Herman D. Haustein
Center for Heat and Mass Transfer, RWTH Aachen
Direct-contact rapid-boiling has many applications in the chemical and thermal fields when speed and timing are of essence, as in high-speed multiphase flow. One such application is the Marine Ramjet (MRJ) employing compressed air as a “fuel”: the gas expansion work is converted to thrust in a nozzle. A novel method is examined here using the rapid boiling of a liquefied-gas (Refrigerant) as an alternative “fuel” source.
For evaluation of the boiling rate occurring in the MRJ, the fundamental problem of rapid boiling (timescale 10-100 milliseconds) of a single droplet in an immiscible liquid medium was investigated experimentally, within a new range of superheat (0.26<Ja*<0.43). The boiling rate was found to be roughly constant and quite insensitive to instantaneous conditions such as rise velocity, due to radial-flow dominating the convective heat transfer. The high-Reynolds flow field in this problem was approximated by the potential-flow solution. This well-known transient solution was rescaled, to require one less assumption. It was then found that due to high expansion-rate an asymmetrical Rankine-body is formed, with inertial flow-separation occurring at the rear, resulting in much higher drag than previously predicted. Following a parametric study, an empirical bubble growth correlation (BGC) was proposed for the prediction of the boiling rate based on: liquid properties, droplet geometry, ambient pressure and ambient temperature. The BGC equation is demonstrated to have higher accuracy than classical theory at high superheats, incorporating the effects of higher pressures (as occur in the MRJ).
For the MRJ application several liquids and outlet nozzles were examined in a modified tow-pool, at velocities up to 18m/s. At lower velocities thrust was similar to compressed air, while at higher velocities a significant advantage in thrust was found due to boiling. This advantage presented a maximum in a narrow range of velocity that is interpreted as the completion of boiling of a typical droplet near the end of the nozzle. By comparing to boiling-time predicted by the BGC equation, typical droplet sizes could be extracted, showing agreement with those measured in the MRJ under static conditions.
Use of a boiling liquid in the MRJ leads not only to a compact self-sustained unit, suitable for underwater operation, but has potential for even higher thrust (expansion-work) at higher velocities than those examined here. While the down side of this method is the limited operation time due to limited liquid storage capacity, it seems suitable for implementation in a surface-torpedo.
ההרצאה תתקיים ביום שני 21.06.10
שעה 13:30
בנין אוירונוטיקה חדר 241
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