摘要 :
The results of the experimental study of the internal characteristics of the pool boiling process of the refrigerant R141b, solution R141b/surfactant Span-80 and nanofluid R141b/Span-80/ TiO2 nanoparticles on the surfaces of stain...
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The results of the experimental study of the internal characteristics of the pool boiling process of the refrigerant R141b, solution R141b/surfactant Span-80 and nanofluid R141b/Span-80/ TiO2 nanoparticles on the surfaces of stainless steel and teflon have been presented.The measurement of the vapor bubble departure diameter, the vapor bubble departure frequency and the nucleation site density has been performed at atmospheric pressure and in the range of heat fluxes from 3.0 to 7.5 kW·m-2.The study showed that the vapor bubble departure diameter in nanofluid boiling on the stainless steel surface is 0.7 mm and on the teflon surface – 0.45 mm. Besides, the additives of nanoparticles to the solution of R141b/Span-80 lead to a decrease in the vapor bubble departure diameter in boiling on the teflone surfaces. The opposite effect was detected in boiling on the stainless steel surface.It is shown that the additives of TiO2 nanoparticles to the solution R141b/Span-80 lead to a decrease in the number of nucleation sites by 2–8 times. This effect depends on the heat flux and type of heaters surface.It was found that the rise of the heat flux leads to an increase in the difference between the magnitudes of nucleation site density for the teflon and stainless steel surfaces in boiling of R141b and R141b/Span-80.The number of nucleation sites on the teflon surface is 2 times lower compared with boiling on the stainless steel surface at a heat flux of 7.5 kW·m-2. The type of surfaces does not affect the number of nucleation sites and vapor bubble departure frequency in nanofluid boiling in the entire investigated range of heat fluxes.Based on the results of the study, it was found that the vapor bubble departure frequency in boiling of R141b and solution R141b/Span-80 on the teflon surface is 1.5–2 times lower compared with boiling on the stainless steel surface.The obtained experimental data can be used in predicting the heat transfer coefficient in boiling of the solution of R141b/Span-80 and nanofluid R141b/Span-80/TiO2.
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In this work, bubble departure diameters and bubble departure frequencies on saturated nucleate pool boiling of methane were studied. The experiments were conducted on the upper surface of a smooth vertical copper cylinder, at pre...
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In this work, bubble departure diameters and bubble departure frequencies on saturated nucleate pool boiling of methane were studied. The experiments were conducted on the upper surface of a smooth vertical copper cylinder, at pressures of 0.15 MPa, 0.2 MPa, 0.3 MPa and 0.4 MPa with heat fluxes varying from 10.64 kW m~(-2) to 79.25 kW m~(-2). Bubble departure diameters were measured from the images captured by a high-speed digital camera at lower heat fluxes less than 79.25 kW m~(-2), at which isolated bubbles were obtained. Bubble departure frequencies were calculated by counting the numbers of the detachment bubbles and the corresponding time intervals. Their relationship with Jacob number (Ja) was analyzed. With an increase in Ja at a given pressure, bubble departure diameter increases while bubble departure frequency tends to decrease. After the comparisons with six most used correlations for bubble departure diameter, a new correlation was developed within ±20% deviation from most of the experimental data. Additionally, a new correlation for the relationship between bubble departure diameter and departure frequency was also proposed within ±20% deviation from most of the experimental fD_d~2.
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The relationship between bubble departure frequency and diameter is fundamental to the boiling process and needs to be fully understood for prediction of overall boiling heat transfer performance. Hydrody-namic models for bubble d...
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The relationship between bubble departure frequency and diameter is fundamental to the boiling process and needs to be fully understood for prediction of overall boiling heat transfer performance. Hydrody-namic models for bubble departure were developed in previous studies. However, these models could not explain the dependence of bubble frequency on properties of the heating substrate and surrounding liquid, which was observed in many experiments. In this work, we develop a unified bubble departure theory for saturated nucleate pool boiling. The heat transfer in the bubble base region after bubble depar-ture is taken into consideration. Two characteristic timescales, representing two dominant heat transfer processes for different regimes of bubble sizes, are extracted. These timescales, which depend on sub-strate and liquid properties, are used to determine bubble departure frequency. The results from our the- ory show reasonably good agreement with existing experimental data. The proposed model provides a unified relationship between bubble departure frequency and diameter for various combinations of heat-ing substrates and working fluids in the saturated nucleate pool boiling regime.
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Numerical simulation of bubble growth during pool boiling under the influence of low frequency vibration was performed to understand the influence of common vibrations such as those induced by wind, highway transportation, and nea...
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Numerical simulation of bubble growth during pool boiling under the influence of low frequency vibration was performed to understand the influence of common vibrations such as those induced by wind, highway transportation, and nearby mechanical devices on the performance of thermal systems that rely on boiling. The simulations were done for saturated R123 boiling at 277.6K with a 15 K wall superheat. The numerical volume-of-fluid method (fixed grid) was used to define the liquid-vapor interface. The basic bubble growth characteristics including the bubble departure diameter and the bubble departure time were determined as a function of the bubble contact angle (20°-80°), the vibration displacement (10μm-50μm), the vibration frequency (5 Hz-25 Hz), and the initial vibration direction (positive or negative). The bubble parameters were shown to be strongly dependent on the bubble contact angle at the surface. For example, both the bubble departure diameter and the bubble departure time increased with the contact angle. At the same vibration frequency and the initial vibration direction, the bubble departure diameter and the bubble departure time both decreased with increasing vibration displacement. In addition, the vibration frequency had a greater effect on the bubble growth characteristics than did the vibration displacement. The vibration frequency effect was strongly influenced by the initial vibration direction. The pressure contour, the volume fraction of vapor phase, the temperature profile, and the velocity vector were investigated to understand these dynamic bubble behaviors. The limitation of the computational fluid dynamics approach was also described.
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This work investigates the mechanism of adiabatic gas bubble growth from submerged orifices with the view of elucidating the interdependence of the bubble shape and the pressure field during growth and departure. Air injection flo...
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This work investigates the mechanism of adiabatic gas bubble growth from submerged orifices with the view of elucidating the interdependence of the bubble shape and the pressure field during growth and departure. Air injection flow rates between 10 mlph and 100 mlph have been tested for orifices of diameters 0.58 mm, 1.05 mm and 1.6 mm. The force field around single isolated bubbles during growth and detachment has been investigated experimentally with the aid of high speed photography and detailed image processing and measurement of the internal bubble pressure. Different flow rates have been compared in order to elucidate dynamic effects. Combining measurements of the instantaneous gas pressure, measurements of the local curvature and the Young-Laplace equation has allowed the estimation of the liquid pressure field acting on the gas-liquid interface over the entire bubble surface. These have subsequently been decomposed into constitutive components such as buoyancy, contact pressure, capillary and the dynamic pressure forces.
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The effects of input parameters such as pressure, mass flux, and inlet subcooling on bubble departure diameter, frequencies, and quadrupole magnetic field have been investigated. It was observed that process of changes in diameter...
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The effects of input parameters such as pressure, mass flux, and inlet subcooling on bubble departure diameter, frequencies, and quadrupole magnetic field have been investigated. It was observed that process of changes in diameter and bubble departure frequency was similar with and without magnetic field, and the bubbles proved a regulated growth in the presence of a magnetic field. Compared to alike working conditions without a magnetic field, with subcooled, mass flux and pressure increase, the bubble departure diameters decreased by an average of 9.55%, 15.18% and 15%, and the bubble departure frequencies increased by 45.45%, 30.38% and 21.71%, respectively.
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Forced convective subcooled boiling flow experiments were conducted in a vertical upward annular channel. Water was used as the testing fluid, and the tests were performed at atmospheric pressure. A high-speed digital video camera...
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Forced convective subcooled boiling flow experiments were conducted in a vertical upward annular channel. Water was used as the testing fluid, and the tests were performed at atmospheric pressure. A high-speed digital video camera was applied to capture the dynamics of the bubble nucleation process. Bubble departure frequencies were obtained from the video for a total of 58 test conditions. The non-dimensional analysis was performed on the current data as well as available data from literature. Existing models and correlations were compared with the experimental data of bubble waiting time, growth time, and departure frequency. The correlations developed for pool boiling flow do not work well for forced convective subcooled boiling flow, while the models proposed for subcooled boiling flow cannot predict the bubble departure frequency in wide experimental ranges. Dimensionless bubble departure frequency is correlated with non-dimensional nucleate boiling heat flux. The new correlation agrees reasonably well with existing experimental data at lower wall superheat.
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In the present investigation, adiabatic gas bubble growth from a submerged orifice has been numerically simulated. The growth of the bubble is described by the full Navier-Stokes equations which have been solved by the finite-volu...
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In the present investigation, adiabatic gas bubble growth from a submerged orifice has been numerically simulated. The growth of the bubble is described by the full Navier-Stokes equations which have been solved by the finite-volume method using the commercial software package TransAT. The numerical simulations have been validated against detailed experimental measurements including the position of the centre of gravity, the curvature profiles and the departure volume. Subsequent to this the CFD platform was used to study some aspects of the influence of gravity level on bubble growth dynamics. In particular, the influence of gravity in the range of 0.1 ≤ g/g~* ≤ 1.5 is investigated and the subsequent influence on the bubble formation and departure characteristics are discussed.
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In the present work the influence of electric fields on the formation of single isolated bubbles growing under quasi-static conditions in HFE-7000 has been investigated experimentally with the aid of high speed photography and det...
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In the present work the influence of electric fields on the formation of single isolated bubbles growing under quasi-static conditions in HFE-7000 has been investigated experimentally with the aid of high speed photography and detailed image processing. An uncomplicated experimental apparatus has been commissioned which can simultaneously measure the internal gas pressure and shape of the bubble during its lifetime. The effect of uniform electric field has been investigated by applying high voltage potentials across the region within which the bubbles grew. In house image processing and analysis tools facilitated the estimation of the local instantaneous Young-Laplace stress over the entire bubble surface. By combining this information with the measurement of the instantaneous gas pressure, the electrically induced stress field acting on the gas-liquid interface was calculated and its influence on the bubble shape is discussed.
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The numerical method was used to study bubble sliding characteristics and dynamics of R134a during subcooled flow boiling in a narrow gap. In the numerical method, the volume of fraction (VOF) model, level set method, Lee phase ch...
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The numerical method was used to study bubble sliding characteristics and dynamics of R134a during subcooled flow boiling in a narrow gap. In the numerical method, the volume of fraction (VOF) model, level set method, Lee phase change model and the SST k ? ω turbulent model were adopted for the construction of the subcooled flow boiling model. In order to explore bubble sliding dynamics during subcooled flow boiling, the bubble sliding model was introduced. The bubble velocity, bubble departure diameter, sliding distance and bubble sliding dynamics were investigated at 0.2 to 5 m/s inlet velocities. The simulation results showed that the bubble velocity at the flow direction was the most important contribution to bubble velocity. Additionally, the bubble velocity of 12 bubbles mostly oscillated with time during the sliding process at 0.2 to 0.6 m/s inlet velocities, while the bubble velocity increased during the sliding process due to the bubble having had a certain inertia at 2 to 5 m/s inlet velocities. It was also found that the average bubble velocity in flow direction accounted for about 80% of the mainstream velocities at 0.2 to 5 m/s. In the investigation of bubble sliding distance and departure diameter, it was concluded that the ratio of the maximum sliding distance to the minimum sliding distance was close to two at inlet velocities of 0.3 to 5 m/s. Moreover, with increasing inlet velocity, the average sliding distance increased significantly. The average bubble departure diameter obviously increased from 0.2 to 0.5 m/s inlet velocity and greatly reduced after 0.6 m/s. Finally, the investigations of the bubble sliding dynamics showed that the surface tension dominated the bubble sliding process at 0.2 to 0.6 m/s inlet velocities. However, the drag force dominated the bubble sliding process at 2 to 5 m/s inlet velocities.
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