Numerical modelling of melt droplet interaction with water

A collapse of the vapour film separating a hot melt droplet from the surrounding water due to sudden ambient pressure rise is considered. The pressure peak causes a direct contact between water and melt, leading to significant disturbances of the melt droplet surface. Results of numerical simulations performed by the VOF method are presented. Parametric analysis of the interaction process is performed for a molten tin droplet with initial temperature of 950 K, immersed in subcooled water having the temperature of 353 K. The interaction is initiated by sudden rise of the ambient pressure to as much as 8 MPa, imitating the arrival of a thermal detonation wave, with its gradual decrease towards the initial pressure of 0.1 MPa. Simulations reveal the collapse of the vapour film, impingement of water on the droplet surface, and subsequent expansion of vapour due to rapid water evaporation. Significant disturbances of the melt droplet surface are obtained, and implications for the steam explosion problem are discussed.

Self-propelled Leidenfrost droplets on a heated glycerol pool

The development of contactless sample manipulation for microfluidic purposes has attracted significant attention within the physicochemical fields. Most existing studies focus on the interactions of unheated liquid substrates and on heated/unheated solid substrates. Therefore, the dynamics of droplets on heated liquid pools have yet to be explored. Here, we present an experimental investigation on the levitated and self-propelled droplets on a heated pool. We aim to identify the effect of the pool temperature and the thermophysical properties of droplets on the dynamics of a self-propelled Leidenfrost droplet on a heated pool. The motion of droplets after levitation on the heated pool is visualized. To elucidate the self-propulsion of Leidenfrost droplets, we quantify the thickness of the vapour film between the approaching droplet and the pool surface. Our experimental results show a quantitative agreement with the simple model prediction for self-propelled Leidenfrost droplets. Our results provide deeper physical insights into the dynamics of Leidenfrost droplets on a heated pool for contactless and contamination-free sample manipulation.

Thermocapillary instability as a mechanism for film boiling collapse

We construct a model to investigate the interfacial stability of film boiling, and discover that instability of very thin vapour films and subsequent large interface superheating is only possible if thermocapillary instabilities are present. The model concerns horizontal saturated film boiling, and includes novel features such as non-equilibrium evaporation based on kinetic theory, thermocapillary and vapour thrust stresses and van der Waals interactions. From linear stability analysis applied to this model, we are led to suggest that vapour film collapse depends on a balance between thermocapillary instabilities and vapour thrust stabilization. This yields a purely theoretical prediction of the Leidenfrost temperature. Given that the evaporation coefficient is in the range 0.7–1.0, this model is consistent with the average Leidenfrost temperature of every fluid for which data could be found. With an evaporation coefficient of 0.85, the model can predict the Leidenfrost point within 10 % error for every fluid, including cryogens and liquid metals where existing models and correlations fail.

Phase diagram for droplet impact on superheated surfaces

We experimentally determine the phase diagram for impacting ethanol droplets on a smooth, sapphire surface in the parameter space of Weber number $\mathit{We}$ versus surface temperature $T$. We observe two transitions, namely the one towards splashing (disintegration of the droplet) with increasing $\mathit{We}$, and the one towards the Leidenfrost state (no contact between the droplet and the plate due to a lasting vapour film) with increasing $T$. Consequently, there are four regimes: contact and no splashing (deposition regime), contact and splashing (contact–splash regime), neither contact nor splashing (bounce regime), and finally no contact, but splashing (film–splash regime). While the transition temperature $T_{L}$ to the Leidenfrost state depends weakly, at most, on $\mathit{We}$ in the parameter regime of the present study, the transition Weber number $\mathit{We}_{C}$ towards splashing shows a strong dependence on $T$ and a discontinuity at $T_{L}$. We quantitatively explain the splashing transition for $T<T_{L}$ by incorporating the temperature dependence of the physical properties in the theory by Riboux & Gordillo (Phys. Rev. Lett., vol. 113(2), 2014, 024507; J. Fluid Mech., vol. 772, 2015, pp. 630–648).

Investigation of Transport Phenomena in a Vapour Film Formed in Contact Between Hot Metallic Sphere and Water

Contact between hot objects and liquids occurs in many industries, such as nuclear reactors, metal casting industries and paper production. In such cases growth of vapour film leads to steam explosion that may cause human and financial damages. It is obvious that possibility of these phenomena can be judged by comparing vapour film radius growth and pressure inside vapour film. In this paper vapour film growth and pressure inside the vapour film formed, on hot sphere interaction with water, are investigated. The numerical simulation of problem is obtained and then validated using experimental test and other available results. The effects of the variations of different parameters such as hot sphere diameter, temperature, immersion depth into water and bulk water temperature are investigated on the vapour film radius, vapour pressure inside vapour film and the saturation temperature of phase interface surface. Finally, the overall results show that the effect of hot sphere interaction with water would be the same pressure inside vapour film suddenly increases up to 5 times more than initial pressure which would lead to hazard and put the safety of the system at risk.

Simulation of cavitation in outward-opening piezo-type pintle injector nozzles

The onset and development of cavitation in the annular needle seat passage of piezo-driven outward-opening pintle injector nozzles used with spray-guided direct-injection gasoline engines are studied using a Eulerian-Lagrangian computational fluid dynamics cavitation model. Cavitation is formed because of the fluid acceleration taking place at the needle sealing area and it has been found to be affected by its geometric details. Various submodels for nucleation and bubble formation, further bubble growth and collapse, as well as bubble break-up and transport are incorporated into the model. Qualitative model validation is performed against experimental data reported elsewhere in large-scale nozzle replicas, showing similar cavitation patterns to be formed. These consist of vapour pockets rather than a continuous vapour film and develop transiently in a rather chaotic manner around the circumferential needle sealing area, even under stationary geometry and fixed-flowrate conditions. Further transient effects associated with the fast opening and closing of the piezo-controlled needle valve are also presented.