Combustion mechanism of liquid fuel spray in a gaseous flame

Abstract

Two-dimensional direct numerical simulation is applied to spray flames stabilized in a laminar counterflow, and the detailed behavior is studied in terms of the droplet group combustion. The stretch ratio of the laminar counterflow is 40 l/s. n-decane (C10H22) is used as a liquid spray fuel, and a one-step global reaction is employed for the combustion reaction model. The results show that with increasing the issued liquid fuel mass fraction, two types of spray combustion appear in front of and inside the high gaseous temperature region, i.e., “premixed-like combustion” and “diffusion-like combustion,” respectively. A droplet group combustion behavior is observed in the diffusion-like combustion region. This diffusion-like combustion, however, disappears when the issued droplet size becomes small, because the droplets complete their evaporation before entering into the high gaseous temperature region. The droplet group combustion tends to reduce the gaseous temperature. This is caused mainly by the suppression of combustion reaction due to the lack of oxygen and partially by the energy exchange through the convective heat transfer between droplets and gaseous phase. The gaseous temperature reduction is promoted by the latent heat of vaporization of the droplets. The use of the parcel approach has a risk of causing a delay of combustion reaction, since the partial fuel vapor pressure increases at limited locations, which suppresses the global droplet evaporation rate.

Flame Structure and Propagation in Turbulent Flame-Droplet Interaction: A Direct Numerical Simulation Analysis

“…Wacks et al [19] used the DNS data to analyse the effects of initial droplet diameter, a d , droplet equivalence ratio, φ d , and turbulence intensity, u , on the flame structure and the statistical behaviour of flame propagation into droplet-laden mixtures. It has been found based on numerical results that the resulting spray flame structure exhibits a considerable extent of premixed combustion in addition to an expected diffusion mode of burning [15][16][17][18][19]. The extent of premixed mode of combustion has been found to increase with decreasing droplet diameter [15][16][17][18][19].…”

“…Two-dimensional unsteady laminar counter-flow simulations have also been used by Nakamura et al [15] and Watanabe et al [16,17] to analyse the effects of equivalence ratio, droplet diameter and droplet group number on n-decane spray flame structure. Fujita et al [18] analysed the same effects using two-dimensional jet flame simulations.…”

“…It has been found based on numerical results that the resulting spray flame structure exhibits a considerable extent of premixed combustion in addition to an expected diffusion mode of burning [15][16][17][18][19]. The extent of premixed mode of combustion has been found to increase with decreasing droplet diameter [15][16][17][18][19]. Moreover, the burned gas temperature can be different from a gaseous diffusion flame due to the combined effects of latent heat of evaporation and premixed combustion [15][16][17][18][19].…”

Numerical modelling of unsteady spray behaviour in moderately high-pressure regime

“…In the following section, the governing equations are presented and explained for both phases. Similar formulations are also available from other recent numerical studies [19][20][21].…”

Evaporation and combustion of multicomponent fuel droplets

“…Recently, the spray combustion behavior has been studied by means of two-or three-dimensional direct numerical simulations (DNSs) (e.g., [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]) or largeeddy simulations (LESs) (e.g., [19][20][21][22][23]). However, the mechanism of spray combustion has not been fully understood yet.…”