The effects of water addition on the laminar flame speeds of CO/H2/O2/H2O mixtures

• The laminar flame speeds of CO/H2/O2/H2O were measured under various H2O content.
• The combined effects of H2O addition determine the laminar flame speeds.
• The direct reaction effects of H2O addition facilitate the CO/H2 combustion.
• The three-body effects of H2O addition inhibit the CO/H2 combustion.

The effects of H2O addition and H2 content on the laminar flame speeds of CO/H2/O2/H2O mixtures were studied experimentally and theoretically. The laminar flame speeds were measured at various CO/H2 ratios (100/0, 95/5, 85/15, 75/25, 50/50) and H2O concentrations (0–60% in the mixtures) using an improved Bunsen burner method. The addition of water affected the laminar flame speeds of the mixtures by changing its thermal properties and altering the chemical kinetics of the combustion reaction. When the H2 content was lower than 15%, the laminar flame speeds first increased and then decreased as the H2O content increased. When the H2 content was greater than 15%, the laminar flame speeds decreased with increasing H2O content. The chemical effects (direct reaction effects and three-body effects) of H2O addition on the laminar flame speeds were analyzed using CHEMKIN package. The direct reaction effects of H2O addition present a promoting effect on CO/H2 combustion (for H2 content of 0–50%). However, adding large amounts of H2O had a complex effect overall because of its impact on the three-body reaction which inhibited combustion. The three-body effect of H2O addition decreased the concentration of free radicals (H, OH, and O) in the chemical reaction zone, caused the flame front to move downstream, and changed the HO2 consumption pathway.