Simulating Gas-Air Mixture Formation for Dual-Fuel Applications

Transcription

1 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

2 Presentation Outline Background Research Questions Mixing Simulation Mechanisms & Metrics What We ve Learned

3 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 (

4 Background (2/4): Natural Gas Engine Research NG-air mixture formation NG combustion NG engine performance

5 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 TDC

6 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

7 Injection Timing for a Medium Speed Engine (Part Load)

8 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?

9 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)

10 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)

11 How Can We Limit Computational Cost? Nozzle-equivalent mesh density in entire cylinder Cell quantity ~ 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

12 How Do We Know Our Simulations Are Realistic?

13 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!

14 Mixing: How and How Well?

15 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?

16 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...

17 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!)

18 Thank You! Please do ask questions! Further information: Thermodynamics & Combustion Technology Research Group Puumiehenkuja 5 A Espoo