Simulation of interaction of fuel jets with an air swirl and walls

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Calculation is illustrated on the example:
Numerical research of influence of a swirl intensity on a fuel consumption of automobile diesel engine.

     It is known, that the increase of an air swirl   intensity  causes increase of  evaporation and combustion rate which has favorable effect on a cycle efficiency. However, in experiments it is noticed that the increase of a swirl intensity over the optimum value causes decrease of heat release ratio and growth of fuel consumption.

The swirl intensity level is set by swirl ratio H. It is the relation between rotational velocity of air swirl and crankshaft.

The purpose of research: to calculate optimum value of a swirl ratio H at which the fuel consumption is minimum

jamz_bok.gif (1061 bytes) The calculation of parameters of a compartment of an automobile diesel YaMZ-238 was carried out  at change of swirl ratio H from 1 to 4.
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H=1. H=2.62 H=4.

   The results of calculation are shown in diagrams. In accordance with growth H there is an increase of deformation of stains of wall surface flows. Stains are extended on a circle of a combustion chamber and cross each other. The zones of crossing of stains are increased.
( The zone of crossing on the plan views is not completely visible. It is caused by the fact that it is located on a lateral surface of the piston bowl.)

Value of specific fuel consumption "be" and shares of fuel in some characteristic zones at different intensity of a swirl H (swirl ratio).

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The experimental data are obtained by Galgovsky V. on one-cylinder compartment of a diesel engine.

rpm=2100;
BMEP=7.0 bar,
gc = 0.0876 g/cycle.

 

     The calculations results allow to describe the mechanism of increase of fuel consumption at increase of a swirl ratio over optimum value.


    At small intensity of swirl the wall surface flows from the next jets either do not absolutely adjoin each other, or their contact is not significant (share of fuel in zones of crossing " s w" is not great). However, with the growth of angular speed of a swirl, the wall surface flows are heavily extended on a circle of a combustion chamber. The area of zones in which the stains of wall surface flows are crossed steadily grows. Share of fuel in zones of crossing is increased ( s w > 12 - 15 %) and, as a consequence, intensity of formation of a rare environment " s " above these places is reduced. There are best conditions of evaporation in the rare environment of jet. Thus, the more fuel will be sprayed in the environment, the more the heat release rate and efficiency of cycle will be. The negative effect of reduction of amount of fuel allocated in environment reduces in the end the favorable influence of growth of swirl intensity. Eventually it results the increase the combustion duration and specific fuel consumption.

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