Dynamic regimes of electrified liquid filaments

We investigate the dynamics of an electrified liquid filament in a nozzle-to-substrate configuration with a close separation. The interplay between compressive viscous and electrostatic stresses dictates previously undocumented transitions between dynamic regimes of “jetting,” “coiling,” and “whipping.” In particular, the onsets of both coiling and whipping instabilities are significantly influenced by the minimum radius along the liquid filament. Using a low-interfacial-tension system, we unravel the physics behind the transitions between jetting, coiling, and whipping of an electrified filament for a range of liquid properties and geometric parameters. Our results enrich the overall physical picture of the electrically forced jets, and provide insights for the emerging high-resolution instability-assisted printing of materials such as folded assemblies and scaffolds.

Effects of Flame Lift-Off Height on Soot Processes in Strongly Forced Methane-Air Laminar Diffusion Flames

Previous work has shown that for sufficiently high periodic forcing amplitudes, laminar diffusion flames can burn in an effectively partially premixed mode. Experimental observations show that the luminosity and sooting properties of the forced flames are significantly modified by the presence of strong forcing. In this work, simulations are performed to study the effects of strong forcing on flow field development in strongly forced laminar isothermal jets and methane air diffusion flames. Unforced and strongly forced cold-flow jets are simulated using a higher order finite volume CFD code. The jet was forced by varying the jet exit velocity over a range of forcing amplitudes and frequencies and it was found that the jet Strouhal number (St) was the important parameter in characterizing flowfield development. Further, the forced jets showed increased entrainment and increased entrainment rates as compared to the non-forced jets. The computations are performed for laminar methane–air diffusion flames. The combustion reactions were modeled using detailed gas-phase chemistry and complex thermo-physical properties. The radiation heat transfer was modeled using the S-6 Discrete Ordinates Method. A 2 equation soot chemistry model for soot nucleation, surface growth and oxidation was used. First an unforced flickering methane–air diffusion flame was modeled and then the flame was forced by varying the amplitude and frequency of the fuel velocity in the nozzle. Cases where the peak velocity in the fuel stream reached 6 times the mean velocity are examined. The internal nozzle flow was also simulated since the near-nozzle region was of particular interest due to the strong mixing processes occurring there and the subsequent effect on the flame properties. Lifted forced flames were also examined, and it was found that the partial premixing in the near nozzle region and increased oxygen entrainment in the forced flames can explain the reduction in soot production for the strongly forced flames.

Experimental Investigation of Mixed Convection Heat Transfer Caused by Forced-Jets in Large Enclosure

This research investigates experimentally mixed convection and heat transfer augmentation by forced jets in a large enclosure, at conditions simulating those of actual passive containment cooling systems and scales approaching those of actual containment buildings or compartments. The experiment was designed to measure the key parameters governing the heat transfer augmentation by forced jets and investigate the effects of geometric factors, including the jet diameter, jet injection orientation, interior structures, and enclosure aspect ratio. The tests cover a variety of injection modes leading to flow configurations of interest that contribute to reveal the nature of mixing and stratification phenomena in the containment under accident conditions of interest. By nondimensionalizing the governing equations, the heat transfer of mixed convection can be predicted to be controlled by jet Archimedes number and geometric factors. Using a combining rule for mixed convection and appropriate forced and natural convection models, the correlations of heat transfer augmentation by forced jets are developed and then tested by experimental data. The effects of jet diameter, injection orientation, interior structures, and enclosure aspect ratio on heat transfer augmentation are illustrated with analysis of experimental results.