Feature 6 | PROPULSION
Study tests potential of air-augmented waterjet
Speaking at the second International Symposium on Marine Propulsors, which took place in Hamburg, Germany in June 2011, Scott Gowing from the Naval Surface Warfare Center, Carderock Division (NSWCCD) described an experimental study of an air-augmented waterjet propulsor.
Air can be added to the flow in a waterjet in order to increase thrust.
A 34
s Mr Gowing explained in his paper, compressed air can be added to the flow in a waterjet
pump to increase the thrust. The gas bubbles expand as the pump mixture passes through the pressure gradient of the convergent discharge nozzle, imparting energy into the flow. In this experiment, air was injected
into a pump fitted to a model boat and static thrust was measured for a range of void fractions using two nozzles. Air was injected between the rotor and stator and downstream of the stator stage. Measurements of the pump and air flow
rates, thrust, pressures, and torque showed how the injected air affected thrust, pump head rise and power.
Te results showed that thrust can be
increased by 12%, and the pump operates at a lower flow coefficient and higher headrise. Te energy balance shows that thrust can be increased with less shaft power than required for the same thrust increase using higher pump speed, but the energy required for the air injection offsets the savings in shaſt power. Underwater
jet propulsion using
compressed gas is a concept that has been tested or modelled in various forms for over the last 50 years. Te basic principle involves gas bubbles expanding and exerting pressure-volume work on the surrounding water as the bubbles travel through the pressure gradient in a nozzle. Te simplest version of this propulsor is a
ramjet in which air is mixed into the high stagnation pressure behind a diverging inlet, and the compressed gas accelerates the mixture out the contracting nozzle in the back. Such propulsors are simple and lightweight, having no moving parts, and are not limited by cavitation. Te extensive work of Gany (2008) and Mor & Gany (2007) have focused on this concept and produced a working prototype.
Gas-augmented waterjet An extension of this concept is the gas-augmented waterjet, in which gas is injected downstream of a waterjet pump and upstream of a flow-accelerating nozzle. Tis waterjet variation, relative to the simple ramjet, should have a higher power density
Warship Technology October 2011
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