Pallabazzer ' ■ ■• ■ 



DISCUSSION 



W. van Gent 

 Netherlands Ship Model Basin ,. 

 Wageningen, Netherlands 



Our main comments on this interesting paper are referred to the analysis 

 of the propulsor, presented in Sec. 3. In this section, first an adiabatic gas 

 expansion in the nozzle is assumed. This causes an overestimation of the effi- 

 ciency of hot gas injection and, in addition, an underestimation of the efficiency 

 of gas injection at water temperature. Whereas this assumption is physically 

 more or less acceptable, next, another assumption on the mixing process is 

 made, which, however, is physically unacceptable, because it violates the en- 

 ergy balance. 



It is stated in Sec. 3.1 that during the mixing process the water is acceler- 

 ated at quasi-constant pressure and stated following Eq. (12) that the mixing 

 section has a constant cross- sectional area. The increase in velocity of the 

 water phase is derived from the equation of continuity, but there is no check on 

 the energy equation. The energy balance given above Eq. (5) shows that, at con- 

 stant pressure, the increase in kinetic energy can only be supplied by a decrease 

 in enthalpy of the gas, but there is no mechanism by which energy transfer can 

 take place. The gas will not expand, but will even contract in consequence of heat 

 losses. Thus, especially at high bulk gas-water ratios, this assumption gives a 

 very unrealistic picture of the mixing process. 



Fortunately, it seems that this assumption has not influenced the calculations 

 of efficiency and specific thrust, defined in Sec. 4. In calculating the efficiency 

 and the thrust, the correct energy balance is used and the specific thrust is the 

 thrust per unit area of the undisturbed water flow cross section. However, the 

 thrust coefficient, defined in Sec. 4 as the thrust per unit area of the largest 

 cross section of the propulsor, is much too high, because the cross- sectional 

 area of the mixing section cannot be constant. 



In our opinion this area must increase in such a way that the water is not 

 accelerated during gas injection. Energy transfer can only take place after com- 

 plete mixing. Our own experiments have shown that a higher efficiency is 

 achieved when the cross- sectional area of the mixing chamber is enlarged after 

 gas injection. 



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