Injector Dynamics Assumptions and their Impact on Predicting Cavitation and Performance Frank Husmeier, Cummins Fuel Systems Presented by Laz Foley, ANSYS
Outline Overview Computational Domain and Boundary Conditions Results Low Outlet Pressure (worst case for cavitation) Comparison of Opening and Closing Event Variable Outlet Pressure Conclusions 2012 Automotive Simulation World Congress 2
Overview Internal fluid dynamics of injector can influence spray pattern and, in turn, impact combustion performance and emissions Critical factors in internal fluid dynamics are: Cavitation behavior Sac filling/sac pressure Spray hole velocity/momentum How can these factors be reliably predicted to improve Analysis Led Designs (ALD)? 2012 Automotive Simulation World Congress 3
Overview Process Flow for Nozzle Analysis Steady State CFD Force Efficiency 1D Simulation Lifting Profile Transient CFD Final Result Rig Test Validation Force Efficiency Roughing of Nozzle/Plunger Design with Steady State CFD Transfer of Force Efficiency to 1D Simulations to Obtain Lifting Profile for Transient CFD Fine Tuning of Design and Force Efficiency with Transient CFD Force Efficiency critical to realistically predict Injector Performance 2012 Automotive Simulation World Congress 4
Computational Domain and Boundary Conditions Focus on injector tip inlet Fluid Volume Based on symmetry assumption, only half a spray hole in computational domain Pressure described at Inlet and Outlet (normalized with inlet pressure) To simulate fully closed valve, wall is introduced at time T=1.40667 Liquid compressibility accounted for, but isothermal Flow properties set according to kerosene type calibration fluid to accentuate cavitation behavior outlet symmetry wall 2012 Automotive Simulation World Congress 5
Computational Domain and Boundary Conditions Lifting Profile and Variable Outlet Pressure Profile Prescribed profile from 1D simulation Time normalized with injection on time Linear pressure drop prescribed at outlet boundary for variable outlet pressure simulations 2012 Automotive Simulation World Congress 6
Computational Domain and Boundary Conditions 2 phase vs. 3 phase flow 2 phase flow: Phase 1: liquid fuel Phase 2: fuel vapor 3 phase flow: Phase 1: liquid fuel Phase 2: fuel vapor Phase 3: air Note: Mass transfer from liquid fuel to fuel vapor based on cavitation model Initially, 100% air downstream of sealing diameter. 100% backflow of air at outlet 2012 Automotive Simulation World Congress 7
Low Outlet Pressure Pressure Opening Event Pressure Closing Event 3 phase flow 2 phase flow Lift = 10% Lift = 100% Lift = 7% Lift = 1% During opening and for full lift, sac pressure considerably higher for 2 phase flow resulting in an unrealistic high force efficiency Only minimal differences in sac pressure during entire closing event 2012 Automotive Simulation World Congress 8
Low Outlet Pressure Vapor Opening Event Velocity Opening Event 3 phase flow 2 phase flow Lift = 1% Lift = 7% Lift = 1% Lift = 7% Vapor generation delayed for 3 phase flow for opening event Less vapor formations for 3 phase flow for opening event Change in cavitation behavior can be attributed to changes in velocity field 2012 Automotive Simulation World Congress 9
Low Outlet Pressure Vapor Closing Event Velocity Closing Event 3 phase flow 2 phase flow Lift = 7% Lift = 1% Lift = 7% Lift = 1% 3 phase flow exhibits higher vapor formation in sac during closing Flow path into spray hole similar in both cases Higher flow velocity in spray hole results in slightly different vapor formations 2012 Automotive Simulation World Congress 10
Low Outlet Pressure 3 phase flow V e l o c it y 2 phase flow Opening Full Lift Closing For 3 phase flow, spray hole exit momentum is higher during opening and closing, but lower for full lift Impacting Spray and Combustion Simulations 2012 Automotive Simulation World Congress 11
Opening vs. Closing Event Vapor 3 phase Velocity 3 phase Closing Opening Lift = 1% Lift = 7% Lift = 1% Lift = 7% During opening, incoming flow must fill sac resulting in lower mass flow rate at the spray hole exit Increased vapor formations in sac and spray hole during closing can be linked to the increased mass flow rate at the spray hole exit 2012 Automotive Simulation World Congress 12
Opening vs. Closing Event Pressure 3 phase closing event opening event Lift = 1% Lift = 7% Lift = 10% Lift = 100% while pressure contours are comparable during opening and closing 2012 Automotive Simulation World Congress 13
Variable Outlet Pressure Pressure 3 phase Variable P out Time = 1.98333 Time = 2.01667 Time = 2.08333 Time = 1.98333: Because pressure drop over needle seat results in sac pressure below outlet pressure (prior to valve closing), incoming pressure wave travels through spray hole Time = 2.01667: Incoming pressure wave reflects off sac wall and needle. Small gap at seat sac interface results in a traveling wave Time = 2.08333: Traveling wave moves towards sac bottom Time independent: Uniform pressure distribution in spray holes while sac pressure reveals large variations 2012 Automotive Simulation World Congress 14
Variable Outlet Pressure Pressure 3 phase Variable P out Time = 2.28333 Time = 2.41667 Time = 2.59167 Time = 2.28333: Pressure variations in sac decay Time = 2.41667: Uniform sac pressure equaling spray hole pressure Time = 2.59167: Constant low pressure is reached at outlet and throughout sac and spray hole Constant Outlet Pressure: Pressure in sac and spray hole equalize quickly and stay constant over time Constant Outlet P 2012 Automotive Simulation World Congress 15
Variable Outlet Pressure Vapor 3 phase Variable P out Constant P out Time = 1.98333 Time = 2.18333 Time = 2.41667 Time = 2.59167 For constant P out, constant vapor formation in sac For variable P out, initial vapor collapses until constant pressure is reached again 2012 Automotive Simulation World Congress 16
Variable Outlet Pressure Air 3 phase Variable P out Constant P out Time = 1.98333 Time = 2.18333 Time = 2.41667 Time = 2.59167 For constant P out, constant air formation in sac For variable P out, increasing air fraction in sac until constant pressure is reached 2012 Automotive Simulation World Congress 17
Conclusions 2 phase vs. 3 phase flow simulation Different velocity field in sac and spray hole creates different vapor formations, i.e. different cavitation behavior Sac pressure for lifts > 7% considerably lower for 3 phase flow simulation, thus resulting in different force efficiencies for plunger, impacting injector performance simulations For 3 phase flow, momentum at spray hole exit higher for low lifts and lower for full lift, therefore impacting combustion performance simulations 2012 Automotive Simulation World Congress 18
Conclusions Opening vs. Closing Event While pressure distribution does not vary significantly, strong hysteresis effect on vapor formations and velocity field Stronger velocity gradients observed during closing event resulting in higher vapor formations High momentum flow along the bottom of the spray hole during closing event potentially impacting spray targeting 2012 Automotive Simulation World Congress 19
Conclusions Constant vs. Variable Outlet Pressure With constant outlet pressure, more vapor is generated in the sac volume With variable outlet pressure, vapor only occurs late in the simulation when pressure is almost constant Variable outlet pressure results in a higher air concentration in sac volume throughout simulation Thus, decreasing outlet pressure results in a slightly higher liquid mass flow rate at the spray hole exit after needle closure 2012 Automotive Simulation World Congress 20
Conclusions In order to reliably predict injector dynamics with respect to cavitation behavior and performance: Run transient simulations with liquid compressibility Use 3 phase flow Prescribe a meaningful inlet and outlet pressure 2012 Automotive Simulation World Congress 21
APPENDIX 2012 Automotive Simulation World Congress 22
Low Outlet Pressure Opening Event Pressure 3 phase flow 2 phase flow Lift = 1% Lift = 7% Lift = 10% Lift = 100% Sac pressure considerably higher for 2 phase flow resulting in significant higher force efficiency 2012 Automotive Simulation World Congress 23
Low Outlet Pressure Closing Event Pressure 3 phase flow 2 phase flow Lift = 100% Lift = 10% Lift = 7% Lift = 1% For low lifts, difference in sac pressure less pronounced for closing event 2012 Automotive Simulation World Congress 24
Low Outlet Pressure Opening Event Vapor 3 phase flow 2 phase flow Lift = 1% Lift = 7% Lift = 10% Lift = 100% Less vapor is formed at higher lifts for 3 phase flow 2012 Automotive Simulation World Congress 25
Low Outlet Pressure Closing Event Vapor 3 phase flow 2 phase flow Lift = 100% Lift = 10% Lift = 7% Lift = 1% Vapor in sac region considerably higher for 3 phase flow during closing 2012 Automotive Simulation World Congress 26
Low Outlet Pressure Opening Event Velocity 3 phase flow 2 phase flow Lift = 1% Lift = 7% Lift = 10% Lift = 100% Air has high impact on velocity field for low lifts 2012 Automotive Simulation World Congress 27
Low Outlet Pressure Opening Event Velocity 3 phase flow 2 phase flow Lift = 1% Lift = 7% Lift = 10% Lift = 100% Smaller momentum in spray holes at full lift due to lower sac pressure for 3 phase flow 2012 Automotive Simulation World Congress 28
Low Outlet Pressure Closing Event Velocity 3 phase flow 2 phase flow Lift = 100% Lift = 10% Lift = 7% Lift = 1% 3 phase flow exhibits higher momentum in spray holes for low lifts 2012 Automotive Simulation World Congress 29
Opening vs. Closing Event 3 phase Pressure closing event opening event Lift = 1% Lift = 7% Lift = 10% Lift = 100% Pressure contours similar during opening and closing 2012 Automotive Simulation World Congress 30
Opening vs. Closing Event 3 phase Vapor closing event opening event Lift = 1% Lift = 7% Lift = 10% Lift = 100% But, considerably more vapor in sac and spray holes during closing event 2012 Automotive Simulation World Congress 31
Opening vs. Closing Event 3 phase Velocity closing event opening event Lift = 1% Lift = 7% Lift = 10% Lift = 100% Differences in vapor mostly attributed to small variations in pressure due to distinctive flow paths 2012 Automotive Simulation World Congress 32
Variable Outlet Pressure Pressure Contours Variable P out constant P out Time = 1.98333 Time = 2.01667 Time = 2.08333 Time = 2.18333 Due to gap between plunger and sac wall, incoming pressure wave creates travelling wave 2012 Automotive Simulation World Congress 33
Variable Outlet Pressure Pressure Contours Variable P out constant P out Time = 2.28333 Time = 2.3 Time = 2.41667 Time = 2.59167 Traveling wave decays and sac pressure equalizes towards end of simulation 2012 Automotive Simulation World Congress 34
Variable Outlet Pressure Vapor Contours Variable P out constant P out Time = 1.98333 Time = 2.01667 Time = 2.08333 Time = 2.18333 Variable outlet pressure shows decaying vapor formations in sac 2012 Automotive Simulation World Congress 35
Variable Outlet Pressure Vapor Contours Variable P out constant P out Time = 2.28333 Time = 2.3 Time = 2.41667 Time = 2.59167 Only when outlet pressure is constant, vapor is generated again in the sac 2012 Automotive Simulation World Congress 36
Variable Outlet Pressure Air Contours Variable P out constant P out Time = 1.98333 Time = 2.01667 Time = 2.08333 Time = 2.18333 Variable outlet pressure results in a higher air entrainment 2012 Automotive Simulation World Congress 37
Variable Outlet Pressure Air Contours Variable P out constant P out Time = 2.28333 Time = 2.3 Time = 2.41667 Time = 2.59167 When outlet pressure is constant, air concentration decreases to some extent 2012 Automotive Simulation World Congress 38