Triangular type pyramid surfaces are made as a new type of heat transfer surface, and its sub-atmospheric pressure pool boiling performance is investigated experimentally. Pure water is cited as a ...working fluid with liquid height of 25 mm and saturation pressures of 15 kPa and 10 kPa. Triangular type pyramid surfaces have pyramid sizes of 2.5 mm, 5.0 mm and 7.5mm, respectively. Performance of the triangular type pyramid surfaces is compared with square type pyramid surfaces whose pyramids sizes are 2.5 mm and 5.0 mm. Heat transfer performance of the triangular type pyramid surface with size of 2.5 mm is the highest among them for both saturation pressures. Heat transfer performance for triangular type pyramid surfaces with real surface area basis is higher by about 1.5 times for size of 2.5 mm, and higher by about 1.2 times for size of 5.0 mm compared with the smooth surface at 15 kPa, and higher by about 1.7 times for size of 2.5 mm, and higher by about 1.3 times for size of 5.0 mm at 10 kPa. When comparing the heat transfer performance of triangular type pyramid surfaces with the square type pyramid surfaces, the triangular type pyramid surfaces have higher performance than the square type pyramid surfaces. Observation of bubbles generated from pyramid surfaces is also conducted for pyramid surfaces with size of 2.5 mm, as well as measurement of bubble growth rates and bubble release times. The results qualitatively show that triangular type pyramid surfaces have good bubble foaming characteristics than the square type pyramid surface.
•A hybrid VOF-Eulerian multifluid model is presented.•A turbulent rough wall function is proposed to enhance the prediction of the velocity profile.•The hybrid model reasonably predict the features ...of low void fraction flow boiling.•The hybrid model could captures large scale interface in high void fraction flow boiling.•Need to extend the wall boiling model to accommodate heat transfer beneath vapor slugs.
The improvement of a hybrid (a combination of Volume-of-Fluid (VOF) and Eulerian model) multiphase flow solver for the numerical prediction of high- and low-void fraction flow boiling regimes is presented in this article. These flow regimes could simultaneously occur in core-catcher and in-vessel retention-external reactor cooling systems during a severe accident in nuclear power plant. To enhance the prediction of the low void fraction boiling regime, a turbulent rough wall function model is implemented in the hybrid model to reproduce the impact of coarseness induced by the existence of growing bubbles along the heating wall on the liquid velocity profile. With this wall function, a more accurate prediction of the radial velocity profile is achieved within the uncertainty of the velocity measurements. Moreover, an improved prediction of the radial void fraction is achieved using the model proposed by Lopez de Bertodano for turbulence dispersion force without compromising the prediction of the radial gas velocity profile and radial liquid temperature profile. Although the hybrid model shows potential in capturing the interface and dynamic behavior of large-scale bubbles (vapor slug) for high void fraction regime, the predicted wall superheat is higher than the measured values. This highlighted the need for the extension of the present wall boiling model to cover flow boiling involving sliding vapor slugs on the heated wall.
•An improved bubble growth model is formulated.•The model is validated against the high resolution experimental results.•Role of heater surface characteristics on the bubble growth (Ceff) is ...derived.•The values of Ceff are found lower for the low-wetting surfaces.•Ceff is the lowest at the Sq of around 0.12 μm for the low-wetting surfaces.
For the modelling of nucleate boiling heat transfer, the bubble growth dynamics is of key importance. The study reported in this paper focuses on a qualitative assessment of the role of heater surface parameters on the bubble growth and the effective microlayer thickness constant, Ceff. The latter is part of a recently derived improved bubble growth model, which we utilize in our analysis along with high-resolution experimental data of the steam bubble growth. The bubble growth model is formulated considering the evaporation of microlayer beneath the bubble, heat diffusion at the bubble surface and condensation at the bubble cap. We found that the values of Ceff are lower and the growth rates of bubble prior to departure are greater at the root mean square roughness of around Sq = 0.12 μm for low-wetting surfaces. For well-wetting surfaces Ceff and the bubble growth rates are also found lower and greater, respectively at Sq = 0.15 μm. Finally, a generalized equation for Ceff is proposed which comprises the effects of surface roughness and wettability on the bubble growth. The findings are useful for improving the bubble growth models and in designing the heater surface in future.
Pool boiling experiments are performed with degassed water on highly smooth surfaces of two different wettabilities: hydrophilic and hydrophobic cases. Boiling curves and visual observations on the ...boiling have been performed. The onset of nucleate boiling (ONB) has been measured and the influence of the wettability has been quantified. As the inherent mean roughness of the glass substrates was lower than one nanometer it was possible to show the sole effect of the wettability. No hysteresis in the boiling curve was observed for both cases. The ONB was observed after 3.5°C superheat on the hydrophobic case and the heat transfer coefficient (HTC) changed suddenly from the one of a convection regime (1.5kW/m2K) to the one of a nucleate boiling regime (4kW/m2K). On the contrary for the hydrophilic case, despite superheat above 37°C and presence of boiling, the HTC was kept as the one of the convection regime.
•Subatmospheric pool boiling is experimentally studied with bubble dynamics.•Dynamic effect of subatmospheric boiling was analyzed.•A complex subatmospheric boiling regime of calcium chloride ...solution was observed.•A new bubble departure diameter correlation was proposed.
The objective of this research is to compare the nucleate boiling characteristic of the calcium chloride aqueous solution with distilled water at sub-atmospheric pressure by analyzing the growing bubble dynamics in order to provide some data for optimizing the design of the dehumidification system. The experiments were carried out with water and calcium chloride solution on the upper surface of a polished stainless steel plate, under sub-atmospheric pressure ranging from 3.6 kPa to 22.0 kPa. An artificial nucleation site was created in the center of the plate to generate the isolated bubbles. A high-speed camera was used to capture the images of dynamic bubbles, and the relevant parameters on bubble dynamics were measured and calculated by frame-by-frame image treatment.
Experimental results show that bubble diameter tends to increase with the pressure decrease which means the lower vapor density and stronger surface tension force at sub-atmospheric boiling. The influence of superheat and sub-cooling degree were also analyzed. Additionally, a complex boiling regime of calcium chloride solution with irregular bubble dynamic parameters was observed. Finally, bubble growth dynamics under sub-atmospheric were analyzed and the force balance equation were established. It is shown that the dynamic effect especially the inertial force dominated the growth stage under sub-atmospheric boiling. A new bubble departure diameter correlation within ±20% deviation was proposed.
•Bubble dynamics during nucleate boiling of FC-72 are studied experimentally and numerically.•The dynamical interface is captured by the three-dimensional VOSET method.•Three dimensional transient ...thermal response of solid wall during boiling is considered.•The numerical results match very well with experimental data.
In this paper, we study the bubble dynamics and heat transfer during a growing and departing bubble of FC-72 on horizontal silicon surfaces experimentally and numerically. In the numerical method, the dynamical interface is captured by the three-dimensional coupled volume of fluid and level set (VOSET) method. The Navier-Stokes equations for the incompressible two-phase flows with surface tension and gravity, along with the continuity and energy equations are solved using the 3D-IDEAL algorithm. The three dimensional solid wall is also coupled into the solver to study the response of a heated solid surface during nucleate boiling. An experiment with working fluid FC-72 was set up to investigate the bubble behavior and heat transfer during the nucleate boiling process. Good agreement is observed between the simulation results and experimental data in prediction of bubble shape during the growth time. It is also found that the departure diameter of saturated FC-72 predicted by the proposed method matches very well with Phan’s correlation. The temperature distribution near the bubble and the spatial averaged heat flux are obtained and analyzed.
In the present work, a new simulation of nanofluid/vapor two-phase flow inside the 2-D rectangular boiling chamber was numerically investigated. The Eulerian–Eulerian approach used to predict the ...boiling curve and the interaction between two phases. The surface modification during pool boiling of silica nanofluid represented by surface roughness and wettability is put into the account in this simulation. New closure correlations regarding the nucleation sites density and bubble departure diameter during boiling of silica nanofluid were inserted to extend the boiling model in this work. Besides, the bubble waiting time coefficient which involved in quenching heat flux under heat flux partitioning HFP model was corrected to improve the results of this study. The numerical results validated with experimental works in the literature, and they revealed good agreements for both pure water and nanofluids. The results found that when improving the heat flux partitioning model HFP by considering the surface modification of nucleate pool boiling parameters, it will give more mechanistic sights compared to the classical model, which is used for predicting of boiling heat transfer of pure liquid.
In this study, the effects of surface roughness (R
a
) on single-phase and two-phase cooling performances of HFE-7100 and water under different coolants, pressures, and subcoolings were studied. The ...eight surfaces were unidirectionally roughened. A general trend was that the heat transfer increased with increasing R
a
. Owing to the larger contact angle and fewer nucleate sites, the smoother surface had poorer heat transfer performance. Visualization was performed to reveal secondary nucleation on the smooth surface. Finally, correlations were developed to predict the heat transfer performance for different values of R
a
.
Boiling is known as an effective mode of heat transfer at low-temperature differences, due to the rapid vaporization of liquid. The performance of boiling heat transfer can be enhanced by reducing ...surface wettability and modifying surface structures. In this study, polished copper (bare) substrates were modified by composite coatings of TiO
2
and SiO
2
nanoparticles. Four different samples were prepared using electrophoretic deposition and by varying the coating duration to 5, 10, 15, and 20 min, which were named S5, S10, S15, and S20, respectively. The surface characteristics of bare and coated samples, such as morphology, wettability, surface roughness, and coating layer thickness, were investigated. The contact angle measurements of the bare surface were 65.7°, whereas the coated samples S5, S10, S15, and S20 were 112.9°, 103.6°, 100.3°, and 96.8°, respectively. The coating layer thickness of the samples S5, S10, S15, and S20 was 2.46, 4.76, 9.86, and 14.58 microns, respectively. The pool boiling performance was examined in demineralized (Milli-Q) water on bare and coated surfaces. The onset of nucleate boiling (ONB) temperature was reduced for all composite coated surfaces. The largest reduction in ONB was observed for S15, which was ~ 3.5 °C less than the bare surface. The maximum enhancements in the boiling heat transfer coefficient (BHTC) recorded for S10 and S15 were 38% and 62%, respectively. The optimum coating layer thickness was observed to be ~ 10 µm, up to which heat transfer performance was improved.
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