Numerical investigation of nozzle length effects on Rayleigh jet breakup behaviors and droplet formation properties at different modulation amplitudes in liquid droplet radiator

Uniform droplet stream generation is required for a liquid droplet radiator to achieve the efficient heat dissipation of space nuclear power systems. In this study, Rayleigh jets breaking into continuous droplet streams at different modulation amplitudes are numerically simulated to investigate the nozzle length effects on jet breakup behaviors and droplet formation properties. First, simulated results confirm that the growth of secondary instability wave on jet free-surface causes the jet breakup pattern transition from downstream pinching to upstream pinching. As the modulation amplitude is from 0.04 up to 0.16, the nozzle length range where the upstream pinching occurs in the jet breakup is broadened from null to 1-5. The essential reason for the increase of jet breakup length is the reduction of fundamental amplitude and the increase of full velocity relaxation length. Then, a comprehensive prediction model for the velocity-modulated jet breakup length is developed in agreement with the experiments, and the average relative error is below 7.38 %. Finally, two types of satellite droplet formation regimes including rear-merging and forward-merging satellite droplets are identified by the oscillation mode of the jet tip. It is found that the primary droplet generation frequency is equal to the external disturbance frequency and independent of nozzle length and modulation amplitude. Increasing the nozzle length and modulation amplitude can both increase the satellite droplet diameter and primary droplet spacing, while the primary droplet diameter is almost constant at around 1.85.