An investigation of a turbulent spray flame using Large Eddy Simulation with a stochastic breakup model

Abstract

A computational investigation of a turbulent methanol/air spray flame in which a poly-dispersed droplet distribution is achieved through the use of a pressure-swirl atomiser (also known as a simplex atomiser) is presented. A previously formulated stochastic approach towards the modelling of the breakup of droplets in the context of Large Eddy Simulation (LES) is extended to simulate methanol/air flames arising from simplex atomisers. Such atomisers are frequently used to deliver fine droplet distributions in both industrial and laboratory configurations where they often operate under low-pressure drop conditions. The paper describes improvements to the breakup model that are necessary to correctly represent spray formation from simplex atomisers operated under low-pressure drop conditions. The revised breakup model, when used together with the existing stochastic models for droplet dispersion and evaporation, is shown to yield simulated results for a non-reacting spray that agree well with the experimentally measured droplet distribution, spray dynamics and size-velocity correlation. The sub-grid scale (sgs) probability density function (pdf) approach in conjunction with the Eulerian stochastic field method are employed to represent the unknown interaction between turbulence and chemistry at the sub-filter level while a comprehensive kinetics model for methanol oxidation with 18 chemical species and 84 elementary steps is used to account for the gas-phase reaction. A qualitative comparison of the simulated OH images to those obtained from planar laser-induced fluorescence (PLIF) confirms that the essential features of this turbulent spray flame are well captured using the pdf approach. They include the location of the leading-edge combustion (or lift-off height) and the formation of a double reaction zone due to the polydisperse spray. In addition, the influence of the spray flame on the structure of the reacting spray in respect of the mean droplet diameters and spray velocities is reproduced to a good level of accuracy

Comparison of droplet evaporation models for a turbulent, non-swirling jet flame with a polydisperse droplet distribution

“…Additional unclosed terms in the mass conservation equation are thus prevented. Further details on the filtered governing equations of mass, momentum and scalars with liquid-phase source terms can be found in [26,30]. The unknown contribution to the filtered chemical source term (ρω α ) is taken into account using a joint sub-grid scale pdf for all the scalar variables required to describe chemical reaction as proposed in [32].…”

“…It is necessary only that the random variable has a mean of zero and standard deviation of unity so providing this property is maintained; the choice of distribution is of little consequence. The formulation has been used successfully over various spray flames in the past -for example, [30]. The solutions of Eq.…”

“…The Eulerian-Lagrangian approach for particulate flows has been successfully applied to study a wide range of turbulent spray flames such as gas turbine combustors [25,26], swirl-stabilised flames [27], a methanol spray flame [28] and an ethanol flame under MILD conditions [29]. The liquid fuel injection is characterised by the use of a stochastic breakup model [30], which does not require an approximation of initial droplet distributions in the vicinity of the nozzle exit. To the authors' knowledge, the spray burner of interest here has been numerically investigated only in the work of Shum-Kivan et al [31] where the focus was made on the analysis of the flame structure and stabilisation.…”

Experimental study on the effect of spray cone angle on the characteristics of horizontal jet spray flame under sub-atmospheric pressure

“…Previous scholars have conducted in-depth theoretical and experimental studies on this issue, and found that the spray cone angle has a decisive influence on the spray THERMAL SCIENCE: Year 2020, Vol. 24 liquid's flow rates [3], the droplet size distribution [4], the final flame shape [5], and temperature characteristics [6][7][8][9]. It is generally accepted that, with an increase in the spray cone angle at the same flow rate, the spray penetration distance decreases and the flame becomes wider and shorter.…”

A Model for Fuel Spray Formation with Atomizing Air

“…Large eddy simulations (LES) of atomizing spray were reported by Apte et al [13], Jones and Lettieri [14], Irannejad et al [15], Jones et al [16], Khan et al [17] and Noh et al [18]. Li et al [19] performed Large-Eddy simulations of ethanol spray combustion.…”