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3 pages, 142 KiB

Open AccessEditorial

Numerical Simulations of Turbulent Combustion

by Andrei N. Lipatnikov

Fluids 2020, 5(1), 22; https://doi.org/10.3390/fluids5010022 - 10 Feb 2020

Cited by 2 | Viewed by 2128

Abstract

Turbulent burning of gaseous fuels is widely used for energy conversion in stationary power generation, e [...] Full article

(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion)

Research

Jump to: Editorial

20 pages, 2247 KiB

Open AccessArticle

Investigations of Evaporative Cooling and Turbulence Flame Interaction Modeling in Ethanol Turbulent Spray Combustion Using Tabulated Chemistry

by Fernando Luiz Sacomano Filho, Louis Dressler, Arash Hosseinzadeh, Amsini Sadiki and Guenther Carlos Krieger Filho

Fluids 2019, 4(4), 187; https://doi.org/10.3390/fluids4040187 - 31 Oct 2019

Cited by 10 | Viewed by 3568

Abstract

Evaporative cooling effects and turbulence flame interaction are analyzed in the large eddy simulation (LES) context for an ethanol turbulent spray flame. Investigations are conducted with the artificially thickened flame (ATF) approach coupled with an extension of the mixture adaptive thickening procedure to [...] Read more.

Evaporative cooling effects and turbulence flame interaction are analyzed in the large eddy simulation (LES) context for an ethanol turbulent spray flame. Investigations are conducted with the artificially thickened flame (ATF) approach coupled with an extension of the mixture adaptive thickening procedure to account for variations of enthalpy. Droplets are tracked in a Euler–Lagrangian framework, in which an evaporation model accounting for the inter-phase non-equilibrium is applied. The chemistry is tabulated following the flamelet generated manifold (FGM) method. Enthalpy variations are incorporated in the resulting FGM database in a universal fashion, which is not limited to the heat losses caused by evaporative cooling effects. The relevance of the evaporative cooling is evaluated with a typically applied setting for a flame surface wrinkling model. Using one of the resulting cases from the evaporative cooling analysis as a reference, the importance of the flame wrinkling modeling is studied. Besides its novelty, the completeness of the proposed modeling strategy allows a significant contribution to the understanding of the most relevant phenomena for the turbulent spray combustion modeling. Full article

(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion)

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12 pages, 499 KiB

Open AccessArticle

Modelling of Self-Ignition in Spark-Ignition Engine Using Reduced Chemical Kinetics for Gasoline Surrogates

by Ahmed Faraz Khan, Philip John Roberts and Alexey A. Burluka

Fluids 2019, 4(3), 157; https://doi.org/10.3390/fluids4030157 - 17 Aug 2019

Cited by 3 | Viewed by 4892

Abstract

A numerical and experimental investigation in to the role of gasoline surrogates and their reduced chemical kinetic mechanisms in spark ignition (SI) engine knocking has been carried out. In order to predict autoignition of gasoline in a spark ignition engine three reduced chemical [...] Read more.

A numerical and experimental investigation in to the role of gasoline surrogates and their reduced chemical kinetic mechanisms in spark ignition (SI) engine knocking has been carried out. In order to predict autoignition of gasoline in a spark ignition engine three reduced chemical kinetic mechanisms have been coupled with quasi-dimensional thermodynamic modelling approach. The modelling was supported by measurements of the knocking tendencies of three fuels of very different compositions yet an equivalent Research Octane Number (RON) of 90 (ULG90, PRF90 and 71.5% by volume toluene blended with n-heptane) as well as iso-octane. The experimental knock onsets provided a benchmark for the chemical kinetic predictions of autoignition and also highlighted the limitations of characterisation of the knock resistance of a gasoline in terms of the Research and Motoring octane numbers and the role of these parameters in surrogate formulation. Two approaches used to optimise the surrogate composition have been discussed and possible surrogates for ULG90 have been formulated and numerically studied. A discussion has also been made on the various surrogates from the literature which have been tested in shock tube and rapid compression machines for their autoignition times and are a source of chemical kinetic mechanism validation. The differences in the knock onsets of the tested fuels have been explained by modelling their reactivity using semi-detailed chemical kinetics. Through this work, the weaknesses and challenges of autoignition modelling in SI engines through gasoline surrogate chemical kinetics have been highlighted. Adequacy of a surrogate in simulating the autoignition behaviour of gasoline has also been investigated as it is more important for the surrogate to have the same reactivity as the gasoline at all engine relevant

p - T

conditions than having the same RON and Motored Octane Number (MON). Full article

(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion)

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20 pages, 2188 KiB

Open AccessArticle

Numerical Investigation of Pressure Influence on the Confined Turbulent Boundary Layer Flashback Process

by Aaron Endres and Thomas Sattelmayer

Fluids 2019, 4(3), 146; https://doi.org/10.3390/fluids4030146 - 1 Aug 2019

Cited by 18 | Viewed by 4267

Abstract

Boundary layer flashback from the combustion chamber into the premixing section is a threat associated with the premixed combustion of hydrogen-containing fuels in gas turbines. In this study, the effect of pressure on the confined flashback behaviour of hydrogen-air flames was investigated numerically. [...] Read more.

Boundary layer flashback from the combustion chamber into the premixing section is a threat associated with the premixed combustion of hydrogen-containing fuels in gas turbines. In this study, the effect of pressure on the confined flashback behaviour of hydrogen-air flames was investigated numerically. This was done by means of large eddy simulations with finite rate chemistry as well as detailed chemical kinetics and diffusion models at pressures between

0.5

bar and 3 bar. It was found that the flashback propensity increases with increasing pressure. The separation zone size and the turbulent flame speed at flashback conditions decrease with increasing pressure, which decreases flashback propensity. At the same time the quenching distance decreases with increasing pressure, which increases flashback propensity. It is not possible to predict the occurrence of boundary layer flashback based on the turbulent flame speed or the ratio of separation zone size to quenching distance alone. Instead the interaction of all effects has to be accounted for when modelling boundary layer flashback. It was further found that the pressure rise ahead of the flame cannot be approximated by one-dimensional analyses and that the assumptions of the boundary layer theory are not satisfied during confined boundary layer flashback. Full article

(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion)

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11 pages, 1393 KiB

Open AccessArticle

Closure Relations for Fluxes of Flame Surface Density and Scalar Dissipation Rate in Turbulent Premixed Flames

by Andrei N. Lipatnikov, Shinnosuke Nishiki and Tatsuya Hasegawa

Fluids 2019, 4(1), 43; https://doi.org/10.3390/fluids4010043 - 7 Mar 2019

Cited by 7 | Viewed by 3016

Abstract

In this study, closure relations for total and turbulent convection fluxes of flame surface density and scalar dissipation rate were developed (i) by placing the focus of consideration on the flow velocity conditioned to the instantaneous flame within the mean flame brush and [...] Read more.

In this study, closure relations for total and turbulent convection fluxes of flame surface density and scalar dissipation rate were developed (i) by placing the focus of consideration on the flow velocity conditioned to the instantaneous flame within the mean flame brush and (ii) by considering the limiting behavior of this velocity at the leading and trailing edges of the flame brush. The model was tested against direct numerical simulation (DNS) data obtained from three statistically stationary, one-dimensional, planar, premixed turbulent flames associated with the flamelet regime of turbulent burning. While turbulent fluxes of flame surface density and scalar dissipation rate, obtained in the DNSs, showed the countergradient behavior, the model predicted the total fluxes reasonably well without using any tuning parameter. The model predictions were also compared with results computed using an alternative closure relation for the flame-conditioned velocity. Full article

(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion)

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13 pages, 1582 KiB

Open AccessArticle

DNS Study of the Bending Effect Due to Smoothing Mechanism

by Rixin Yu and Andrei N. Lipatnikov

Fluids 2019, 4(1), 31; https://doi.org/10.3390/fluids4010031 - 19 Feb 2019

Cited by 6 | Viewed by 2834

Abstract

Propagation of either an infinitely thin interface or a reaction wave of a nonzero thickness in forced, constant-density, statistically stationary, homogeneous, isotropic turbulence is simulated by solving unsteady 3D Navier–Stokes equations and either a level set (G) or a reaction-diffusion equation, respectively, with [...] Read more.

Propagation of either an infinitely thin interface or a reaction wave of a nonzero thickness in forced, constant-density, statistically stationary, homogeneous, isotropic turbulence is simulated by solving unsteady 3D Navier–Stokes equations and either a level set (G) or a reaction-diffusion equation, respectively, with all other things being equal. In the case of the interface, the fully developed bulk consumption velocity normalized using the laminar-wave speed S_L depends linearly on the normalized rms velocity u′/S_L. In the case of the reaction wave of a nonzero thickness, dependencies of the normalized bulk consumption velocity on u′/S_L show bending, with the effect being increased by a ratio of the laminar-wave thickness to the turbulence length scale. The obtained bending effect is controlled by a decrease in the rate of an increase δA_F in the reaction-zone-surface area with increasing u′/S_L. In its turn, the bending of the δA_F(u′/S_L)-curves stems from inefficiency of small-scale turbulent eddies in wrinkling the reaction-zone surface, because such small-scale wrinkles characterized by a high local curvature are smoothed out by molecular transport within the reaction wave. Full article

(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion)

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24 pages, 6099 KiB

Open AccessArticle

Effects of Lewis Number on the Evolution of Curvature in Spherically Expanding Turbulent Premixed Flames

by Ahmad Alqallaf, Markus Klein and Nilanjan Chakraborty

Fluids 2019, 4(1), 12; https://doi.org/10.3390/fluids4010012 - 16 Jan 2019

Cited by 14 | Viewed by 4704

Abstract

The effects of Lewis number on the physical mechanisms pertinent to the curvature evolution have been investigated using three-dimensional Direct Numerical Simulation (DNS) of spherically expanding turbulent premixed flames with characteristic Lewis number of

L e = 0.8

, 1.0 and 1.2. It [...] Read more.

The effects of Lewis number on the physical mechanisms pertinent to the curvature evolution have been investigated using three-dimensional Direct Numerical Simulation (DNS) of spherically expanding turbulent premixed flames with characteristic Lewis number of

L e = 0.8

, 1.0 and 1.2. It has been found that the overall burning rate and the extent of flame wrinkling increase with decreasing Lewis number

L e

, and this tendency is particularly prevalent for the sub-unity Lewis number (e.g.,

L e = 0.8

) case due to the occurrence of the thermo-diffusive instability. Accordingly, the

L e = 0.8

case has been found to exhibit higher probability of finding saddle topologies with large magnitude negative curvatures in comparison to the corresponding

L e = 1.0

and 1.2 cases. It has been found that the terms in the curvature transport equation due to normal strain rate gradients and curl of vorticity arising from both fluid flow and flame normal propagation play pivotal roles in the curvature evolution in all cases considered here. The net contribution of the source/sink terms of the curvature transport equation tends to increase the concavity and convexity of the flame surface in the negatively and positively curved locations, respectively for the

L e = 0.8

case. This along with the occurrence of high and low temperature (and burning rate) values at the positively and negatively curved zones, respectively acts to augment positive and negative curved wrinkles induced by turbulence in the

L e = 0.8

case, which is indicative of thermo-diffusive instability. By contrast, flame propagation effects tend to weakly promote the concavity of the negatively curved cusps, and act to decrease the convexity of the highly positively curved bulges in the

L e = 1.0

and 1.2 cases, which are eventually smoothed out due to high and low values of displacement speed

S_{d}

at negatively and positively curved locations, respectively. Thus, flame propagation tends to smoothen the flame surface in the

L e = 1.0

and 1.2 cases. Full article

(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion)

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25 pages, 15752 KiB

Open AccessArticle

Investigation of the Turbulent Near Wall Flame Behavior for a Sidewall Quenching Burner by Means of a Large Eddy Simulation and Tabulated Chemistry

by Arne Heinrich, Guido Kuenne, Sebastian Ganter, Christian Hasse and Johannes Janicka

Fluids 2018, 3(3), 65; https://doi.org/10.3390/fluids3030065 - 6 Sep 2018

Cited by 3 | Viewed by 4521

Abstract

Combustion will play a major part in fulfilling the world’s energy demand in the next 20 years. Therefore, it is necessary to understand the fundamentals of the flame–wall interaction (FWI), which takes place in internal combustion engines or gas turbines. The FWI can [...] Read more.

Combustion will play a major part in fulfilling the world’s energy demand in the next 20 years. Therefore, it is necessary to understand the fundamentals of the flame–wall interaction (FWI), which takes place in internal combustion engines or gas turbines. The FWI can increase heat losses, increase pollutant formations and lowers efficiencies. In this work, a Large Eddy Simulation combined with a tabulated chemistry approach is used to investigate the transient near wall behavior of a turbulent premixed stoichiometric methane flame. This sidewall quenching configuration is based on an experimental burner with non-homogeneous turbulence and an actively cooled wall. The burner was used in a previous study for validation purposes. The transient behavior of the movement of the flame tip is analyzed by categorizing it into three different scenarios: an upstream, a downstream and a jump-like upstream movement. The distributions of the wall heat flux, the quenching distance or the detachment of the maximum heat flux and the quenching point are strongly dependent on this movement. The highest heat fluxes appear mostly at the jump-like movement because the flame behaves locally like a head-on quenching flame. Full article

(This article belongs to the Special Issue Numerical Simulations of Turbulent Combustion)

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