Papers by Keyword: Bubble Formation

Abstract: Computational modelling of dissolved gas bubble formation and growth in supersaturated solution is essential for various engineering applications, including flash vaporisation of petroleum crude oil. The common mathematical modelling of bubbly flow only caters for single liquid and its vapour, which is known as cavitation. This work aims to simulate the bubble nucleation and growth of dissolved CO₂ in water across a cavitating nozzle. The dynamics of bubble nucleation and growth phenomenon will be predicted based on the hydrodynamics in the computational domain. The complex interrelated bubble dynamics, mass transfer and hydrodynamics was coupled by using Computational Fluid Dynamics (CFD) and bubble nucleation and growth model. Generally, the bubbles nucleate at the throat of the nozzle and grow along with the flow. Therefore, only the region after the throat of the nozzle has bubbles. This approach is expected to be useful for various types of bubbly flow modelling in supersaturated condition.

Abstract: A solution could be made supersaturated when the physical conditions was altered. The supersaturated gas would form bubble to bring the solution back to the equilibrium condition. Bubble would first nucleated and then growth in the solution. This work was aimed to simulate the bubble formation across an orifice, with sufficient pressure drop. The CO₂ was assumed to be made saturated at 6 atm in water, and formed bubble when the solution flowed across the orifice. As the CO₂ bubble formed across the orifice, the volume of the CO₂ gas occupied almost the whole space in the orifice. The mass of the dissolved CO₂ in water had decreased too as it had transferred into gas CO₂.

Abstract: The dynamics of bubble generating in carboxymethylcellulose (CMC) aqueous solution were studied using chaos method. The results show that for low position (P) and gas flowrate (Q_g), the characteristic frequency of axial and radial velocities are both consistent to bubble periodic shear frequency; but for high position, power spectrums are of broad coverage within low frequency range. The magnitudes of radial characteristic peaks increase with gas flowrate. Phase portraits of reconstructed phase space of velocity time series display certain features of chaotic attractor. For low P and Q_g, phase portrait takes on uniform elliptical shape due to liquid periodic motion caused by bubble periodic rising. Whereas for high P and Q_g, the liquid turbulence is reinforced by bubble increasing velocity and large-scale turbulent intensity around bubble increases, consequently, leading to significant compression of phase portrait.

Abstract: Kinetic Theory of Granular Flow (KTGF) has been successfully incorporated and widely implemented in the Eulerian simulation models in many multiphase cases. The KTGF theory involves many parameters and is applied in the multiphase simulation for the purpose of hydrodynamic properties modeling of the granular phase. This paper is focused on granular viscosity which is a parameter in the KTGF that incorporates three different viscosities arising from the inter-phase and intra phases interaction in a bubbling fluidized bed (BFB). The 2D BFB model of 0.2 m width and 0.8 m length having a 13-hole orifice plate has been modeled for this purpose. The model was constructed using Gambit software version 2.4.6 and then simulated using ANSYS Fluent version 14. Two models of granular viscosity, namely Syamlal-Obrien model and Gidaspow model, were compared based on its effect to the pressure drop and bed expansion of the BFB. The results depicted that the simulation based on Syamlal-Obrien model tends to produce larger bubbles and contributing to a higher pressure drop across the distributor plate as compared to the Gidaspow model.

Abstract: Water model experiments were carried out to understand the behavior of the bubble formation near the immersion nozzle, bubble rising velocity in the liquid. The critical state appeared when Fr number changed from 5 to 6 was described. The character of the critical phenomenon was whether the evident separation between two continuous bubbles or air masses appeared.

Abstract: This paper presents the results of an experimental study of heat transfer in a pool boiling evaporator with porous insert. Different types of graphite foams were tested with the phase change coolant FC-72 in a designed thermosyphon. Comparisons between the graphite foams and a solid copper block show that the porous structure enhances pool boiling significantly. The boiling thermal resistance of the tested graphite foams was found to be about 2 times lower than that of the copper block. The bubble formation recorded by a high speed camera indicates that boiling from a graphite foam is more vigorous than from a copper block. The designed thermosyphon with graphite foam insert can remove heat fluxes of up to 112 W/cm2 with the maximum heater temperature maintained below 100°C.

Abstract: A new strategy for the treatment of positron annihilation lifetime (PAL) spectra in viscous liquids is proposed, enabling to extract values of the Ps reaction rate constants with intratrack radiolytic products as well as parameters of the free volume distribution in viscous media.

Abstract: The passive oxidation mechanism of CVD SiC was discussed from experimental results with high-temperature thermogravimetry and thermodynamic analyses. The bubble formation temperature around 1900 K could be too low for an oxygen inward diffusion limited process but conform to a CO outward diffusion limited process. The parabolic rate constant (kp) had weak oxygen partial pressure (PO2) dependence, kp ∝ PO2 n where n = 0.09 to 0.12. These n values may be consistent with the CO outward diffusion limited process. The activation energy of kp obtained in the present study, 210 kJ/mol, could suggest a different mechanism from the well-approved oxygen molecule permeation limited process at lower temperatures below 1600 K. Amorphous phase was significantly contained in SiO2 scales formed in an N2-O2 atmosphere. No effect of the amorphous formation on kp was identified.