PhD defence by Uffe Sjølund Freiberg on Cyclical Varying Pitch Propeller
23.10.2020 kl. 13.00 - 16.00
Uffe Sjølund Freiberg, Department of Energy Technology, will defend the thesis "Cyclical Varying Pitch Propeller"
Cyclical Varying Pitch Propeller
Uffe Sjølund Freiberg
Professor Torben Ole Andersen
Jens Ring Nielsen, MAN Energy Solutions
Professor Henrik C. Pedersen
Associate Professor Thomas Condra, Dept. of Energy Technology, Aalborg University (Chairman)
Associate Professor Poul Andersen, Technical University of Denmark
Dirk Büche, MAN Energy Solutions
The cyclical varying pitch (CVP) propeller is a propeller which pitches the propeller blades cyclically. This is to compensate for the non-uniform wake field that the propeller operates in behind the ship hull. Conventional propellers on the market today are either a fixed pitch (FP) propeller or a controllable pitch (CP) propeller. Common for both of these propellers is that the blades pitch cannot be adapted in a cyclical manner. The CVP propeller is not available on the market today, but it has the potential to improve the performance of the propeller with respect to efficiency, shaft vibrations, cavitation, pressure pulses and noise.
Through a state-of-art review four challenges, in realising the CVP propeller, are identified. These challenges are: how to design the optimum propeller blades for the CVP propeller, how to determine the optimum pitch trajectory for the CVP propeller blades, how to design the cyclical pitch mechanism and how to ensure the reliability of the CVP propeller. These different challenges are, to some degree, coupled with each other. For example the design of the propeller blades is a trade-off between the propeller efficiency and the cavitation extent. This trade-off depends on the cyclical pitch trajectory and the power consumption for the cyclical pitch mechanism. In this dissertation some of the problems associated with the identified challenges
One of the problems addressed in this dissertation is how to determine the required power and torque to pitch the propeller blades according to a desired pitch trajectory. This is in order to be able to account for the power consumption of the cyclical pitch mechanism in the design of the CVP propeller and make requirements for the cyclical pitch mechanism. This is addressed by making a model which is able to determine the forces and torques acting on the propeller blades during the cyclical pitching. The model established is applied to a CVP propeller case previously considered in another study. Two different pitch trajectories are investigated and compared to the propeller blades being fixed. Using the model no gain in the propulsion efficiency is obtained because the blade geometry is the same for each of the pitch trajectories. Furthermore, it was found that the pitch trajectory, which should minimise the variation in the blade thrust, increases the variations instead.
Another problem addressed in this dissertation is how to determine the optimum pitch trajectory for the CVP propeller. The optimum pitch trajectory investigated in this dissertation is the pitch trajectory which minimises the variation in the hydrodynamical forces and torques acting on the propeller blades when the propeller geometry and operating conditions are known. Ideally, the efficiency, cavitation, etc. should also be included when determining the optimum pitch trajectory but they are not, because of the limitations in the model established. Using the model established in an iterative manner to determine the optimum pitch trajectory, is computationally expensive and therefore four alternative models are proposed. These models require fewer computational resources to evaluate and are therefore more suitable for determining the optimum pitch trajectory. Each of the models has their own pros and cons, but the most appropriate models are used to determine a series of optimum pitch trajectories. The validation of these optimum pitch trajectories is to be made in the future.
Experimental open-water and self-propulsion tests are made with a CVP propeller and a CP propeller to compare the relative difference between the two propellers. The experiments are made in model scale to reduce the financial costs. The test setup used for the experiments was designed and fabricated for this project. There were two purposes for making the experiments. One was to validate the performance improvement of the CVP propeller experimentally. The results showed an improvement in the efficiency, but the improvement was generally not large enough to be outside the uncertainty bounds of the experiments. The other purpose of the experiments was to get experimental data which could be used to validate the model established for the CVP propeller. The reliability of the experimental results was not good enough to be able to validate the different model components for the CVP propeller. This has to be addressed in the future.
The defence will be in english - all are welcome
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