1. Advanced Vaporization System for Small Jet Engines
Valery Sherbaum, Alexander Roizman, Vladimir Erenburg, Igor Gaisinsski, Yeshayahou Levy
Turbo and Jet Engine Laboratory, Technion, IIT
A topic of the presented study is Advanced Vaporization System for Small Jet Engines
14th ISRAELI SYMPOSIUM ON JET ENGINES & GAS TURBINES

2. Application of vaporizers
In recent time vaporizers found wide application in small jet engines
due to their simplicity. But they have several drawbacks, the main being
high sensitivity to operation load,
possible overheating of a vaporizer wall,
start-up problems.
In recent time vaporizing fuel injection systems have found a wide application in small jet engines. A main drawback of the vaporizer is that they do not provide full evaporation; from the other hand, during some operation modes the vaporizer wall can be overheated and destroyed. The alternative evaporation system was studied

3. TYPICAL VAPORIZER DESIGN
It can be seen that almost all models have a shape of Hebrew “Het” letter. A fresh air is delivered to the combustion zone through holes in front wall of the liner. The air also cools the front wall.
Fresh air cools the vaporizer
Design of vaporizer for large engine. Inlet flow is splitted and this enables to reduce number of vaporizers

4. TYPICAL VAPORIZER DESIGN (cont.)
Combustor with 6 vaporizers,
small jet engine
Combustor for small jet engine,
Turbo&Jet Laboratory test rig
Several types of the combustors with vaporizers were tested in the Turbo&Jet Laboratory . Here one can see a small combustor with 6 vaporizers with outer diameter of a liner equal to 105mm. The combustor was studied at the Laboratoty
Several types of the combustors with vaporizers were tested in the Turbo&Jet Laboratory

5. THE OBJECTIVES OF THE PRESENTED STUDY:
Qualitative analysis of influence of thermodynamic and aerodynamic parameters on evaporation rate of a single droplet.
CFD study of effect of two-phase flow rotation on evaporation rate and wall temperature in a straight tube.
CFD study of rotation effect on evaporation in the generic vaporizer.

6. THEORETICAL BACKGROUND
Vaporizer operation field
Typical boiling curve of liquid at one atmosphere presents specific heat flux qs as a function of a difference temperatures of wall and boiling liquid , Te = Ts – Tb*
As the vaporizer is located inside the combustion zone, evaporation process occurs to the right from the red line; heat transfer in this field is not studied enough.
*Nucleate boiling, Wikipedia,

7. Single droplet evaporation near the wall*
THEORETICAL BACKGROUND (cont.)
Single droplet evaporation near the wall*
Heat flux to droplet
High temperature wall
The recent study of evaporation process was carried out by Myers and Chaprin. When droplet impinges to a flat surface it rebounds from the surface and this is unsteady-state process. The simplified model considers steady state process of evaporation. It is assumed that droplet located over the heated surface and elevated pressure under the droplet keeps the droplet over the surface.
Accepted geometry of the droplet:
The droplet is above hot surface
*Myers T.G. and Charpin J.P.F., (2009), "A Mathematical model of the Leidenfrost effect on an axisymmetric Droplet", Phys. of Fluids, Vol. 21, , pp –

8. DROPLET – WALL INTERACTION
If a surface is superheated 2 regimes are possible and this depends on droplet energy. Relations in FLUENT are empirical, and taken from internal combustion engines model.
vaporizer wall temperature, Tw>1000K

9. EVAPORATION PROCESS SCHEME (FLUENT)
Scheme of droplet impingement on a superheated surface. Drops impinge on liquid film or surface and splash. Process depends on droplet kinetic energy, its size, and physical properties such as surface tension and viscosity and state of the surface (film or soli surface). Quantitative relationships are very rough and based on few tests only.
Liquid film or vapor layer
heat flux from wall

10. EVAPORATION PROCESS test results
There are very limited results of interaction between superheated wall and droplets during impinging.
The photographs below demonstrate difference between models and actual interaction process*
There are very limited results of interaction between superheated wall and droplets during impinging.
Large droplet evolution during impingement shows difference between accepted models and actual process. From the beginning droplet has spheroid shape and then it is transformed to small droplets with different diameters.
Droplet evolution during impingement at the wall, Twall = 195deg. C, time in ms
*Eitan Kompinsky Single droplet impact and droplet-droplet collision on a solid, non-heated and heated surfaces.
Doctorat thesis, 2015

11. To accelerate the evaporation process we propose to swirl air inside the vaporizer What are the expected effects of flow rotation?
Example
Inner vaporizer diameter D = 10 mm
Droplet tangential velocity Vt= 10m/s
Centrifugal acceleration of a droplet is equal to a=V2/R=20,000m/s2
this is in factor 2000 more than gravity acceleration
Benefits
It is proposed to swirl air inside the vaporizer for enhancement of evaporation rate. Here I ‘d like to give a simple example
Droplets move to the vaporizer inner wall and cool the vaporizer case.
Spray distribution does not depend on the vaporizer orientation in space.
Droplets are pressed against the vaporizer wall and that leads to acceleration of vaporization process.
Drawbacks
Swirler should be added
Pressure drop of a vaporizer increases

12. Typical vaporizer design was chosen for the simulations
CFD SIMULATION
Swirling and non-swirling case
Typical vaporizer design was chosen for the simulations
During CFD simulations swirling and non-swirling case assume to compare. The simulation can give a quantitative comparison as used model in FLUENT is far from the actual process.

13. SIMULATION MODEL-1 (straight tube)
INITIAL CONDITIONS:
mfuel = kg/s
T=298K
mair = 0.004kg/s
T=480K
Vair around = 20 m/s
Tair around=2500K
P=400kPa
A straight tube presents initial part of the vaporizer. A straight tube is surrounded by volume to imitate combustor flow around the actual vaporizer
Geometrical scheme
A tube is surrounded by volume to imitate flow around the actual vaporizer installed in
a combustor
A straight tube presents an initial part of the vaporizer

14. SIMULATION RESULTS OF SMALL JET ENGINE*
were used as initial conditions around the vaporizer
Temperature around the vaporizer was accepted = 2500K
Gas velocities range outside the vaporizer is equal 10-40m/s
2500K
vaporizer
The simulations did not consider processes inside the vaporizer (gaseous fuel was used), but give a reference for conditions outside the vaporizer.
*Simulations were carried out by Alex Dolnik

15. T=480K
T=2500K
Mesh consists of 2,085,587 polyhedral cells; red arrows - fuel inlet, lilac – external heating flow, blue – air inlet, white – outlet,
K-ε turbulence model for air flow and wall-film model for discrete phase were used in a final simulations. Wall-jet discrete model showed close results

16. SIMULATION MODEL (straight tube)
The following options were simulated:
Spray angle α =5 deg.:
Axial air flow
Tangential air flow velocity is equal to axial component
Spray angle α =60 deg.:
Tangential air flow equal to axial component
The following parameters were obtained during simulation process
Evaporation rate
Temperature distribution in the vaporizer wall
Air velocity distribution
Air temperature distribution
Average residence time of droplets

17. Temperature distribution in the wall, inlet region (1-10)mm
Outer part
Wall
Inner part
axial inlet flow
Outer part
Wall
Inner part
axial & swirl inlet flow
Wall temperature higher for axial flow
In case of rotating air, maximum wall
temperature is lower in 40 deg.
It can be seen according to color of the wall

18. Spray propagation inside the vaporizer
axial inlet flow
axial & swirl inlet flow
Droplets spread in a whole volume of the tube and evaporate more Intensive

19. Main results (straight tube)
AIR SWIRLING PROVIDES BETTER EVAPORATION,

20. Full scale vaporizer simulation scheme
Volume outside the vaporizer
(conditions are taken from
the combustor simulations
vaporizer
Vaporizer model with surrounding volume for
imitation conditions outside the vaporizer
Mesh consists of 2,098,512 polyhedral cells; red arrows - fuel inlet, blue – air inlet, light blue – outer flow

21. Full scale vaporizer, simulation results
Wall temperature nearby inlet
Tmax = 1900K
The highest temperature region
Tmax=2200K
Wall temperature nearby outlet
Tmax = 2000K
The results are not shown actual temperature values but give reference about
temperature distribution as the evaporation model has serious assumptions

22. Full scale vaporizer, simulation results (cont.)
Air flow is pressed to the outer side of the tube
Air velocity field
Air flow is pressed to the outer side of the tube

23. Full scale vaporizer, simulation results (cont.)
In a straight region nearby inlet droplets are concentrated at the exit and so the vaporizer’s wall is not cooled. does not. Downstream the droplets concentrated near outer wall of the vaporizer. This decreases evaporation and wall cooling.
Droplets propagation inside the vaporizer

24. Conclusions
Rotation effect on the vaporizer operation is considered. The qualitative analysis and CFD studies of interaction of two-phase rotating flow with superheated wall is carried out. The following results are obtained:
1. Rotation of the two-phase flow increases force which presses droplets to the superheated wall. This force outperforms gravity force by many times, so the vaporizer operation does not depend on its spacial orientation.
2. The qualitative analysis shows that due to rotation the droplet becomes flatter or disintegrated.

25. In both cases heat transfer by conductivity and radiation increases.
3. The CFD simulations of two-phase flow in a straight hot tube and generic vaporizer showed that rotation leads to increasing of evaporation rate and decreasing of maximum temperature of wall.
4. The next stages of the study are improvement of vaporization model using theoretical analysis and tests simulations of a full scale generic vaporizer with different operation modes, experimental study of rotation effect, recommendations for optimal vaporizer design.

26. THANK YOU FOR
YOUR ATTENTION!

27. Swirler option and manufactured models
Optional air swirler design which better joints to the vaporizer.