|Ph.D Thesis||Department of Mechanical Engineering|
|Supervisors:||Prof. Emeritus Gany Alon|
|Prof. Elias Ezra|
|Full Thesis text|
The basic problem of fast direct-contact boiling and its application to propulsion in a Marine Ramjet (MRJ) was studied. The boiling of a multiphase droplet in an immiscible liquid was investigated experimentally, within a new range of superheats, leading to the definition of "Rapid Boiling". At these high superheat levels a unique boiling curve with several distinct stages was observed. The transition time-points between stages were predicted through a simplified model with good accuracy.
The majority of boiling occurs at a roughly constant rate ("nominal boiling rate" - NBR), occurring after “radial” convection dominates the heat transfer. This may explain why the NBR was found to be quite insensitive to instantaneous conditions (such as rise velocity). A parametric study was conducted, from which an empirical correlation (employing only initial parameters) was found for the prediction of the NBR, revealing that it is most dependent on the superheat level. Prediction using the proposed correlation is demonstrated to be more accurate than classical two-phase theory, especially at high pressures and/or high superheats.
As Reynolds number is high and time-duration short, the transient flow around the rising/expanding droplet can be approximated by the potential flow solution. This classic solution is extended to several basic cases of physical relevance. The drag coefficient is found to be dependent entirely on the dimensionless velocity ratio (growth rate/ rectilinear velocity). Additionally, pressure profiles for several cases predict flow-separation delay/prevention in equivalent cases of viscous flow. Later, this solution is reexamined and it is suggested that the instantaneous flow-form is different from that in the literature. Consequently, the pressure profile is recalculated leading to a drag coefficient much higher than that previously found.
Finally, use of Rapid Boiling of a pressure liquefied gas (PLG) as a "fuel" in the MRJ propulsion method is examined in a tow-pool facility. PLG boiling is compared directly to results of compressed air (currently used). Generally, it was found that while air had the advantage at lower velocities, boiling PLG outperformed it at higher ones. The slower boiling PLG, n-Butane, displayed a local maximum in the thrust (about 40% above air, at 16m/s), above which, only partial boiling occurs in the nozzle and thrust is reduced. R134a outperformed air increasingly, with increase in velocity, showing great promise for obtaining a maximum at velocities yet beyond reach. This modification to the MRJ should enable time-limited, shallow, underwater operation, such as in a torpedo.