The dynamics of the piezo inkjet printhead operation

This article presents a comprehensive review of numerical methods and models for interface resolving simulations of multiphase flows in microfluidics and micro process engineering. The focus of the paper is on continuum methods where it covers the three common approaches in the sharp interface limit, namely the volume-of-fluid method with interface reconstruction, the level set method and the front tracking method, as well as methods with finite interface thickness such as color-function based methods and the phase-field method. Variants of the mesoscopic lattice Boltzmann method for two-fluid flows are also discussed, as well as various hybrid approaches. The mathematical foundation of each method is given and its specific advantages and limitations are highlighted. For continuum methods, the coupling of the interface evolution equation with the single-field Navier–Stokes equations and related issues are discussed. Methods and models for surface tension forces, contact lines, heat and mass transfer and phase change are presented. In the second part of this article applications of the methods in microfluidics and micro process engineering are reviewed, including flow hydrodynamics (separated and segmented flow, bubble and drop formation, breakup and coalescence), heat and mass transfer (with and without chemical reactions), mixing and dispersion, Marangoni flows and surfactants, and boiling.

Numerical modeling of multiphase flows in microfluidics and micro process engineering: a review of methods and applications

Mass transport phenomena in direct methanol fuel cells

Influence of Bubbles on the Energy Conversion Efficiency of Electrochemical Reactors

Platinum (Pt)-based nanoparticle metals have received a substantial amount of attention and are the most popular catalysts for direct methanol fuel cell (DMFC). However, the high cost of Pt catalysts, slow kinetic oxidation, and the formation of CO intermediate molecules during the methanol oxidation reaction (MOR) are major challenges associate with single-metal Pt catalysts. Recent studies are focusing on using either Pt alloys, such as Fe, Ni, Co, Rh, Ru, Co, and Sn metals, or carbon support materials to enhance the catalytic performance of Pt. In recent years, Pt and Pt alloy catalysts supported on great potential of carbon materials such as MWCNT, CNF, CNT, CNC, CMS, CNT, CB, and graphene have received remarkable interests due to their significant properties that can contribute to the excellent MOR and DMFC performance. This review paper summaries the development of the above alloys and support materials related to reduce the usage of Pt, improve stability, and better electrocatalytic performance of Pt in DMFC. Finally, discussion of each catalyst and support in terms of morphology, electrocatalytic activity, structural characteristics, and its fuel cell performance are presented.

https://link.springer.com/content/pdf/10.1186/s11671-018-2799-4.pdf

Platinum-Based Catalysts on Various Carbon Supports and Conducting Polymers for Direct Methanol Fuel Cell Applications: a Review

Computational fluid dynamics (CFD) software tools for microfluidic applications – A case study

A new concept that enables fully passive CO2 gas bubble removal in micro direct methanol fuel cells (μDMFCs) is presented. The original concept behind the presented degassing structure (flowfield) is based on microchannels with a T-shaped cross section. These channels have defined tapering angles over their cross section (α) and along their axis (β). The tapered channel design creates an intrinsic transport mechanism that removes the gas bubbles from the electrodes by capillary forces only. Computational fluid dynamic (CFD) simulations have been used to determine applicable opening angles of α = 5° and β = 1.5°. The experimental verification was done by using a transparent flowfield to show the passive bubble removal as well as with a fully operational μDMFC. During the operation, the fuel cell delivered an output of up to 8 mW cm−2 without the need for external pumping in short-term measurements. During the long-term measurements, discontinuous pumping showed the highest fuel cell efficiency compared to the continuously pumped fuel supply.

Increasing μDMFC efficiency by passive CO2 bubble removal and discontinuous operation

In this paper we present a new concept of creating and using capillary pressure gradients for passive degassing and passive methanol supply in direct methanol fuel cells (DMFCs). An anode flow field consisting of parallel tapered channels structures is applied to achieve the passive supply mechanism. The flow is propelled by the surface forces of deformed CO2 bubbles, generated as a reaction product during DMFC operation. This work focuses on studying the influence of channel geometry and surface properties on the capillary-induced liquid flow rates at various bubbly gas flow rates. Besides the aspect ratios and opening angles of the tapered channels, the static contact angle as well as the effect of contact angle hysteresis has been identified to significantly influence the liquid flow rates induced by capillary forces at the bubble menisci. Applying the novel concept, we show that the liquid flow rates are up to thirteen times higher than the methanol oxidation reaction on the anode requires. Experimental results are presented that demonstrate the continuous passive operation of a DMFC for more than 15 h.

/pdf/capillary-driven-pumping-for-passive-degassing-and-fuel-54zz339c4e.pdf

Capillary-driven pumping for passive degassing and fuel supply in direct methanol fuel cells

Preventing micro-channels from clogging is a major issue in most micro and nanofluidic systems (Gravesen et al., J Micromech Microeng 3(4):168–182, 1993; Jensen et al., In: Proc. of MicroTAS 2002, Nara, Japan, pp 733–735, 2002; Wong et al., J Fluid Mech 292:71–94, 1995). The T-shaped channel first reported by Kohnle et al. (In: IEEE MEMS, the 15th international IEEE micro electro mechanical conference (ed), Las Vegas, pp 77–80, 2002) prevents micro-channels from clogging by the aid of the equilibrium bubble position in such a geometry. This work is concerned with the static and dynamic behaviour of bubbles in such T-shaped micro-channels. The aspect ratio of a rectangle enclosing the T-shaped channel and the contact angle of the walls are the main parameters influencing the static and dynamic bubble behaviour. It is investigated in this article how these parameters relate to the equilibrium bubble shape and how optimum bubble velocities can be achieved inside the channel. An analytical model depending on the contact angle and the channel geometry is presented that allows to determine the bubble configuration inside the channel by minimizing the bubble’s surface energy. A second model is derived to predict the velocity of gas bubbles driven by buoyancy in vertical T-shaped channels. The model is applied to design T-shaped channels with a maximum mobility of gas bubbles. Experiments with MEMS fabricated devices and CFD simulations are used to verify the models. Furthermore design rules for an optimum non-clogging channel geometry which provides the highest gas bubble mobility are given.

/pdf/static-and-dynamic-behaviour-of-gas-bubbles-in-t-shaped-non-47ibddkoh7.pdf

Static and dynamic behaviour of gas bubbles in T-shaped non-clogging micro-channels

In this paper a micro direct methanol fuel cell (muDMFC) is presented, which is operated in a completely passive way, i.e. the cell does not require an external pump for fuel supply. The surface energy of deformed CO2 bubbles, generated as a reaction product during DMFC operation, is employed to supply methanol to the anode. In contrast to a digital valve based approach presented earlier by Meng et. al. [1], a tapered channel is applied to achieve a pumping mechanism. This way the pump rates can be adapted to the requirements of a specific cell. The presented study reveals that this concept is able to maintain the supply for all typical DMFC operation conditions. Experimental results are presented that demonstrate the continuous operation of a passive muDMFC for more than 15 hours.

/pdf/fully-passive-degassing-and-fuel-supply-in-direct-methanol-4f4m0kricr.pdf

Christian Litterst

Papers

Computational fluid dynamics (CFD) software tools for microfluidic applications – A case study

Increasing μDMFC efficiency by passive CO2 bubble removal and discontinuous operation

Capillary-driven pumping for passive degassing and fuel supply in direct methanol fuel cells

Static and dynamic behaviour of gas bubbles in T-shaped non-clogging micro-channels

Fully passive degassing and fuel supply in direct methanol fuel cells