Effect of H2 addition on laminar burning velocity of NH3/DME blends by experimental and numerical method using a reduced mechanism

Abstract

Blending dimethyl ether (DME) into ammonia (NH3) can efficiently enhance the combustion of pure NH3, and has attracted increasing attention. Partial dissociation of NH3 can convert NH3/DME to NH3/DME/H2 mixtures. This work measured the Markstein length and laminar burning velocity of NH3/DME/air mixtures for different blend ratios (100/0, 80/20, 40/60, and 0/100) with various H2 additions (0%, 20%, and 40%) at ϕ = 0.7–1.7, 0.1 MPa, and 298 K using a spherical constant-volume combustion method. A reduced kinetics mechanism was developed and optimized based on own prior detailed mechanism. This model gives more accurate predictions for NH3, DME, NH3/DME, and NH3/DME/H2 compared to previous models in terms of laminar burning velocity, ignition delay time, and species concentration. The experimental and modeling results show that the measured Markstein length of NH3/DME/H2 blends as a function of equivalence ratio presents different states for different NH3/DME blend ratios. The existence of H2 can effectively increase the laminar burning velocity of NH3/DME/air mixtures. Relative increase in laminar burning velocity (ESL) shows a non-monotonic tendency with increasing equivalence ratio for different H2 additions. The peak value of ESL for NH3/DME/air mixtures with various H2 additions appears at around ϕ = 1.5. This can be attributed to the dominant effect of C-contain and N-contain reactions rather than HO reactions at the rich burn side. Similar to ESL, the free radicals that dominate laminar burning velocity may transit from H, O, and OH to CH and NH radicals at around ϕ = 1.3 ∼ 1.6. The thermal effect also contributes to the ESL and laminar burning velocity for rich fuel.