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AuthorAttili, Antonio (3)Bisetti, Fabrizio (3)Samtaney, Ravi (3)Chung, Suk Ho (2)Lu, Tianfeng (2)View MoreDepartmentMechanical Engineering Program (8)Physical Sciences and Engineering (PSE) Division (8)Clean Combustion Research Center (5)Mechanical Engineering (2)Reactive Flow Modeling Laboratory (RFML) (2)View MoreJournalCombustion and Flame (4)Physics of Fluids (2)Magnetohydrodynamics (1)Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences (1)KAUST Acknowledged Support Unit

Supercomputing Laboratory (8)

Competitive Research (1)Shaheen (1)Shaheen II (1)KAUST Grant Numberk1052 (1)project K1052 (1)Project No. k1052 (1)PublisherElsevier BV (4)AIP Publishing (2)Latvijas Universitate32 Miera StreetSalaspils-1LV-2169 (1)The Royal Society (1)SubjectSoot (3)Direct numerical simulations (2)DNS (2)Turbulent flames (2)Biodiesel (1)View MoreTypeArticle (8)Year (Issue Date)2019 (3)2016 (1)2014 (3)2013 (1)Item AvailabilityMetadata Only (6)Embargoed (2)

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Scaling and spatial intermittency of thermal dissipation in turbulent convection

Bhattacharya, Shashwat; Samtaney, Ravi; Verma, Mahendra K. (Physics of Fluids, AIP Publishing, 2019-07-16) [Article]

We derive scaling relations for the thermal dissipation rate in the bulk and in the boundary layers for moderate and large Prandtl number (Pr) convection. Using direct numerical simulations of Rayleigh-Bénard convection, we show that the thermal dissipation in the bulk is suppressed compared to passive scalar dissipation. The suppression is stronger for large Pr. We further show that the dissipation in the boundary layers dominates that in the bulk for both moderate and large Pr. The probability distribution functions of thermal dissipation rate, both in the bulk and in the boundary layers, are stretched exponential, similar to passive scalar dissipation.

Large eddy simulation of hydrodynamic turbulence using renormalized viscosity

Vashishtha, Sumit; Samuel, Roshan; Chatterjee, Anando Gopal; Samtaney, Ravi; Verma, Mahendra K. (Physics of Fluids, AIP Publishing, 2019-06) [Article]

We employ renormalized viscosity to perform large eddy simulations (LESs) of decaying homogeneous and isotropic turbulence in a cubical domain. We perform a direct numerical simulation (DNS) on 5123 and 2563 grids and LES on 323, 643, and 1283 grids with the same initial conditions in the resolved scales for a flow with Taylor Reynolds number Reλ = 210. We observe good agreement between LES and DNS results for the temporal evolution of turbulence kinetic energy Eu(t), kinetic energy spectrum Eu(k), and kinetic energy flux Πu(k). Also, the large-scale structures of the flow in LES are similar to those in DNS. These results establish the suitability of our renormalized viscosity scheme for LES.

Cross Helicity sign reversals in the dissipative scales of magnetohydrodynamic turbulence

Titov, V.; Stepanov, R.; Yokoi, N.; Verma, M.; Samtaney, Ravi (Magnetohydrodynamics, Latvijas Universitate32 Miera StreetSalaspils-1LV-2169, 2019-04) [Article]

Direct numerical simulations of magnetohydrodynamic turbulence with kinetic energy and cross helicity injections at large scales are performed. It was observed that cross helicity changes its sign as we go from large and intermediate scales to small scales. In addition, the magnetic reconnections are strongest in the regions, where the cross helicity changes the sign and becomes smallest in magnitude. Thus, our simulations provide an important opportunity to explore the regions of magnetic reconnections in nonlinear MHD.

Effects of non-unity Lewis number of gas-phase species in turbulent nonpremixed sooting flames

Attili, Antonio; Bisetti, Fabrizio; Mueller, Michael E.; Pitsch, Heinz (Combustion and Flame, Elsevier BV, 2016-02-13) [Article]

Turbulence statistics from two three-dimensional direct numerical simulations of planar n-heptane/air turbulent jets are compared to assess the effect of the gas-phase species diffusion model on flame dynamics and soot formation. The Reynolds number based on the initial jet width and velocity is around 15, 000, corresponding to a Taylor scale Reynolds number in the range 100 ≤ Reλ ≤ 150. In one simulation, multicomponent transport based on a mixture-averaged approach is employed, while in the other the gas-phase species Lewis numbers are set equal to unity. The statistics of temperature and major species obtained with the mixture-averaged formulation are very similar to those in the unity Lewis number case. In both cases, the statistics of temperature are captured with remarkable accuracy by a laminar flamelet model with unity Lewis numbers. On the contrary, a flamelet with a mixture-averaged diffusion model, which corresponds to the model used in the multi-component diffusion three-dimensional DNS, produces significant differences with respect to the DNS results. The total mass of soot precursors decreases by 20-30% with the unity Lewis number approximation, and their distribution is more homogeneous in space and time. Due to the non-linearity of the soot growth rate with respect to the precursors' concentration, the soot mass yield decreases by a factor of two. Being strongly affected by coagulation, soot number density is not altered significantly if the unity Lewis number model is used rather than the mixture-averaged diffusion. The dominant role of turbulent transport over differential diffusion effects is expected to become more pronounced for higher Reynolds numbers. © 2016 The Combustion Institute.

Direct numerical simulations of the ignition of a lean biodiesel/air mixture with temperature and composition inhomogeneities at high pressure and intermediate temperature

Luong, Minhbau; Lu, Tianfeng; Chung, Suk Ho; Yoo, Chun Sang (Combustion and Flame, Elsevier BV, 2014-11) [Article]

The effects of the stratifications of temperature, T, and equivalence ratio, φ{symbol}, on the ignition characteristics of a lean homogeneous biodiesel/air mixture at high pressure and intermediate temperature are investigated using direct numerical simulations (DNSs). 2-D DNSs are performed at a constant volume with the variance of temperature and equivalence ratio (T′ and φ{symbol}′) together with a 2-D isotropic velocity spectrum superimposed on the initial scalar fields. In addition, three different T s(-) φ{symbol} correlations are investigated: (1) baseline cases with T′ only or φ{symbol}′ only, (2) uncorrelated T s(-) φ{symbol} distribution, and (3) negatively-correlated T s(-) φ{symbol} distribution. It is found that the overall combustion is more advanced and the mean heat release rate is more distributed over time with increasing T′ and/or φ{symbol}′ for the baseline and uncorrelated T s(-) φ{symbol} cases. However, the temporal advancement and distribution of the overall combustion caused by T′ or φ{symbol}′ only are nearly annihilated by the negatively-correlated T s(-) φ{symbol} fields. The chemical explosive mode and Damköhler number analyses verify that for the baseline and uncorrelated T s(-) φ{symbol} cases, the deflagration mode is predominant at the reaction fronts for large T′ and/or φ{symbol}′. On the contrary, the spontaneous ignition mode prevails for cases with small T′ or φ{symbol}′, especially for cases with negative T s(-) φ{symbol} correlations, and hence, simultaneous auto-ignition occurs throughout the entire domain, resulting in an excessive rate of heat release. It is also found that turbulence with large intensity, u′, and a short time scale can effectively smooth out initial thermal and compositional fluctuations such that the overall combustion is induced primarily by spontaneous ignition. Based on the present DNS results, the generalization of the effects of T′, φ{symbol}′, and u′ on the HCCI combustion is made to clarify each effect. These results suggest that temperature and composition stratifications together with a well-designed T s(-) φ{symbol} correlation can alleviate an excessive rate of pressure rise and control the ignition-timing in homogeneous charge compression-ignition (HCCI) combustion. © 2014 The Combustion Institute.

Advancing predictive models for particulate formation in turbulent flames via massively parallel direct numerical simulations

Bisetti, Fabrizio; Attili, Antonio; Pitsch, Heinz G. (Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, The Royal Society, 2014-07-14) [Article]

Combustion of fossil fuels is likely to continue for the near future due to the growing trends in energy consumption worldwide. The increase in efficiency and the reduction of pollutant emissions from combustion devices are pivotal to achieving meaningful levels of carbon abatement as part of the ongoing climate change efforts. Computational fluid dynamics featuring adequate combustion models will play an increasingly important role in the design of more efficient and cleaner industrial burners, internal combustion engines, and combustors for stationary power generation and aircraft propulsion. Today, turbulent combustion modelling is hindered severely by the lack of data that are accurate and sufficiently complete to assess and remedy model deficiencies effectively. In particular, the formation of pollutants is a complex, nonlinear and multi-scale process characterized by the interaction of molecular and turbulent mixing with a multitude of chemical reactions with disparate time scales. The use of direct numerical simulation (DNS) featuring a state of the art description of the underlying chemistry and physical processes has contributed greatly to combustion model development in recent years. In this paper, the analysis of the intricate evolution of soot formation in turbulent flames demonstrates how DNS databases are used to illuminate relevant physico-chemical mechanisms and to identify modelling needs. © 2014 The Author(s) Published by the Royal Society.

Formation, growth, and transport of soot in a three-dimensional turbulent non-premixed jet flame

Attili, Antonio; Bisetti, Fabrizio; Müeller, Michael E.; Pitsch, Heinz G. (Combustion and Flame, Elsevier BV, 2014-07) [Article]

The formation, growth, and transport of soot is investigated via large scale numerical simulation in a three-dimensional turbulent non-premixed n-heptane/air jet flame at a jet Reynolds number of 15,000. For the first time, a detailed chemical mechanism, which includes the soot precursor naphthalene and a high-order method of moments are employed in a three-dimensional simulation of a turbulent sooting flame. The results are used to discuss the interaction of turbulence, chemistry, and the formation of soot. Compared to temperature and other species controlled by oxidation chemistry, naphthalene is found to be affected more significantly by the scalar dissipation rate. While the mixture fraction and temperature fields show fairly smooth spatial and temporal variations, the sensitivity of naphthalene to turbulent mixing causes large inhomogeneities in the precursor fields, which in turn generate even stronger intermittency in the soot fields. A strong correlation is apparent between soot number density and the concentration of naphthalene. On the contrary, while soot mass fraction is usually large where naphthalene is present, pockets of fluid with large soot mass are also frequent in regions with very low naphthalene mass fraction values. From the analysis of Lagrangian statistics, it is shown that soot nucleates and grows mainly in a layer close to the flame and spreads on the rich side of the flame due to the fluctuating mixing field, resulting in more than half of the total soot mass being located at mixture fractions larger than 0.6. Only a small fraction of soot is transported towards the flame and is completely oxidized in the vicinity of the stoichiometric surface. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames. Finally, given the difficulties in obtaining quantitative data in experiments of turbulent sooting flames, this simulation provides valuable data to guide the development of models for Large Eddy Simulation and Reynolds Average Navier Stokes approaches. © 2014 The Combustion Institute.

Direct numerical simulations of the ignition of lean primary reference fuel/air mixtures with temperature inhomogeneities

Luong, Minhbau; Luo, Zhaoyu; Lu, Tianfeng; Chung, Suk Ho; Yoo, Chun Sang (Combustion and Flame, Elsevier BV, 2013-10) [Article]

The effects of fuel composition, thermal stratification, and turbulence on the ignition of lean homogeneous primary reference fuel (PRF)/air mixtures under the conditions of constant volume and elevated pressure are investigated by direct numerical simulations (DNSs) with a new 116-species reduced kinetic mechanism. Two-dimensional DNSs were performed in a fixed volume with a two-dimensional isotropic velocity spectrum and temperature fluctuations superimposed on the initial scalar fields with different fuel compositions to elucidate the influence of variations in the initial temperature fluctuation and turbulence intensity on the ignition of three different lean PRF/air mixtures. In general, it was found that the mean heat release rate increases slowly and the overall combustion occurs fast with increasing thermal stratification regardless of the fuel composition under elevated pressure and temperature conditions. In addition, the effect of the fuel composition on the ignition characteristics of PRF/air mixtures was found to vanish with increasing thermal stratification. Chemical explosive mode (CEM), displacement speed, and Damköhler number analyses revealed that the high degree of thermal stratification induces deflagration rather than spontaneous ignition at the reaction fronts, rendering the mean heat release rate more distributed over time subsequent to thermal runaway occurring at the highest temperature regions in the domain. These analyses also revealed that the vanishing of the fuel effect under the high degree of thermal stratification is caused by the nearly identical propagation characteristics of deflagrations of different PRF/air mixtures. It was also found that high intensity and short-timescale turbulence can effectively homogenize mixtures such that the overall ignition is apt to occur by spontaneous ignition. These results suggest that large thermal stratification leads to smooth operation of homogeneous charge compression-ignition (HCCI) engines regardless of the PRF composition. © 2013 The Combustion Institute.

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