LES/PDF modeling of autoignition in a lifted turbulent flame: Analysis of flame sensitivity to differential diffusion and scalar mixing time-scale

Continuous reaction calorimetry in microreactors is a powerful technology for the investigation of fast and exothermic reactions regarding thermokinetic data. A Seebeck element based reaction calorimeter has been designed, manufactured, and its performance has been shown in previous works using neutralization reaction in a microreactor made from PVDF-foils [1]. The Seebeck elements allow for spatial and temporal resolution of heat flux profiles across the reactor. Therefore, hot spots and regions of main reaction progress are detected. Finally, heat of reaction has been determined in good agreement with literature data [1]. However, more information can be retrieved related to chemical transformations using the continuously operated reaction calorimeter. In this work, mixing time scale is determined for instantaneous and exothermic reactions. Volumetric flow rate is varied and the region of main reaction progress is shifted within the microreactor. The reaction occurs near the reactor outlet for low flow rates. Here, mixing is dominated by diffusion. However, the reaction and hot spot are shifted towards the reactor inlet for high flow rates as convective mixing regime is reached and secondary flow profile with Dean vortices develop due to curvature of the reaction channel. Finally, mixing time scales can be derived from the location of heat flux peaks. Results display a decrease in mixing time at increased flow rates. Additionally, passive micromixers can be evaluated regarding their efficiency and comparison can be drawn. Moreover, pumps can be characterized and evaluated regarding low-pulsation dosing using the Seebeck element based reaction calorimeter.

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A model for the mixing time scale of a turbulent reacting scalar

Physics of Fluids ◽

10.1063/1.1565333 ◽

2003 ◽

Vol 15 (6) ◽

pp. 1375 ◽

Cited By ~ 16

Author(s):

Chong M. Cha ◽

Philippe Trouillet

Keyword(s):

Time Scale ◽

Mixing Time ◽

Mixing Time Scale

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Mixing Time Scale Models for Multiple Mapping Conditioning with Two Reference Variables

Flow Turbulence and Combustion ◽

10.1007/s10494-020-00188-0 ◽

2020 ◽

Author(s):

C. Straub ◽

A. Kronenburg ◽

O. T. Stein ◽

S. Galindo-Lopez ◽

M. J. Cleary

Keyword(s):

Time Scale ◽

Mixing Time ◽

Scale Models ◽

Mixing Time Scale

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Effect of Vertical Vibration on the Mixing Time of a Passive Scalar in a Sparged Bubble Column Reactor

Fluids ◽

10.3390/fluids5010006 ◽

2020 ◽

Vol 5 (1) ◽

pp. 6 ◽

Cited By ~ 1

Author(s):

Shahrouz Mohagheghian ◽

Afshin J. Ghajar ◽

Brian R. Elbing

Keyword(s):

Void Fraction ◽

Time Scale ◽

Temporal Evolution ◽

Bubble Column ◽

Mixing Time ◽

Passive Scalar ◽

Bubble Column Reactor ◽

Mean Flow ◽

Vertical Vibrations ◽

Mixing Time Scale

The present study used a sparged bubble column to study the mixing of a passive scalar under bubble-induced diffusion. The effect of gas superficial velocity (up to 69 mm/s) and external vertical vibrations (amplitudes up to 10 mm, frequency <23 Hz) on the mixing time scale were investigated. The bubble-induced mixing was characterized by tracking the distribution of a passive scalar within a sparged swarm of bubbles. Void fraction and bubble size distribution were also measured at each test condition. Without vibrations (static), the bubble column operated in the homogenous regime and the mixing time scale was insensitive to void fraction, which is consistent with the literature. In addition, the temporal evolution of the static column mixing was well approximated as an error function. With vertical vibrations at lower amplitudes tested, the bubble-induced mixing was restrained due to the suppression of the liquid velocity agitations in the bubble swarm wake, which decelerates mixing. Conversely, at higher amplitudes tested, vibration enhanced the bubble-induced mixing; this is attributed to bubble clustering and aggregation that produced void fraction gradients, which, in turn, induced a mean flow and accelerated the mixing. The vibration frequency for the range studied in the present work did not produce a significant effect on the mixing time. Analysis of the temporal evolution of the concentration of the passive scalar at a fixed point within the column revealed significant fluctuations with vibration. A dimensionally reasoned correlation is presented that scales the non-dimensional mixing time with the transient buoyancy number.

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Lagrangian characteristics of turbulence and scalar transport in direct numerical simulations

Journal of Fluid Mechanics ◽

10.1017/s0022112000002391 ◽

2001 ◽

Vol 427 ◽

pp. 241-274 ◽

Cited By ~ 84

Author(s):

P. K. YEUNG

Keyword(s):

Reynolds Number ◽

Correlation Function ◽

Numerical Simulations ◽

Time Scales ◽

Time Scale ◽

Cross Correlation ◽

Scalar Fields ◽

Direct Numerical Simulations ◽

Cross Correlation Function ◽

Differential Diffusion

A study of the Lagrangian statistical properties of velocity and passive scalar fields using direct numerical simulations is presented, for the case of stationary isotropic turbulence with uniform mean scalar gradients. Data at higher grid resolutions (up to 5123 and Taylor-scale Reynolds number 234) allow an update of previous velocity results at lower Reynolds number, including intermittency and dimensionality effects on vorticity time scales. The emphasis is on Lagrangian scalar time series which are new to the literature and important for stochastic mixing models. The variance of the ‘total’ Lagrangian scalar value (ϕ˜+, combining contributions from both mean and fluctuations) grows with time, with the velocity–scalar cross-correlation function and fluid particle displacements playing major roles. The Lagrangian increment of ϕ˜+ conditioned upon velocity and scalar fluctuations is well represented by a linear regression model whose parameters depend on both Reynolds number and Schmidt number. The Lagrangian scalar fluctuation is non-Markovian and has a longer time scale than the velocity, which is due to the strong role of advective transport, and is in contrast to results in an Eulerian frame where the scalars have shorter time scales. The scalar dissipation is highly intermittent and becomes de-correlated in time more rapidly than the energy dissipation. Differential diffusion for scalars with Schmidt numbers between 1/8 and 1 is characterized by asymmetry in the two-scalar cross-correlation function, a shorter time scale for the difference between two scalars, as well as a systematic decrease in the Lagrangian coherency spectrum up to at least the Kolmogorov frequency. These observations are consistent with recent work suggesting that differential diffusion remains important in the small scales at high Reynolds number.

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Passive scalar mixing: Analytic study of time scale ratio, variance, and mix rate

Physics of Fluids ◽

10.1063/1.2214704 ◽

2006 ◽

Vol 18 (7) ◽

pp. 075101 ◽

Cited By ~ 25

Author(s):

J. R. Ristorcelli

Keyword(s):

Time Scale ◽

Passive Scalar ◽

Analytic Study ◽

Scalar Mixing ◽

Scale Ratio

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Fuel Variation Effects in Propagation and Stabilization of Turbulent Counter-Flow Premixed Flames

Volume 4B: Combustion, Fuels, and Emissions ◽

10.1115/gt2018-77139 ◽

2018 ◽

Author(s):

Ehsan Abbasi-Atibeh ◽

Sandeep Jella ◽

Jeffrey M. Bergthorson

Keyword(s):

High Speed ◽

Laminar Flame ◽

Turbulent Flame ◽

Mass Diffusion ◽

Flame Stabilization ◽

Turbulent Flames ◽

Flame Speed ◽

Flame Surface ◽

Differential Diffusion ◽

Turbulent Flame Speed

Sensitivity to stretch and differential diffusion of chemical species are known to influence premixed flame propagation, even in the turbulent environment where mass diffusion can be greatly enhanced. In this context, it is convenient to characterize flames by their Lewis number (Le), a ratio of thermal-to-mass diffusion. The work reported in this paper describes a study of flame stabilization characteristics when the Le is varied. The test data is comprised of Le ≪ 1 (Hydrogen), Le ≈ 1 (Methane), and Le > 1 (Propane) flames stabilized at various turbulence levels. The experiments were carried out in a Hot exhaust Opposed-flow Turbulent Flame Rig (HOTFR), which consists of two axially-opposed, symmetric turbulent round jets. The stagnation plane between the two jets allows the aerodynamic stabilization of a flame, and clearly identifies fuel influences on turbulent flames. Furthermore, high-speed Particle Image Velocimetry (PIV), using oil droplet seeding, allowed simultaneous recordings of velocity (mean and rms) and flame surface position. These experiments, along with data processing tools developed through this study, illustrated that in the mixtures with Le ≪ 1, turbulent flame speed increases considerably compared to the laminar flame speed due to differential diffusion effects, where higher burning rates compensate for the steepening average velocity gradient, and keeps these flames almost stationary as bulk flow velocity increases. These experiments are suitable for validating the ability of turbulent combustion models to predict lifted, aerodynamically-stabilized flames. In the final part of this paper, we model the three fuels at two turbulence intensities using the FGM model in a RANS context. Computations reveal that the qualitative flame stabilization trends reproduce the effects of turbulence intensity, however, more accurate predictions are required to capture the influences of fuel variations and differential diffusion.

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