Numerical Study On Combustion Characteristics Of Ammonia/methane Swirling Flame

Combustion is the most important way for humans to utilize energy,and the global energy supply still depends on fossil fuels.As a carbon-free fuel,ammonia（NH₃）has received intense research interest recently because of its potential to reduce CO₂emission in energy utilization.However,ammonia combustion applications have been discouraged recently,mainly because of low flammability and high fuel NO_X emissions.The previous studies show that mixing methane with ammonia can effectively improve the flammability of ammonia and maintain low CO₂ emission.The research of the combustion characteristics of ammonia/methane blended fuels can provide theoretical support for ammonia-containing fuels applications,reducing the dependence of energy supply on fossil fuels.It is essential for the gradual transition to a carbon-free society.Currently,swirling combustion is the most widely used enhanced combustion technology in industrial combustion equipment.This study aims to perform numerical simulation to investigate the effects of different working conditions（including ammonia mixing ratio,air preheating temperature,and swirl number）on the combustion characteristics in swirling ammonia/methane flames,especially flame structure,combustion stability and pollutant emissions.This study is performed by large eddy simulation based on the open source CFD toolbox,Open FOAM.Using the structured mesh and dynamic Smagorinsky model to simulate the turbulent flow process.The detailed reaction mechanism proposed by Okafor is used to predict chemical reactions.The subgrid-corrected partially stirred reactor（Pa SR）model is used to solve the turbulence-chemistry interaction.The combustion flow field measured by the particle image velocimetry（PIV）technique will be used to verify the accuracy of the numerical results.The main conclusions are shown as follows:（1）The flow field obtained by LES is in good agreement with the flow field obtained by the PIV technique.As the ammonia blending ratio increases,the flame height shortens,the flame color gradually changes from blue to orange,and NO emission increases.The increase of ammonia blending ratio will reduce flame propagation speed and enhance turbulent-chemical interaction,thereby reducing the flame stability.When the ammonia blending ratio is lower than 30%,the increase of ammonia blending ratio cannot effectively reduce CO emission.（2）As the air temperature increases,the thermal diffusion instability induced by preferential diffusion will be suppressed,and the sensitivity of NO to OH radical decreases.Air preheating increases flame propagation speed and reduces the probability of local quenching at the flame root,thereby enhancing flame stability.SNCR（Selective Non-Catalytic Reduction）reactions are the key to controlling NO emission.Air preheating changes the temperature and the residence time in the central recirculation zone,and NO emission will increase with the temperature and decrease with the residence time.（3）As the swirl number increases,the flame height shortens,the recirculation velocity increases,the central recirculation zone expands radially and contracts axially,and NO emission decreases.Flame stability decreases with the swirl number,however,low swirl intensity cannot generate the central recirculation zone to stabilize the flame,while high swirl intensity will lead to local quenching at the flame root.