Cavitation erosion experiments in blocked flow with two ice-class propeller models

Abstract

Ships that operate in ice-infested waters often experience momentary increased propeller cavitation because ice pieces can block the How into the propeller. Model tests have shown that the presence of blockages in the flow field can lead to regions of violent cloud cavitation. For ducted propellers, this additional cavitation is more significant than it is for open propellers: ice pieces may become lodged against and within the duct and subject the propeller to longer periods of increased cavitation due to the blocked How. Associated with this cavitation is the possibility of cavitation erosion. -- The extent and severity of cavitation erosion that a marine propeller may experience cannot presently be determined theoretically. To gain an understanding of how this phenomenon affects a full scale propeller, it is necessary to perform model tests. -- This thesis presents the results of an erosion study that was completed in the cavitation tunnel at the Institute for Marine Dynamics (IMD), a facility of the National Research Council (NRC) of Canada. Paint tests provided the medium through which the erosion was studied. A limited number of tests were attempted with pressure sensitive films to estimate the intensity of the cavitation. Two model propellers, both having a diameter of 200 mm, were used. The first model, an open, fixed pitch propeller, was of the type that is fitted to the Canadian Coast Guard (CCG) R-CIass (1200-Scries) icebreakers. The second model, a variable pitch ducted propeller, was of the type that is fitted to the Canadian Marine Drilling (CANMAR) Lid. vessel MV Robert LeMeur. -- During testing, a simulated ice blockage was installed upstream of the model such that the nominal blade/blockage clearance was approximately I nun. Experiments were completed over a range of advance coefficients for various test conditions, including tests at reduced pressure and at atmospheric pressure. The effects of changes in the proximity of the blockage were also examined. A lest was performed in open flow for each propeller to provide a baseline for the erosion results. For each experiment, the resulting types of cavitation and the erosion patterns were recorded. V7/.V video equipment and }5 mm still photography were used lo document both the experiments and the results. In subsequent analysis, the areas of erosion were estimated using an image analysis program and comparisons between each test were made. -- Generally, given the same test conditions, the dueled propeller experienced more erosion than the open propeller. Cavitation persisted longer on the ducted propeller due to the influence of the nozzle. -- For both propellers, the amount of face erosion increased with increasing advance coefficient. Back erosion was minimally affected by changes in the advance coefficient. -- Concerning proximity effects, erosion increased, for both propellers, as the blade/block clearance was increased, until it reached a peak. This peak occurred at a gap of 5 mm. Further increases in clearance resulted in reduced erosion. -- Pressure sensitive film tests were unsuccessful as the films were torn from the blades by the violent cavitation. For future tests under such adverse conditions, a more robust fastening system is required