Computational Investigation of Weakly Turbulent Flame Kernel Growths in Iso-Octane Droplet Clouds in CVC Conditions

Abstract

Numerical simulations of turbulent flame kernel growths in monodisperse clouds of iso-octane liquid droplets are conducted in conditions relevant to constant volume combustors. The simulations make use of a low-Mach number Navier-Stokes solver and a thermodynamic pressure evolution model has been implemented to reproduce the pressure variation that may be issued from either experiments or from a standard (i.e., analytical) compression law. Chemistry is described with a representative skeletal mechanism featuring 29 species and 48 elementary reaction steps. The computational results clearly confirm the enhancement of flame propagation in constant volume combustion conditions. The impact of the droplet diameter on the turbulent flame development is scrutinized for two distinct values of the Stokes number St equal to 0.1 and 1.0. Significant influence on the flame dynamics is put into evidence. This is a direct outcome of the equivalence ratio and temperature heterogeneities, which are themselves very sensitive to the choice of the Stokes number value. Then, small-scale turbulence-scalar interactions (TSI) are studied by analyzing the fields of the scalar gradients and strain-rate. Their dynamics is investigated for both non-reactive and reactive two-phase flows conditions. The TSI analysis is performed on the basis of time evolution equations written for quantities that characterize the couplings between the velocity gradient tensor and scalar gradients vectors. Special emphasis is placed on the possible influence of mass exchange terms between the liquid and gaseous phases.

Statistical Behaviour and Modelling of Fuel Mass Fraction Dissipation Rate Transport in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation study

“…Due to the high volatility of n-heptane, evaporation commences on entry and the droplet diameter decreases by at least 40%, 30% and 25% by the time it reaches the most reactive region of the flame for the initial a d /δ th = 0.06, 0.08, 0.10 cases respectively, such that the volume of even the largest droplets remains smaller than half that of the cell volume, which validates the sub-grid point source treatment of droplets adopted for flame-droplet interactions analysed here. The droplet diameter to grid size used in the current analysis remains comparable to several previous DNS analyses [18][19][20][21][22][23]31].…”

“…Furthermore, pyrolysis does not directly affect the statistics of scalar gradients and scalar dissipation rate which is the main focus of the current analysis. A recent DNS analysis [31] with comparable simulation parameters as that of the current analysis demonstrated with the help of Kerstein-Law parameter that either group combustion or individual burning are unlikely under these conditions. It should also be noted that detailed discussion of the flame-droplet interaction under both laminar and turbulent flow conditions for the current DNS database can be found elsewhere [9][10][11][12] and thus is not repeated here.…”

An extended flame index partitioning for partially premixed combustion

“…Long chained primary fuel molecules may completely dissociate well before they reach the flame reaction zone and, as a consequence, the primary fuel mass fraction gradient may become irrelevant. There were therefore some attempts to generalize the existing definitions by considering the mass fractions of unburnt carbon and oxygen atoms instead of fuel and oxidizer [24]…”

“…(4) or Eq. (5), remain still very popular today [23][24][25][26]. However, previous analyses from Fiorina et al [8] and Knudsen and Pitsch [16] put some of their limitations into evidence.…”