La propulsion biphasique à dilution étagée EDE

Click to increase image sizeClick to decrease image size

Injection of high-velocity gas flow at a small incidence angle into water flowing at relatively low velocity has been found to result in an efficient energy-transfer process. The EDE stagewise emulsion-dilution process has been developed as an application of this principle to marine jet-propulsion craft. Principles and characteristics of this type of propulsion unit form the subject of the first part of this report. Primary energy is supplied by a gas under pressure. Original features of the process are 1) creation of a pressure cycle (water admitted to the system is recompressed by deceleration at the initial stage), 2) gas injection at a small incidence angle, and 3) expansion of the emulsion in a non-converging outlet stage. Relevant experimental work was done on a small-scale model in a hydrodynamic tunnel, with propulsion unit intake velocities up to 30 m/sec. The tests showed the unit to be adaptable for a variety of utilization conditions, and, more precisely, emphasized its potential usefulness for high-speed applications, with thrust in the neighbourhood of 6 tons/m2 and efficiency around the 0.5 mark at V₀ - 25 m/sec. Because of the heterogeneous flow structures at both phase and velocity levels, it is difficult to construct a complete mathematical model and determine adequate scaling relationships for this type of propulsion unit. * In its second part, this paper analyzes the interest of such a propulsion device for high-speed ships and describes briefly some experiments on a ship-model. The ship which has been thought as the most suitable is a large-displacement surface effect ship with sidewalls. This choice is based on the following grounds : 1) the speed has to be very high : if not, this kind of propulsion does not present advantages over the existing systems (screw-propellers or water-jet). 2) if the displacement is a large one, the propulsive power (even for non-conventional ships with a good lift-drag ratio) may be very considerable : about 200 000 hp for a 4000 T - 70 knot SES. In that case, the mechanical transmission of power is troublesome, if not unrealizable with today technology : and a propeller without mechanical transmission is of great interest, 3) a good efficiency occurs when the specific thrust is moderate ; then the frontal area of the propeller has to be large and the added drag resulting of the propeller position affects the performances much more for an hydrofoil craft than for a SES. The ship model was that of an 4000 T SES (length/breadth ratio of the air cushion = 5) at the scale 1/55 th . A 4000 T SES requires, at the speed of 70 knots, a thrust of about 200 T ; with a specific thrust of 5.6 T/sq meter the frontal area of the propeller is then - 40 sqmeter. The best way to obtain such an area seems to use the entire breadth between the sidewalls. In that aim : 1) - the aft closing (by flexible skirts) has been suppressed 2) - the hullbottom, near the stern, has been modified : it has been brought narrower of the free-surface in the aft part of the air cushion (in order to minimize the leakage) and reaches the upper stern with a moderate upward slope. An orientable wing, whose span is the breadth between the sidewalls, forms (with the sidewalls and the hullbottom) a duct ; air-injection is operated in the middle part of this duct, using injectors located on the hullbottom and the back of the wing. Such a propeller acts then in two different ways : it gives the required thrust by the mixing of air with the waterflow over the wing ; - it forms the aft closing of the cushion. The aim of the experiments on this model was not to find the optimum configuration for the propeller, but rather to test the possibility of auto-propulsion. The auto-propulsion was obtained in a speed-range up to 4.5-5 m/s (corresponding to speed ≤ 70 knots). The propulsive efficiency was, in the experiment conditions, of about 50 % : a value very similar to that of a non-integrated propeller of the same kind.