Simulation of Cryogenics Cavitation

Sean Kelly and Corin Segal

University of Florida

OrlandoJanuary 2010

• Experimental Setup

• Theoretical Description

• Results

– High and low cavitation numbers

• Discussion

– Comparisons to water

– Comparison to CFD

Overview

Motivation

• Cavitation is known to cause large volume changes and high pressures that can damage equipment

• Large performance losses in rocket engine turbopumps

Test Facility

• 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

• NACA 0015 hydrofoil

–Chord Length: 50.8 mm

– Span: 100 mm

– Angle of Attack: ± 10 °

–Nine Pressure Transducers

oSeven on suction side

o Two on pressure side

•High speed camera

–500 images per second

–10μs exposure time

Working Fluid

• Primarily a function of density ratio• Water shows a low T* even at high

temperatures

• 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

Fluoroketone

The Cavitation Process

•Turbopumps• High liquid-vapor

density ratio

• 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

Cavitation Number

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

• Strouhal Number

f = frequency of oscillation of formation and collapse of bubbles

• Reynolds Number • 780,000 - 2,110,000• 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

Additional Parameters of Interest

Cavitation Modes

• Mode I: Incipient Cavitation• σ = σi

• Mode II: Cloud Cavitation• σ < σi

• Mode III: Supercavitation• σ << σi

Images from Arndt, 1981

Test Conditions

• Correlation of Strouhal Number versus σ/2α in previous studies

• Indication of the values at which different cavitation modes occur

I

II

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

0 AOAU∞ = 5.9 m/s

0 AOAU∞ = 5.9 m/s, T = 50 C

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

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

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

Low Cavitation Number

• Cp along chord length• σ = 0.7 and 0 both supercavitation

– Nose of profile has been engulfed in the cavityo Cp at x/c = o nearly zero

– Oscillation of cloud front farther down chord cause average Cp drop below Pv

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

σ =1.00, U∞ = 5.9 m/s, T=40 °C, α =7.5

Cavitation Video

Top View

• Cloud cavitation• Bubbles leaving cloud and collapsing downstream• Cloud surges from line of peak –Cp to ~15% of chord

1

6

2 3

4 5

Still image series

• Series of images at σ = 1, T=40 °C • Δt = 0.002 s

• Frames 1-5 show 1 cycle of bubble growth and separation from the main cloud, over 0.01 s

Courtesy University of Michigan Aerospace Engineering CFD Group

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°

Comparisons to tests in water

Water at σ =1.25, T = 25 ° C

Fluoroketone at σ =2.11, T = 25 °C

Top View Top View

• 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

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

Acknowledgements

• NASA Constellation project• Combustion and Propulsion Laboratory

at University of Florida