Development of a diesel jet on the basic stage
On the basic stage of development of a jet each elementary portion of injected fuel moves in an axial core of a jet down to its top. Reaching the top this portion is superseded on periphery of a jet, is sharply braked till the complete loss of initial speed and fills in an environment of a jet. The part of an elementary portion of fuel dissipates in an environment of a jet on the way to forward front. At approach of a jet to a wall the fuel getting in forward front gradually passes to the zone of wall surface flow. The trajectory of a jet and, accordingly, time and place of its clash with a wall are determined in view of influence of swirl. The process of interaction of a fuel jet with a wall is rather complicated. During stacking of forward front of a jet on a wall, conical condensed gas-fuel layer is formed on it in borders of a stain. The layer formed by crossing of a cone of jet with the surface of the wall. After fast stacking of jet front on a wall fuel begins to distribute out of borders of initial stain. The high-speed axial flow of a jet flying up to a wall condenses the wall surface layer widens its borders. Part of the flow moves above this layer to its periphery. The form of a wall surface stain and velocity of its growth in various directions depend on the value of the clash angle of the jet with the wall. On a fig. 1 a typical film-gramme of the development of a jet in the combustion chamber is shown .
|Film-gramme is obtained by K.Koptev, V.Gavrilov, V.Plotnikov
(St.-Petersburg ship-building institute). Fuel was injected into the bomb with the model
of the piston. The nozzles size is 7 x 0.4 mm. Speed of shooting 3700 frame / sec.
At approach of a jet to an inclined wall, small deviation of the top of the jet from the nozzles axis is observed. The deviation occurs in the part of a blunt corner of a clash of a jet to a wall. It is caused by formation of a condensed air flow before a jet. The flow first enters the interaction with a wall and causes preliminary turn of the top of a jet. Having flow to a wall, the jet is distributed upwards and downwards along its surface. The flow directed upwards on a wall quickly gets in a clearance above the piston, and in constrained conditions is distributed both on the top of the piston, and on the surface of the head of the cylinder. The progress of fuel along a wall is slowed down in comparison with free development of a jet because of flow friction against the wall, of loss of the kinetic energy of a jet with drops reflected from a wall, etc. Moreover, the movement of a wall surface flow is influenced by an air swirl in the combustion chamber. The wall surface flow is non-uniform in structure, density, temperature. It complicates calculation of fuel evaporation. Therefore, it is expedient in a wall surface flow, as well as in a free jet, to allocate three characteristic zones with average parameters of heat- and mass-exchange (Fig.2).
|The diagram of a diesel jet.
1 - Rare environment of a jet.
2 - Dense axial core of free jet.
3 - Dense forward front.
4 - Rare environment of a wall surface flow.
5 - Dense core of a wall surface flow on a piston bowl surface.
6 - Forward front of a wall surface flow.
7 - Axial conical core of a wall surface flow.
The first zone is an axial conic nucleus on a wall (7). It is formed at stacking of the front of a jet on a wall. Further, the structure of this nucleus is continuously updated at the expense of new weights of fuel, flying up to a wall. The second zone is a wall surface flow of fuel, extending out of borders of an initial stain (5). The third zone is a rare environment above the wall surface flow (4), where a part of fuel braked in forward front of a wall surface flow (6) passes. At distribution of fuel on a wall extending in all directions the wall surface flow (deformed by a swirl) can cross any characteristic border dividing the zones with various conditions of evaporation and burning of fuel. For example, flow can proceed from a lateral inclined surface of piston bowl to horizontal crown, or can be distributed further: on a surface of the cylinder wall, etc. Partial crossing of the wall surface flows formed by the next jets is also possible. In all these cases amount of fuel crossing the border, is calculated from the solution of the problem of crossing of characteristic zones.