Spray-wall-interaction with wall film forming and evaporation Florian Schulz (Group Prof. Schmidt)
Institute of Fluid Dynamics and Thermodynamics, Faculty of Process and Systems Engineering, Otto-von-Guericke-University Magdeburg
Problem Definition Objectives Cooperation
Spray Characterization Spray-Wall Interaction
The strong influence of the surface temperature on the
droplet-wall interaction, the resulting liquid deposition
and the film evaporation is known. But the quantitative
forecast of wall films caused by dense sprays is usually
insufficient because of the various parameters
influencing the occurring heat fluxes which reduce the
surface temperature during the injection. With the help
of infrared thermography, it is possible to visualize
these processes.
The aim of the experimental wall film investi-
gation is, to get a deeper understanding of the
governing processes of spray-wall interaction,
wall film forming and evaporation. With the
help of the developed semi empirical models,
prediction of the deposited fuel mass under
various conditions can be made. With these
knowledge the wall film and soot emissions
can be minimized.
M. Sc. Kutelova (Group Prof. Tomas)
• Determination of surface energy and surface tension
Dipl.-Ing. Vorhauer (Group Prof. Tsotsas)
• Support at infrared thermography measurements
Dipl.-Ing. Baer, M.Sc. Dragomirov (Group Prof. Schmidt)
• exchange of experimental experience for PDA, IR and
HS measurements
Dr. Nallathambi (Group Prof. Specht)
• Development of an inverse heat transfer algorithm
Macroscopic Microscopic
DFG-Graduate School 1554 “Micro-Macro-Interactions in Structured Media and Particle Systems”
Introduction.
Modern gasoline engines use the principle of direct injection. During the warm-up and for early injections the spray droplets contact the piston surface.
As a result of the spray-wall interaction a wall film occurs. The liquid film as a fuel rich zone is one important reason of high soot emissions. Therefore it
is necessary to perform investigations on wall film forming. It is aimed to reduce the wall film mass.
The general effects are based on the particle systems and structured media. On the on hand the microscaled properties of a gasoline spray were
described by the distribution parameters of particle systems. On the other hand the wetting and cooling process of the piston can be characterized by
macroscaled material properties. Various methods were used to analyze the occurring processes. These methods are shown below.
Conclusions Results and Discussion
Until now the spray and wall film properties can be
described as a function of the analyzed boundary
conditions. That already shows potential to reduce
the wall film mass in real homogeneous charged
engines.
In a next step the microscale properties are
correlated with the macroscale properties, also the
spray physics with the wall film behavior. This will
lead to an new empirical model to predict wall films.
The macroscale behavior of the spray could be described by HS-Imaging. Also the microscaled droplet properties were
found by using Phase-Doppler-Anemometry. With the help of infrared technology, the heat fluxes on top and within the
wall can be described as a function of different boundary conditions. This is especially important for the determination of
the changing wall temperatures, which most influence the film evaporation process. LIF measurements enable the
determination of the wall film mass for varied boundary conditions.
The first evaluation of the measurement results shows that a reduction of the wall film mass can be achieved by an
increase of the injection pressure and the wall temperature. At the same time the wall film mass reaches a maximum with
changing ambient pressure and nozzle-wall distance. The analysis by the Design of Experiments method emphasizes that
the position of the maximum wall film is affected by interaction with the other boundary conditions.
Injection Pressure
Cham
ber
Pre
ssure
50bar 150bar 300bar
1bar
3bar
5bar
film height
[µm]
pRail = 50 bar pRail = 150 bar pRail = 300 bar
pchamber
= 1bar
pchamber
= 3bar
pchamber
= 6bar
79
11131517
15 20 25 30 35 40Dro
ple
t d
iam
eter
[µ
m]
Distance [mm]
Tchamber = 80°C pRail = 150 bar p_K=1bar d32
p_K=3bar d32
p_K=6bar d32
p_K=1bar d10
p_K=3bar d10
p_K=6bar d10
20
30
40
50
60
15 20 25 30 35 40
Dro
ple
t ve
loci
ty
[m/s
]
Distance [mm]
Tchamber = 80°C pRail = 150 bar
p_K=1bar
p_K=3bar
p_K=6bar
Fields of temperature differences for different injector
positions (varied angles and distances); at 12ms
after start of injection, pinj
=15MPa, Twall
= 80°
335
340
345
350
355
0 5 10 15 20
tem
pe
ratu
re [K
]
time [ms]
coated back 0,14mm0,12mmboundary layer 0,1mm0,06mm0,02mmtop surface 0,0mm
Infrared Thermography
Laser – Induced – Fluorescence
Field of temperature differences of the
fuel wall film caused by a six hole nozzle
pinj
=15MPa, a=35mm, Twall
= 80°
Simulated temperatures within a 0.1mm
nickel alloy sheet with a 40µm coating on
the back, due to a constant heat flux of
3.1 MW/m2 with a duration of 1.5ms
Heat flux at the spray impact zone
Twall
= 80°
Schematic experimental setup
Phase-Doppler Anemometry measurements to
derive droplet diameters and droplet velocities
High-Speed-Imaging to characterize the
spray propagation in space and time
Figure below: Shadowgraphy images
(average image derived from 30 single
images ) at a chamber temperature of
80° at 500µs after start of injection
The parameters effecting the wall
film forming and evaporation are
illustrated in the left figure. The
initial spray properties generated
by the injector are fundamental.
Depending on the droplet velocity,
diameter and the fuel properties
the liquid deposition will be cha-
racteristic. The wall material influ-
ences the occurring heat fluxes
and therefore the spray-wall inter-
action, the film propagation and
the film evaporation. The time of
evaporation depends on the distri-
bution of the wetted area and the
heat inserted through the wall.
Simplified sectional view
of the pressure chamber
Injector
Laser
Chamber
Window
Wall with
Quartz Plate
Camera
Diffusor
Filter
The wall film resulting from the impingement of one spray jet onto
the quartz plate is exited by the laser radiation of 266nm. The
measured fluorescence signal contains the film height information,
due to a calibration and a post processing.
Figure right:
Wall film height
images at 12ms
after start of
injection for
Tchamber
= 80°
As substitute for
gasoline iso-octane
is used with 5%vol
of 3-Pentanone
as tracer.
C
C
C
C
C