Designing a Marine Waterjet with 3D Inverse Design and CFD

The main goal in the design of the waterjet was to achieve a compact design and meet the stringent cavitation performance and propulsive efficiency requirements of the EFV. The compact design was important both because of space limitations on the EFV and the positive effects of having less entrained water in the waterjet system. Past attempts have been made to design compact pump units for waterjet applications, but many of these fail because of the deterioration of the suction performance with a more compact pump.

The suction performance is especially important for the EFV, which needs maximum waterjet power to be applied at low craft speed so that it can get on plane. This is typical of planning hull arrangements. In addition, the new pump could not exceed the present waterjet diameter, even though it needed to absorb 12-percent more horsepower and match the existing gearbox speed requirements.

In order to achieve these challenging objectives for the EFV waterjet, the 3D Inverse Design code, TURBOdesign1, was used to generate blade design geometries that met the pump specifications. Both rotor and stator blades were devised. Suitable blade geometries from TURBOdesign1 were then evaluated with a 3D Navier-Stokes CFD code to determine their performance and cavitation limits. This step indicated any needed refinements to meet the waterjet requirements. These tools were first directed at the evaluation of three options: a single rotating blade row plus a stator; a rotating blade set consisting of main blades and splitter blades, plus a stator; and two co-rotating blade rows (an inducer and a kicker) plus a stator. In the second step of the design process, the single rotor/stator concept was optimised to maximize the efficiency while matching a given design point. The resulting design is predicted to have much improved cavitation performance compared with a design accomplished with conventional design methods.

Rotors for waterjets have previously been designed with the assumption of free-vortex radial blade loading. This loading distribution simplifies the design process, because it means equal energy input to the flow at each radius and a uniform axial velocity in the exit flow of a blade row. However, it results in rather long axial blade lengths for the hub sections. In order to reduce weight and volume, and increase hydraulic efficiency and therefore thrust, it was desirable that a non-uniform radial blade loading be used for the new design. By using TURBOdesign1 it was possible to easily specify non-free vortex work distributions, as the code can directly compute a blade geometry, which would satisfy the specified mass flow rates and work distributions.

TURBOdesign1 allows the designer to specify both the spanwise rVθ (Or work) distribution and the blade loading distribution. By varying both the spanwise rVθ and blade loading distribution (which directly controls the blade surface pressure distribution) it was possible to control secondary flows and also minimize regions of low static pressure on the blade surfaces.

These features of TURBOdesign1 allow the designer to develop high efficiency designs with high suction specific speed performance. For each type of rotor considered, a computational grid was prepared such as the one shown below.

The overall pump dimensions were based on a 1D design sizing code but TURBOdesign1 was then used to modify the meridional geometry interactively. The designs were first analysed in TURBOdesign1 to check the surface static pressures and diffusion factors. Once the design satisfied the basic criteria, the design geometry was analysed using a 3D CFD code to further establish hydrodynamic performance. Lastly, maximum and minimum principal stresses on the pressure and suction sides of the blades were compared with the allowable material values by 3D FEA analysis, which also checked that blade deflections were not a problem.

In an effort to reduce the rotor weight, decrease the manufacturing cost, and improve efficiency, the Type 1 rotor was chosen for optimization, and the number of blades was reduced from seven to five. Iterations between TURBOdesign1 and CFD were done with these goals in mind:

• Match the design point
• Increase the efficiency
• Maintain a positive margin above vapor pressure throughout the blades
• Keep the blade stresses low

A final run with the rotor blade alone was done using the complete inlet velocity profile, which had been obtained previously for a specific waterjet inlet. In this case a 360° model of the rotor blade was set up and run in CFD. The final rotor design was further analysed for cavitation performance results. The predictions were compared with those for the prior waterjet unit designed with conventional techniques. The new blade design generated with TURBOdesign1 should be much more resistant to cavitation.

Additional Analyses

CFD was also used in a parametric study on clearance, incidence angle, and surface finish for the rotor. An off-design analysis indicated that the pump efficiency would drop only about 1 point over the useful speed range for this marine vehicle application.

Conclusions

This project demonstrated how a combination of TURBOdesign1 and a flexible 3D CFD analysis tool could be used to optimise a waterjet pump design. This rotor/stator/nozzle is compact, lightweight, and easy to manufacture. The predicted cavitation performance is much better than that of the prior design, even though the new design has increased power for the same impeller diameter. Cavitation predictions from CFD for an older and a new rotor design.