Kwang-Il Choi

An Experimental Investigation of Condensation Heat Transfer Coefficients and Pressure Drops of Refrigerants Inside Multiport Mini-channel Tubes

The condensation heat transfer coefficients and pressure drops of R410A and R22 flowing inside a horizontal aluminum multiport mini-channel tube having 18 channels are investigated. Experimental data are presented for the range of vapor quality from 0.1 to 0.9, mass flux from 50 to 500kg/m2s, heat flux from 3 to 15kW/m2 and the saturation temperature at 48∘C. The pressure drop across the test section was directly measured by a differential pressure transducer. At a small scale, the noncircular cross-sections can enhance the effect of the surface tension. The average heat transfer coefficient increased with the increase of vapor quality, mass flux and heat flux. Under the same test conditions, the heat transfer coefficients of R22 are higher than those for R410A, the pressure drops for R410A are 7–19% lower than those of R22. The lower pressure drop of R410A has an important advantage as an alternative working fluid for R22 in air-conditioning and heat pump systems.

Experiment and CFD Simulation of Boiling Heat Transfer Coefficient of R410A in Minichannels

This study performed a comparison between experimental and computational fluid dynamic (CFD) simulation results of boiling heat transfer coefficient of R410A in a small tube. The experimental data were obtained in the horizontal circular tubes of 3.0mm inner diameter, the length of 3000mm including: mass flux and heat flux in a range from 300kg/m2s to 600kg/m2s and from 5kW/m2 to 10kW/m2, respectively, and the saturation temperature constantly kept at 20∘C. In the simulation procedure, the Eulerian multiphase with wall boiling were obtained. The effects of mass flux and heat flux on the heat transfer coefficient of R410A were analyzed. The comparative data between CFD and experiment was also illustrated.

SiO2 나노입자가 분산된 TiO2 나노섬유의 제작 및 광촉매 특성 분석

본 연구에서는 전구체 각각의 독립제어가 가능한 이성분계 금속산화물을 얻기 위해 졸-겔법으로 합성한 실리카 나노입자를 TiO2 전구체와 교반시켜 전기방사법을 이용하여 실리카가 고르게 분산된 TiO2 나노섬유를 성공적으로 제작하였다. 제작된 나노섬유는 FE-SEM, XRD, EDS를 이용해 구조적 특성분석과 UV-VIS, 광촉매 반응기를 통해 광촉매특성 분석을 하였다. 그 결과, 실리카가 분산된 TiO2 나노섬유는 실리카가 분산되지 않은 TiO2 나노섬유 보다 광촉매효율이 10% 가량 향상되었다. 이는 실리카 나노입자가 첨가됨으로써 TiO2가 흡수하지 못하는 380~440 nm 가시광선영역을 흡수하여 광학적 특성 향상되었으며 Ti와 Si 두 금속산화물간에 Bronsted acid site가 생성되어 OH 라디칼을증가시킴으로써 광조사에 의해 여기된 전자를 잡아 재결합 손실을 억제하는 역할을 하여 화학적 특성이 개선되어 광촉매 효율이 증가되었을 것으로 사료된다.− In this study, we suggest a facile method to control conditions of single component independently when preparing consisting two-component metal oxides nanofiber by simply dispersing nanoparticles in precursor solution. The well dispersed SiO 2 nanoparticles in TiO 2 nanofibers were successfully synthesized through a simple electrospinning process. The as-synthesized nanodfibers were investigated via FE-SEM, XRD and EDS for structural studies, furthermore, the analysis of UV-VIS and photocatalytic activity were carried out for demonstrate the effect of SiO 2 nanoparticles dispersed in TiO 2 nanofibers. As a result, TiO 2 nanofibres dispersed with SiO 2 nanoparticles have enhanced photocatalytic activity than that of TiO 2 nanofibres only. In this strategy, the introduction of SiO 2 nanoparticles in TiO 2 nanofibers were attribute to enlarge absorption in the visible region (380~440 nm). Additionally, Brønsted acid sites generated in each metal oxide of Ti and Si increase OH radicals efficiently as well as it limit recombination loss by holding photogenerated electrons for high efficient photocatalytic activity.

Boiling Heat Transfer of Ammonia inside Horizontal Smooth Small Tube

ABSTRACT:This paper is presented an experimental study of flow boiling heat transfer charac-teristics of ammonia, and is focused on pressure gradient and heat transfer coefficient of the refri-gerant flow inside horizontal small tube with inner diameter of 3.0mm and length of 2000mm. The direct heating method is applied for supplying heat to the refrigerant, where the test tube is uni-formly heated by electric current. The local heat transfer coefficients were obtained over a heat flux range of 20 to 80 kW/㎡, a mass flux range of 50 to 500 kg/㎡s, a saturation temperature range of 0 to 10°C, and quality up to 1.0. The pressure drops increase with increasing mass flux and heat flux, and with decreasing saturation temperature. The heat transfer coefficients increase with increasing mass flux and saturation temperature in middle and high quality region. And the local heat transfer coefficient increase with increasing heat flux in low quality region. The heat transfer coefficient of the experimental result was compared with six existing heat transfer coeffi-cient correlation. A new boiling heat transfer coefficient correlation based on the superposition model for ammonia in small tubes is developed average deviation of -0.17% and mean deviation of 10.85%.Keywords:Horizontal small tube(수평미세관), Ammonia(암모니아), Flow boiling(흐름비등), Heat transfer coefficient(열전달계수), Pressure drop(압력강하), Correlation(상관식)

Heat Transfer Coefficient during Evaporation of R-1234yf, R-134a, and R-22 in Horizontal Circular Small Tubes

Experimental data of heat transfer coefficient during evaporation of R-1234yf, R-134a, and R-22 in horizontal circular small tubes are compared. The local heat transfer coefficient is obtained for heat fluxes ranging from 10 to 35 kW m−2, mass fluxes ranging from 100 to 650 kg m−2 s−1, saturation temperatures of 5, 10, and 15°C, and quality up to 1.0. The test sections are made of stainless steel tubes with inner diameters of 1.5 and 3.0 mm, the lengths of 1000 and 2000. Effects of heat flux, inner tube diameter, and saturation temperature on heat transfer coefficient are reported in the present study. Nucleate boiling heat transfer contribution is predominant, especially at low quality region, and laminar flow appears in the evaporative small tubes. The experimental results are compared against four existing heat transfer coefficients, and the modified correlation of heat transfer coefficient is developed with good prediction.

Flow condensing heat transfer of R410A, R22, and R32 inside a micro-fin tube

ABSTRACT An experimental study of condensation heat transfer characteristics of flow inside horizontal micro-fin tubes is carried out using R410A, R22, and R32 as the test fluids. This study especially focuses on the influence of heat transfer area upon the condensation heat transfer coefficients. The test sections were made of double tubes using the counter-flow type; the refrigerants condensation inside the test tube enabled heat to exchange with cooling water that flows from the annular side. The saturation temperature and pressure of the refrigerants were measured at the inlet and outlet of the test sections to defined state of refrigerants, and the surface temperatures of the tube were measured. A differential pressure transducer directly measured the pressure drops in the test section. The heat transfer coefficients and pressure drops were calculated using the experimental data. The condensation heat transfer coefficient was measured at the saturation temperature of 48°C with mass fluxes of 50–380 kg/(m2s) and heat fluxes of 3–12 kW/m2. The values of experimental heat transfer coefficient results are compared with the predicted values from the existing correlations in the literature, and a new condensation heat transfer coefficient correlation is proposed.

Pressure Drop and Boiling Heat Transfer Characteristics of R410A in Macro-Scale and Mini-Scale Channels

This chapter demonstrates the two‐phase flow pressure drop and heat transfer of R410A during boiling in various tube types. The pressure drop and local heat transfer coefficients were obtained for heat fluxes ranging from 10 to 40 kW/m2, mass fluxes ranging from 100 to 600 kg/m2s, the vapour quality up to 1.0 and the saturation temperatures of 5–15°C. The test sections were made of various tube diameters of 1.5, 3.0, 6.61 and 7.49 mm, respectively. The effect of mass flux, heat flux, saturation temperature and inner tube diameter on pressure drop and heat transfer coefficient was analysed. The experimental results were compared against several existing pressure drop and heat transfer coefficient correlation. New correlations of pressure drop and boiling heat transfer coefficient were also developed in this present study.

Effect of ultrasonic waves on anti-freezing for plate-type heat exchanger

This study attempted to examine a method of anti-freezing on a plate-type heat exchanger in a low-temperature environment. Freezing condition was observed after ultrasonic waves were generated. Data were recorded to determine the optimal conditions for freezing. Ethylene glycol, which is commonly used in antifreeze formulations, was used as the brine, and the temperature was varied between –8 and –16°C. The water for freezing provided by the thickness of 1–3 mm. In addition, experiments were conducted by adjusting the output to identify the changes that occurred due to the incidence of ultrasonic energy. The results of the anti-freezing effect were brine temperature, freezing thickness, and frequency band of ultrasonic waves.

Characterization of Low-temperature Conductive Films Bonded PV Modules and Its Field Test

In this paper, PV modules using a low-temperature conductive film(LT-CF) as a bonding material between a cell and a solder free ribbon were produced and chracterized, which is more environmental-friendly, cost effective and high efficient. Mainly, filed electrical performance of PV modules using three different types of bonding material; a convetional solder ribbon(SR), a LT-CF and a light-capturing Ribbon(LCR) were compared to comfirm the feasibility of LT-CF as a bonding material. The filed test were conducted for 3 months and results were discussed in terms of amount of output energy production and efficiency.

Characteristics of Two-Phase Flow Boiling Heat Transfer and Pressure Drop of NH3, C3H8 and CO2 in Horizontal Circular Small Tubes

An experimental investigation on the characteristics of two-phase boiling heat transfer of NH3 , C3 H8 and CO2 in horizontal small stainless steel tubes of 1.5 and 3.0 mm inner diameters are presented in this paper. Experimental data were obtained over a heat flux range of 5 to 70 kW/m2 , mass flux range of 50 to 600 kg/m2 s, saturation temperature range of 0 to 12°C, and quality up to 1.0. The test section was heated uniformly by applying an electric current to the tubes directly. Nucleate boiling heat transfer was the main contribution, particularly at the low quality region. Laminar flow was observed in the small tubes. The heat transfer coefficient of the present working refrigerants was compared with other correlations. A new boiling heat transfer coefficient correlation based on the superposition model for refrigerants in small tubes was developed.© 2010 ASME