Study Of The Mixing Process Of Multi-hole GDI Fuel Sprays Under Superheated Conditions Based On Advanced Optical Diagnostic Tools

As is known that the break-up and mixing process of normal liquid fuelspray happens in the liquid phase region in a P-T diagram, and is governed bythe forces such as inertia force, viscous force, surface tension force and air dragforce. However when a heated fuel is injected into an atmospheric with ambientpressure lower than the saturation pressure of the fuel, the spray injection couldreach the two-phase region and the spray is called flash-boiling spray orsuperheated spray. A large number of bubbles inside the liquid core nucleate,grow up and explode, improving the spray atomization and evaporationsignificantly. Therefore it is desirable to apply the superheated spray in SIDIengines which could potentially improve the engine performance. However itsbreak-up mechanism is still unclear up to now.In this work, advanced laser diagnostic tools were applied to investigate thespray structure transformation and the interaction between the fuel spray andambient gas under various superheated conditions. The spray structuretransformation was explained in the view of spray flow-field distribution. Themomentum transfer and the mixing process between the two phases werecharacterized.To quantify the superheat degree of the testing fuel, a pure substance wasused instead of real gasoline. N-hexane was chosen as the testing fuel because itis one of the most popular substitutes for gasoline in numerical simulation and isless toxic. Both the ratio of ambient pressure to saturation pressure (Pa/Ps) andthe difference between fuel temperature and boiling-point temperature (Tf-Tb)can be used as characteristic parameters to describe the structure behaviorsobserved for flash-boiling sprays in an equivalent way. To simplify thefollowing analysis, the non-dimensional pressure ratio was taken as thesuperheat degree indicator in this work. Three characteristic superheated regionswere identified, namely the non-superheated region, transition region and the fully-superheated region. The spray structure, flow-field distribution and theinteraction between two phases in each region were analyzed separately.In this first part, the macroscopic spray structure in both axial andcross-sectional directions were investigated using laser-sheet imaging. Inaddition, shadowgraphic microscopic imaging technique was used to observe th enear-field spray break-up process and the bounce spray after the end of injection.A relationship between the microscopic and macroscopic spray structure wasbuilt. The difference of spray break-up mechanism between normal liquid spraysand superheated sprays were identified. The characteristics of spray structure ineach superheated region were summarized.To understand the driving force of the spray structure transformation, thehigh-speed PIV technique was successfully applied to the spray flow-fieldmeasurement. Both axial and cross-sectional flow-fields were studied. A mixingzone was found in between spray plumes. Strong plume to plume interaction andobvious vortex motion were observed with the increase of superheat degree. Theidentification and tracking of vortex structures was based on the critical-pointanalysis of the local velocity gradient tensors. The correlation between thevortex motion and the flow-field distribution was found.As for the simultaneous fuel spray-ambient gas two-phase flow-fieldmeasurement, only limited publication and data can be found due to thecomplicated hardware requirement and optical arrangement etc., restricting theapplication of those optical diagnostic tools and theoretical development ofair-fuel two-phase flow. Therefore it is desirable to develop a two-phase flowdiagnostic tool based on a relatively simple apparatus. In this background,one-laser one-camera based high-speed two-color PIV was developed andsuccessfully applied to the air-fuel two-phase flow measurement in this work.This methodology used the Mie-scattering signal to tag the spray particle andthe fluorescent signal to tag the ambient gas tracer particles. Two separationtimes was introduced in this technique to accommodate both the dynamiccharacteristic of fuel spray and ambient gas. An image-doubler, with differentoptical filters mounted, was installed in front of a high-speed camera to recordthe two phases separately on the same camera chip.Based on the high-speed two-color PIV, the two-phase flow and air-fuel mixing process were studied. The ambient gas flow was characterized into threeregions. The first is the “entrainment zone” where ambient gas entrainment isinduced by the spray injection wave near the nozzle. The second is the “headvortex zone” at the spray tip area where the ambient gas is push out/engulfed bythe following fuel spray. The third zone is called “recirculation zone” lying inbetween the above two zones. The ambient gas motion in this zone results fromthe continuity nature of the gas flow to compensate the entrainment zone. Theambient gas kinetic energy distribution in these three zones and the momentumtransfer between the fuel spray and ambient gas were analyzed under varioussuperheated conditions. A non-dimensional ambient gas entrainment model wasput forward.In summary, this work investigated systematically the spray structure,spray flow-field and air-fuel mixing characteristics under various superheatedconditions. Compared to normal liquid spray, the superheated spray can becharacterized with flexible spray structure, improved atomization quality andstrengthened mixing process. It provides a potential solution for technicalproblems such as fuel impingement, oil dilution, and piston carbonizationduring the development process of SIDI engines. This work provides importanttechnical references for relative combustion system design and data support forthe control strategy and simulation of superheated sprays.