Bouncing Regimes Research Articles

Energy balance of droplets impinging onto a wall heated above the Leidenfrost temperature

This work is an experimental study aiming at characterizing the heat transfers induced by the impingement of water droplets (diameter 80–180μm) on a thin nickel plate heated by electromagnetic induction. The temperature of the rear face of the nickel sample is measured by means of an infrared camera and the heat removed from the wall due to the presence of the droplets is estimated using a semi-analytical inverse heat conduction model. In parallel, the temperature of the droplets is measured using the two-color Laser-Induced Fluorescence thermometry (2cLIF) which has been extended to imagery for the purpose of these experiments. The measurements of the variation in the droplet temperature occurring during an impact allow determining the sensible heat removed by the liquid. Measurements are performed at wall conditions well above the Leidenfrost temperature. Different values of the Weber numbers corresponding to the bouncing and splashing regimes are tested. Comparisons between the heat flux removed from the wall and the sensible heat gained by the liquid allows estimating the heat flux related to liquid evaporation. Results reveal that the respective level of the droplet sensible heat and the heat lost due to liquid vaporization can vary significantly with the droplet sizes and the Weber number.

Heat transfer of droplets impinging onto a wall above the Leidenfrost temperature

In aero-engines, droplet/wall interaction phenomena have a considerable influence on the mixture formation process and on wall heat fluxes. Impinging droplets may rebound, splash into secondary droplets or form a liquid film onto the solid surface. Droplet rebound and splashing is also a mechanism for the back penetration of the fuel vapor in the central region of the combustion chamber where the gas temperature is high enough for ignition. This work is an experimental study aiming at characterizing the heat transfers induced by the impingement of water droplets (diameter 80–180 μm) on a thin nickel plate heated by electromagnetic induction. The temperature of the rear face of the nickel sample is measured by means of an infrared camera and the heat removed from the wall due to the presence of the droplets is estimated using a semi-analytical inverse heat conduction model. In parallel, the temperature of the droplets is measured using the two-color Laser-Induced Fluorescence Thermometry which has been extended to imagery for the purpose of these experiments. The measurements of the variation in the droplet temperature occurring during an impact allow determining the liquid sensible heat. Measurements are performed at surface conditions well above the Leidenfrost temperature. A wide range of Weber numbers corresponding to the bouncing and splashing regimes are tested. Comparison between the heat flux removed from the wall and the sensible heat gained by the liquid allows estimating the heat flux related to liquid evaporation. Results reveal that the respective level of the droplet sensible heat and the heat lost due to liquid vaporization can vary significantly with the droplet sizes and the Weber number.

Heat transfer for Leidenfrost drops bouncing onto a hot surface

When droplets impinge onto a hot wall, different regimes can be observed depending on the wall temperature and Weber number. In the case of interest, the temperature of the wall is more than the Leidenfrost temperature and the Weber number based on normal velocity of the droplet is less than the threshold value leading to the splashing regime (We<80). We particularly focused on the perfect bouncing regime (We<30) for which an impinging droplet levitates on a thin layer of its own vapour. This vapour is instantaneously created between the base of the deforming droplet and the heated surface so that direct contact with the hot solid is avoided. For these low Weber numbers, the droplet surface energy is high enough compared to its kinetic energy to permit the rebound, so that the droplet recovers its initial shape without breaking up after the bounce. Although the generated vapour insulates the droplet, some heat is exchanged with the wall during the interaction (i.e. during the resident time).In this paper, we report on experimental measurements of heat transfer due to droplet impact in the Leidenfrost regime. The energy released by the wall and measured using an inverse conduction method leads to an estimation of the heat transfer coefficient during impact in the Leidenfrost regime. For that, the time evolution of the droplet base surface is estimated using a simple modelling validated on experimental data. Finally, the energy measured is compared to existing models.

Bouncing, coalescence, and separation in head-on collision of unequal-size droplets

The dynamics of head-on collision of unequal-size droplets were experimentally and theoretically investigated, with emphasis on identifying distinct collision outcomes and interpreting the size-ratio dependence. A unified regime diagram in terms of bouncing, permanent coalescence, and separation after coalescence was identified for hydrocarbon and water droplets in the parameter space of the size ratio and a collision Weber number. Experimental results show that the transition Weber number, Web-c, that separates the bouncing and permanent coalescence regimes, weakly depends on the size ratio, while the transition Weber number, Wec-s, that separates permanent coalescence and separation regimes, significantly increases with the size ratio. A theoretical model based on energy balance and scaling analysis was developed to explain the size-ratio dependence of Wec-s. The theoretical results show good agreement with the experimental data for tetradecane and decane droplets, with a moderate discrepancy for water droplets.

Collision of two spin-polarized fermionic clouds

We study the collision of two spin polarized Fermi clouds in a harmonic trap using a simulation of the Boltzmann equation. As observed in recent experiments we find three distinct regimes of behavior. For weak interactions the clouds pass through each other. If interactions are increased they approach each other exponentially and for strong interactions they bounce off each other several times. We thereby demonstrate that all these phenomena can be reproduced using a semiclassical collisional approach and that these changes in behavior are associated with an increasing collision rate. We then show that the oscillation of the clouds in the bounce regime is an example of an unusual case in quantum gases: a nonlinear coupling between collective modes, namely the spin dipole mode and the axial breathing mode which is enforced by collisions. We also determine the frequency of the bounce as a function of the final temperature of the equilibrated system.

Impact attrition of brittle structured particles at low velocities: rigorous use of a laboratory vibrational impact tester

Brittle particle impact attrition was measured over three orders of magnitude of impact velocity by use of both single- and multiple-impact testers. The multiple-impact tester was a resonant cantilever impactor with dynamic amplitude control and small-gap cavities, designed to ensure operation near the transition between the bouncing and resonant impact regimes. For this impactor, a novel technique using a trajectory simulation was developed to elucidate average impact velocities, effective particle restitution behavior, and average losses per impact from a set of nominal attrition rates (loss/time). This allowed direct comparison of data from the single-impact and multiple-impact testers. Results were obtained for three brittle, porous pharmaceutical particles with significantly-different, well-characterized internal structures. Results reveal a relatively unexplored mode of attrition that is distinguished by lack of gross fragmentation at low velocities, with a steep velocity dependence. This regime is expected for any brittle particle for which simple chipping is not seen—such as rounded solid particles and many agglomerates. At higher impact velocity, gross particle fragmentation is observed. The transition between these attrition regimes appears connected to particle structure, such as the size of attrition-resistant primary particles in an agglomerate or the point at which dominant flaws (that lead to fragmentation) are no longer critically active. The ranking of particles according to damage in high-velocity impacts was not predictive of damage in low-velocity impacts, because the particle attrition did not necessarily exhibit the same velocity-dependence in the two regimes. Such differences are critical for predicting performance in operations such as pneumatic conveying or fluidized beds, respectively.

Experiments on the motion of a solid sphere toward a wall: From viscous dissipation to elastohydrodynamic bouncing

In the present study, we investigate experimentally the motion of a macroscopic (non-Brownian and noncolloidal) solid sphere falling under gravity into a viscous liquid toward a solid wall. We observe the transition from a nonbouncing to a bouncing regime when increasing the Stokes number St which characterizes the particle inertia. In the bouncing regime, the recording of the particle trajectories allows us to determine the coefficient of “elastic” restitution e above the transition. We observe that e first increases with St as predicted by Davis, Serayssol, and Hinch (1986) and seems to reach a plateau at high St.

Dynamics of a ball bouncing on a rough inclined line

We present here a simple theoretical model that describes the motion of ball bouncing on a rough inclined line. The rough line consists of microfacets whose orientation can be different from the line inclination. We examine the behavior of the ball as a function of the orientation of the microfacets and determine the conditions under which the jumps of the ball are decreasing or increasing in their amplitude. In particular we show that when the facet inclination varies along the line with a well-defined spatial periodicity the ball can reach a steady bouncing regime that leads ultimately to chaotic behavior via a period-doubling scenario. Furthermore, we find that the presence of noise associated with facet inclination destroys the structure of the chaotic regime.