Simulation of Cryogenics Cavitation

petnamelessUrban and Civil

Nov 15, 2013 (4 years and 11 months ago)

124 views

Simulation of Cryogenics Cavitation

Sean Kelly
*

and Corin Segal


University of Florida, Gainesville, Florida, 32611, USA


Cavitation in cryogenic fluids was simulated on a NACA0015 hydrofoil in a
closed facility

filled with
perfluorinated ketone

2
-
trifluor
omethyl
-
1,1,1,2,4,4,5,5,5
-
nonafluoro
-
3
-
pentanone
1
, hereafter referred to as
fluoroketone
, which ex
hibits a strong

thermodynamic effect
at ambient conditions
.

Static pressures were
measured at seven taps along the chord of
the

hydrofoil and
along the wall o
f the

test se
ction. Frequency
analysis
coupled with high
-
speed photography
at several angles of attack, speeds up to 9.6 m/s and
temperatures up to 50°C

showed

the formation and collapse of vapor bubbles in the regimes from incipient
cavitation

to supercav
itation
. Fl
u
o
rketone exhibits
cavitation characteristics similar to cryogenics.

C
avitation
can be observed
before static pressure drop
s

to saturation pressure.


Nomenclature


α

= angle of attack

C
p

= pressure coefficient

C
p,l

= s
pecific heat of liquid phase

c

= chord length

ΔH
vap


= heat of vaporization

ΔT

= local temperature depression due to thermodynamic effect

ΔT*

= nondimensional thermodynamic effect

Cp

= pressure coefficient

f

= frequency





*

Graduate Research Assistant, Mechanical and Aerospace Engineering, MAE
-
A 231, PO Box 116250,

University
o
f Florida, Gainesville FL 32611, Member AIAA



Asso捩慴c mrof敳eorI
䵥捨ani捡氠lnd A敲osp慣攠bngin敥r楮gI MAb
J
A 2P1I m传Box 11S
2R0I

啮楶敲s楴y of 䙬cr楤愬
䝡楮敳v楬汥lci P2S11I Asso捩慴c 䙥汬cw Af䅁K

k

= thermal conductivity


μ

= dynamic viscosity

v

=
kinematic viscosity

P

= static pressure

P
o

= stagnation pressure

P


= freestream static pressure

Pr

= Prandtl number

[
C
p,l

μ/k
]

Re

= Reynolds
number

[
U

c/v
]

ρ

= density

ρ
l

= density of liquid phase

ρ
v


= density of vapor phase

σ

= cavitation number

Stc

= Strouhal number, based on chord length
[
fc/U

]

U


= freestream velocity


I.

Introduction

A.

Fundamentals of Cavitation

AVITATION
in tur
bopumps is a major concern to performance and longevity
,

si
nce it leads to vibration,
unsteady flow and potentially destructive erosion of the surfaces.
A main parameter describing the process is the
cavitation number,

defined as



2
1
2
v
P P
U



 




Cavitation can be formally described as the formation of vapor bubbles due to
a drop in local static
pressure
2
. It occurs when the

local static pressure equals the saturation vapor pressure, which corresponds to the
cavitat
ion number being equal to

Cp.
Since

the vapor pressure
is
a function of temperature, a t
hermal

depression
C

(1)

occurs
when the fluid vaporizes

at the given

flow conditions
. This
effect is strong
in
fluids sens
itive to this
temperature drop
, such as cryogenics,

and leads to a cavitation number not equal to

Cp.

For water

this temperature change is insignificant which makes it a poor fluid to simulate the cavitation
inside of turbopumps
;

for liquid propellants and other cryogenics that exhibit a strong thermodyna
mic effect, this
temperature change must be considered.


This
effect
can be estimated
based on a scaling suggested by Franc et al
2
:


* 0.7 0.2
Pr Re
T T
    

And

*
,
vap
v
p l
H
T
C



 


At ambient conditions water exhibits a
very low
ΔT
*

approximat
ely

10 times
smaller

than flouroketone
and
two orders of magnitude smaller than
the rocket propellant
liquid hydrogen
at operating conditions
. This low ΔT
*
is
due to the large density ratio ρv/ρl, and is hard to increase due to the inaccessibility of water
’s critical point.

Experiments u
sing cryogenics present

considerable

difficulties
due to

low temperatures, such as
condensation on viewing windows, thermal stresses and the danger of ignitio
n
. Fluids that exhibit a low ΔT
*

show
discreet vapor and liquid ph
ases, whereas cryogenics and other fluids with a high ΔT
*

tend to contain a frothy, two
-
phase mixture.

Due to the compressibility of these mixtures,
their
low speeds of sound, often only a few m/s
,

can
produce effects such as choking and shock waves.
Flour
oketone
a
t

ambient conditions has a

low kinematic viscosity
,
2
.3x10
6

times smaller than

water’s
,

which allows fo
r high
-
Re testing at low speeds.


Fluoroketone alleviates cryogenic testing
difficulties
and

allows us to study cavitation effects in
thermosens
itive fluids
at ambient conditions
. When heated to 70

C, flouroketone has a ΔT
*

identical

to liquid
hydrogen
. Furthermore, w
hen cooled

it

matches water, thus
allowing some
degree

of comparison

between
tests in
water and cryogenics.
Values of ΔT
*
for these fluids are given in Table 1.
Cavitation in flouroketon
e occurs at
higher

cavitation numbers than
in
water,
and the fluid is less reactive and hence less likely to suffer inaccuracies in the
results due to impurities.


(2)

(3)


II.

Experimental Setup






II.

Experimemtal Setup

A.

Facility


A
closed
-
loop
tunnel with a 100

m
m
x

100

mm test section was constructed for these experiments and
filled with
fluoroketone
.

It

is

designed to operate at pressures up to 0.5

MPa
. The tunnel shown in Fig.

1

is driven by
a 25HP pump

controlled by a Danfoss VLT 5022 controller that allows the sp
eed of the pump to be controllably
varied
, capable of delivering 0.11

m
3
/s, which can sustain 10

m/s at ambient conditions. A stagnation chamber

upstream of the test section
eliminates possible

bubbles formed in the pump and

is

fitted with mesh screens of

380,
100
,

and 75 micron pore size that filter and help to straighten the flow.
Downstream of the test section there is a
settling chamber designed to inhibit large
-
scale
low
-
frequency
oscillations in the flow due to mass
-
redistribution
.



The tunnel was e
quipped with Omega PX303 pressure transducers in the stagnation chamber, settling
chamber and along the bottom wall of the test section upstream of the hydrofoil.
The 200 mm diameter pump outlet
is equipped with a Prandtl probe to monitor flow speed. All s
ensor outputs were recorded at 1 kHz during the tests.

The tunnel is fitted with a 7.5

kW
submersion

heater and a
degassing

system
wherein the facility is held under
vacuum at 50

kPa for several hours to remove dissolved oxygen
,

then repressurized with arg
on.


The test section has optical access via glass windows on the top and front side and
laser access

through the
hydrofoil. The hydrofoil can be positioned at an angle of attack of +/
-
10 degrees.

Images were taken at 500
frames
per second by a Cook PCO
12
00S CCD camera with a Navitar 50

mm

f/0.95 lens set to an aperture of
f/
1.2.
Exposure times were 10

μs.

B.

Hydrofoil


Tabl
e 1. K
ey cavitation properties for various fluids
.


This

demonstrating the relatively strong
thermodynamic effect in saturated liquid hydrogen under

low
-
pressure turbopump inlet conditions and fluoroketone
under modest temperature and pressure.



A NACA0015 hydrofoil with chord c=50.8

mm and span of 100

mm was

used in the study. Pressure taps
are located
at x/c= 0.0, 0.0
5
6, 0.11
2
, 0.
168
, 0.21
2
, 0.3
17
,

and 0.88
3

along the suction side and x/c = 0.40 and 0.75
along the pressure side.
To reduce response time, e
ach is fi
tted to a 200

kPa

Omega PX303

absolute pressure
transducer using tubing filled with the working fluid.
The analysis indicated that frequencies in the range f interest,
ie below 500Hz are not damped in these tubes.


III.

Test
conditions

A table of experimental conditions is shown in Table 2.

The tests were designed to isolate the effects of several
experimental parameters. Comparison of tests 1 and 2 show the effect of angle of attack while holding constant the
temperature, vel
ocity and cavitation number. Similarly, comparing results from tests 2 and 3 indicate the effect of
cavitation number while the velocity, temperature and angle of attack are held constant.


Tests 3 and 4 show the
effects of velocity only while 4,5, and 6 c
ompare only changes in temperature.



Figure 1
. Sketch of
tunnel facility

used in experiments
.









References

1.

Gustavsson, J. P. R., Denning, K., Segal, C., “Experimental study of Cryogenic Cavitation Using Fluoroketone”, AIAA
2008
-
0576,
46
th

AIAA Aerospace Sciences Meeting and Exhibit
, Reno, NV, Jan 7
-
10, 20
08

2.

Brennen, C. E.,
Cavitation and Bubbl
e Dynamics
, Oxford University Press, New York, 1994.

3.

Franc, J
-
P, Rebattet, C, Coulon, A, “An experimental investigation of thermal effects in a cavitating
inducer”,
5th International Symposium on Cavitation

(Cav2003),

Osaka, Japan, 2003.


AoA [°]
Temp [°C]
U∞ [m/s]
σ
Press. [psi]
Case #
5
25
6
1.5
12.1
1
7.5
25
6
1.5
12.1
2
7.5
25
6
0.78
9.1
3
7.5
25
7.5
0.78
11
4
7.5
30
7.5
0.78
12.2
5
7.5
40
7.5
0.78
15.6
6


Table 2.

Table showing experimental conditions.

Each test is desi
gned to isolate the effect of
one individual experimental condition, including velocity, temperature, angle of attack, and
cavitation number.