EFFECTS OF DROPLET BREAKUP, HEATING AND EVAPORATION ON AUTOIGNITION OF DIESEL SPRAYS

1
EFFECTS OF DROPLET BREAKUP, HEATING AND
EVAPORATION ON AUTOIGNITION OF DIESEL SPRAYS
S. Martynov1, S. Sazhin2, C. Crua2, M.
Gorokhovski3, A. Chtab4, E. Sazhina2, K. Karimi2,
T. Kristyadi2, M. Heikal2 1 Department of
Mechanical Engineering, University College
London, Torrington Place, London, WC1E 7JE, UK 2
Sir Harry Ricardo Laboratories, Internal
Combustion Engines Group, University of Brighton,
Brighton, BN2 4GJ, UK 3 LMFA UMR 5509 CNRS Ecole
Centrale de Lyon, 36 avenue Guy de Collongue,
69131 Ecully Cedex, France 4 CORIA UMR 6614 CNRS
University of Rouen, 76 801 Saint-Etienne du
Rouvray, France
Methodology
Models for heating and evaporation of droplets
Shell autoignition model
Spray breakup models

• The Eulerian (gas) - Lagrangian (liquid) CFD code
  KIVA-II is applied
• Droplet parcels, representing the liquid phase,
  are injected using the blob injection method
• Several sub-models for the spray breakup, heating
  and evaporation of droplets are implemented into
  the customised version of the KIVA-II code
• Autoignition is described, based on the Shell
  model
• Sensitivity and parametric studies of the
  autoignition delay in Diesel sprays are performed

Liquid phase
Initiation CnH2m O2 ? 2R Propagation R ? R
P R ? R B R ? R Q R Q ? R
B Branching B ? 2R Termination R ?
out 2R ? out where R is the radical, B
is the branching agent, Q is the intermediate
product, P is the final product, consisting
of CO, CO2, H2O. The pre-exponential constant
for the reaction rate for the production of the
branching agent was set to Af4 3 106 (Sazhina et
al, 2000).

• TAB (ORourke and Amsden 1987),
• WAVE KH-RT (Patterson Reitz, 1998),
• Stochastic (Gorokhovski Saveliev, 2003)
• The modified version of the WAVE model, which
  takes into account the damping effect of
  injection acceleration on the break-up rate
  constant

• Infinite thermal conductivity (ITC) model
  (default in KIVA II),
• Effective thermal conductivity (ETC) model,
  taking into account both finite liquid thermal
  conductivity and re-circulation inside droplets.

Gas phase

• Model 0 (default in KIVA II)
• Model AS (Abramzon Sirignano, 1989), taking
  into account for the finite thickness of the
  boundary layer around the droplet

where B1,eq 10 is the break-up time of the
conventional WAVE model, and a is a
dimensionless acceleration parameter.
Validation test cases

• VCO type Diesel single-hole injection nozzle of
  200 µm in diameter
• Injection pressures 60 160 MPa, fuel
  temperature 350 400 K
• In-cylinder pressures 5 9 MPa, gas temperature
  750 800 K.

Results autoignition delay time
Effect of droplet heating and evaporation
models Effect of breakup model Effect of the
Shell model constant Effect of the fuel
temperature Effect of the gas temperature Effect
of grid size
Predicted and experimentally measured ignition
delay times versus in-cylinder pressure in
base-line computations mesh with 20 x 48 cells
was used, 1000 droplet parcels were injected at
pressure 160MPa into a cylinder with initial
pressure 6.2MPa. If not stated otherwise, the
temperature of injected fuel was 375K, the
initial gas temperature wais 750K, the modified
WAVE breakup model, ETC liquid phase model and AS
gas phase model were used.
Conclusions
Results local spray properties

• The predicted decrease in the autoignition delay,
  with increasing in-cylinder gas pressure in the
  approximate range 5.5 MPa to 7 MPa, agrees with
  experimental observations.
• The choice of the gas phase model has only a
  minor effect on the predicted autoignition delay,
  which can be safely ignored in practical
  engineering computations.
• The difference in the autoignition delay times,
  predicted by the ITC and ETC models is noticeable
  and needs to be taken into account in practical
  computations. The application of the ETC model is
  recommended as a more physical one.
• The choice of the spray breakup model is shown to
  have a small effect on the autoignition delay
  time.
• The Shell model kinetic rate constant Af4
  variation by 100 can cause reduction in the
  autoignition delay time by 20.
• An increase in the initial gas or liquid fuel
  temperature by 20 - 25 K can noticeably reduce
  the autoignition delay time

time 0.98 ms time 1.49 ms time 1.73 ms
time 1.98 ms
Publications

• Crua, C. (2002) Combustion processes in a diesel
  engine. PhD thesis, University of Brighton.
• Sazhin, S.S. (2006) Advanced models of fuel
  droplet heating and evaporation, Progress in
  Energy and Combustion Science, 32 162-214.
• Sazhin, S.S., Abdelghaffar, W.A., Krutitskii,
  P.A., Sazhina, E.M., Heikal, M.R. (2005) New
  approaches to numerical modelling of droplet
  transient heating and evaporation, Int. J Heat
  Mass Transfer, 48. 4215-4228.
• Sazhin, S.S., Abdelghaffar, W.A., Sazhina, E.M.,
  Heikal, M.R. (2005) Models for droplet transient
  heating effects on droplet evaporation,
  ignition, and break-up, Int. J Thermal Science,
  44, 610-622.
• Sazhin, S.S., Abdelghaffar, W.A., Sazhina, E.M.,
  Mikhalovsky, S.V., Meikle, S.T. and Bai, C.
  (2004) Radiative heating of semi-transparent
  diesel fuel droplets, ASME J Heat Transfer, 126,
  105-109. Erratum (2004) 126, 490-491.
• Sazhin, S.S., Kristyadi, T., Abdelghaffar, W.A.
  and Heikal, M.R. (2006) Models for fuel droplet
  heating and evaporation comparative analysis,
  Fuel, 85(12-13), 1613-1630.
• Sazhin, S.S., Krutitskii, P.A., Abdelghaffar,
  W.A., Sazhina, E.M., Mikhalovsky, S.V., Meikle,
  S.T., Heikal, M.R. (2004) Transient heating of
  diesel fuel droplets, Int. J Heat Mass Transfer,
  47. 3327-3340.
• Sazhina, E.M., Sazhin, S.S., Heikal, M.R.,
  Babushok, V.I., Johns, R.A. (2000) A detailed
  modelling of the spray ignition process in Diesel
  engines. Combustion Science and Technology, 160,
  317-344.

Spatial distribution of droplets (top row) and
gas temperature field (bottom row) at four
moments of time. Mesh with 20 x 48 cells was
used 1000 droplet parcels with initial
temperature 375K were injected at 160MPa into air
at 6.2MPa and 750K the modified WAVE breakup
model, ETC liquid phase model and AS gas phase
model were used. Droplets are shown with
diameters magnified 500 times.
Acknowledgements
The authors are grateful to the European Regional
Development Fund Franco-British INTERREG IIIa
(Project Ref 162/025/247) and the Indonesian
Government (TPSDP, Batch III) for financial
support