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