A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames

Abstract

Data obtained in 3D direct numerical simulations of statistically planar, 1D weakly turbulent flames characterised by different density ratios σ are analysed to study the influence of thermal expansion on flame surface area and burning rate. Results show that, on the one hand, the pressure gradient induced within a flame brush owing to heat release in flamelets significantly accelerates the unburned gas that deeply intrudes into the combustion products in the form of an unburned mixture finger, thus causing large-scale oscillations of the burning rate and flame brush thickness. Under the conditions of the present simulations, the contribution of this mechanism to the creation of the flame surface area is substantial and is increased by σ, thus implying an increase in the burning rate by σ. On the other hand, the total flame surface areas simulated at σ = 7.53 and 2.5 are approximately equal. The apparent inconsistency between these results implies the existence of another thermal expansion effect that reduces the influence of σ on the flame surface area and burning rate. Investigation of the issue shows that the flow acceleration by the combustion-induced pressure gradient not only creates the flame surface area by pushing the finger tip into the products, but also mitigates wrinkling of the flame surface (the side surface of the finger) by turbulent eddies. The latter effect is attributed to the high-speed (at σ = 7.53) axial flow of the unburned gas, which is induced by the axial pressure gradient within the flame brush (and the finger). This axial flow acceleration reduces the residence time of a turbulent eddy in an unburned zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy for wrinkling the flamelet surface (e.g. the side finger surface) is weakened owing to a shorter residence time.

KEYWORDS:

• premixed turbulent flame
• density ratio
• Darrieus–Landau mechanism
• DNS
• modelling

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. As discussed in detail elsewhere [46], the present DNSs address the flamelet regime of premixed turbulent combustion.

2. It is worth noting that, under the conditions of the present study, the axial acceleration of the lighter combustion products is even stronger and results in and countergradient transport of c, as discussed in detail elsewhere [37,45].

3. The results discussed in the previous sections were conditioned to the local combustion progress variable c, rather than the mean .

4. These lines do not go to zero at because they show the results of integrating an incorrect transport equation (term T₃ is skipped).

5. Akkerman and Bychkov [85] and Bychkov et al. [87] noted that the response of a premixed flame to ‘vortices perpendicular to the direction of flame propagation’ could be more pronounced at low density ratios.

Additional information

Funding

A.N.L. gratefully acknowledges financial support by the Combustion Engine Research Center and Chalmers Transport and Energy Areas of Advance. V.A.S. gratefully acknowledges financial support by ONERA and by the Ministry of Education and Science of the Russian Federation [Contract No. 14.G39.31.0001 of 13.02.2017].