Comparison of combustion efficiency of micro hydrogen jet flames confined in cylindrical tubes of different diameters

In the present study, micro hydrogen jet flames confined in cylindrical tubes without air co-flow were numerically investigated under zero gravity. The effects of tube diameter (D-2 = 5, 10 and 20 mm) and fuel velocity on combustion efficiency were scrutinized. The results show that under an identical fuel velocity, the larger the tube diameter, the higher the combustion efficiency can be achieved. Moreover, for D-2 = 5 mm, the combustion efficiency increases gradually with the increase of fuel velocity, whereas that of D-2 = 10 and 20 mm first increases rapidly, then remains constant, and finally decreases with further increase of fuel velocity. Analysis demonstrates that under D-2= 5 and 10 mm, flame tip opening occurs under all fuel velocities, while for D-2= 20 mm, flame tip is closed at medium fuel velocity. Moreover, the reaction zone expands in a larger tube. Furthermore, the air entrainment ratio under an identical fuel velocity decreases as the tube diameter is reduced. For D-2= 5 mm, the air entrainment ratio increases gradually with an increasing fuel velocity, whereas that of D-2 = 10 and 20 mm increases non-monotonically and peaks at medium fuel velocity. In summary, flame structure and air entrainment ratio are the two major factors affecting the combustion efficiency.