 |
Highly-resolved numerical simulations and high-speed camera measurements have been carried out for a generic test rig with an air-assisted, external mixing nozzle. Objective of the study is to access the effect of elevated pressure on the primary atomization of a high-viscous liquid jet. The viscosity of the liquid was varied by 1 and 100 mPas and the pressure from 1 to 6 bar. Different nozzle geometries were used to keep the gas-to-liquid ratio and exit velocity of the gas constant. The numerical and experimental results showed reasonably good agreement regarding morphological behaviors of the primary breakup process. A pulsating mode instability of the primary breakup followed by a fiber type disintegration of ligaments from the liquid core has been confirmed for all cases. An increased viscosity of the liquid phase leads to larger liquid ligaments and a smaller cone angle. At elevated pressure, finer liquid fragments are disintegrated and the liquid core contracts, causing a shorter breakup length and narrower liquid cone. The simulations reveal a strongly increased kinetic energy of the liquid at higher pressure, particularly in the high frequency range, which indicates a reinforced transfer of momentum from gas to liquid phase. For all considered cases, an exponential growth of the specific kinetic energy of liquid with decreasing volume fraction of liquid has been found, with an exponential decay rate close to unity. The enhanced momentum transfer is attributable to the increased Weber number with pressure or gas density, respectively. In addition, small-scale, high-frequency turbulent fluctuations is enhanced due to an increased Reynolds number at elevated pressure. The behavior of strengthened momentum transfer is in accordance with an increased gas-to-liquid momentum flux ratio, which should be considered as a relevant parameter for characterizing primary atomization.
