A study of flame dynamics and structure in premixed turbulent planar NH₃/H₂/air flames

Abstract

Ammonia (NH3) has received considerable attention as a near future carbon-free synthetic fuel due to its economic storage/transportation/distribution, and its potential to be thermally decomposed to hydrogen (H2). To promote the low burning velocity and heat of combustion of ammonia, one viable option is to enrich pure ammonia with hydrogen. In this study, two quasi direct numerical simulations (quasi-DNS) with detailed chemistry and the mixture-averaged transport model are examined to study stoichiometric planar ammonia/hydrogen/air flames under decaying turbulence. The reactants temperature and pressure are set to 298 K and 1 atm, respectively. The initial turbulent Karlovitz number is changed from 4.3 to 16.9, implying that all the test conditions are located within the thin reaction zones combustion regime. The results indicate that the density-weighted flame displacement speed ([Formula: see text]), on average, is higher than the unstrained premixed laminar burning velocity ([Formula: see text]) value for both test cases. This suggests that the flame elements propagate faster than their laminar flame counterpart. With increasing the Karlovitz number, the turbulent burning velocity and the wrinkled flame surface area increase by about 35%. Furthermore, the mean flame stretch factor defined as the ratio of the turbulent to the laminar burning velocity divided by the ratio of the wrinkled to the unwrinkled flame surface area is equal to 1.08. This indicates that the local flamelet velocity value, on average, is higher than the unstrained premixed laminar burning velocity. In addition, the results show that the mean value of the local equivalence ratio for the turbulent conditions is higher than its laminar counterpart due to the preferential diffusion of hydrogen and turbulent mixing. Furthermore, the net production rate of hydrogen is shown to be negatively correlated with the flame front curvature suggesting that the local burning rate is intensified in positively curved regions.

Editorial –‘Thermo- and fluid-dynamics of clean propulsion powerplants’: THIESEL 2022 Special Issue by Xandra Margot

“…Indeed, five out of the nine papers presented here, deal with studies on combustion with diverse types of fuels, including ammonia and hydrogen. [5][6][7][8][9] Closely linked to this topic is naturally the fuel injection process since this is paramount to achieve a controlled and efficient combustion. 10 This has been one of the important topics of the conference too and three papers on this have been selected for this special issue.…”

“…As stated above, this special issue includes two papers about the use of ammonia as eco-fuel, 5,8 one on dual-fuel combustion for heavy-duty applications, 6 two on investigation related to hydrogen engines, 7,9 two on ducted fuel injection, 11,12 plus one on needle and armature dynamics of direct injectors. 13 Finally, the last one is on the fuel consumption of mobile air-conditioning systems in electrified vehicles.…”

“…14 Pele´et al 8 investigated experimentally combustion and emissions characteristics of a spark-ignition engine resulting from the injection of pure ammonia and of different ethanol-ammonia blends, with different strategies to reach homogeneous or stratified conditions at various intake pressures. Tamadonfar et al 5 on the other hand performed some quasi-DNS simulations to study the flame dynamics and structure of stoichiometric planar ammonia /hydrogen/air flames under different turbulent conditions. Longo et al 6 have addressed the impact on heavy-duty engines emissions associated to the use of Diesel-hythane dual-fuels.…”