Non-premixed lean flame characteristics depending on air hole positions in a baffled micro combustor

Abstract

Compared to a premixed micro combustor, non-premixed micro combustor has the advantage of being able to control more easily the fuel-air mixing with reducing flame flashback. To find an efficient micro combustor based on the non-premixed combustion, non-premixed flame characteristics of CH4-air in a baffled micro combustor are numerically studied. Among many factors of the combustor with a seven-hole baffle, the air hole position and air-fuel ratio are important parameters in understanding the combustion characteristics of non-premixed micro combustor. So, to investigate the relation of the combustion efficiency and flame characteristics under different geometrical conditions, six air hole positions of the baffle and five air-fuel ratios are selected. Various simulations based on the Reynolds Stress Model and detailed chemical mechanism of GRI 3.0 are performed. The results show combustion characteristics depending on recirculating flows and the relation between baffle configurations and flame structures for efficient combustion. The stoichiometric zone strongly depends on the relative positions of air and fuel holes. As the air holes of the baffle are away from the fuel hole, the center of dominant vortices is moved from near the inner combustor wall to the combustor center. So, the combustion efficiency can be maximized for the baffles of air hole positions having the center recirculation in addition to non-negligible wall recirculations. To examine this feature, turbulent flows for many combustors with different baffles are analyzed. Even though the air hole diameter changes, the efficient condition of air hole position is nearly fixed at two-thirds of the combustor radius. Another crucial factor in a non-premixed micro combustor is the equivalence ratio which can be linked to the operating condition. So, the geometrical effects are examined for different equivalence ratios. As the equivalence ratio decreases, the flame shape varies from the jet flame to the M−shape flame. As a result, the flame length decreases and the flame temperature increases. From such results, flame characteristics and various flame shapes are discussed with the global equivalence ratio and the effects of air hole position. So, the best baffle configuration is explored for the combustion efficiency. Also, the development of wall recirculations is analyzed with the wall heat transfer from the viewpoint of a heat emitter.