Simulating Gas-Air Mixture Formation for Dual-Fuel Applications Karri Keskinen, Ossi Kaario, Mika Nuutinen, Ville Vuorinen, Zaira Künsch and Martti Larmi Thermodynamics and Combustion Technology Research Group Aalto University School of Engineering Department of Energy Technology
Presentation Outline Background Research Questions Mixing Simulation Mechanisms & Metrics What We ve Learned
Background (1/4): Natural Gas-Based Dual-Fuel Engines State of the art Port gas injection + diesel (direct) injection Gas mixtures are lean and homogeneous Diesel injection required for ignition (DF engines) Spark plug in (single-fuel) SG engines Why make research on novel gas / dual-fuel technologies? Improving the competitiveness of gas engines in a changing engine market environment Wärtsilä 20DF dual-fuel marine engine (www.wartsila.com)
Background (2/4): Natural Gas Engine Research NG-air mixture formation NG combustion NG engine performance
Background (3/4): Optimal Mixtures for Gas Engines Localized mixture remains clear of peripheral HC slip-inducing zones Remaining mixture is sufficiently homogeneous for a high quality combustion process Little limitation of surplus air (through charge air pressure / throttling) is required (Stratification!) Gaseous, lean mixture is within reach of a pilot diesel spray AFR distribution, cylinder sector simulation model @ TDC
Background (4/4): Direct Injection of Gas May Be a Solution to These Problems... Why Is It Not Yet Implemented? Producing high gaseous injection pressure leads to losses in engine total efficiency Mixture formation is a challenge Gas jets have worse intrinsic mixing capability than liquid sprays Low-pressure jets = long injection duration
Injection Timing for a Medium Speed Engine (Part Load)
Research Questions What are the potential mixing mechanisms related to direct-injected gas jets? How low can injection pressure be taken? What is required of injection equipment and control? What is required of the combustion chamber geometry?
Gas Jets 101 High mach number flows at moderate pressure ratios Compressible flow phenomena: Shock and sound wave formation, supersonic jets (de Laval nozzles) Low momentum density (versus liquid sprays) Vastly different jets from different geometries Single-orifice nozzles Multiorifice nozzles Hollow-cone nozzles Density gradient in an underexpanded, straight-orifice gas jet (LES) (Vuorinen, 2012)
Gas Jets in Engine Conditions Nozzle length scales << Cylinder length scales Resolving in-nozzle & near-nozzle flow phenomena is a challenge! Lift: 2 mm Computational mesh: Hollow-cone nozzle Concentration contours in the early stages of a collapsing hollow-cone jet (LES) Concentration isosurface during injection: Multiorifice-type nozzle (RANS)
How Can We Limit Computational Cost? Nozzle-equivalent mesh density in entire cylinder Cell quantity ~ 100 000 000 Time-dependent condensing of a moving mesh Moving meshes: Additional complexity Deformation & removal of cell layers Remedy: Time-dependent moving mesh condensing Many gas jets are low-penetrating Control of near-nozzle region density Significant decrease in cell number Injection and jet advancment during the compression stroke
How Do We Know Our Simulations Are Realistic?
Simulations & Experimental Research Experimental investigation Dialogue Computational investigation Simulation (RANS) PLIF Experiment (Künsch, 2013) Goal Knowing what we re doing in an actual engine Phenomenological & quantitative validation Gas jets are sensitive to in-cylinder conditions Research in the field of engines is still fresh!
Mixing: How and How Well?
Identifying & Quantifying Mixture Formation Mechanisms (1/2) Mixing Mechanisms in Engines Intrinsic momentum-induced mixing (diesel sprays) Intake flow-induced mixing (port-injected engines) Spray/jet guiding & piston guiding (gasoline direct injection) What role do these mechanisms have within a Gas-DI framework? How do we evaluate mechanism efficacy?
Identifying & Quantifying Mixture Formation Mechanisms (2/2) Flexibility from CFD! PDF-type mixture distributions Proportions of rich and lean mixtures Critical region observation (HC slip tendency) Mixture distribution - ignition source distance Turbulence quantities...
What We ve Learned Why Gas-DI mixture formation is challenging How not to do things Knowing what we re doing: Jet formation physics Particularly important (and sometimes unintuitive!) in complex jets Promising mixture formation mechanisms Efficacy & efficiency in simulation Robustness, parametric adjustability Computational requirements ( & mesh optimization!)
Thank You! Please do ask questions! Further information: karri.keskinen@aalto.fi Thermodynamics & Combustion Technology Research Group Puumiehenkuja 5 A 02150 Espoo