Water jet propulsion is a kind of thruster which uses the reaction force generated by the water jet to drive the ship forward. By the pump body, runner, water inlet and nozzle and other components, and can change the direction of water jet through the nozzle to achieve the ship's manoeuvrability, good manoeuvrability, especially for shallow waterways, water jet thruster has a good adaptability.
The basic principle of water jet propulsion: through the ship and other navigational vehicles to its movement in the opposite direction of the jet with a certain speed of water, according to the force and reaction, the ship will be subjected to the reaction force of the water, and this force is the thrust.
Water jet propulsion technology originated from the water pump: people will be placed on the pump on board, when the pump works to the stern of the water jet, you can achieve a simple ship propulsion process. And then will achieve this kind of propulsion pump with water inlet and outlet mechanism and control mechanism, is the most primitive water jet propulsion device. Therefore, the efficiency of the early water jet propeller is not as good as the propeller.
The main factors affecting the efficiency of the water jet propulsion: the efficiency of the propeller is the product of mechanical, volumetric and hydraulic three kinds of efficiency. Axial water jet propeller has no volume loss, the impact of mechanical efficiency is mainly related to the design and manufacturing quality, hydraulic losses, including hydraulic friction and local resistance losses, due to the water jet propulsion compared to the water pump to increase the inlet runners and outlet nozzles, hydraulic losses are the core of the impact of water jet propulsion efficiency.
Types of hydraulic loss:
1, along the loss, including friction loss in the entire flow channel.
2, the attached surface layer (boundary layer) separation, the mainstream flow velocity changes caused by pressure changes along the flow direction, the formation of vortices in the separation zone, hydraulic loss is very large;
3, the flow in the bend and the secondary flow, the bending of the flow, not only to cause its own part of the hydraulic loss, but also a sharp change in the flow velocity distribution, destroying the downstream of a very long period of Straight pipe section of the flow, so that the loss increases, sharp bends in the flow is easy to form a secondary flow, secondary flow and the main stream superposition to form a complex spiral movement, hydraulic loss is greater.
4, the loss of the pump body impeller, the vortex formed by the inlet of the winding vane and the vortex caused by the vortex in the flow channel between the impeller blades.
The key to reduce hydraulic losses is the flow channel and impeller hydrodynamic design.