1994-03-01

Initial Comparisons of Computed and Measured Hollow-Cone Sprays in an Engine 940398

Efforts are reported to reproduce the distribution of liquid and vapor fuel from a pulsating hollow-cone liquid-only injector measured by the planar exciplex technique within the head cup of a motored ported single-cylinder engine operated at 1600 rpm with high swirl and a squish ratio of 75%. The injector, cup and cylinder were coaxial. The measurements show that shortly after the beginning of the injection the maximum liquid and vapor fuel concentrations are along the axis but also that the spray achieves substantial radial and axial penetrations. The engine flowfield without injection had previously been characterized by LDV and PIV and so had been the injector and its spray in constant pressure environments so that little arbitrariness was left in reproducing the spray in the engine. Two spray models were used. In one the large drops produced by the break up of the liquid sheet were introduced into the numerical field at the injector exit nearly with the poppet seat angle. In the other, the large drops produced by the liquid sheet were introduced at the end of the liquid sheet nearly with the poppet seat angle and small drops from stripping of the liquid sheet surface were introduced between the injector and the end of the liquid sheet. In neither model was the propagation and motion of the liquid sheet itself explicitly computed. Extensive parametric studies were performed varying: initial drop size and size distribution function, initial drop temperature, amount of residual fuel in the engine, drop drag coefficient and Nusselt number, gas swirl velocity, gas tumble, gas turbulence and numerical resolution but the accumulation of fuel along the axis was never reproduced. The conclusion is always the same: large drops coming in at nearly the poppet seat angle are needed to reproduce the radial and axial penetrations of the spray and they induce a gas flowfield that sweeps along most of the small drops and prevents significant liquid fuel from accumulating along the axis of the spray. It is concluded that one must consider the actual motion of the liquid sheet and its gradual collapse toward the axis so that the large drops that it generates start with nearly the poppet seat angle but end being projected right along the axis.

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