Comparison of air and EGR with different water fractions dilutions on the combustion of hydrogen-air mixtures

Lean combustion and exhaust gas recirculation (EGR) are key strategies for controlling NO and heat release process of hydrogen engines. This paper firstly compared these two strategies on combustion behaviors of hydrogen-air mixtures within dilution ratio ranges from 0 to 50%. Then, the water vapor effects in EGR on combustion of hydrogen-air mixtures are further investigated. All tests are conducted in a constant volume combustion vessel at the initial temperature of 373 K and pressure of 1 bar. CHEMKIN PRO is adopted to predict the hydrogen-air mixtures combustion behaviors under different conditions with one-dimensional premixed adiabatic planner flame model. The unburnt hydrogen concentration and the NO mole fraction in the final products were measured. The results show that laminar burning velocity decreases more obviously with the EGR ratio than air dilution ratio. EGR dilution has much obvious influence on reducing NO mole fraction in the final products. However, both dilution strategies adversely cause the increase of incompletely combusted H2. To balance the NO control and unburned H2 emission, 30% EGR rate with 20% water vapor fraction is recommended based on the testing ranges of this study.