Gas-Turbine Powerplants for Two-Phase Hydropropulsion 



at T = 1150°K about quadruples k^ and, from Fig. (19), triples or quadruples 

 <y at same speed v^ and same flow rate ratio e. Such comparison will appear 

 clearly in the following part, where a will be calculated. It is also noteworthy 

 that rip decreases while o- increases with k^. 



This behavior, which is well known, will be pointed out later in 

 comparison with the behavior of the overall coefficient, to stress the fact that 

 the analysis of the propulsive efficiency can suggest completely wrong 

 conclusions. r- . . , ;. 



(b) A propulsive comparison among hydrojectors, pumpjectors, and 

 water jets can be made only in terms of actual propulsive feasibility. Since the 

 hydrojector and the pumpjectors can provide any thrust and any propulsive ef- 

 ficiency, two other design parameters must be examined, that is, the inflow 

 ratio 7 = iTia/mH and the cross- section ratio A^/A^. However, the parameter 7 

 depends on the actual powerplant, and it will be discussed in the following para- 

 graphs; here only a discussion on the cross- section ratio can be developed. As 

 has been pointed out, the specific thrust of the water jet is really limited, but 

 there are no problems about the exit cross section, because it will always be 



'^u ''^o "^ 1 (Fig. 14); that is, the chamber cross section is actually the signifi- 

 cant propulsor cross section. It will be not so for two-phase propulsors, as 

 can be seen from Figs. (11) and (14), because A^/A^ often becomes >1; nay, the 

 highest thrust is always obtained at A^/A^, > 1. If one decides to function at 

 A^ Aq not higher than 1, a design limit on o- immediately descends. At A^ A^ = 

 1 and v^, = 50 m/s, the hydrojector is poorer than water jets at /3p > 11, at 

 v„ = 30 m/s this happens for water jets at /Sp > 9. This means, for example, 

 that the PG-H(2)-type water jet, which provides an advance speed of 26 m/s 

 with /3p = 17 (see Ref . [23j) shows a thrust of a = 1.4, while the hydrojector 

 which has been limited by A^^o = 1 offers a = 0.62 at the same speed. 



In Fig. (14) the behavior of a (hydrojector) corresponding to 

 A^/Aj, = 1 has been represented. 



(c) In Figs. (13) and (15) the graph of a^, the thrust obtained by trun- 

 cation of a supersonic nozzle at the throat, is also plotted. It is remarkable 

 that a very faint decrease of thrust is associated with a strong decrease of the 

 discharge cross section, which is now A^,. This behavior is emphasized in Fig. 

 (16), where the curve of a^ for A^/A„ = 1 is plotted. It appears that now the 

 thrust available for A^/A^, = 1 is increased especially at high speed, where the 

 thrust produced by hydrojector can be higher than that produced by water jet. 



It is also noteworthy (Fig. 15) that a^ becomes practically insensitive to the 

 velocity v^, for any given value of A^/A^. 



(d) The presence of a pump strongly improves performance [Figs. (14) 

 and (15)]. A pumpjector at /Sp = 2 and A^/A^, = 1 provides a specific thrust 

 comparable with the one produced by a /3p = 18 water jet. If the nozzle is 

 truncated at ^c/K = 1, the thrust can be more than double that for a /Sp = 21 

 water jet (Fig. 16). On the other hand, a /Sp = 2 water jet moving at V^ = 15 

 m/s (Fig. 14) shows a = 0.47 with \,/Ao = 0.22. If the same nozzle (that is 

 the same A^/A^) is adopted in such a way that the exit cross section could 

 represent a critical section for two-phase flow (Fig. 15b) a thrust of 0.64 can 



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