Effect of wall surface wettability on collective behavior of hydrogen microbubbles rising along a wall

This paper presents an experimental study of the influence of wall surface wettability on the behavior of hydrogen microbubbles rising along a nearly vertical wall. Multiple optical diagnostics, including particle tracking velocimetry, have been employed for the study. The microbubble behavior observed along three different kinds of wall surfaces (hydrophobic, hydrophilic, and super-hydrophilic) was characterized by the microbubble-wall attachment, bubble size distribution, bubble coalescence, and microbubble layer formation. Microbubbles rising along the wall with poor wettability soon attach to the wall and grow to millimeters in size as a result of bubble coalescence. Such millimeter-sized bubbles detach from the wall because of their increased buoyancy, and eventually enhance transverse diffusion of microbubbles, which is known as the sweep-out effect. In contrast, in the case of very good wettability, almost no microbubbles attach to the wall and smoothly form a thin microbubble layer in the wall proximity. The observed phenomena contradict our intuitive expectation of the effect of surface wettability on gas bubbles, and hence may be regarded as a feature of microbubbles that distinguishes them from large bubbles

Effect of heated wall inclination on natural convection heat transfer in water with near-wall injection of millimeter-sized bubbles

Natural convection heat transfer from a heated wall in water with near-wall injection of millimeter-sized bubbles is studied experimentally. Velocity and temperature measurements are conducted in the nearwall region. In the range of the heated wall angles from 0 to 40 degrees from the vertical, the heat transfer coefficient increases by up to an order of magnitude with bubble injection. The ratio of the heat transfer coefficient with bubble injection to that without injection increases with the wall inclination angle. Based upon measured liquid temperature distributions and liquid flow velocity profiles, enhancement of heat transfer by bubble injection is explained by two mechanisms. First, wall-parallel transport of cold liquid into the thermal boundary layer is enhanced by the bubble-driven flow. Second, wall-normal mixing of warm liquid and cold liquid occurs, as a result of wall-normal velocity fluctuations of the liquid phase activated by a combination of bubble rising motion, vortex shedding from the bubbles, and unsteady vortices formed within the boundary layer. The unsteady vortices travel along the wall together with the bubbles, primarily contributing to the enhancement of heat transfer at higher wall inclination angles

Computational investigation of prolonged airborne dispersion of novel coronavirus-laden droplets

We have performed highly accurate numerical simulations to investigate prolonged dispersion of novel coronavirus-laden droplets in classroom air. Approximately 10,900 virus-laden droplets were released into the air by a teacher coughing and tracked for 90 min by numerical simulations. The teacher was standing in front of multiple students in a classroom. To estimate viral transmission to the students, we considered the features of the novel coronavirus, such as the virus half-life. The simulation results revealed that there was a high risk of prolonged airborne transmission of virus-laden droplets when the outlet flow of the classroom ventilation was low (i.e., 4.3 and 8.6 cm/s). The rates of remaining airborne virus-laden droplets produced by the teacher coughing were 40% and 15% after 45 and 90 min, respectively. The results revealed that students can avoid exposure to the virus-laden droplets by keeping a large distance from the teacher (5.5 m), which is more than two times farther than the currently suggested social distancing rules. The results of this study provide guidelines to set a new protection plan in the classroom to prevent airborne transmission of virus-laden droplets to students

Numerical model for cough‐generated droplet dispersion on moving escalator with multiple passengers

To investigate the motion of virus‐laden droplets between moving passengers in line, we performed numerical simulations of the distribution of airborne droplets within a geometrically detailed model similar to an actual escalator. The left and right sides and the ceiling of the escalator model were surrounded by walls, assuming a subway used by many people every day with concern to virus‐laden droplets. Steps and handrails were incorporated in the model to faithfully compute the escalator‐specific flow field. The ascending and descending movements of the escalator were performed with 10 or 5 passengers standing at different boarding intervals. To resolve the unsteady airflow that is excited by a moving boundary consisting of passengers, steps, and handrails, the moving computational domain method based on the moving‐grid finite‐volume method was applied. On the basis of the consideration that the droplets were small enough, droplet dispersion was computed by solving the equation of virus‐laden droplet motion using a pre‐computed velocity field, in which the flow rate of a cough, diameter distribution, and evaporation of droplets are incorporated. The simulation resolved the detailed motion of droplets in flow, and therefore, we were able to evaluate the risk of viral adhesion to following passengers. As a result, we found that the ascending escalator had a higher risk of being exposed to virus‐laden droplets than the descending escalator. We also reported that the chance of viral droplet adhesion decreases as the distance from the infected person increases, emphasizing the importance of social distancing

Pulsatory rise of microbubble swarm along a vertical wall

Based on the experimental finding that microbubble swarms dramatically promote heat transfer from a vertical heated wall, despite their potentially adiabatic nature, tests of microbubble fluid mechanics in the isothermal state are performed to clarify the unique motion characteristics of microbubble swarms. At constant bubble flow rate, the microbubble swarm shows a significant pulsatory rise along a vertical flat wall, particularly for small bubbles. Particle tracking velocimetry applied to the microbubbles shows that a two-way interaction between the microbubbles and the liquid flow self-excites the pulsation during their co-current rise. The sequence consists of the following processes: (i) increase in the bubble number density close to the wall as a result of the liquid velocity gradient driven by the microbubbles themselves; (ii) wave generation inside the microbubble swarm to induce the pulsatory rise of the swarm; and (iii) amplification of the waves, which results in void-bursting motion in the final stage

Natural convection heat transfer enhancement using bubble injection between vertical parallel plates

We experimentally study natural convection heat transfer enhancement using bubble injection between a vertical heated plate and a flow guiding plate in water. While heat transfer is more than tripled with bubble injection, the introduction of the flow guiding plate parallel to the heated plate further increases heat transfer by up to 12% despite attenuating the wall-normal liquid fluctuating velocity. We attribute further heat transfer enhancement to increased wall-parallel liquid velocity driven by bubbles rising at high speeds. We also perform temperature calculations incorporating experimentally measured mean liquid velocities, which permit a quantitative evaluation of the wall-parallel mean liquid velocity and wall-normal liquid fluctuating velocity contributions to heat transfer. The calculations show that the fractional contribution of wall-parallel mean liquid velocity with bubble injection to heat transfer is high, especially in the upstream region of the heated section, and the presence of the flow guiding plate near the heated wall increases the contribution by up to 30% compared with the case without the plate

Computational investigation of prolonged airborne dispersion of novel coronavirus-laden droplets

We have performed highly accurate numerical simulations to investigate prolonged dispersion of novel coronavirus-laden droplets in classroom air. Approximately 10,900 virus-laden droplets were released into the air by a teacher coughing and tracked for 90 min by numerical simulations. The teacher was standing in front of multiple students in a classroom. To estimate viral transmission to the students, we considered the features of the novel coronavirus, such as the virus half-life. The simulation results revealed that there was a high risk of prolonged airborne transmission of virus-laden droplets when the outlet flow of the classroom ventilation was low (i.e., 4.3 and 8.6 cm/s). The rates of remaining airborne virus-laden droplets produced by the teacher coughing were 40% and 15% after 45 and 90 min, respectively. The results revealed that students can avoid exposure to the virus-laden droplets by keeping a large distance from the teacher (5.5 m), which is more than two times farther than the currently suggested social distancing rules. The results of this study provide guidelines to set a new protection plan in the classroom to prevent airborne transmission of virus-laden droplets to students