• Journal of Mechanical Science and Technology
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• v.15 no.1
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• pp.98-107
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• 2001

Continuing efforts to achieve increased circuit performance in electronic package have resulted in higher power density at chip and module level. As a result, the thermal management of electronic package has been important in maintaining or improving the reliability of the component. An experimental investigation of thermosyphonic boiling in vertical tube and channel made by two parallel rectangular plates was carried out in this study for possible application of the direct immersion cooling. Fluorinert FC-72 as a working fluid was used in this experiment. Asymmetric heated channel of open periphery with gap size of 1, 2, 4 and 26mm and uniformly heated vertical tubes with diameter of 9, 15 and 20mm were boiled at saturated condition. The boiling curves from tested surfaces exhibited the boiling hysteresis. It was also found that the gap size is not a significant parameter for the thermosyphonic boiling heat transfer with this Fluorinert. Rather pool boiling characteristics appeared for larger gap size and tube diameter. The heat transfer coefficients measured were also compared with the calculation results by Chens correlation.

• 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.

• Nuclear Engineering and Technology
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• v.54 no.3
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• pp.1126-1135
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• 2022

The MARS code has been assessed for the prediction of onset of flow instability (OFI) in a vertical channel. For assessment, we built an experiment database that consists of experiments under various geometry and thermal-hydraulic condition. It covers pressure from 0.12 to 1.73 MPa; heat flux from 0.67 to 3.48 MW/m²; inlet sub-cooling from 39 to 166 ℃; hydraulic diameters between 2.37 and 6.45 mm of rectangular channels and pipes. It was shown that the MARS code can predict the OFI mass flux for pipes reasonably well. However, it could not predict the OFI in a rectangular channel well with a mean absolute percentage error of 8.77%. In the cases of rectangular channels, the error tends to depend on the hydraulic diameter. Because the OFI is directly related to the subcooled boiling in a flow channel, we suggest a modified subcooled boiling model for better prediction of OFI in a rectangular channel; the net vapor generation (NVG) model and the modified wall evaporation model were modified so that the effect of hydraulic diameter and heat flux can be accurately considered. The assessment of the modified model shows the prediction of OFI mass flux for rectangular channels is greatly improved.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.16 no.10
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• pp.977-983
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• 2004

An experimental study on saturated flow boiling heat transfer of R113 was peformed in a vertical narrow rectangular channel with offset strip fins. Two-phase pressure gradients and boiling heat transfer coefficients in an electrically heated test section were measured in the range of quality $0{\sim}0.6$, mass flux $17{\sim}43kg/m^{2}s$, and heat flux of $500{\sim}3,000W/m^2$ Two-phase friction multipliers were determined as a function of Lockhart-Martinelli parameter. Local boiling heat transfer coefficients were analysed in terms of mass flux, heat flux and local vapor quality. Correlation for boiling heat transfer coefficients was proposed, which was in good agreement with experimental data.

• Journal of computational fluids engineering
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• v.16 no.2
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• pp.88-93
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• 2011

A numerical study of subcooled onset of nucleate boiling (ONB) in a micro-channel under pulsed heating using volume of fluids (VOF) model was conducted. The VOF simulation adopting the existing experimental condition is compared to the experimental data. The time to ONB was determined when the void fraction at the microheater surface first appeared. The theoretical superheat for homogeneous nucleation relatively predicts the transient ONB results of convective flow of water well based on local temperature distribution. It was found that once heat load increases at the heater, transient flow boiling starts to occur faster.

• 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.34 no.4
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• pp.286-300
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• 2002

An algorithm for the sodium boiling model has been developed for calculation of the void reactivity feedback as well as the fuel and cladding temperatures in the KALIMER core after onset of sodium boiling. Modeling of sodium boiling in liquid metal reactors using sodium as a coolant is necessary because of phenomenon difference comparing with that observed generally in light water reactor systems. The applied model to the algorithm is the multiple-bubble slug ejection model. It allows a finite number of bubbles in a channel at any time. Voiding is assumed to result from formation of bubbies that (ill the whole cross section of the coolant channel except for the liquid film left on the cladding surface. The vapor pressure, currently, is assumed to be uniform within a bubble The present study is focused on not only demonstration of the vapor bubble behavior predicted by the developed model, but also confirmation of a qualitative acceptance for the model. As a result, the model can represent important phenomena in the sodium boiling, but it is found that further effort is also needed for its completition.

• Nuclear Engineering and Technology
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• v.54 no.12
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• pp.4776-4797
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• 2022

In the one-dimensional thermal-hydraulic (TH) codes, a subcooled boiling model to predict the void fraction profiles in a vertical channel consists of wall heat flux partitioning, the vapor condensation rate, the bubbly-to-slug flow transition criterion, and drift-flux models. Model performance has been investigated in detail, and necessary refinements have been incorporated into the Safety and Performance Analysis Code (SPACE) developed by the Korean nuclear industry for the safety analysis of pressurized water reactors (PWRs). The necessary refinements to models related to pumping factor, net vapor generation (NVG), vapor condensation, and drift-flux velocity were investigated in this study. In particular, a new NVG empirical correlation was also developed using artificial neural network (ANN) techniques. Simulations of a series of subcooled flow boiling experiments at pressures ranging from 1 to 149.9 bar were performed with the refined SPACE code, and reasonable agreement with the experimental data for the void fraction in the vertical channel was obtained. From the root-mean-square (RMS) error analysis for the predicted void fraction in the subcooled boiling region, the results with the refined SPACE code produce the best predictions for the entire pressure range compared to those using the original SPACE and RELAP5 codes.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.7 no.1
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• pp.132-141
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• 1995

Characteristics of pressure-drop oscillations(PDO) in a boiling channel were studied numerically and compared with experimental data. Effects of initial and boundary conditions on PDO were investigated in terms of oscillation period and amplitude. The period and amplitude of PDO increased with increasing of the compressible volume in the surge tank and the heat input. PDO occurred within the specific range of the fluid temperature, at which oscillation period and amplitude diminished rapidly with the increase of the fluid temperature. The increase of the loss coefficient in fluid supply line resulted in slightly longer oscillation period and larger amplitude. Numerical results showed good agreement with the experimental data.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.5 no.3
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• pp.179-186
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• 1993

The characteristics of pressure-drop oscillations(PDO) in boiling channel are studied experimentally. The effects of initial and boundary conditions on PDO are investigated in terms of oscillation period and amplitude. The period and amplitude of PDO are increased with the increase in the compressible volume in surge tank and heat input. However the amplitude of PDO is decreased with fluid temperature under low subcooling condition. Higher initial insurge flowrate resulted in almost invariant oscillation period but lower amplitude. At higher heat input the oscillation of heater wall temperature is significant, whose period is the same as that of pressure-drop instability.