• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.12 no.11
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• pp.963-971
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• 2000

In order to investigate the effects of an electric field on EHD(Electro-hydrodynamic) nucleate boiling hat transfer characteristics in a nonuniform electric field under saturated pool boiling, the basic study has been performed experimentally. In the present study, the working fluid is R-113 and the plate-wire electrode system is used to generate a steep electric field gradient. Boiling parameters are investigated by using a high speed camera. The electric field distribution around a wire is obtained to understand the effect of an electric field on bubble departure/movement. The experimental results show EHD effects are much more considerable when the applied voltage increases. Bubbles depart away from the heated wire in radial direction. It is confirmed that the mechanisms of EHD nucleate boiling are closely connected with the dynamic behavior of bubbles. The boiling parameters are significantly changed by the electric field strength. With increasing applied voltages, the bubble size decreases and the nucleation site density, bubble velocity and bubble frequency increase.

• Solar Energy
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• v.1 no.1
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• pp.30-36
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• 1981

Pool Boiling heat transfer from controlled arrays of artificial nucleation sites was studied experimentally. Distilled water were boiled from artificial sites of uniform size, shape and spacing, drilled in superfinished copper horizontal surfaces at site density of 16, 25, 36, 49, 64, 81, 100 per $2.25cm^2$. The results confirm the boiling heat transfer from artificial sites can be improved by increasing the site density N/A or temperature difference ${\Delta}T$ or both. Following experimental correlation were developed for predicting the heat transfer rate from the heating surface which has artificial sites. $$q/A = C(T_s - T_{sat})^{1.811}(N/A)^{0.41}$$

• Proceedings of the Korean Nuclear Society Conference
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• 1996.11a
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• pp.230-235
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• 1996

The new concept of dry area formation based on Poisson distribution of active nucleation sites and the concept of the critical active site density is presented. A simple statistical model is developed to predict the change of slope of the boiling curve up to critical heat flux (CHF) quantitatively. The predictions by the present model are in good agreement with the experimental data. Also it turns out that the present model well explains the mechanism on how the surface wettability influences CHF.

• 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.

• Nuclear Engineering and Technology
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• v.50 no.7
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• pp.1068-1078
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• 2018

Experiments were performed to investigate bubble behaviors and pool boiling Critical Heat Flux (CHF) on a thin flat rectangular copper heater fabricated on Printed Circuit Board (PCB), at various inclination angles. The surface inclination angles were $0^{\circ}$, $45^{\circ}$, $90^{\circ}$, $135^{\circ}$, and $180^{\circ}$. Results showed the Onset of Nucleate Boiling (ONB) heat flux increased with increasing heater orientation from $0^{\circ}$ to $90^{\circ}$, while early ONB occurred when the heater faced downwards ($135^{\circ}$ and $180^{\circ}$). The nucleate boiling was observed to be unstable at low heat flux (1-21% of CHF) and changed into typical boiling when the heat flux was above 21% of CHF. The result shows the CHF decreased with increasing heater orientation from $0^{\circ}$ to $180^{\circ}$. In addition, the bubble departure diameter at the heater facing upwards ($0^{\circ}$, $45^{\circ}$, and $90^{\circ}$) was more prominent compared to that of the heater facing downward ($135^{\circ}$). The nucleation site density also observed increased with increasing heat flux. Moreover, the departed bubbles with larger size were observed to require a longer time to re-heat and activate new nucleation sites. These results proved that the ONB, CHF, and bubble dynamics were strongly dependent on the heater surface orientation.

• Proceedings of the Korean Nuclear Society Conference
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• 1996.05b
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• pp.241-246
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• 1996

An analytical model to calculate rate of vapor generation due to heterogeneous wall nucleation in flashing flow is developed. In the present model, an important parameter of the vapor generation term, i.e. nucleation site density is calculated by integrating its probability distribution function with respect to active cavity radius. The limits of integration are minimum and maximum active cavity radii, and these are formulated using an active cavity model for nucleate boiling. This formulation, therefore. can statistically account for the effect of surface specific thermo-physical and geometric conditions on the vapor generation rate and flashing inception. For verifying the adequacy of the present model, steady state two-fluid and the bubble transport equations are solved with applicable constitutive equations. The applicable region of the bubble transport equation is also extended to churn-turbulent flow regime to predict interfacial area concentration at high void fraction. Predicted results in terms of axial pressure and void fraction profiles along the channels are compared with experimental data of Super Moby Dick and BNL Reasonable agreements have been achieved and this shows the applicability of the present model to flashing flow analysis.

• Proceedings of the Korean Nuclear Society Conference
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• 1997.10a
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• pp.546-551
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• 1997

This paper presents a prediction of critical heat flux (CHF) in bubbly flow regime using dry-spot model proposed recently by authors for pool and flow boiling CHF and existing correlations for forced convective heat transfer coefficient, active site density and bubble departure diameter in nucleate boiling region. Without any empirical constants always present in earlier models, comparisons of the model predictions with experimental data for upward flow of water in vertical, uniformly-heated round tubes are performed and show a good agreement. The parametric trends of CHF have been explored with respect to variations in pressure, tube diameter and length, mass flux and inlet subcooling.

• Journal of Advanced Marine Engineering and Technology
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• v.40 no.4
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• pp.275-281
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• 2016

Subcooled boiling under low pressure was numerically investigated using computational fluid dynamics(CFD). The wall boiling model was used for simulating the subcooled boiling; this model requires sub-models consisting of bubble departure diameter, nucleation site density and bubble departure frequency. The CFD code CFX provides the default models based on experimental data. Because these models are mostly developed under high pressure conditions, it would not be predicted well in low pressure conditions. Thus in this study, CFD validation for subcooled boiling under low pressure was analyzed. The numerical results were compared with experimental data from published paper. Simulations were performed with mass flux ranging from 250 to $750kg/m^2s$, heat flux ranging from 0.37 to $0.77MW/m^2$ and constant outlet pressure of 0.11 MPa. Employing the empirical correlation developed under low pressures could increase the accuracy of numerical analysis.

• Nuclear Engineering and Technology
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• v.48 no.4
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• pp.859-869
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• 2016

Component-scale modeling of boiling is predominantly based on the Eulerian-Eulerian two-fluid approach. Within this framework, wall boiling is accounted for via the Rensselaer Polytechnic Institute (RPI) model and, within this model, the bubble is characterized using three main parameters: departure diameter (D), nucleation site density (N), and departure frequency (f). Typically, the magnitudes of these three parameters are obtained from empirical correlations. However, in recent years, efforts have been directed toward mechanistic modeling of the boiling process. Of the three parameters mentioned above, the departure diameter (D) is least affected by the intrinsic uncertainties of the nucleate boiling process. This feature, along with its prominence within the RPI boiling model, has made it the primary candidate for mechanistic modeling ventures. Mechanistic modeling of D is mostly carried out through solving of force balance equations on the bubble. Forces incorporated in these equations are formulated as functions of the radius of the bubble and have been developed for, and applied to, low-pressure conditions only. Conversely, for high-pressure conditions, no mechanistic information is available regarding the growth rates of bubbles and the forces acting on them. In this study, we use direct numerical simulation coupled with an interface tracking method to simulate bubble growth under high (up to 45 bar) pressure, to obtain the kind of mechanistic information required for an RPI-type approach. In this study, we compare the resulting bubble growth rate curves with predictions made with existing experimental data.