Numerical simulation of oxy-fuel jet flames using unstructured LES–CMC

Flow Turbulence Combust

2015

The Large Eddy Simulation (LES)/three-dimensional Conditional Moment Closure (CMC) model with detailed chemistry is applied to predict the operating condition and dynamics of complete extinction (blow-off) in swirling non-premixed methane flames. Using model constants previously selected to provide relatively accurate predictions of the degree of local extinction in the piloted jet flames Sandia D−F, the error in the blow-off air velocity predicted by LES/3D-CMC in short, recirculating flames with strong swirl for a range of fuel flow rates is within 25 % of the experimental value, which is considered a new and promising result for combustion LES that has not been applied before for the prediction of the whole blow-off curve in complex geometries. The results also show that during the blow-off transient, the total heat release gradually decreases over a duration that agrees well with experiment. The evolution of localized extinction, reactive scalars and scalar dissipation rate is analyzed. It has been observed that a consistent symptom for flames approaching blow-off is the appearance of high-frequency and high-magnitude fluctuations of the conditionally filtered stoichiometric scalar dissipation rate, resulting in an increased fraction of local extinction over the stoichiometric mixture fraction iso-surfaces. It is also shown that the blow-off time changes with the different blow-off conditions.

Section: Les and Cmc Modelsmentioning

confidence: 99%

Prediction of Global Extinction Conditions and Dynamics in Swirling Non-premixed Flames Using LES/CMC Modelling

Zhang

Flow Turbulence Combust

2015

“…[1] for details) was used for the solution of the spray and gas-phase evolution whereas the CMC equations were solved using a superimposed in-house unstructured code [11, 34]. The coupling between the two solvers is achieved through density and temperature and is based on the strategy described in [11] with additional features due to the presence of the spray. The CMC solver receives flow field quantities (including spray source terms) from the LES code and, after the solution of the CMC equations, gives back to the LES solver the unconditional density and temperature fields (together with additional quantities required by the spray sub-models).…”

Section: Methodsmentioning

confidence: 99%

LES/CMC Simulations of Swirl-Stabilised Ethanol Spray Flames Approaching Blow-Off

Giusti

Kotzagianni

Flow Turbulence Combust

2016

Large Eddy Simulations (LES) with the Conditional Moment Closure (CMC) combustion model of swirling ethanol spray flames have been performed in conditions close to blow-off for which a wide database of experimental measurements is available for both flame and spray characterization. The solution of CMC equations exploits a three-dimensional unstructured code with a first order closure for chemical source terms. It is shown that LES/CMC is able to properly capture the flame structure at different conditions and agrees reasonably well with the measurements both in terms of mean flame shape and dynamic behaviour of the flame evaluated in terms of local extinctions and statistics of the lift-off height. Experimental measurements of the overall (liquid plus gaseous) mixture fraction, performed using the Laser-Induced Breakdown Spectroscopy technique, are also included allowing further assessment and validation of the numerical method. The sensitivity of the simulation results to the various boundary conditions is discussed.

“…Here we 140 used the Kronecker delta, δ αF = 1 if α is the fuel species, zero otherwise; and similar for δ ψF . The transport in physical space was re-arranged as an advective and a dilatation term, as in Refs [49,12]. In addition to turbulent transport, the molecular diffusion in physical space was retained, since both terms can be of similar size in finely resolved LES.…”

Section: Doubly Conditional Moment Closurementioning

confidence: 99%

“…The DCMC equation is discretised using a finite volume formulation as detailed in Refs [12,49], where CMC is solved on a grid that is significantly coarser than then LES resolution. Unconditionally filtered quantities are computed by integrating their conditional moments with the FDF.…”

Section: Doubly Conditional Moment Closurementioning

confidence: 99%

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Large Eddy Simulation of a spray jet flame using Doubly Conditional Moment Closure

Sitte

2019

Combustion and Flame

A spray jet flame is modelled using Large Eddy Simulation (LES) with Doubly Conditional Moment Closure (DCMC). Since turbulent spray flames may include multiple combustion modes, the DCMC model uses both mixture fraction and reaction progress variable as conditioning variables. Conditional spray terms were included in the DCMC model to consider the coupling between evaporation and the flame structure. In the case of spatial homogeneity and in the limit of negligible mixture fraction scalar dissipation rate (SDR), the DCMC equation is shown to reproduce the flame structure of freely propagating laminar flames. For the spray jet flame, a good agreement between the simulation results and the experiments is achieved, in terms of the spray statistics, as well as the instantaneous and mean flame shape. The simulation shows important differences in the flame structure between the turbulent inner and the quasilaminar outer flame branch. The doubly-conditional parametrisation appears to be advantageous for resolving small scale effects related to droplet evaporation. Analysis of the DCMC equation suggests that the behaviour of the flame at its anchoring point is strongly influenced by non-premixed burning modes. The solution appears to be weakly affected by terms of convective transport in the DCMC equation, but significant spatial variations and temporal fluctuations of the conditional reaction rate, around 10 % of the time-based mean, persist. the effect of temperature inhomogeneity on ignition [24] and have so far demonstrated a great potential of the modelling approach for predicting complex, transient combustion phenomena. To the knowledge of the authors, the only simulation of a lab-scale flame using DCMC, coupled with a Reynolds-Averaged Navier Stokes (RANS) computation, has been performed by Sitte and Mas-50 torakos [25]. From a broader perspective, the strategy of double-conditioning has also been recently employed in the CMC-related modelling approach of Conditional Source-term Estimation (CSE) [26].In this work, we present an application of the LES-DCMC approach, based on mixture fraction and reaction progress variable as conditioning variables. 55This allows parametrisation of the whole range from non-premixed to fully premixed flames. Moreover, the effect of liquid droplet evaporation on the flame structure is considered by modelling the spray terms in conditional space. Inclusion of conditional evaporation terms in the model equation is challenging and