Modelling droplet motion and interfacial tension in filters collecting liquid aerosols

Extensive experimental investigations have shown some of the differences between the behaviours of the barrel and the clamshell shapes of droplets on filter fibres in flow fields. Realistic flow velocities (such as those used in many industrial filter systems) were utilised. The forces acting are air drag, interfacial tension and gravity. The properties of the interfacial restoring force are modelled, and show agreement with the experimental results, at least in the linear extension region before the onset of oscillatory behaviour of the droplets (induced by instability of the flow field). The model for the oscillatory behaviour is explored, and the natural frequencies of oscillation in the radial and transverse directions are shown to be the same, for the barrel shape. The clamshell shape also has the same natural frequencies, but they are different to those of the barrel shape. The coupling of the radial and transverse oscillation modes is explored for both the barrel and clamshell shape. Some contact angle results are given, both without airflow acting on the droplet and with increasing airflow

High Flash-point Fluid Flow System Aerosol Flammability Study and Combustion Mechanism Analysis

The existence of flammable aerosols creates fire and explosion hazards in the process industry. Due to the operation condition of high pressure circumstances, heat transfer fluids tend to form aerosols when accidental leaking occurs on pipelines or storage vessels. An aerosol system is a complicated reactive system; there are neither systematic flammability data similar to the case with pure gases, nor clearly described ignition-to-combustion process of a droplet-air mixture system. The flammable regions of three main, widely-used commercial heat transfer fluids: Paratherm NF (P-NF); Dowtherm-600 (D-600); and Plate Heat Exchange Fluid (PHE), were analyzed by electro-spray generation with laser diffraction particle analysis method. The aerosol ignition behavior depends on the droplet size and concentration of the aerosol. From the adjustment of differently applied electro-spray voltages (7-10 kV) and various liquid feeding rates, a flammable condition distribution was obtained by comparison of droplet size and concentration. All of the fundamental study results are to be applied to practical cases with fire hazards analysis, pressurized liquid handling, and mitigation system design once there is a better understanding of aerosols formed by high-flash point materials. On the other hand, the process of combustion from initial stage to global flame formation was simulated with COMSOL-multi-physics in terms of heat transfer, droplet evaporation, and fluid dynamics of liquid-air interaction. The local temperature change through time, as an indicator of luminous flame appearance, was analyzed to describe the flame development and ignition delay time of aerosols. We have conducted a series of simulation regarding physical formula in description of this combustion process, and will conclude with how temperature distribution influenced the appearance of luminous flames, which was the symbol of successful ignition of aerosol. The mitigation implementing timing and location can be characterized with further understanding of this combustion process. The potential application of the ignition delay will be beneficial to the mitigation timing and detector sensor setting of facilities to prevent aerosol cloud fires. Finally, the scientific method of aerosol flammability study was discussed for its potential impacts on experimental results. A modeling point of view was introduced, with the analysis of electric field application on fuel droplets, and the related fundamental study of the ignition phenomenon on aerosol system. Existing charges from electrospray is beneficial for the monodispersity and control of aerosols for fundamental study. However, the additional charges accumulated on the droplet surfaces are likely to have impacts on flammability due to the excess energy they applied to the aerosols system and droplet-droplet distraction or turbulences. This is a re-visit of aerosol flammability study method, with a conclusion that charges did have positive impact on droplets’ ignition concentration range with a balancing effect on turbulence increase to reduce the ignition chance

Modeling Fluid Interactions with Granular and Fibrous Surfaces

Understanding the interactions between a body of liquid and a curvy surface is important for many applications such as underwater drag force reduction, droplet filtration, self-cleaning, and fog harvesting, among many others. This study investigates ways to predict the performance of granular and fibrous surfaces for some of the above applications. More specifically, our study is focused on 1) modeling the mechanical stability of the air-water interface over submerged superhydrophobic (SHP) surfaces and their expected drag reduction benefits, and 2) predicting the mechanical stability of a droplet on a fiber in the presence of an external body force. For the first application, we modeled the air–water interface over submerged superhydrophobic coatings comprised of particles/fibers of different diameters or Young–Laplace contact angles. We developed mathematical expressions and modeling methodologies to determine the maximum depth to which such coatings can be used for underwater drag reduction as well as the magnitude of the depth-dependent drag reduction effect of the surface. For the second application, we studied the force required to detach a droplet from a single fiber or from two crossing fibers. The results of our numerical simulations were compared to those obtained from experiment with ferrofluid droplets under a magnetic field, and excellent agreement was observed. Such information is of crucial importance in design and manufacture of droplet–air and droplet–fluid separation media, fog harvesting media, protective clothing, fiber-reinforced composite materials, and countless other applications