At TMMFL, we study the flow dynamics, instabilities, and breakup mechanisms of free surface flows such as liquid jets, liquid sheets, droplets, and bubbles. In these flows, surface tension plays an important role. Most of the studies are related to the convection heat transfer of these flows, and the results and findings are useful for developing forced convection cooling with flow enhancement.
Capillary instability of liquid jets:
The disturbance on a capillary jet can be imposed by radius modulation, velocity modulation, or jet vibration. The objective of the study is to understand the equivalence between the three types of disturbances. Theoretical analysis based on Bernoulli equation for unsteady flows is conducted. It is found that a radius-modulated disturbance is equivalent to a velocity-modulated disturbance with the same wave number, if the non-dimensional amplitude of the radius disturbance is 1.5 times that of the velocity disturbance. This is validated by carrying out numerical simulation based on velocity modulation and comparing with the linear theory based on radius modulation. It is also revealed that disturbance generated by vibrating nozzle with small amplitude is equivalent to velocity disturbance. The non-dimensional amplitude of the equivalent velocity disturbance is a function of non-dimensional vibration amplitude and vibration wave number. The wave number of the velocity disturbance is shown to be twice of the vibration wave number. Validated by experimental observation, if the vibration wave number is less than 0.5, each nozzle vibration cycle generates two droplets. If the vibration wave number is between 0.5 and 1, each vibration cycle generates one droplet.Bubbles in curved tube flows:
The heat transfer of boiling flows in helically-coiled tubes has significant relation with the bubble dynamics in the curved flows. To understand the bubble dynamics in curved flows, we experimentally study air bubbles in water flows in a coiled tube, which is approximately considered as a toroidal tube. The torus is perpendicular to the ground. Air bubbles are generated by injecting constant air flow at varied locations of the torus. Focus is put on the relationship of the bubble departure and traveling path with the water flow and injection location. The force analysis includes the gravitational force, the drag forces by the main and secondary flows, and the centrifugal force. Injection at the outer side of the torus shows higher bubble departure rate than the inner side, which is caused by the higher drag force of main flow at the outer side. The displacement of bubble traveling path between the inner and outer sides is mainly affected by the gravitational force for slow curved flows, and is dominated by the centrifugal force for fast curved flows.
Droplet dynamics on surfaces:
We study the impact of single droplets impacting surfaces with low and high velocities and on surfaces ranging from hydrophilic to super-hydrophobic. We also study the coalescence of two droplets on surfaces.
Interaction of free surface flows on surfaces:
The research on jet impingement at TMMFL focuses on the fluid dynamics aspect of jet implement, which includes the hydraulic jump of single jet impingement, and interaction of two jets impinging on the surface.