Discussion of the Fluid Acceleration Quality of a Ducted Propulsion System on the Propulsive Performance

Abstract

This paper focuses on the ducted propulsion with the accelerating nozzle, and discusses the influence of its fluid acceleration quality on its propulsive performances, including the hull efficiency, the relative rotative efficiency, the effective wake, and the thrust deduction factor. An actual ducted propulsion system is used as an example for computational analysis. The computational conditions are divided into four combinations, which are provided with different propeller pitches, cambers, and duct lengths. The method applied in this study is the Computational Fluid Dynamics (CFD) technology, and the contents of the calculation include the hull's viscous resistance, the wave-making resistance, the propeller performance curve, and the self-propulsion simulation in order to obtain the ship's effective wake, thrust deduction factor, hull efficiency, and relative rotative efficiency. The performance curve of the propeller and resistance estimation results are compared with the experimental values for determining the correctness of the self-propulsion simulation. According to the computational analysis, it is known that increasing the propeller pitch cannot effectively increase the hull efficiency. The duct acceleration quality can be reduced by shortening the duct length; hence, when the effective wake fraction and thrust deduction factor decrease, the hull efficiency is increased. In addition, the pressure inside the duct is relatively low if the acceleration quality of the duct is too high, which is unfavorable for controlling the propeller cavitation. Moreover, if the hull bottom in front of the propeller is tapered up from the front to the back at an overly steep angle, the thrust deduction factor will be too large and lead to a relatively low hull efficiency.