Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Research on Loss Mechanism of Water Jet Propulsion Inlet Duct
Guorui Ji 1,2, Luyi Wang1,2, Xiaohui Yin 1,2 , Jianguo Liu 1,2
1. Science and Technology on Water Jet Propulsion Laboratory, Shanghai, China;
2. Marine Design & Research Institute of China, Shanghai, China

section of the water jet propulsion the inlet duct is mainly used totransport sufficient, uniform, high efficiency and low cavitation water
to the water jet propulsion pump (Jin, PZ, 1997). It is an important
flow-through component of the water jet propulsion. The loss of theinlet duct will directly affect the hydraulic performance of the water jet
propulsion unit. Therefore, enough attention should be paid to the inlet
duct in the design.

ABSTRACT
Inlet duct is an important flow passage component of water jet
propulsion unit, the hydrodynamic performance of which has great
effect on propulsion performance of propulsion pump. Therefore,
special attention should be paid to the design of inlet duct. Generally,
efficiency is the key index to measure its hydrodynamic performance.
Wang (2012) proposed three different kinds of inlet duct efficiency
formulas to assess the hydrodynamic performance of the inlet duct,
according to different energy definitions from the 21st，23rd and 24th
ITTC. The research result indicates that the formula including kinetic、
pressure and potential energies is most reasonable. However, whenevaluating the inlet duct hydrodynamic performance, there are someshortcomings in efficiency, as including or excluding reference
pressure and position of potential energy reference surface will affectinlet duct efficiency. Chang (2010) defined hydraulic loss to measure
inlet duct loss, but it was not dimensionless, which was not conduciveto the comparison of different inlet duct hydrodynamic performance.
In this paper, the dimensionless loss coefficient is adapted to replace
efficiency. And the loss coefficient is not affected by the reference
pressure and potential energy reference surface at all. According to thecauses of inlet duct loss, it is divided into friction loss and local loss.
Correspondingly, the friction loss coefficient and local loss coefficientare defined. And also the calculation method of loss coefficient isproposed in this paper. So the variation laws of these two kinds of loss
with inlet velocity ratio (IVR) are obtained. Besides, the effects of shaft
and grille on the hydrodynamic performance of inlet duct will also be
studied, which can provide reference for inlet duct design.

ANALYSIS OF LOSS MECHANISM
According to the causes of inlet passage loss, the loss can be divided
into friction loss and local loss.

Friction Loss
When water flows through the wall, there will be friction between
water molecules and the wall as well as between water molecules. Theloss caused by friction is called friction loss. The friction loss is related
to the wall area and flow velocity.

Local Loss
Local loss is due to the change of inlet duct shape or water flow
direction, as well as the blocking loss caused by the grille, shaft andshaft protection tube set inside the inlet duct.

Mathematical Expression of Loss

KEY WORDS: Water Jet Propulsion; Inlet Duct; Efficiency; Loss
Mechanism.

INTRODUCTION
When the water jet propulsion ship is voyaging, the water flows
through the bottom of the ship through the inlet duct into the impeller
house, and spews out from the nozzle through the impeller acceleration.
The thrust generated by the momentum difference between the nozzle
and the inlet drives the water jet propulsion ship forward. As the initialFig.1 Control volume representing the hydrodynamic model of the3911