Gas-Turbine Powerplants for Two-Phase Hydropropulsion 



Therefore, as already outlined, two kinds of analysis must be developed: 

 With the first one, the general powerplant has to be considered with a simpli- 

 fied analytical model of ejector to evaluate the most efficient configuration 

 and its absolute performance with svifficient approximation; the second kind of 

 investigation requires a particle-exchange analysis of the pure ejector, from 

 both the theoretical and the experimental points of view, to identify the best 

 propulsor design and mixing technique. 



3 PROPULSOR ANALYSIS 



3.1 Description 



Actual hydrojet (IG) configurations are represented in Fig. 2. Cases (a) 

 and (b) refer to surface vehicles (pump jets) and case (c) refers to an under- 

 water vehicle (ducted propeller). No detailed configurations of propulsive ejec- 

 tors are available in the literature, but appropriate configurations can be 

 easily represented (Figs. 1 and 3). Figure 3 is deduced from the Mar jet [8], 

 where the chamber is obtained between two separated parallel foils, while Fig. 

 1 considers a circular chamber. This propulsor, where the water stream is 

 accelerated just by the gas (which has been sometimes called "water ramjet") 

 will be designated here as hydrojector (IR). No example exists of hybrid sys- 

 tems (that is, a pump associated with an ejector) apart from the Foa propulsor 

 (Fig. 4a), where a gas pseudoblade takes the place of the conventional pump, 

 being followed by a mixing phase. However, new types of hybrid propulsor with 

 mechanical pump can be easily imagined (Fig. 4b, 4c). This kind of propulsor will 

 be designated as pump-jector (IB). The most general fluid-dynamic model of 

 the propulsor is presented in Figs. 5 and 6. In Fig. 5 the propulsor is axially 

 symmetric and rectilinear. This model can be applied to underwater propul- 

 sors. In Fig. 6 an S-propulsor model is shown which can be applied to surface 

 (water-air) propulsion. 



The water stream velocity v^ is slowed down partially outside and partially 

 inside the diverging inlet to the chamber value Vj. Here the water is first com- 

 pressed by the pump at constant velocity, then accelerated by the mixing at 

 quasi -constant pressure to the value Vq at the nozzle inlet, where a two-phase 

 homogeneous compressible flow enters, expanding to the external pressure at 

 the exit. [The cylindrical shape of the chamber is not a statement; the experi- 

 mental analysis must recommend the best shape. Therefore section (o) can be 

 considered just as a reference.] 



In underwater models, this pressure is equal to the free- stream one, while 

 in S-models the exit pressure is the atmospheric one. In underexpanded noz- 

 zles, the pressure depends on the internal flow. 



In the following, a simplified analysis of the propulsor is developed under 

 the hypotheses of one-dimensional, homogeneous, ideal flow. 



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