propeller cavitation
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Ship Hydrodynamics2
Propeller Cavitation
Ship Hydrodynamics-1
Alexandria University
2/28/2010 2
Recall EGL and HGL for a propeller
HGL
124 3
HGL
EGL
EGL
g
V
2
2
1
4p
1p
01 z04 z
1V2V4V
Ship Hydrodynamics-1
Alexandria University
2/28/2010 3
Let us look at the pressure just beind the propellerWhen propeller is in action, water is acceleratedV2 increases and P2 decreases
2V
4V
02 z
124 3
HGL
EGL
g
V
2
2
1
1p
01 z
1V
g
V
2
2
2
2p
Ship Hydrodynamics-1
Alexandria University
2/28/2010 4
Pending on V2 we have three possibilities for P2
P2/γ above the system centerline (above atmosphere +ve gage pressure)P2/ γ drops to system centerline (atmosphere zero gage pressure)P2/ γ falls below the system centerline (sub atmosphere -ve gage pressure)
Ship Hydrodynamics-1
Alexandria University
2/28/2010 5
2V
4V
02 z
124 3
HGL
EGL
g
V
2
2
1
1p
01 z
1V
g
V
2
2
2
2p
Ship Hydrodynamics-1
Alexandria University
2/28/2010 6
What is the limit for water to stay in a liquid state?The limit is that when the pressure is equal to the pressure at which vapor starts to form Or when the pressure is equal to the water vapor pressure
PV=1.7kN/m2 = 0.174 m H20
PA=98kN/m2 = 10 m H20
+ve(gage)
Zero Gage-ve(gage)
+ve
Zero Absolute
P/ (m)
0.17
0.0
8.09.010.0
In absoulte scale:P > Pvapour water is liquid
When P= Pvapour vapor formsi.e. bubble formsi.e. water boils i.e. flow cavities form
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Alexandria University
2/28/2010 7
Types of cavitation
Sheet Bubble Cloud Tip vortex Hub vortex
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2/28/2010 8
consequences of cavitation occurrence
Bubble forms, then collapse close to the blade boundaries causingVibrationNoisePittingMaterial damageLoss of thrust,Lower propulsion efficiency
Conclusion:Cavitation should be avoided by proper design of propeller and proper operation
Ship Hydrodynamics-1
Alexandria University
2/28/2010 9
Effect on thrust coefficient
KT
Non Cavitating Propeller
J
Cavitating Propeller
Ship Hydrodynamics-1
Alexandria University
2/28/2010 10
Effect on propeller efficiency
h0
Non Cavitating Propeller
J
Cavitating Propeller
Ship Hydrodynamics-1
Alexandria University
2/28/2010 11
Propeller Cavitation ChartsAt any blade section position r,
margin against cavitation is:
define s as cavitation number
)()( rhPrP A
vA PrhP )(
( )pressureDynamic
cavitationagainstinMr
arg
( )( )
25.0 R
vA
V
PrhPr
At 0.7R blade section
( )( )
RR
vA
V
PrhPR
7.0
25.07.0
VA
RPMR
PA
Thrust
h
r
r
Ship Hydrodynamics-1
Alexandria University
2/28/2010 12
Burrill cavitation diagram
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Alexandria University
2/28/2010 13
Required Blade AreaA useful formula for obtaining a first indication as to the required expanded blade area ratio was derived by Keller (1966),
whereT thrustZ number of propeller bladesPA atmosheric pressure PV vapour pressureh propeller centerline immersionk constant varying from 0 (for transom stern naval vessels) to 0.2
(for high powered single screw vessels)
( )( )
kDphp
TZEAR
vA
2
3.03.1
Ship Hydrodynamics-1
Alexandria University
2/28/2010 14
Example: The propeller in the previous example was designed using B4.40 propeller series chart. It is required to Check whether this propeller will or will not cavitate. Use :1- Keller criterion.2- Burril cavitation chart
Vs=21 knots
RT=888.64 kN
7.5 m
Ship Hydrodynamics-1
Alexandria University
2/28/2010 15
The results obtained are:
Estimated QPC 0.75
Wake fraction, w 0.20
Thrust deduction fraction, t 0.20
Relative rotative efficiency, hR 1.00
RPM 102
Expanded Blade Area Ratio 0.4
Value of d 95
19.56 feet
(5.96 m)
P/D 1.26
Open water Efficiency ho 0.77
feetn
VD A
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Alexandria University
16
Solution
The required thrust is
The Keller area criterion for a single screw vessel gives:
Hence, the used blade area is not enough to avoid cavitation
2/28/2010
( ) ( ) smwVVA /64.82.018.101
( ) ( ) ( ) ( )kN
Vt
P
Vt
RV
t
RT E 9.1044
8.1015.01
9592
111
( )( )
2.03.03.1
2
Dphp
TZEAR
vA
( )( )
633.02.096.51000*72.15.7*81.9*10251000*98
1000*9.10444*3.03.12
EAR
Ship Hydrodynamics-1
Alexandria University
2/28/2010 17
If Burril cavtitation limit is given by:
( )( )
( )
( )22
7.0
2
7.0
7.0
2
7.02
7.0
7.0
5.0
7.0
ln03892.02533.05.0
2
RVV
V
PRhP
V
AT
AR
R
vatmR
R
R
b
( )
22
2
22
7.0
22
7.0
/47.56496.2*60
102**2*7.064.8
7.02
smV
RVV
R
AR
Ship Hydrodynamics-1
Alexandria University
2/28/2010 18
( )
( )
1446.0
47.564*1025*5.0
1000*72.198.2*7.05.781.9*10251000*98
5.0
7.0
7.0
7.0
2
7.0
7.0
R
R
R
vatmR
V
PRhP
( )( )
( )( )
( ) ( )
( ) ( )
placetakewillCavitationHence
V
AT
V
AT
V
AT
V
AT
V
AT
itR
b
actualR
b
actualR
b
itR
b
R
R
b
lim
2
7.0
2
7.0
2
2
7.0
2
lim
2
7.0
2
7.02
7.0
5.05.0
32.047.564*1025*5.0
98.2*4.0/1000*1045
5.0
0317.01446.0ln03892.02533.05.0
ln03892.02533.05.0
0.144
0.32
0.032
( )
R
R
vAR
V
PRhP
7.0
27.0
2
1
7.0
2
2
1R
P
V
AT
Cِavitati
on ِ
Limit
Nِo Cavitation
Ship Hydrodynamics-1
Alexandria University
2/28/2010 19
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