), the turn-induced single vortex cell becomes identical regardless of the rib configuration so that similar local heat/mass transfer distributions are observed in the fuming region for the cross- and parallel-ribbed case.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.32 no.4
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• pp.290-299
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• 2008

Evaporation heat transfer characteristics of $CO_2$/propane mixtures in horizontal smooth and micro-fin tubes have been investigated by experiment. The experiments were carried out for several test conditions of mass fluxes, heat fluxes, compositions of $CO_2$/propane refrigerant mixtures and tube geometries. Direct heating method was used for supplying heat to the refrigerant where the test tube was uniformly heated by electric current which was applied to the tube wall. Heat transfer coefficient data during evaporation process of $CO_2$/propane mixtures were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 mm, respectively. The tests were conducted at mass fluxes of 318 to 997 $kg/m^2s$, heat fluxes of 6 to 20 $kW/m^2$ and for several mixture compositions (100/0, 75/25, 50/50, 25/75, 100/0 by wt% of $CO_2$/propane). The differences of heat transfer characteristics between smooth and micro-fin tubes for various compositions of $CO_2$/propane refrigerant mixtures and the effect of mass flux, and heat flux on enhancement factor (EF) and penalty factor (PF) were presented.