Set-off length reduction by backward flow of hot burned gas surrounding high-pressure diesel spray flame from multi-hole nozzle

Abstract

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.

Axis-symmetric diesel spray flame model coupled with momentum flux distribution measurement and improved soot phi-T map

“…Measurements are made under non-evaporating conditions at room temperature. Diesel spray flames were observed in an optically accessible engine 18,22,28 to evaluate the soot oxidation/ formation regions calculated by the axis-symmetric spray flame model. A single-orifice nozzle was located in a metal blank fitted to the top-view window, inclined by 10°, as shown in Figure 16.…”

“…The spray velocity U ( x ) can be calculated as a function of the x -coordinate, using Wakuri’s spray momentum theory. 21 In this study, we used the extended spray momentum theory which takes the backward flow surrounding sprays into account, 22 instead of Wakuri’s theory. The residence time of the fuel elements after passing through the nozzle can be calculated by integrating the reciprocal of U ( x ) from the nozzle exit to x .…”

“…Increase in residence time of fuel element in soot formation region as oxygen concentration decreases. (a) Trajectories of fuel elements in three O 2 concentration cases, (b) comparison of residence time for three cases, (c) definition of residence time of fuel elements after passing through the nozzle as determined by the extended spray momentum theory 22.…”