Detailed chemistry LES/CMC simulation of a swirling ethanol spray flame approaching blow-off

Flow Turbulence Combust

2017

Simulations of a pilot-stabilised flame in a uniformly dispersed ethanol spray are performed using a Doubly Conditional Moment Closure (DCMC) model. The DCMC equation for spray combustion is derived, using the mixture fraction and the reaction progress variable as conditioning variables, including droplet evaporation and differential diffusion terms. A set of closure sub-models is suggested to allow for a first, preliminary application of the DCMC model to the test case presented here. In particular, the DCMC model is used to provide complete closure for the Favre-averaged spray terms in the mean and variance equations of the conditioning variables and the present test case is used to assess the importance of each term. Comparison with experimental data shows a promising overall agreement, whilst differences are related to modelling choices.

Section: Methodsmentioning

confidence: 99%

Modelling of Spray Flames with Doubly Conditional Moment Closure

Sitte

Flow Turbulence Combust

2017

“…This presumed shape of the conditional spray term is scaled, such that Π |η, ζ les integrates with the FDF to give the filtered value Π . As 185 pointed out in previous work for singly-conditioned CMC [14,27], this source term needs to be limited to avoid numerical instability in the case where the probability associated with [Q F (η, ζ) = Y Fs les ] is very low. At the CMC resolution Π |η, ζ cmc is obtained by volume averaging over the LES cells in the same way as for the conditional SDRs.…”

Section: Doubly Conditional Moment Closurementioning

confidence: 99%

“…This can be achieved using Large Eddy Simulation (LES) and the Conditional Moment Clo- 15 sure (CMC) framework for turbulent combustion modelling. CMC modelling is well-established (see reviews by Klimenko and Bilger [7] and Kronenburg and Mastorakos [8]) and LES-CMC [9] particularly has so far been successfully applied to various combustion problems, including forced ignition [10], the stabilisation of lifted non-premixed jet flames [11], local extinction and blow-off 20 in non-premixed [12,13] and spray flames [14], and the behaviour of premixed flames approaching blow-off [15]. In all these cases a single conditioning variable was used to parametrise the flame structure; either the mixture fraction or the reaction progress variable in the rare cases where CMC was applied to a premixed flame.…”

Section: Introductionmentioning

confidence: 99%

“…To date, DCMC has primarily been applied at the level of fundamental feasibility studies, where DCMC was tested against a Direct Numerical Simulation (DNS) and closure for the DCMC equation was provided from DNS results. These studies focused on predicting extinction [21,22,23] and has so far only been attempted for singly-conditional CMC in a small number 60 of publications [14,27,28,29,30,31,32]. A model for the doubly-conditional spray terms has previously been proposed but was not included in the DCMC model equation [25].…”

Section: Introductionmentioning

confidence: 99%

“…The spray source term in Eqn. 4 is exact and the following modelling choices were made for the other transport equations: the evaporation source in the ξ 2 -equation is not considered in the present work, since currently available sub-models for N ξ seem unsuitable to balance the production of variance in regions with low turbulence level -evaporation in regions 105 with lower turbulence intensity constitutes a main difference between the present case and the flame studied by Giusti and Mastorakos [14] who successfully included this source term. The effect of evaporation on c 2 was neglected and the source to thec-equation was modelled as discussed by Sitte and Mastorakos [25].…”

mentioning

confidence: 99%

See 2 more Smart Citations

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

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.