Influence of hydrogen blending on the heat transfer characteristics of premixed methane/air jet impinging flame

• Relative position between the pipe-to-wall distance (H) and flame height dominates heat transfer. • Hydrogen blending causes opposite efficiencies for lower/higher relative positions. • Optimal heat transfer efficiency is achieved when H is slightly below the flame height. • A correlation between the Nusselt number and the effectiveness factor was established. This study experimentally investigates the effects of hydrogen blending on the heat transfer characteristics of premixed methane/air jet impinging flames. The effect of hydrogen blending ratio ( α H 2 = 0%, 20%, 40%), pipe-to-wall distance ( H / d = 1.5–4), equivalence ratio ( ϕ = 0.9–1.3), and Reynolds number ( Re = 600–900) on the heat transfer characteristics are systematically examined. The results show that the effect of hydrogen blending strongly depends on the relative position between the pipe-to-wall distance ( H / d ) and the flame height. When H / d is lower than the flame height, increasing the hydrogen blending ratio enhances heat transfer in the stagnation region, raising thermal efficiency from 43.8% to 46.4%. Conversely, when H / d is higher than the flame height, increasing the hydrogen blending ratio enhances the thermal dilution effect, leading to a decrease in thermal efficiency from 44.2% to 42.7%. Within the entire range of hydrogen blending ratios, the thermal efficiency reaches its optimum when H / d is slightly below the flame height. Near the stoichiometric ratio ( ϕ = 1.0), the Nusselt number reaches its maximum due to the highest flame temperature. As Re increases, the jet momentum is enhanced, leading to higher Nusselt numbers and effectiveness factors but lower thermal efficiency. Based on the experimental data, a correlation between the Nusselt number and the effectiveness factor at the stagnation-point has been established.