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Simultaneous mapping of single bubble dynamics and heat transfer rates for SiO <sub/>2</sub> /water nanofluids under nucleate pool boiling regime
摘要： Dependence of single vapor bubble dynamics and heat transfer rates on varying concentration of SiO2 nanoparticles for a range of subcooled conditions (0–9 ?C) has been experimentally studied under nucleate pool boiling configuration. Non-invasive measurements have been carried out using rainbow schlieren deflectometry. Results on bubble dynamics showed that the bubble diameter and aspect ratio decrease with increasing subcooling levels as well as concentration of nanofluids. The frequency of bubble oscillations was found to increase first and then decrease with increasing subcooling levels while it decreases monotonically with increasing nanofluid concentration. Bubble departure frequency increased significantly for nanofluids, while it decreased with increasing subcooling levels. Condensation effects at the bubble interface were reflected in the form of redistribution of colors around it. Schlieren images clearly revealed a spread in the spatial extent of the thermal boundary layer region caused by the suspended nanoparticles around the vapor bubble as well as near the heated substrate. This phenomenon has been considered as one of the factors that tends to alter the condensation effects and, in turn, affects the bubble dynamics. Quantitative analysis of schlieren images revealed that the natural convective heat flux increases with increasing subcooling levels, while it decreases with increasing nanoparticle concentration. Deterioration in the natural convection phenomenon in the presence of suspended nanoparticles has been attributed to the reduced strength of thermal gradients adjacent to the heater substrate. On the other hand, evaporative heat flux was observed to decrease with increasing subcooling levels and increase with increasing concentration of nanofluids.
关键词： nucleate pool boiling,bubble dynamics,schlieren deflectometry,subcooling,heat transfer,nanofluids
更新于2025-09-23 15:23:52
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Clear observation of the formation of nanoparticles inside the ablation bubble through a laser-induced flat transparent window by laser scattering
摘要： To understand the formation mechanism of nanoparticles via laser ablation in liquids direct observation of the dynamics inside the ablation bubble is essential. One powerful technique for this is small-angle X-ray scattering. Applying the laser-scattering technique in a similar context faces some difficulties, because, firstly the probe laser does not easily go into the bubble due to the curved bubble surface, and secondly the extremely weak scattering signal from inside the bubble is overwhelmed by the very strong reflection at the bubble surface. In this paper we solve those two problems by sending the probe laser into the bubble through a laser-induced flat transparent window, and demonstrate the clean observation of laser-scattering signals from the inside of the ablation bubble. We observe the signature of the formation of nanoparticles around the two areas inside the bubble, i.e., around the central area and apex inside the bubble. The scattering signals originating from the centre of the bubble persist throughout the entire growth and shrinkage stages of the bubble, while practically no scattering signals are found outside the bubble and this clearly implies that almost all nanoparticles are formed inside the bubble. Interestingly, the scattering signals originating from the apex inside the bubble gradually emerge after the bubble reaches the maximum size, and they become brighter as it further shrinks. Those findings are consistent with the scenario of nanoparticle formation obtained by small-angle X-ray scattering experiments.
关键词： nanomaterial synthesis,light scattering,nanoparticles,laser ablation,bubble dynamics
更新于2025-09-23 15:19:57
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The Vibration Behavior of Suba??Micrometer Gas Vesicles in Response to Acoustic Excitation Determined via Laser Doppler Vibrometry
摘要： The ability to monitor sub-micrometer gas vesicles’ (GVs) vibration behavior to nonlinear buckling and collapse using laser Doppler vibrometry is reported, providing a precise noncontact technique for monitoring the motion of sub-micrometer objects. The fundamental and first harmonic resonance frequencies of the vesicles are found to be 1.024 and 1.710 GHz, respectively. An interparticle resonance is furthermore identified at ≈300 MHz, inversely dependent upon the agglomerated GV size of around 615 nm. Most importantly, the vesicles amplify and broaden input acoustic signals at far lower frequencies—for example, 7 MHz—associated with medical and industrial applications, and they are found to transition from a linear to nonlinear response at 150 kPa and to collapse at 350 kPa or greater.
关键词： bubble dynamics,gas vesicles,sub-micrometer object metrology,acoustofluidics,laser Doppler vibrometry
更新于2025-09-19 17:13:59
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Laser induced cavitation: Plasma generation and breakdown shockwave
摘要： Laser induced cavitation is one of the effective techniques to generate controlled cavitation bubbles, both for basic study and for applications in different fields of engineering and medicine. Unfortunately, control of bubble formation and symmetry is hardly achieved due to a series of concurrent causes. In particular, the need to focus the laser beam at the bubble formation spot leads, in general, to a conical region proximal to the light source where conditions are met for plasma breakdown. A finite sized region then exists where the electric field may fluctuate depending on several disturbing agents, leading to possible plasma fragmentation and plasma intensity variation. Such irregularities may induce asymmetry in the successive bubble dynamics, a mostly undesired effect if reproducible conditions are sought for. In the present paper, the structure of the breakdown plasma and the ensuing bubble dynamics are analyzed by means of high speed imaging and intensity measurements of the shockwave system launched at breakdown. It is found that the parameters of the system can be tuned to optimize repeatability and sphericity. In particular, symmetric rebound dynamics is achieved almost deterministically when a pointlike plasma is generated at the breakdown threshold energy. Spherical symmetry is also favored by a large focusing angle combined with a relatively large pulse energy, a process which, however, retains a significant level of stochasticity. Outside these special conditions, the elongated and often fragmented conical plasma shape is found to be correlated with anisotropic and multiple breakdown shockwave emission.
关键词： high speed imaging,Laser induced cavitation,bubble dynamics,breakdown shockwave,plasma generation
更新于2025-09-12 10:27:22
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Simulation of laser-produced single cavitation bubbles with hybrid thermal Lattice Boltzmann method
摘要： Using a hybrid thermal Lattice Boltzmann method, the dynamics of laser-produced single cavitation bubbles in bulk liquid and the bubble collapse process near a solid boundary are numerically investigated. The simulated bubble evolutions satisfyingly agree with the theoretical calculations and the previous experimental results. The simulated bubble radius changes in bulk liquid are in good accordance with the calculations of a revised 2-D Rayleigh–Plesset equation that incorporates an extra thermal effect term. The maximum bubble radius is linearly proportional to the bubble collapse time and the input laser energy, which is consistent with the experimental data and bubble dynamics theory. Processes of a single cavitation bubble collapse at various distances from a solid boundary are analyzed in detail. The velocity vectors, density, pressure, and temperature fields are presented. The retarding effect of a solid boundary is successfully reproduced in the LBM simulations and leads to bubble elongation, the formation of micro-jet, bubble toroidal deformation, and the attraction of the bubble to the boundary during the collapse phase. The attraction effect, maximum jet velocity, and maximum pressure at the solid boundary all increase with reduced non-dimensional distance. A critical non-dimensional distance of 2.2 is validated by both the simulation and experiment. The hybrid thermal Lattice Boltzmann method is a reliable tool to investigate non-isothermal cavitation bubble dynamics.
关键词： Thermal Lattice Boltzmann method,Bubble collapse near boundary,Laser-produced bubble,Cavitation bubble dynamics
更新于2025-09-11 14:15:04