Design and Analysis of Contra-Rotating Propeller Blade

Contra rotating propellers are the two propellers rotating in opposite directions with respect to each other.

This type of propulsive system has the hydrodynamic advantage of recovering the slip stream energy, thus maximizing the thrust power and the propulsive efficiency.

The main aim is to analyze the contra-rotating propeller blade.

This involves modeling of blade in CATIA, Meshing in HYPERMESH, Static analysis in ANSYS. Theoretically by applying Bernoulli’s principle and Newton’s second law on the flow of water through the blade, its performance is analysed.

Abstract

The Contra rotating propeller (CRP) is a type of propulsion system which consists of two propellers on the same line of shaft, spaced a short axial distance apart and rotating in opposite directions.

contra rotating propellers can be classified broadly by virtue of its location for installation in a marine vehicle into two of its kinds

Single end CRP – Both propellers placed at single end of marine vehicle.

Dual end CRP-propellers placed at both exterme ends of the vehicle.

Contra-rotating propeller

Focus on Single end CRP Single end CRP is considered where the

rotational slip stream energy of front propeller is utilized by the aft propeller thus increasing thrust force.

Rotational speed- The aft propeller is directly installed on main engine shaft, its RPM and optimum diameter are determined. The front propeller RPM is reduced and its rotating direction is reversed by the gearing system.

Distance between propellers – To effectively utilize the slip stream energy, the propellers are placed at closest distance.

Propeller diameter-The diameter of aft propeller is reduced by a few mm since the slip stream get contracted leaving the front propeller.

Design Characteristics of CRP

The material that is considered for the analysis is forged alluminium AL-24345.

It is high corrosive resistant, light in weight and easy to maintain.

Propeller material

Property Value

Density (gm/cm3) 2710

Young’s modulus (N/mm2) 7.00*104

Poisson’s Ratio 0.33

Before modeling the blade, the data is considered for modeling.

Modeling of CRP blade

Sections r/R LLE (m) LTE (m)Pitch angle (Deg)

82.0 0.4439 0.0537 -0.0392 46.583.1 0.45 0.0541 -0.0393 46.892.4 0.5 0.0571 -0.0408 47.9

101.6 0.55 0.0589 -0.0427 48.1110.8 0.6 0.0593 -0.0452 47.6120.1 0.65 0.0579 -0.049 46.5129.3 0.7 0.0551 -0.0522 45.1138.5 0.75 0.0513 -0.0544 43.4147.8 0.8 0.0463 -0.0562 41.3157.0 0.85 0.0396 -0.0568 39.2166.2 0.9 0.0313 -0.0562 36.5170.8 0.925 0.0267 -0.054 35175.5 0.95 0.02 -0.0496 33.2180.1 0.975 0.0104 -0.0386 31.5184.7 1 -0.0019 -0.0256 29.2

Open CATIA icon. Start – shape-Generative shape design. Open macro file (Excel file) options-enable

this content and then go to view option- Macros-Click.

Select a plane and offset plane to blade radius

Plane type : offset from plane Reference plane : XY plane Offset distance : blade radius

Procedure of modeling

Select that offset plane and then go to sketcher Project all points by using project 3D element Select spline and join all the projected points Join the corner with the line after that draw the

tri tangent circle for radius of the trailing edge Trim the circle and spline to get a shape

Draw a line beside the blade section and give constraint of distance between Leading edge and generator line.

Select all the splines, curves and click on translate and drag the selected lines to the point of intersection taking origin as base.

Select the translated section and rotated it with respect to origin (click rotate and click on origin and give pitch angle)

Select circle center-support (ZX plane)-radius (take circle center as (0,0and 0))

Click on extrude-profile: circle. Direction: Y- axis and then click ok.

Click on develop-Wire to develop: profile Hide all except develop Then disassemble the developed profile in to 4 curves Repeat the process for all the sections Select spline and join all the sections. Select all the lower sections and guide curves after

clicking multi section. And repeat it for upper sections and corners

After that double click on any plane or surface and go to GSD then the four multi sections should be extrapolated.

After that all extrapolated surfaces are to be joined by using join command. Once they joined the unnecessary part is eliminating by using split command.

After the splitting the split surface is filled by using fill option.

Once they are joined, by using close surface command, the blade is to be made solid

Blade is imported to HYPERMESH Environment Select the import sub panel on the files panel. Select the Geom option. Select the appropriate file format to be imported from

the pop-up menu.

Importing of Blade to Mesh

Select the solid map panel on the 3-D page. Select the volume tetra sub-panel. With the surf selector active, select one of the surfaces in the

model. The rest of the connected surfaces are selected automatically. Set 2D: to trias and 3D: to tetras and specify element size. Click mesh to create the hexa mesh.

Meshing

The propeller blade is assumed as radial vane and the water is assumed to flow tangentially at the inlet of blade.

From Bernoulli’s theorem, for an incompressible fluid like water, the total

energy at any point is constant P1/ρg + v1

2/2g + z1 = P2/ρg + v22/2g + z2

P1, v1, z1 - initial conditions of water before striking the propeller

P2, v2, z2 – final conditions of water after striking the propeller

Theoretical calculations

P1/ρg – P2/ρg = v22/2g

Power required to accelerate the water Power (P) = w*Q*H P = w*Q*(P1/ρg – P2/ρg) P = ρ*A*v2

3/2 Since the blade is assumed as radial vane, its

velocity triangle is as shown

To calculate the thrust force, the data consideration is as shown

Single

Propeller

Contra rotating

Propeller

Front Propeller Aft Propeller

Engine Power(hp) 20,000 10,000 10,000

Engine Speed(rpm) 800

550

800

Propeller

diameter (m)

1

1

0.74

PropellePitch (inches)

27 28

20

Angleof

deflection(degrees)

120

120

120

The blade angle is calculated by treating the propeller as screw

Tan α = Pitch / 2πr Considering the angle of deflection as 1200

From the velocity triangles, Thrust force Fx = ρav1 (vw1 – vw2) Thrust power = Fx x V ( where v is the

average velocity of ship i.e., 15 knots)

Propulsive efficiency η p = T.P / P

From the procedure, the obtained results for both single propeller and contra rotating propellers is as shown

Single

Propeller

Thrust

Force (KN)

1174.298

Thrust

Power (KW)

8861.056

Propulsive

Efficiency (%)

59

Contra Rotating

Propellers

Front

Propeller

Aft

PropellerThrust Force

(KN)

617.87 646.944

Thrust Power

(KW)

9544.05

Propulsive

Efficiency

(%)

65

Static analysis is performed to determine deflection and stresses experienced by the CRP blade.

From the theoretical calculations, the force acting on CRP is assumed to be uniformly distributed at all sections.

Meshed blade is imported to ANSYS for analysis as shown

Static Analysis

One end is fixed and loads are applied at every section from the load data shown

SECTIONSIN Z Direction

SECTION LOAD (THRUST) (N)

SECTION LOAD (TORQUE) (N)

FX FY

82.103 - -

83.25 16.58 12.33

92.5 16.58 12.33

101.75 16.58 12.33

111 16.58 12.33

120.25 16.58 12.33

129.5 16.58 12.33

138.75 16.58 12.33

148 16.58 12.33

157.25 16.58 12.33

166.5 16.58 12.33

171.125 16.58 12.33

175.75 16.58 12.33

180.375 16.58 12.33

185 0 0

TOTAL 646.42 480.29

The displacement vector sum and von-mises stress are as shown

Maximum deflection is 0.641mm Maximum stress is 103.264N/mm2

Prototype

More power can be transmitted for a given propeller radius.

The propeller efficiency is increased by recovering energy from the tangential (rotational) flow from the leading propeller.

The mechanical installation of coaxial contra-rotating shafts is complicated, expensive and requires more maintenance.

Suitable for Tankers, Cargo vessels, LNG carriers, Ferries, Cruise vessels and various types of Naval vessels.

Suitable for wind turbine and tidal turbine for ocean energy utilisation.

Advantages, Disadvantages and Applications

The results show the contra rotating propeller found to be as expected superior to the traditional single propeller.

The propulsive efficiency of the contra rotating propeller at the design point of view is about 6% higher than the single propeller design (65% and 59% respectively).

The Ansys results proves that the obtained deflections and stresses induced in the after propeller are within permissible limits.

Conclusion

Single

propeller

Contra Rotating

Propellers Front Propeller After Propeller

Thrust force

(KN)

1174.298 617.87 646.944

Thrust Power

(KW) 8861.056 9544.07

Propulsive

Efficiency

(%)

59 65

USUM U X UY UZ

0.641 0. 456 0. 451 0. 031