Search for: All records

Abstract Engines have been a crucial element in powering our world since their invention. New decarbonization technologies are needed, and hydrogen fuel, often referred to as the fuel of the future, has gained significant interest in this regard. The relatively high flame temperature of hydrogen fuel is associated with higher NOx emissions, and exhaust gas recirculation (EGR) can be used to address this. However, exhaust gas recirculation changes the reactivity and the laminar burning velocity of the mixture, which plays a significant role in flame stability and fuel burning rates. Hence, investigating the effect of EGR on laminar burning velocity is crucial for efficient hydrogen engines. Laminar burning velocity measurements were performed by observing the outwardly propagating flames in a constant volume combustion vessel. Measurements were taken at 2 bar, 373 K, equivalence ratios of 0.7 and 1.0, and dilution ratios of 0% to 50%. Chemical kinetic simulations were combined with the experimental measurements to quantify the effects of EGR (35% H2O+65%N2) on the laminar burning velocity and Markstein length. Results regarding Lewis number, flame thickness, and expansion ratio across the flame showed that adding higher EGR dilution can change flame stability and reduce cellularity of the hydrogen flames. 

Since its implementation, exhaust gas recirculation has proven to be a reliable technique to control NOx emissions by lowering combustion temperature. Dilution with exhaust gas recirculation, whether in internal combustion engines or sequential-staged gas turbine combustors, affects flame reactivity and stability, which are related to the heat release rate and engine power. Another way to control emissions is to use hydrogen as a carbon-free alternative fuel, which is considered a milestone in the energy-decarbonization journey. However, the high reactivity of hydrogen is one of its hurdles and understanding this effect on laminar burning velocity is important. Flame propagation and burning velocity control the mixture reactivity and exothermicity and are related to abnormal combustion phenomena, such as flashback and knock. Therefore, understanding the effect of exhaust gas addition on the laminar burning velocity of hydrogen/air mixtures is imperative for engine design. In this work, a constant volume combustion chamber was used to observe the laminar burning velocity of stoichiometric hydrogen/air mixtures diluted with combustion products at 1 bar and 423K. Actual combustion products (35 % H₂O + 65 % N₂, by mole) were used for dilution at rates of 0-50%. The burned gas Markstein length was calculated for all mixtures. Experimental results of the laminar burning velocities for all mixtures were compared with kinetic modeling results. These measurements showed the monotonic reduction of reactivity and the laminar burning velocity with dilution. The reduced burning rates at higher dilution were reflected on the pressure gradient inside the vessel. Markstein length values decreased with dilution, meaning that flame instabilities increased with dilution.