1. Fuel Spray Dynamics Modelisation
For liquid fuels, the treatment of fuel spray dynamics in NastComb is based on a Monte
Carlo method and adopts the concept of “discrete particles”. Strategy is that of sampling
randomly from assumed probability distributions that govern droplet properties at injection
and droplet behavior subsequent to injection. Turbulent correlation times associated with
proper distributions of turbulent displacements for spray droplets are considered. The
stochastic particle method is suitable to calculate evaporating liquid sprays including the
effect of droplet collisions, coalescence and aero-dynamic breakup (TAB model, Taylor
Analogy Break-up) [19]. In Appendix 2 the relevant equations of the method are given.
In order to reach a better agreement with available experimental results in situations more
similar Development, implementation, and calibration of a sophisticated, organic set of
self-complementing two-phase flow modelisations suitable to increase the fuel-spray
behaviour prediction capabilities of solver NastComb. This latter is the fully timedependent, rapid-transient, numerical tool which, developed in the last 14 years within this
UNIGE-DIMSET research group, was selected as the best tool to be utilised in order to
support from a theoretical perspective the parallel experimental investigations being
pursued, always at UNIGE-DIMSET, within Subtask 2.3.1. More in detail, the NastComb's
previously available TAB ("Taylor Analogy Break-up") spray-model, has been
complemented by a new model named RT-KH ("Rayleigh Taylor - Kelvin Helmholtz"),
rooted on instability analyses performed at the inter-phase between the liquid and the gas.
Kelvin Helmholtz instabilities are considered as responsible for the droplets primary breakup whereas the Rayleigh Taylor instability governs, together with the former, the
secondary break-up. The two models, introduced into NastComb, have been compared in
connection with a few reference test situations in order to have guidance toward the
respective calibrations. Fig.1 presents an example of comparisons among the prediction
capabilities of the different models.
FIG. 1
TAB Model
•SMR= 43 micron
•Droplet N°= 6000
RTKH model
•SMR= 25 micron
•Droplet N°= 20000
1
2. LPP-System Parametric Optimisation
Parametric application of NastComb in correspondence of several geometrical and
functional configurations of the Avio-Group LPP swirl premixer, in order to progressively
achieve its optimisation according to the imposed design specifications of: complete flow
stability, adequate levels of fuel prevaporisation and air-fuel premixing in the outlet section,
combined with an outlet swirl number of order 0.5. The optimisation strategy, involving
many successive fully 3D flow-field predictions, both in single- and in two-phase flow
situations (at the design preheating conditions), has been pursued successfully. According
to it, the final configuration has been assessed of the physical model which was then
manufactured and has undergone extensive experimental testing. This activity has
produced the Deliverable D 2.19:"Calculations of time-dependent, isothermal flow within
several designs of LPP swirl premixers”, transmitted, as required, at Month 12.
3. Premixed-Combustion Modeling Experimental Validation
In order to help validate the time-dependent, fully-reactive predictions, in connection with
the real-scale, Avio-Group overall combustion system, to be pursued by NastComb after
the present phase of unreactive calculations in its LPP device, the solver has been applied
in correspondence of a laboratory combustion-test situation recently performed at UNIGEDIMSET, wherein a rapid-mix combustion system, made up of a radial swirl-premixer
followed by a cylindrical combustor, has undergone detailed experimental testing. The
cross comparisons between the experimental temperature distribution within the cylindrical
combustion chamber and NastComb predictions have turned out at all positive. See Fig.2.
The tests are proceeding in order to cross compare predictions with the corresponding
experimental data in specific connection with the flame process stability as well as the
emissions.
C
B
1350
A
1300
1250
°C
1200
1150
1100
1050
1000
Radial position (Section A)
Experimental data
°C
1350
1350
1300
1300
1250
1250
1200
1150
NastComb
1200
°C
1150
1100
1100
1050
1050
2
FIG. 2
4. Unsteady Two-Phase Numerical/Experimental Comparisons in the LPP System
Particularly in the last six months, the research activity has taken full advantage of the
steps performed in the previous 18 months of the Project, which were mainly addressed to
the numerical "modelisation" and "calibration" phases of the solver, in order to proceed
with the phase of its extensive "applications", in terms of detailed time-dependent
simulations directly corresponding to the unsteady numerical/experimental comparisons in
connection with the preheated, two-phase (air-ethanol), unreactive flow field taking place,
at atmospheric conditions, within and downstream the Avio-Group enlarged-scale (4.73 to
1) premixer model-rig under parallel experimental investigation at UNIGE-DIMSET.
A first important outcome of all the numerical investigations has been the clear evidence
that no steady conditions could be achieved even for significant parametric variations both
in the geometrical and the functional parameters of the premixer, pointing out an intrinsic
fluid dynamic instability of the swirling flow, of periodically "snaking" behaviour, showing
frequencies ranging from 180 to 260 Hz, of the same order as the experimentally
measured. Interesting is also the observation, coming from the numerical analyses
performed in parametric mode, that the said instability showed a marked increase in its
intensity in dependence of the length the discharge chamber as well as of the increase of
the mass flow in the inner swirler.
All the detailed time-dependent results of the numerical investigations performed in
unreactive mode (enlarged scale model) are presented in the required Deliverable D2.20:
"Report on simulation of non-reacting two-phase flow and validation of turbulence and
droplet formation models" which, due at month 24, will be delivered before the mid-term
meeting. In it, wherever possible, also the detailed, azimuthally-averaged as well as timedependent, theoretical/experimental comparisons related to both the one-phase (air) and
the multi-phase (air-ethanol) flow fields are reported and discussed, which have turned out
significantly positive. From this document, the following pictures Figs. 3, 4, 5 have been
taken, respectively showing the locations of the measuring traverses, the unsteady
trajectories of the droplets as predicted by NastComb, as well as the numericalexperimental cross comparisons with reference to the averaged radial distributions of the
droplets’ diameters. In Fig.5, the accuracy of the predictions is remarkably good, which
has resulted from the imposition of droplets’ elastic rebounds from the premixer walls in
correspondence of the points of their impingements. No such precision could be obtained
by excluding wall rebounds and imposing the condition of droplets’ adherence to the walls,
with corresponding formation of a liquid boundary layer.
3
FIG. 3 : Locations of measuring traverses
FIG. 4 : Unsteady droplets’ trajectories with wall rebounds
in premixer as well as in combustor (color scale is velocity)
4
50
45
40
35
30
25
20
15
10
5
0
Experimental
NastComb prediction
D32 [um]
Traverse 1
50
45
40
35
30
25
20
15
10
5
0
Traverse 3
Experimental
NastComb prediction
20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80
20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80
radial distance [mm]
radial distance [mm]
Traverse 2
Experimental
NastComb prediction
D32 [um]
D32 [um]
D32 [um]
50
45
40
35
30
25
20
15
10
5
0
50
45
40
35
30
25
20
15
10
5
0
Traverse 4
Experimental
NastComb prediction
20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80
20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80
radial distance [mm]
radial distance [mm]
FIG. 5 : Averaged radial distributions of the droplets’ diameters:
numerical-experimental cross comparisons
5. Unsteady, fully reactive, numerical predictions within the real-scale prototype
Recently, a preliminary unsteady numerical investigation has been performed, in fully
reactive conditions, within and downstream of the real-scale LPP prototype, at operating
pressures, with kerosene fuel. The reactive scheme adopted was an extended partial
oxidation mechanism (EPOM, 12 species, 28 reactions), but, very recently, implementation
in NastComb of an advanced, detailed-chemistry mechanism (ADCM, 68 species, 260
reactions), keeping fully unsteady interaction with turbulence and radiation, has been
successfully performed. Contrarily to expected, the intensity of pulsations appeared as
somehow decreasing, for the same air mass flow, entering the reactive conditions with
respect to the unreactive one. As a first outcome, it can thus be stated that the basic fluiddynamic instability of the swirling jet is still present but it does not “link” with the chemical
kinetic process so that no significant thermo-acoustical waves (humming) are produced. Of
course, these results are preliminary and new future evidence, possibly coming from the
adoption of ADCM mechanism, could point toward different conclusions. In Fig.6 a timedependent picture (one out of a sequence of about 500) of the temperature distribution in
the overall combustion system (real scale, real pressure) is given.
5
FIG. 6 : Preliminary reactive calculation (time-dependent temperature distribution)
To be noticed as a potentially dangerous situation, the tendency of the flame front to
protrude back into the premixer duct, with risk of inducing both unsustainable wall thermal
stresses as well as metal surface damage. Flame flash-back risk appears also present.
6. Next six months activities
According to work-plan, after the above positive cross comparisons in unreactive mode,
during the next six months the theoretical/numerical investigations will proceed with a
progressive emphasis into the fully time-dependent reactive calculations within the real
scale Avio-Group prototype.
6