Stefan Herbert

Propulsion mechanisms for Leidenfrost solids on ratchets

We propose a model for the propulsion of Leidenfrost solids on ratchets based on viscous drag due to the flow of evaporating vapor. The model assumes pressure-driven flow described by the Navier-Stokes equations and is mainly studied in lubrication approximation. A scaling expression is derived for the dependence of the propulsive force on geometric parameters of the ratchet surface and properties of the sublimating solid. We show that the model results as well as the scaling law compare favorably with experiments and are able to reproduce the experimentally observed scaling with the size of the solid.

The Influence of System Pressure on Bubble Coalescence in Nucleate Boiling

Boiling is one of the most effective heat transfer mechanisms. In spite of a long time of research, the physical fundamentals are still not sufficiently understood. Pursuing the objective to predict heat transfer based on physical and geometrical properties, experimental and numerical investigations are conducted at the institute of the authors. The focus of the presented research is the coalescence of two single bubbles under varying pressure conditions. In the experiment a thin stainless-steel foil is used as a Joule heater. The experiments were performed in a pressure range of 300–1000 mbar using FC72 as working fluid. Two types of heaters with a distance between two artificial nucleation sites of 300 μm (type 3) and 500 μm (type 5) were used. The experimental results indicate a strong dependence of the occurrence of bubble coalescence on pressure. For the type 5 heater, a Gaussian distribution for the coalescence frequency when plotted over pressure is observed. Experimental results with the type 3 heater show a similar distribution of the frequency with a shifted maximum. Further, it is shown that during bubble coalescence a small droplet can remain inside the bubble and enhance the heat transfer, which is attributed to an additional thin film region. The formation of this remaining droplet is sensitive to system pressure. Numerical investigations of bubble coalescence were conducted with the computational fluid dynamics (CFD) software OpenFOAM. In OpenFOAM, dynamic mesh handling allows high spatial resolution at the phase boundary, which is captured with the volume-of fluid method. Evaporation and a subgrid microscale model were implemented in the flow solver to account for evaporation at the phase boundary and the three-phase contact line. The results show a strong dependence of bubble dynamics and coalescence on contact angle and bubble growth rate. Although it was possible to observe the creation of the residual droplet, more effort needs to be put into finding appropriate initial conditions.

Local Heat Flux Investigation during Pool Boiling Single Bubble Cycles under Reduced Gravity

In the present work the bubble diameter, heater surface temperature distribution, and local heat flux during different stages of single bubble cycles during pool boiling of PF-5060 at a pressure of p = 600 mbar have been investigated in several stable low g levels during the 1st Joint European Partial-g Parabolic Flight (JEPPF) Campaign. In previous parabolic flight campaigns, microgravity conditions were achieved by following a parabolic trajectory with the specially equipped A-300-Zero-G Aircraft. In this recent JEPPF campaign, the parabolic trajectories were slightly shifted, to establish—apart from microgravity conditions—also stable gravity levels of 0.16 g (lunar gravity) and 0.38 g (Martian gravity). High-resolution measurements of the heater surface temperature were performed using high-speed infrared thermography. An infrared (IR)-transparent sputtered heater design was employed in order to allow temperature measurements by IR thermography at a distance of approximately 800 nm to the heater/fluid interface. From the acquired temperature data, the local heat flux distribution was calculated numerically. Bubble shape and interaction were recorded with a high-speed black-and-white camera. In contrast to previous investigations, the stable low gravity levels enabled performance of measurements during single bubble (ebullition) cycles without the influence of residual flows induced by boiling under a different gravity level, as is the case in the beginning of a regular microgravity parabola. The accuracy of the measurement technique could be drastically enhanced compared to earlier publications. A local temperature drop and corresponding heat flux peak have been observed close to the three-phase contact line.