• International Journal of Air-Conditioning and Refrigeration
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• v.16 no.1
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• pp.22-29
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• 2008

A sequence of visually experimental observations was conducted to investigate the flow boiling and two-phase flow in a coiled tube. Different boiling modes and bubble dynamical evolutions were identified for better recognizing the phenomena and understanding the two-phase flow evolution and heat transfer mechanisms. The dissolved gases and remained vapor would serve as foreign nucleation sites, and together with the effect of buoyancy, centrifugal force and liquid flow, these also induce very different flow boiling nucleation, boiling modes, bubble dynamical behavior, and further the boiling heat transfer performance. Bubbly flow, plug flow, slug flow, stratified/wavy flow and annular flow were observed during the boiling process in the coiled tube. Particularly the effects of flow reconstructing and thermal non-equilibrium release in the bends were noted and discussed with the physical understanding. Coupled with the effects of the buoyancy, centrifugal force and inertia or momentum ratio of the two fluids, the flow reconstructing and thermal non-equilibrium release effects have critical importance for flow pattern in the bends and flow evolution in next straight sections.

• Journal of Ocean Engineering and Technology
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• v.35 no.1
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• pp.50-58
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• 2021

The demand for eco-friendly energy is expected to increase due to the recently strengthened environmental regulations. In particular, the flow inside the pipe used in a cargo handling system (CHS) or fuel gas supply system (FGSS) of hydrogen transport ships and hydrogen-powered ships exhibits a very complex pattern of multiphase-thermal flow, including the boiling phenomenon and high accuracy analysis is required concerning safety. In this study, a feasibility study applying the boiling model was conducted to analyze the multiphase-thermal flow in the pipe considering the phase change. Two types of boiling models were employed and compared to implement the subcooled boiling phenomenon in nucleate boiling numerically. One was the "Rohsenow boiling model", which is the most commonly used one among the VOF (Volume-of-Fluid) boiling models under the Eulerian-Eulerian framework. The other was the "wall boiling model", which is suitable for nucleate boiling among the Eulerian multiphase models. Moreover, a comparative study was conducted by combining the nucleate site density and bubble departure diameter model that could influence the accuracy of the wall boiling model. A comparison of the Rohsenow boiling and the wall boiling models showed that the wall boiling model relatively well represented the process of bubble formation and development, even though more computation time was consumed. Among the combination of models used in the wall boiling model, the simulation results were affected significantly by the bubble departure diameter model, which had a very close relationship with the grid size. The present results are expected to provide useful information for identifying the characteristics of various parameters of the boiling model used in CFD simulations of multiphase-thermalflow, including phase change and selecting the appropriate parameters.

• 한국전산유체공학회:학술대회논문집
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• 2008.03b
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• pp.472-473
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• 2008

Flow boiling in parallel microchannels has received attention as an effective heat sink mechanism for power-densities encountered in microelectronic equipment. the bubble dynamics coupled with boiling heat transfer in microchannels is still not well understood due to the technological difficulties in obtaining detailed measurements of microscale two-phase flows. In this study, complete numerical simulation is performed to further clarify the dynamics of flow boiling in microchannels. The level set method for tracking the liquid-vapor interface is modified to include the effects of phase change and contact angle. The method is further extended to treat the no-slip and contact angle conditions on the immersed solid. Also, the reverse flow observed during flow boiling in parallel microchannels has been investigated. Based on the numerical results, the effects of channel shape and inlet area restriction on the bubble growth, reverse flow and heat transfer are quantified.

• Nuclear Engineering and Technology
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• v.43 no.3
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• pp.205-216
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• 2011

Recently, many research studies have investigated the enormous critical heat flux (CHF) enhancement caused by nanofluids during pool boiling and flow boiling. One of the main reasons for this enhancement is nanoparticle deposition on the heated surface. However, in real applications, nanofluids create many problems when used as working fluids because of sedimentation and aggregation. Therefore, artificial surfaces on silicon and metal have been developed to create an effect similar to that of nanoparticle deposition. These modified surfaces have proved capable of greatly increasing the CHF during pool boiling, and good results have also been observed during flow boiling. In this study, we demonstrate that the wetting ability of a surface, i.e., wettability, and the liquid spreading ability (hydrophilic surface property), are key parameters for increasing the CHF during both pool and flow boiling. We also demonstrate that when the fuel surface in nuclear power plants is modified in a similar manner, it has the same effect, producing a large CHF enhancement.

• Journal of Advanced Marine Engineering and Technology
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• v.10 no.4
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• pp.67-77
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• 1986

The information on the heat transfer characteristics, flow pattern and pressure drop, are very important for the desing of general heat exchanger, refrigerating system, air conditioning system and energy recovery system. In these systems, water or lubricating oil contained in working fluid affects greatly the flow and heat transfer condition and this phenomena must be considered in the practical design. An experiment has been performed for studying the flow and heat transfer characteristics of the forced convective horizontal flow of R-113 under the range of the liquid single phase state to the boiling flow state. Basic experimental results are obtained in the case that water or lubricating oil does not contaminate in the test fluid. Experimental results are as follows; (1) The local heat transfer coefficients in the nucleate boiling region and transition boiling region are almostly ten times as large as that of liquid single phase flow. (2) The measured heat transfer coefficient in the present experimental range is relatively agreed well with the predicted value from the various experimental results for the boiling flow.

• 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.

• Proceedings of the KSME Conference
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• 2001.06d
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• pp.181-188
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• 2001

The present research is an experimental study of subcooled flow boiling behavior using flat, microporousenhanced square heater surfaces in pure FC-72. Two $1-cm^{2}$ copper surfaces, one highly polished (plain) and one microporous coated, were flush-mounted into a 12.7 mm square, horizontal flow channel. Testing was performed for fluid velocities ranging from 0.5 to 4 m/s (Reynolds numbers from 18,700 to 174,500) and pure subcooling levels from 4 to 20 K. Results showed both surfaces' nucleate flow boiling curves collapsed to one line showing insensitivity to fluid velocity and subcooling. The log-log slope of the microporous surface nucleate boiling curves was lower than the plain surface due to the conductive thermal resistance of the microporous coating layer. Both, increased fluid velocity and subcooling, increase the CHF values for both surfaces, however, the already enhanced boiling characteristics of the microporous coating appear dominant and require higher fluid velocities to provide additional enhancement of CHF to the microporous surface.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.23 no.11
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• pp.718-725
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• 2011

Characteristics of flow boiling heat transfer in microchannels were investigated experimentally. The microchannels consisted of 9 parallel trapezoidal channels with each channel having 205 ${\mu}m$ of bottom width, 800 ${\mu}m$ of depth, $3.6^{\circ}$ of sidewall angle, and 7 cm of length. Tests were performed with R113 over a mass velocity range of 150~920 $kg/m^2s$, heat flux of 10~100 $kW/m^2$ and inlet pressures of 105~195 kPa. Flow boiling heat transfer coefficient in microchannels was found to be dominated by heat-flux. However the effect of mass velocity was not significant. Contrary to macrochannel trends, the heat transfer coefficient was shown to decrease with increasing thermodynamic equilibrium quality. A new correlation suitable for predicting flow boiling heat transfer coefficient was developed based on the laminar single-phase heat transfer coefficient and the nucleate boiling dominant equation. Comparison with the experimental data showed good agreement.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.13 no.1
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• pp.9-17
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• 2001

In this study, flow boiling experiments were performed using R-134a on a plain tube bundle. Tests were conducted for the following range of variables; quality from 0.1 to 0.9, mass flux from $8\;kg/m^2s$ to $26\;kg/m^2s$ and heat flux from $10\;kW/m^2s$ to $40\;kW/m^2s$. The heat transfer coefficients were strongly dependent on the heat flux. However, they were almost independent on the mass flux or quality. The data are compared with the modified Chen model, which satisfactorily () predicted the data. Original Chen model, however, did not adequately predict the effect of quality. The reason may be attributed to the flow pattern of the present test, where the bubbly flow prevailed for the entire test range. The heat transfer coefficients of the tube bundle were 6~40% higher than those of the single tube pool boiling.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.8
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• pp.748-755
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• 2004

Flow boiling heat transfer in small-diameter round tubes has been experimentally studied. The experimental apparatus consisted mainly of refrigerant pump, condenser, receiver, test section of a 1.67 mm inner-diameter round tube and pre-heater for control of refrigerant quality at the inlet of test section. To investigate the effect of bubble nucleation site characteristics of different tube materials, three different tubes of copper, aluminum and brass were used. The ranges of the major experimental parameters were 5∼30 ㎾/$m^2$ of the wall heat flux, 0.0∼0.9 of the inlet vapor quality and the refrigerant mass flux was fixed at 600 kg/$m^2$s. The experimental results showed that the flow boiling heat transfer coefficients in small tubes were affected only by heat flux, but independent of mass flux and vapor quality. The effect of tube material on flow boiling heat transfer was observed small.