The rate of heat and mass transfer at the surface of acoustically levitated pure liquid droplets is predicted theoretically for the case where an acoustic boundary layer appears near the droplet surface resulting in an acoustic streaming. The theory is based on the computation of the acoustic field and squeezed droplet shape by means of the boundary element method developed in Yarin, Pfaffenlehner & Tropea (1998). Given the acoustic field around the levitated droplet, the acoustic streaming near the droplet surface was calculated. This allowed calculation of the Sherwood and Nusselt number distributions over the droplet surface, as well as their average values. Then, the mass balance was used to calculate the evolution of the equivalent droplet radius in time. The theory is applicable to droplets of arbitrary size relative to the sound wavelength λ, including those of the order of λ, when the compressible character of the gas flow is important. Also, the deformation of the droplets by the acoustic field is accounted for, as well as a displacement of the droplet centre from the pressure node. The effect of the internal circulation of liquid in the droplet sustained by the acoustic streaming in the gas is estimated. The distribution of the time-average heat and mass transfer rate over the droplet surface is found to have a maximum at the droplet equator and minima at its poles. The time and surface average of the Sherwood number was shown to be described by the expression Sh = KB/√ω[Dscr ]0, where B = A0e/(ρ0c0) is a scale of the velocity in the sound wave (A0e is the amplitude of the incident sound wave, ρ0 is the unperturbed air density, c0 is the sound velocity in air, ω is the angular frequency in the ultrasonic range, [Dscr ]0 is the mass diffusion coefficient of liquid vapour in air, which should be replaced by the thermal diffusivity of air in the computation of the Nusselt number). The coefficient K depends on the governing parameters (the acoustic field, the liquid properties), as well as on the current equivalent droplet radius a.For small spherical droplets with a[Lt ]λ, K = (45/4π)1/2 = 1.89, if A0e is found from the sound pressure level (SPL) defined using A0e. On the other hand, if A0e is found from the same value of the SPL, but defined using the root-mean-square pressure amplitude (prms = A0e/√2), then Sh = KrmsBrms/ √ω[Dscr ]0, with Brms = √2B and Krms = K/√2 = 1.336. For large droplets squeezed significantly by the acoustic field, K appears always to be greater than 1.89. The evolution of an evaporating droplet in time is predicted and compared with the present experiments and existing data from the literature. The agreement is found to be rather good.We also study and discuss the effect of an additional blowing (a gas jet impinging on a droplet) on the evaporation rate, as well as the enrichment of gas at the outer boundary of the acoustic bondary layer by liquid vapour. We show that, even at relatively high rates of blowing, the droplet evaporation is still governed by the acoustic streaming in the relatively strong acoustic fields we use. This makes it impossible to study forced convective heat and mass transfer under the present conditions using droplets levitated in strong acoustic fields.

No abstract

For better water management in gas channels (GCs) of polymer electrolyte fuel cells (PEFCs), a profound understanding of the liquid water dynamics is needed. In this study, we propose a novel geometrical setup to conduct a series of direct simulations of the liquid water dynamics in a GC. The conducting pathways in the gas diffusion layer (GDL) are simplified by three cylindrical pipes connected to a liquid water reservoir representing the catalyst layer (CL). The droplet dynamics, corner film dynamics, and the competition between the film and droplet flows in the GC are explored in detail. The results show that the three-phase contact line plays an important role in resisting the gas drag force for a droplet movement in the GC. The gas drag force can dominate the film flow along the GC corners, and a proper selection of the contact angle of the GC sidewalls is necessary to balance two requirements: increasing the film removal ability and removing the water clogging fast. The competing mechanisms of the droplet and film flows give us the possibility to regulate liquid water flow into GCs, and maybe lead to a better water management in GCs. Finally, the results from this work also serve to provide insights into the development of a phenomenological model for the liquid water flooding in GCs.

No abstract

No abstract

No abstract

In a polymer electrolyte membrane (PEM) fuel cell water is produced by electrochemical reactions in the catalyst layer on the cathode side. The water diffuses through the catalyst layer and a fibrous substrate into gas channels where it is transported away by convection. The fibrous substrate represents the gas diffusion media (GDM). Sometimes the GDM has a thin microporous layer on the side facing the catalyst layer. The same layer structure can be found on the anode side. All layers together are the porous layers of a PEM fuel cell. Under certain operating conditions condensation can occur in the porous layers which might lead to flooding conditions and — if the liquid water forms droplets which grow together in the gas channels — the complete blockage of the channels. Both situations can lead to a local starvation of reactant gases with negative impact on fuel cell performance and durability. The void space of the hydrophobic fibrous substrate in a PEM fuel cell can be interpreted as micro channels in a broader sense, especially if liquid phase transport from the catalyst layer towards the gas channels is in focus. Due to the small dimensions with effective channel diameter in the range of micrometer the flow of liquid water is governed by capillary forces. The same applies for the gas channels at low gas velocities since the Bond and Capillary numbers are well below one. Thus the investigation of liquid water flow and distribution under low gas velocities in the hydrophobic fibrous substrate and the spreading of liquid water along the hydrophilic gas channel walls under capillary action is of special interest for PEM fuel cells and investigated here.

In various chemical processes thorough homogeneous mixing is of great importance. Due to their small characteristic dimensions, micromixers have a great potential to achieve fast and uniform mixing. However, in the field of powder synthesis from precipitation processes the use of standard micromixers is severely limited because of rapid clogging of the microchannels. As an alternative, mixing nozzles which are less susceptible to fouling can provide a sufficient mixing quality. The flow field and fluid distribution inside multi-fluid droplets during droplet formation is simulated. Depending on the geometry and flow rates, complex velocity fields and flow distributions are found and the impact on the mixing efficiency is qualitatively deduced. Furthermore, we point out how the tendency of fouling can be further reduced with the help of improved nozzle geometries.