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
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!
Simulation (RANS)
PLIF Experiment (Künsch, 2013)
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:
[email protected]
Thermodynamics & Combustion Technology Research Group
Puumiehenkuja 5 A
02150 Espoo