Experimental and Numerical Studies of High Pressure Multiple Injection Sprays

Characterization of high pressure diesel sprays has been performed both experimentally and numerically. The experimental study was conducted using a fuel injection system which has a capability of producing multiple injection sprays. The fuel sprays were injected from a multi-hole nozzle into a pressurized cylindrical chamber with optical windows. In order to investigate the effects of a multiple injection strategy on spray characteristics, a double injection spray with the mass evenly distributed between the first and second sprays, and a 1 millisecond dwell between sprays was compared with a single injection spray. Both single and double injection cases had nominally the same injection pressure, injection delivery, and ambient gas density. Transient spray tip penetration lengths and spray angles were obtained from high speed photographic spray images. The spray droplet sizes were derived from the images by using a light extinction method.

The numerical study was conducted using the KIVA-II code for the operating conditions used in the spray experiments. In order to improve the agreement between the measurements and the predictions, the KIVA code was modified to include a drag model based on droplet distortions calculated by the DDB model, a breakup model that includes the effect of Rayleigh-Taylor accelerative instabilities on the droplets and a new drop collision model. The effect of the drop drag model was found to be very important in drop size predictions. The new breakup model was found to give more reasonable predictions of the distribution of drop sizes in the spray. The revised drop collision model gave more accurate drop sizes at the end of the injections where the standard model tended to over predict drop coalescences. The KIVA calculations show reasonable agreement with the experimental results, and indicate that the present spray breakup model characterizes the physical phenomena well at pressures and injection velocities currently seen in diesel engines.