Effect of nozzle dimensions and fuel type on flame lift-off length

Flame lift-off length of diesel fuel has significant influence on fuel-air mixture and soot formation. The results of the investigation on flame lift-off length are presented in this study. Experimental and numerical studies were conducted on a constant volume combustion chamber. To investigate the influence of injector nozzle geometry, four different types; namely divergent, straight, straight-rounded and convergent-rounded nozzles, were used in the experiments. Experiments were carried at 825 K and 3.5 MPa ambient conditions. To investigate the effect of fuel type, diesel fuel, n-heptane and a mixture of hexadecane-heptamethylnonane CN65 (cetane number 65) were injected by straight-rounded nozzle. The experimental results showed that the nozzle with smaller diameter and divergent shape has shorter flame lift-off length, that is thought to be related to lesser amount of injected fuel. Other nozzles have almost the same flame lift-off length. The experiments, conducted with fuels of different cetane number but similar heating values, showed that fuel type has insignificant effect on flame lift-off length. This result can be attributed to similar adiabatic flame temperature obtained after flame stabilization. In order to explain the driving parameters of flame lift-off length, KIVA3V CFD code was used in the numerical simulations which were carried under different ambient temperature and oxygen concentrations. It was concluded that the location of flame stabilization is effected by the flame temperature and the reverse motion of high temperature gas of combustion zone.