71,235 research outputs found
A study of liquid droplet disintegration for the development of nanostructured coatings
No full textThermal spray coatings produced from a liquid feedstock are receiving an increasing level of interest due to the advanced, nanostructured coatings which are obtainable by these processes. In this article, a high-velocity oxy-fuel (HVOF) thermal spray system is computationally investigated to make a scientific assessment of the liquid droplet behavior on injection. An existing liquid-fuelled HVOF thermal spray gun is simulated using the computational fluid dynamic approach. The steady-state gas-phase dynamics are initialized by the introduction of liquid kerosene and oxygen which react within the combustion chamber producing a realistic compressible, turbulent jet. Discrete-phase water droplets are injected at the powder injection port. On injection, the water droplets breakup and vaporize, while being entrained through the acceleration barrel of the HVOF system. The results obtained give an insight to the mechanism which control the water droplet sizes and disintegration process, and serve as a fundamental reference for future development of liquid feedstock devices
Modelling of impingement phenomena for molten metallic droplets with low to high velocities
No full textThermal spray coatings are formed by accelerating a stream of powder particles towards a targeted substrate surface where they impact, deform, and adhere. A fundamental understanding of the splat formation can pave the way for future developments in thermal spray technology through better understanding. Numerical modelling is applied in this investigation which simulates the detailed transient flow of a molten metal droplet impacting, deforming, and solidifying on a flat, solid substrate. The computations are carried out on a fixed Eularian structured mesh using a volume of fluid method to simulate the boundary between the metallic and atmospheric-gas phases. The results shed light on the break-up phenomena on impact and describe in detail how the solidification process varies with an increasing impact velocity.<br/
Study of in-flight and impact dynamics of non-spherical particles from HVOF guns
Full text linkHigh velocity oxygen fuel thermal spray has been widely used to deposit hard composite materials such as WC-Co powders for wear-resistant applications. Unlike gas atomized spherical powders, WC-CO powders form a more complex geometry. The knowledge gained from the existing spherical powders on process control and optimization may not be directly applicable to WC-Co coatings. This paper is the first to directly examine nonspherical particle in-flight dynamics and the impingement process on substrate using computational methods. Two sets of computational models are developed. First, the in-flight particles are simulated in the CFD-based combusting gas flow. The particle information prior to impact is extracted from the CFD results and implemented in a FEA model to dynamically track the impingement of particles on substrate. The morphology of particles is examined extensively including both spherical and nonspherical powders to establish the critical particle impact parameters needed for adequate bonding. <br/
Effects of limestone calcination on the gasification processes in a BFB coal gasifier
Full text linkAn Eulerian-Eulerian computational fluid dynamics (CFD) model of the gasification processes in a coal bubbling fluidised bed (BFB) is presented incorporating the devolatilisation, heterogeneous, homogeneous reactions and limestone calcination. The model considers separate phases for the coal, limestone and char and is carried out for different experimental conditions taken from the literature.The results of the exiting gas compositions have been averaged over time and validated with experimental data. The hydrodynamic behaviour as well as temperature and reaction distributions within the bed are presented. The impact of limestone calcination on the gaseous composition is observe
FIGURE 3 in Sphaeropteris guangxiensis Y. F. Gu & Y. H. Yan (Cyatheaceae), a new species of tree fern from Southern China
No full textFIGURE 3. Fiddleheads and sori on pinnule segments.—A & D. Sphaeropteris brunoniana.—B & E. S. guangxiensis.—C & F. S. hainanensis.Published as part of Gu, Yu-Feng, Jiang, Ri-Hong, Liu, Bao-Dong & Yan, Yue-Hong, 2021, Sphaeropteris guangxiensis Y. F. Gu & Y. H. Yan (Cyatheaceae), a new species of tree fern from Southern China, pp. 69-74 in Phytotaxa 518 (1) on page 72, DOI: 10.11646/phytotaxa.518.1.8, http://zenodo.org/record/544801
CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors: Modelling the impact of biomass shrinkage
No full textThe fluid–particle interaction and the impact of shrinkage on pyrolysis of biomass inside a 150 g/h fluidised bed reactor is modelled. Two 500 m in diameter biomass particles are injected into the fluidised bed with different shrinkage conditions. The two different conditions consist of (1) shrinkage equal to the volume left by the solid devolatilization, and (2) shrinkage parameters equal to approximately half of particle volume. The effect of shrinkage is analysed in terms of heat and momentum transfer as well as product yields, pyrolysis time and particle size considering spherical geometries. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of user defined function (UDF).<br/
The influence of multiple tubes on the tube-to-bed heat transfer in a fluidised bed
Full text linkThere have been few studies modelling both flow and heat transfer in fluidised beds. The kinetic theory of granular flow (KTGF) has been used for flow prediction in the past without heat transfer modelling. In the present study, a two-fluid Eulerian-Eulerian formulation incorporating the KTGF was applied first to a tube-to-bed reactor with one immersed tube and compared with the results in the literature. The bed was then modified to introduce two and three heated tubes. The effects on the flow and temperature distribution, local heat transfer coefficients and averaged heat transfer coefficients over a 3.0s time period were carried out. Results showed that increasing the number of tubes promotes heat transfer from tubes to the particles and flow. The heat transfer coefficients extracted from the single-tube to three-tube cases were analyzed in detail, confirming the importance of linking flow/particle and heat transfer calculation
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