Effect of momentum flow ratio on entrainment of a confined coaxial jet in a co-flow

Abstract

Iron ore pellet production processes produce large amounts of CO2 emissions when burning fossil fuels. One example is the grate-kiln process, where normally a coal flame is used to indurate pellets. To mitigate the emissions, coal used in rotary kilns can be replaced with for example hydrogen. However, the fuel is mixed with secondary process air, and replacing a solid fuel with a gaseous one changes the mixing characteristics. This demands different means of injecting the fuel. A coaxial jet can be used to control the mixing of fuel and secondary air, as well as the flow field. The aim is to control the mixing of hydrogen and secondary air to achieve a hydrogen flame that is similar to the reference coal flame. This study numerically investigates different coaxial jet configurations. Steady-state simulations of a simplified model of the real kiln are performed using a Reynolds Stress turbulence model. Results show that decreasing the momentum flow ratio between the outer and inner jet, M jet , to a certain value delays the spread of the hydrogen jet and thus gives a longer jet. Further decreasing this ratio gives an even longer jet, but has the side effect of producing recirculation of hydrogen.