Abstract
This paper presents three-dimensional direct numerical simulations of lean premixed turbulent H2/air flames in the thin and distributed reaction zones, with the Karlovitz numbers at 60, 110, 150 and 1000, and pressures at 1 and 5 atm, respectively. Flame front structures and chemical pathways are examined in detail to investigate the effects of pressure and turbulence on flames. There is an increasing number of finer structures on the flame front with increased Karlovitz number. Eddy structures are observed downstream of the reaction zone under high turbulence intensity and thus Karlovitz number, indicating that the turbulent eddies are small and energetic enough to break through the distributed reaction zone. Statistical analysis indicates that the probability of high curvatures increases with increasing Karlovitz number at a constant pressure. When the Karlovitz number is kept constant, the probability of high curvatures is significantly higher at the atmospheric pressure than at elevated pressure. The approximation of Schmidt number (Sc = 1) in theoretical analysis introduces errors in the estimation of the smallest flow scale and the Karlovitz number. Accordingly, in the turbulent flame regime diagram, the boundary between the thin reaction zone and the distributed reaction zone should be modified at the elevated pressure. Moreover, the decorrelation of heat release and H2 consumption is directly related to turbulence intensity, and the decorrelation is reduced at the elevated pressure. Due to the enhanced radical transport at high Karlovitz number, chemical pathways can be locally changed, which is more significant at elevated pressure.
Highlights
Lean premixed turbulent flames are widely used in industrial combustion devices, where the pressure and turbulence intensity tend to be very high
Aspden et al [4,9] conducted a series of lean premixed H2/air flames and they found that the interface between fuel and products is becoming smooth and there is a decorrelation between heat release and fuel consumption at high Karlovitz numbers
When the eddy size is characterized with lδ < η < δL, where lδ is the thickness of inner layer, the small eddies have the ability to penetrate into the reaction zone
Summary
Lean premixed turbulent flames are widely used in industrial combustion devices, where the pressure and turbulence intensity tend to be very high. Three-dimensional (3D) direct numerical simulations (DNS) with detailed chemistry are used to study the flame characteristics and chemical pathways at high Karlovitz numbers. Savard et al [17] extended DNS to turbulent combustion of heavy hydrocarbons at 20 bar and they found that the chemical pathways remain globally unchanged at different Karlovitz numbers. The aim of present work is to investigate the flame front structures and chemical pathways of turbulent flames at elevated pressures, covering the thin and distributed reaction zones. A series of DNS of lean premixed H2/air flames with the Karlovitz numbers at 60, 110, 150 and 1000, and pressures at 1 and 5 atm are performed with a detailed chemistry.
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