1. Simulation of Cryogenics Cavitation
Sean Kelly and Corin Segal
University of Florida
Orlando
January 2010

2. Overview Experimental Setup Theoretical Description Results
High and low cavitation numbers
Discussion
Comparisons to water
Comparison to CFD

3. Motivation
Cavitation is known to cause large volume changes and high pressures that can damage equipment
Large performance losses in rocket engine turbopumps

4. Test Facility NACA 0015 hydrofoil High speed camera
Chord Length: 50.8 mm
Span: 100 mm
Angle of Attack: ± 10 °
Nine Pressure Transducers
Seven on suction side
Two on pressure side
High speed camera
500 images per second
10μs exposure time
Closed loop watertunnel
Filled with a degased working fluid
Operation at 5-10m/s
and 25-70° C
Pmax = 5 atm
Optical and laser sheet access

5. Working Fluid Fluoroketone
Exhibits a large ΔT* at reasonable temperatures
When heated ΔT* comparable to cryogenics
High density ratio
Liquid at STP
Boiling Point 48 °C at 1 atm
Primarily a function of density ratio
Water shows a low T* even at high temperatures

6. The Cavitation Process
Turbopumps
High liquid-vapor density ratio

7. Critical Cavitation Number σi
Ratio of phase change pressure drop to kinetic energy per unit volume
Describes the potential of the fluid to cavitate
Lower cavitation number indicates more intense cavitation
Critical Cavitation Number σi
Indicates cavitation is present on the hydrofoil
–Cp is equal to vapor pressure
Bubbles begin to form on the line of peak –Cp

8. Additional Parameters of Interest
Strouhal Number
f = frequency of oscillation of formation and collapse of bubbles
Reynolds Number
780, ,110,000
Flow is fully developed, turbulent
σ/2α
Used to normalize cavitation number against α
Mode I transition:
Water: 8.5
Fluoroketone: 17
Mode II transition:
Water: 7.7
Fluoroketone: 14

9. Mode I: Incipient Cavitation
Cavitation Modes
Mode I: Incipient Cavitation
σ = σi
Mode II: Cloud Cavitation
σ < σi
Mode III: Supercavitation
σ << σi
Images from Arndt, 1981

10. Test Conditions II I
Correlation of Strouhal Number versus σ/2α in previous studies
Indication of the values at which different cavitation modes occur

11. Experimental Conditions
Angle of attack 0, 2, 5, 7.5 °
Tests showing effect of independent temperature and velocity changes
Tests at the same cavitation number

12. 0 AOA
U∞ = 5.9 m/s

13. 0 AOA
U∞ = 5.9 m/s, T = 50 C

14. High Cavitation Number
σ =2.11, U∞ = 5.9 m/s T = 25°C, α = 2°
Top View
Side View
Incipient (Mode I) cavitation at a small angle of attack
Bubbles form at line of peak Cp and propagate downstream
No cavity is formed
Streaks of bubbles are thin and close to body

15. High Cavitation Number
Cp along the chord length
σ = 1.5 and 2.8 are incipient cavitation
Upward trend indicates no cavity
σ = 1 is cloud cavitation
Note nose of hydrofoil sees lower pressure due to vapor cavity
Note the change in curve between σ = 1 and 1.5

16. Low Cavitation Number σ = 0.7, U∞ = 7.16 m/s T = 40°C, α = 7.5°
Top View
Side View
Blue vertical lines indicate leading and trailing egdes
Supercavitation shows a cavity over the entire span and chord
Separation of bubble from main cavity
3-D and turbulent nature
Some very fine structures and bubbles

17. Low Cavitation Number Cp along chord length
σ = 0.7 and 0 both supercavitation
Nose of profile has been engulfed in the cavity
Cp at x/c = o nearly zero
Oscillation of cloud front farther down chord cause average Cp drop below Pv

18. Comparison of all Cavitation Numbers
Comparison of Cp for all modes of cavitation
Cloud cavitation tends to stay above –Cp = 1
Cavity is compressed by freestream and collapses before covering entire hydrofoil
Higher cavitation numbers can have –Cp < 1
Cp at nose can indicate cavitation mode

19. Cavitation Video σ =1.00, U∞ = 5.9 m/s, T=40 °C, α =7.5
Top View
Cloud cavitation
Bubbles leaving cloud and collapsing downstream
Cloud surges from line of peak –Cp to ~15% of chord

20. Still image series 1 2 3 4 5 6
Frames 1-5 show 1 cycle of bubble growth and separation from the main cloud, over 0.01 s
Series of images at σ = 1, T=40 °C
Δt = s

21. Comparison to Computation
CFD code showing pressure
Shows similar behavior as experimental data
Bubble growth, separation, and collapse
Re-entrant jet
Cavity covers entire hydrofoil and is periodic
Second cloud forms at trailing edge
Even in supercavitation, parts of the hydrofoil surface see pressures higher than Pv
Including the two-phase cavity and more accurate prediction of thermal effect are necessary
σ=0.33, U∞ =10.43 m/s, T= 40 °C, α= 7.5°
Courtesy University of Michigan Aerospace Engineering CFD Group

22. Comparisons to tests in water
Top View
Top View
Fluoroketone at σ =2.11, T = 25 °C
Water at σ =1.25, T = 25 ° C
Comparison of cavitation in water to that in fluoroketone
Notice incipient cavitation at much higher σ in fluoroketone
Bubble size in water is much larger
Less noticeable streaks in water
Cavity in cloud and supercavitation in water has discreet phase boundaries, in fluoroketone it is a 2-phase, frothy mixture

23. Conclusions Different modes of cavitation and how they are identified
Parameters used to describe cavitating flows and how we relate values to transition between modes
Justification for use of a thermosensitive fluid
Reasons to study cavitation and the negative effects it can have on equipment
Observed what incipient cavitation looks like and how it transitions to cloud cavitation
Behavior of pressure on Cp curves of all modes
How bubbles formed and separated from cavity them moved downstream to collapse
Experimental and computational results align and further motivation to study and predict cavitation
Main differences between tests in water and fluoroketone are bubble size, different critical cavitation numbers

24. NASA Constellation project
Acknowledgements
NASA Constellation project
Combustion and Propulsion Laboratory at University of Florida