Combined measurements of flow field,

1

• Combined measurements of flow field,
• partially oxidized fuel (POF) and soot

2
Combined PIV, LII and LIF of PPC motivation

• Numerical simulations suggest
• bulk flow structures can
• enhance late-cycle mixing rates

Can we observe these structures experimentally?
Simulation performed at University of Wisconsin
for previous work with Paul Miles.
Unburned fuel and air are transported to a common
interface at higher swirl
3
Operating condition
Engine Volvo D5 Speed 1200 RPM Load 4.0
bar O2 12 (56 EGR) ? 1.15 CA50 5
ATDC SOI -12.5 ATDC Fuel 70 n-heptane 30
iso-octane
Smoke (FSN)
4
Ignition
Little evidence of POF despite turn-up in AHRR
First low-temp heat release occurs in lower
central bowl Fuel from head of jet?
5
Start of high temperature heat release
Earlier POF transported upward, inward
Diminished POF suggests initial high-temp HR
occurs in upper bowl
Volume expansion identifies main heat release
asoccurring in upper, outer bowl
Volume expansion near edges of POF
6
Late pre-mixed burning

• Soot first appears after the peak heat release
• Soot POF appear in two distinct spatial zones
• A dual-vortex flow structure is observed, but,
  contrary to simulations of similar engines, the
  inner vortex contains POF!

7
Later mixing-controlled burning
POF above piston top remains spatially
fixed Flow structure in central region is
dominated by axial expansion. This means that
there is little flow motion to promote mixing of
this fluid Heat release remains concentrated
within the bowl, giving the reverse squish flow a
strong vertical component, leading to formation
of a toroidal vortex above the piston top
8
Late-cycle burnout
9
Single-cycle images / flow fields (not
simultaneous)
10
Soot, POF in-cylinder history
11
Summary and conclusions I

• Dual-vortex flow structures similar to those
  observed in numerical simulations are observed.
  Heat release at the interface of the two vortices
  remains to be verified/quantified (more
  accurate/precise measurements are needed).The
  presence of POF within these structures needs to
  be evaluated carefully experimental POF
  distributionsappear to contradict numerical
  predictions.
• Dual-zone distributions of POF are observed
  during the expansion stroke. The dual zone
  distribution is also observed on a single-cycle
  basis. Soot, however, is found mainly within the
  central zone.

12
Summary and conclusions II

• Mean bulk flow structures during expansion do
  little to mix fluid within the central zone with
  fluid elsewhere in the cylinder.
• POF in the outer zone, above the bowl mouth, is
  located (trapped?) within a toroidal vortex
  attached to the piston top.
• Single-cycle flow fields and POF/soot
  distributions retain many features observed in
  the mean fields.

13

• Comparison of the flow fields of
• conventional and late injection timings
• as well as different swirl ratios

14
Operating conditions

• Late timing (below)
• O2 15 (40 EGR)
• SOI - 4.0 ATDC
• CA50 18 ATDC

Conventional timing (above) O2 12 (56
EGR) SOI - 12.5 ATDC CA50 5 ATDC
15
Fuel jet setting up vortex
Late injection SOI -4

• Conventional injection timing
• SOI -12.5

Fuel jet sets up a standing vortex, clearly
visible before main heat release Peak velocities
around 4 Sp
16
Mixing-controlled combustion conv. timing

• Vortex formed at centreline moves
• outwards
• imparts looping motion on reverse-squish flow
• fluid entering squish region comes from uppermost
  regions of the cylinder

17
Late mixing controlled burn conv. timing

• Vortex formed in centre still present, now at
  bowl mouth
• Second vortex formed at centreline
• Upward looping reverse-squish motion still present

18
Premixed combustion, late timing

• Some flow directed into squish volume apparent at
  10 CAD
• Reverse-squish motion stronger than for
  conventional injection timing
• Transport into squish volume mainly from mid-part
  of bowl for conv. timing mostly from outer
  parts

19
Mixing-controlled combustion late timing

• Mushroom-shaped structure formed
• transports fluid both inwards and outwards
• Evolves into looping motion at rim, forms full
  vortex

20
Influence of swirl

• Late timing
• Higher swirl

Late timing Lower swirl
Main difference is the fluid motion entering the
squish region. At higher swirl, this motion has a
dominant vertical component and evolves into a
toroidal vortex by 30 CAD. At lower swirl, fluid
trajectory is much flatter and the formation of
the toroidal vortex is delayed until about 35-40
CAD.
21
Summary and conclusions

• Fuel injection event sets up a clockwise-rotating
  vortex in the bowl region. Peak velocities in
  this flow are up to about 4 times the mean piston
  speed.
• Peak reverse squish flow is found to coincide
  closely with the timing of the peak heat release
• For the higher swirl cases, the location of the
  peak maximum in the reverse squish flow are
  different for the two operating conditions
• Conventional timing has maximum close to bowl rim
• Later timing has maximum closer to bowl
  mid-radius
• Toroidal vortex appears near bowl rim just after
  start of mixing-controlled combustion
• Looping structure at bowl rim is flatter at lower
  swirl andthe formation of toroidal vortex is
  delayed

22

• Investigation of combustion with EGR
• using high speed video

23
(Very) preliminary observations

• Strong correlation between regions of high soot
  luminosity and positive vorticity
• Work in progress
• Results and conclusions will beadded when
  available

24

• In-cylinder quantitative
• soot measurements

25
Soot history
Same later-cycle soot amount, but very different
in-cylinder soot history at different (high) EGR
ratios. Most soot production takes place after
peak of heat release