scholarly journals Flow and deposition characteristics of nanoparticles in a 90° square bend

2015 ◽  
Vol 19 (4) ◽  
pp. 1235-1238 ◽  
Author(s):  
Zhong-Ping Dai ◽  
Zhao-Qin Yin
Keyword(s):  
Residence Time ◽  
Penetration Rate ◽  
Particle Model ◽  
Flow Conditions ◽  
Dean Number ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Discrete Particle Model ◽  
Schmidt Numbers ◽  
Bend Flow

Large eddy simulation and discrete particle model have been used to study the nanoparticles through a 90? square bend flow considering the effects of Brownian motion and turbulence diffusion. The penetration rate and the residence time of particles are evaluated under different flow conditions and various particle sizes. Results show that particles penetration rate increases with an increase in Dean and Schmidt numbers. The particle size and flow Dean number have significantly effects on the particles residence time in the bend.

Analysis of Autoignition Chemistry in Aeroderivative Premixers At Engine Conditions

2021 ◽  
Author(s):  
Sandeep Jella ◽  
Gilles Bourque ◽  
Pierre Gauthier ◽  
Philippe Versailles ◽  
Jeffrey M. Bergthorson ◽  
...  
Keyword(s):  
Residence Time ◽  
Real Life ◽  
Precursor Chemistry ◽  
Mode Analysis ◽  
Safety Factors ◽  
Eddy Simulation ◽  
Reduced Mechanism ◽  
Short Period ◽  
Large Eddy ◽  
Pure Methane

Abstract The minimization of autoignition risk is critical to premixer design. Safety factors based on ignition delays of homogeneous mixtures, are generally used to guide the choice of a residence time for a given premixer. However, autoignition chemistry at aeroderivative conditions is fast (0.5-2 milliseconds) and can be initiated within typical premixer residence times. The analysis of what takes place in this short period involves the study of low-temperature precursor chemistry. By coupling the evolution of the Chemical Explosive Modes to turbulence, it is possible to obtain a measure of spatial autoignition risk where both chemical (e.g. ignition delay) and aerodynamic (e.g. local residence time) influences are unified. In this article, we describe a method that couples Large Eddy Simulation to newly developed, reduced autoignition chemical kinetics to study autoignition precursors in an example premixer representative of real life geometric complexity. A blend of pure methane and dimethyl ether (DME), a common fuel used for experimental autoignition studies, was transported using the reduced mechanism (38 species / 238 reactions) at engine conditions at increasing levels of DME concentration until exothermic autoignition kernels were formed. The Chemical Explosive Mode analysis closely follows the large thermochemical changes in the premixer as a function of DME concentration and identifies where the premixer is sensitive and flame anchoring is likely to occur.

scholarly journals Large Eddy Simulation of Multi-Phase Flow and Slag Entrapment in Molds with Different Widths

Metals ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 374
Author(s):  
Xianjiu Zhao ◽  
Xianglong Li ◽  
Jieyu Zhang
Keyword(s):  
Large Eddy Simulation ◽  
Transverse Velocity ◽  
Flaw Detection ◽  
Interface Velocity ◽  
Dimensionless Number ◽  
Particle Model ◽  
Three Dimensional Model ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Slag Entrapment

Slag entrapment is a critical problem that affects the quality of steel. In this work, a three-dimensional model is established to simulate the slag entrapment phenomenon, mainly focusing on the slag entrapment phenomenon at the interface between slag and steel in molds with different widths. The large eddy simulation (LES) model and discrete particle model (DPM) are used to simulate the movements of bubbles. The interactions between phases involve two-way coupling. The accuracy of our mathematical model is validated by comparing slag–metal interface fluctuations with practical measurements. The results reveal that the average interface velocity and transverse velocity decrease as the mold width increases, however, they cannot represent the severity of slag entrapment at the interface between slag and steel. Due to the influence of bubble motion behavior, the maximum interface velocity increases with mold width and causes slag entrapment readily, which can reflect the severity of slag entrapment. On this basis, by monitoring the change of impact depths in different molds, a new dimensionless number “C” is found to reveal the severity of slag entrapment at the interface between slag and steel. The results show that the criterion number C increases with mold width, which is consistent with the results of flaw detection. Therefore, criterion number C can be used to reflect the severity of slag entrapment in different molds.

scholarly journals Large Eddy Simulation of Multi-Phase Flow and Slag Entrapment in a Continuous Casting Mold

Metals ◽  
2018 ◽  
Vol 9 (1) ◽  
pp. 7 ◽  
Author(s):  
Xianglong Li ◽  
Baokuan Li ◽  
Zhongqiu Liu ◽  
Ran Niu ◽  
Yanqiang Liu ◽  
...  
Keyword(s):  
Large Eddy Simulation ◽  
Continuous Casting ◽  
Vortex Formation ◽  
Three Dimensional ◽  
Particle Model ◽  
Continuous Casting Mold ◽  
Casting Mold ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Slag Entrapment

A transient, three-dimensional mathematical model has been developed to study the slag entrapment in a continuous casting mold. The unsteady turbulent flow is computed using the large eddy simulation (LES). The sub-grid scale structure is modeled by the Smagorinsky–Lilly model. The movements of discrete bubbles, as well as three continuous phases (air–slag–steel), are described by solving the coupled discrete particle model and volume of fraction (DPM+VOF) approach. The bubble transport inside different phases (steel and slag) and the escape near the air–slag interface are well studied. Good agreement is obtained by comparing with the plant observation of the slag eyes on the top surface of the mold. Three main mechanisms of slag entrapment are identified; vortex formation, shear-layer instability, and meniscus fluctuation. Four stages are observed for a slag entrapment: deformation, necking, breaking, and dragging in the mold. The model is helpful for understanding the formation of slag entrapment during continuous casting.

scholarly journals Large-Eddy Simulation of Air Parcels in Stratocumulus Clouds: Time Scales and Spatial Variability

2006 ◽  
Vol 63 (3) ◽  
pp. 952-967 ◽  
Author(s):  
Yefim L. Kogan
Keyword(s):  
Large Eddy Simulation ◽  
Spatial Variability ◽  
Time Scales ◽  
Residence Time ◽  
Turnover Time ◽  
Spatial Inhomogeneity ◽  
Cloud Base ◽  
Eddy Simulation ◽  
Large Eddy ◽  
Stratocumulus Clouds

Abstract Large ensembles of air parcel trajectories driven by the (large-eddy simulation) LES-generated velocity fields from simulations of stratocumulus clouds were analyzed, focusing on statistics of air parcel in-cloud time scales, as well as their spatial variability. In the case of a drizzling stratocumulus cloud the in-cloud residence time is 2–5 times longer than the characteristic cloud eddy turnover time. About 70% of all air parcels cycle in the cloud more than 2 times and about 50% more than 3 times, thus indicating that air cycling is an essential feature of drizzling stratocumulus cloud dynamics. The extent of cycling is different in the case of nondrizzling stratocumulus cloud, where mean in-cloud time scales are on the order of eddy turnover time. Evidently air cycling in cloud depends on boundary layer stability and flow circulation; the latter is affected by cooling of evaporating drizzle and heating by solar radiation. Results show significant inhomogeneity of in-cloud time scales, which leads to inhomogeneity in cloud microphysical parameters. The potential effects of in-cloud residence time spatial inhomogeneity on cloud microstructure are obvious and significant. Older parcels will contain larger droplets and previously processed cloud condensation nuclei (CCN). Nonadiabatic mixing between old and new parcels provides new embryos for coagulation and accelerates drizzle formation. It is hypothesized that mixing of parcels with different histories, that is, with drop size distributions at different stages of their evolution, may contribute to the drop spectrum broadening. The results also suggest a possible positive feedback mechanism between drizzle and decoupling, namely, parcels with long time trajectories will favor enhanced drizzle growth, which, in turn, will lead to stronger evaporation below cloud base followed by a stronger increase in stability of the subcloud layer and stronger decoupling; all resulting in more air parcel cycling in cloud and more drizzle, which may eventually lead to stratocumulus cloud breakup.

Cavitation Modelling in Different Designs of Micro Kaplan Hydro-Turbine

2018 ◽  
Author(s):  
Tarek ElGammal ◽  
Tomoki Sakamoto ◽  
Ryoichi S. Amano
Keyword(s):  
Relative Efficiency ◽  
Water System ◽  
Cavitating Flows ◽  
Flow Conditions ◽  
Hydro Turbine ◽  
Eddy Simulation ◽  
Water Head ◽  
Large Eddy ◽  
Regular Design ◽  
Efficiency Gap

The paper investigates the cavitation in micro-turbomachinery, using a small-sized water system. Unsteady numerical model is architected to predict cavitating flows through a 7.5 cm axial hydro-turbine working at 2.8 m water head. Based on the validated simulations, specific turbine designs (regular design and rim-drive turbines) are simulated with cavitating flow conditions including different rotation speeds (1000–5000 rpm) and outlet pressures (0, -24, -48, -96, kPa gage). Phase change interactions (liquid water and vapor) were considered by adding the physics models of Volume of Fluid (VOF) multiphase, cavitation, and Large Eddy Simulation (LES) turbulence. Records featured spatial variation in the cavitation pattern between the two designs. Rim-drive turbine stands against cavitation along the rim integration lines, but it starts the hub cavitation earlier than the regular turbine. The proposed rim-drive bests the regular geometry before cavitation, and the relative efficiency gap increased to be 16% at extreme cavitation condition.

scholarly journals Analysis of Channel Vortex and Cavitation Performance of the Francis Turbine under Partial Flow Conditions

Processes ◽  
2021 ◽  
Vol 9 (8) ◽  
pp. 1385
Author(s):  
Tao Guo ◽  
Jinming Zhang ◽  
Zhumei Luo
Keyword(s):  
Negative Impact ◽  
Impact Angle ◽  
Francis Turbine ◽  
Scale Model ◽  
Vortex Identification ◽  
Flow Conditions ◽  
Eddy Simulation ◽  
Cavitation Performance ◽  
Large Eddy ◽  
Calculation Results

To realize a multienergy complementary system involving hydropower and other energy sources, hydraulic turbines frequently run under partial flow conditions in which a unique flow phenomenon, the channel vortex, occurs in the runner, causing fatigue failure and even cavitation to the turbine blade. Cavitation severely shortens the service life of the unit and terribly limits the output of the turbine under partial flow conditions. In this paper, a numerical model of a Francis turbine was created with tetrahedral grids; the large eddy simulation (LES) method based on the WALE subgrid scale model and the Schnerr–Sauer cavitation model was adopted to carry out numerical simulation of the Francis turbine; and a vortex identification method based on the Q criterion was used to capture and analyze the channel vortex. The calculation results showed that a negative impact angle at the inlet of the runner occurred when the turbine ran under partial flow conditions, leading to three different types of channel vortexes in the blade channel. Also, different channel vortexes caused cavitation on different positions on the runner, and the volume change of cavitation showed periodic properties.

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