The backward flow of the hot burned gas surrounding a diesel flame was found to be one of the factors reducing the set-off length (also called the lift-off length), that is, the distance from a nozzle exit into which a diffusion flame cannot intrude. In the combustion chamber of an actual diesel engine, the entrainment of the surrounding gas into a spray jet injected from a multi-hole nozzle is restricted by the combustion chamber walls and the adjacent spray jets, thus inducing the backward flow of the surrounding gas toward the nozzle exit. The emergence of this backward flow was measured by particle tracking velocimetry in the non-combusting condition. A new momentum theory for calculating the backward flow velocity was established by extending Wakuri’s momentum theory. Shadowgraph imaging in an optical engine successfully visualized the backward flow of the hot burned gas. The hot burned gas is re-entrained into the spray jet in the region of the set-off position and shortens the set-off length in comparison to that of a single free-spray flame which does not induce the backward flow.
A study of entrainment process in diesel spray based on the momentum theory