Wilko Rohlfs

Film flow on vertical fibers has been studied for the past 30 years. However, it is only recently that the influence of the nozzle on the film and bead dynamics has been demonstrated. Consequently, similar to the regimes observed with a faucet, both a dripping and a jetting regime have been reported for film flow on fibers. This study investigates the dynamics of thin liquid films on vertical fibers near the nozzle outlet using both experimental and numerical approaches, drawing insights from dripping faucet studies. This allows for a clear definition of the recently observed dripping and jetting regimes, with a particular focus on gaining a deeper understanding of the flow and formation of beads in the dripping regime, as well as the transition to jetting.

Analytical investigations of heat transfer during the vertical impingement of an unsubmerged axisymmetric liquid jet on a horizontal plate have been limited to the regions ahead of the jump. This limitation is due to the complex flow physics in the jump region arising from sudden changes in the flow field. This is addressed in here by extending the averaging theory (AT) introduced by Bohr et al. (Phys. Rev. Lett., vol. 79, issue 6, 1997, pp. 1038–1041) which was further developed by Watanabe et al. (J. Fluid Mech., vol. 480, 2003, pp. 233–265), to describe the heat transfer problem in circular hydraulic jumps including separation. The applicability of the resulting theory to determine the temperature field in the jump region is evaluated using the data available in the literature and also by means of fully resolved numerical solutions. Good agreement is observed for moderate Prandtl numbers. However, for sufficiently high Prandtl numbers, deviations become notable. The reasons for the deviations according to their relevance are (i) monotonically decreasing temperature profile inherent to the AT, whereas the fully resolved numerical solutions exhibit a local maximum in the temperature profile away from the plate; and (ii) inapplicability of the concept of dividing the flow field into a region affected and a region unaffected by heat transfer according to the thermal boundary layer thickness. This concept leads to the overestimation of the temperature close to the wall and to the existence of a threshold Prandtl number, for which the thermal boundary layer thickness does not meet the free surface anymore. Around this threshold Prandtl number, the temperature field shows a discontinuous behaviour.

The global increase in CO2 emissions in recent decades has highlighted the development of efficient solid sorbents for CO2 capture from ambient air and flue gases. Amine-based hydrogels (e.g., polyethylenimine) are emerging as a new and interesting option. While the production of amine-based hydrogel sorbents remains in its early stage, in this work, we present a robust technique utilizing in-air microfluidics (IAMF) for an affordable, scalable and highly controllable production of solid hydrogel particles for CO2 capture. By jetting a core-shell compound liquid into air, we fabricate particles which encompass a robust core-shell structure where the core is crosslinked hydrogel and the shell is a porous silica layer. Based on this, this study focuses on three main aspects. First, we demonstrate that the fabricated particles exhibit significant water adsorption, satisfactory CO2 capture capacity, and stable cycling performance. Second, we compare the effects of two crosslinkers, triglycidyl trimethylolpropane (TTE) ether and epichlorohydrin (ECH). Particles crosslinked with TTE show higher CO2 capture capacity and faster kinetics than those crosslinked with ECH. Finally, we investigate the influence of water content on the CO2 capture performance of the particles. Finally, we studied the effect of water content on the CO2 capacity of the particles. Increasing the water content enables the particles to capture more CO2 but reduces the capture efficiency. A sharp “collapse” in capacity is observed when the water content decreases below 40 wt%, which is likely attributed to the collapse of the porous structure of the particles.

The coalescence-induced jumping of droplets on superhydrophobic surfaces is useful in engineering-related applications to enhance condensation-based heat transfer, self-cleaning, and anti-icing and, thus, has attracted extensive attention in research. Some researchers have claimed that superhydrophobic surfaces with protuberant structures can yield droplets with a higher jumping velocity. While the structure of the surface influences droplet dynamics, the concomitant energy transition also needs to be considered. The effects of the geometry on the mechanism of jumping and the energy transition need to be investigated. In this paper, an improved volume-of-fluid method is verified based on experiments and then applied to simulate the jumping behaviors of droplets on superhydrophobic surfaces with cuboid protuberant structures. The effects of repulsion caused by the contributions of the surface tension and the superhydrophobicity of the protuberance are crucial to enhancing the jumping of the droplets. The forces due to them provide a thrust oriented in the direction of jumping of the droplets to increase the value of the positive energy term, while reducing the area and duration of contact between the droplet and the substrate to reduce the negative dissipation term and enhance the efficiency of energy conversion. Surprisingly, an excessively tall structure leads to a sustainable increase in the velocity of jumping of the droplets under the effects of repulsion and the Laplace pressure after piercing the liquid bridge. The work here provides guidance to optimally combine a superhydrophobic substrate with special structures to enhance the jumping of droplets.

Air-source heat pumps save energy and reduce emissions but frost formation on the evaporator is still a major challenge. Effective frost mitigation and de-frosting require a fundamental understanding of frost formation dynamics. Previous studies model frost growth and densification within a 15 % error margin, based on bulk and surface properties. However, efficient frost mitigation requires better prediction of local and internal properties including crystal structure. So far, limited optical accessibility has hindered experimental analysis. In this study, a method is proposed to overcome this limitation, through the use of X-ray microcomputed tomography. To test the overall methodological feasibility, additional experimental complexities related to frost melting and instability were mitigated by substituting with a urea-mixture. A sample visually resembling frost was chosen for analysis. A detailed 3-D image of the air/crystal interface, with 7.3 µm spatial resolution, was captured. The porosity of the sample was determined to be 84 %. After filtering, segmenting, and cropping, the image was exported to MATLAB. The effective thermal conductivity and effective mass diffusivity of the sample were determined by solving the respective steady-state three-dimensional diffusion equations. The effective thermal conductivity was determined as 0.0879 W/m.K, which lay between the ideal series and parallel values. The same is true for the effective mass diffusivity of 11.75 mm²/s. The general methodological feasibility was confirmed by the successful scanning, reconstruction and analysis of the frost substitute.

Since 2017, HeatQuiz is developed as a tool for students in the subject of Heat and Mass Transfer to train their skills and strengthen their knowledge. The topics span from view factors to temperature distributions up to dimensionless numbers. With more that 800 questions, HeatQuiz provides a large variety of questions in it's database. Due to the COVID-19 pandemic and the obligation for social distancing HeatQuiz has evolved from a mobile teaching application with customized subject-specific question types to a tool that combines game-based elements with structured access to an open heat transfer lecture. Combining micro-lectures for the flipped-classroom concept and hundreds of quiz questions in a well structured learning environment is one of the main strengths. This article describes the learning path concept and how it is embedded in HeatQuiz. Secondly, a new question type will be introduced that is especially designed to practise the skill of developing energy balances based on a sketch of the control volume, defining all relevant fluxes, and finally defining the required boundary conditions.

In suspended falling films, i.e., films on the underside of a bounding wall with arbitrary inclination, the surface film topology evolves towards a distinct spanwise structuring of the flow into rivulets, which is potentially accompanied by dripping events. Experimental data suggest a connection between long-term spanwise structuring and primary instabilities of the film surface. However, available experimental data regarding this connection remain nonconclusive. Hence, the present study aims at elucidating the evolution of a suspended falling film from varying imposed initial conditions to the emergence of spanwise modulations and rivulet formation. The study is carried out by means of extended numerical simulations employing a weighted residual integral boundary layer model for falling liquid films. The investigated parameter space covers recent experimental data on the topic. Varying imposed initial conditions, Reynolds number, Kapitza number, as well as wall inclination, several possible causes for a deviation of observed spanwise wavelengths from the one predicted by the primary Rayleigh-Taylor mechanism, are identified. This includes a distinct influence of initial conditions, asynchronous destabilization of consecutive wavefronts, competing short wave capillary instabilities, and rivulet interaction.