Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session U31: Drops: Heat Transfer, Evaporation and Buoyancy Effects I |
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Chair: Vladimir Ajaev, Southern Methodist University Room: 239 |
Tuesday, November 22, 2022 8:00AM - 8:13AM |
U31.00001: Numerical simulation of spray drying using droplet drying kinetics Anurag Bhattacharjee, Aswin Gnanaskandan Spray drying is the most widely used method for producing dry powder particles (e.g., instant coffee, milk, starch, cheese, bio-therapeutics, etc.) by atomizing a liquid or slurry and rapidly drying it using a hot gas as the drying medium. We are developing a numerical model to accurately predict spray droplet evaporation and evolution of particle formation in spray drying. Understanding the evolution of particle formation in a spray dryer is important as it directly influences the final product quality. Experiments inside a spray dryer are often not able to shed light on the physical processes governing the particle formation at the scale of an individual particle. Hence, we are developing a numerical method that uses complex mass and heat transfer equations to account for the change in properties of a single droplet based on advanced droplet drying kinetics. The drying kinetics model consists of two stages of drying. First stage consists of droplet shrinkage with loss of moisture and diffusion of dissolved or suspended solid particles inside the droplet. Second stage starts when the concentration of the dissolved or suspended solid content on droplet surface reaches a critical saturation level at which point a porous crust is formed. The drying stops when the moisture content within the droplet reaches a desired value. The key feature of the model is the inclusion of first order dominating physics at all stages of drying without compromising on speed and efficiency of the model. The model is validated using single droplet drying experiments from literature. Details of the methodology along with simulation results will be discussed. |
Tuesday, November 22, 2022 8:13AM - 8:26AM |
U31.00002: Transient geometrical features of a dense spray puff Lucas Rotily, Emmanuel Villermaux A spray puff is, from a fluid dynamics point of view, an impulsive source of mass and momentum diluting in a quiescent, dry environment. We address the question of its lifetime, namely the time it takes for all the droplets it is composed of, to fully evaporate. This lifetime is much larger than the one of the individual droplets taken in isolation (typically seconds versus milliseconds). The contrast is attributed to the markedly non-Poissonian repartition of the droplets within the puff. Intermittent clusters of concentrated droplets are separated by large voids. Using liquids with very different volatilities, we document the dynamics of the clusters (through the evolution of their `coarse grained scale’, Phys. Rev. Letters (2006) 97, 144506), and of the voids (through the evolution of droplets field Voronoi tessellations). From these measurements emerges a unified picture explaining the substantial evaporation delay of the puff in spite of its initial division into fine droplets. This scenario is furthermore compatible with the evolution of the transient droplets size content of the spray, of which we give a full description up to the complete evaporation. |
Tuesday, November 22, 2022 8:26AM - 8:39AM |
U31.00003: Effects of substrate heating and particle size variation on the contact line pinning-depinning dynamics of evaporating sessile droplets containing mono-/bidispersed colloidal particles Nagesh D Patil, Suryansh Gupta, Mahesh R Thombare We investigate the contact line (CL) pinning and depinning dynamics of evaporating sessile water droplets containing mono-/bidispersed colloidal particles on nonheated/heated hydrophobic polydimethylsiloxane substrate. Transient variation of droplet shapes and the liquid-gas interfacial temperatures are acquired using a high-speed camera and an infrared-thermography, respectively. On the nonheated substrates, it is seen that for droplets with mono- and bidispersed colloidal particles, there exists a notable difference in the CL pinning durations leading to a variation in the particle deposition patterns. A first-order model is illustrated based on the various forces acting on the particles near the CL. For the bidispersed cases, a variation in the inter-particle contact force in the presence of particles of different sizes is unveiled. Further, for the heated substrate cases, a temperature difference exists between the droplet's apex and the CL. Moreover, a stagnation zone develops near the CL region due to the combined effects of outward-driven capillary flow and circulatory thermal Marangoni flow. Consequently, the complete pinning of the CL with a ring-like deposit is manifested for the bidispersed case as opposed to an inner deposit for the monodispersed case. |
Tuesday, November 22, 2022 8:39AM - 8:52AM |
U31.00004: Evaporation of particle-laden drops: The role of particle formation in situ Mario F Cordova Gonzalez, Hossein Hejazi The evaporation characteristics of drops are relevant in many engineering applications including chemical deposition, cooling, spray, and combustion. The addition of particles and surfactants in liquid drops can change the evaporation rate where often the rate is found to be a decreasing function of particle concentration. Here, we place a highly concentrated solution of heptane with sorbitan monooleate (Span 80 at 5.0 wt.%) in contact with a mixture of sodium silicate solution and ammonium bicarbonate (the reagents). Consequently, an emulsion phase is spontaneously formed in heptane where the reagent-filled micelles (emulsion drops) undergo a sol-gel reaction, hence, forming silica particles in situ. We examine the evaporation dynamics of drops where silica particles are formed in situ and study the extent to which the effects are analogous to those of drops with ex situ dispersed silica particles. We monitor the shape of pendant drops of heptane laden with emulsions resulting in the history reduction of the droplet's radius. The drop lifetime depends on the age of the sample and is considerably altered with the origination of colloidal size particles from the emulsion templates. |
Tuesday, November 22, 2022 8:52AM - 9:05AM |
U31.00005: Theoretical and Numerical Comparisons of Evaporation Models for Liquid Droplets Christopher K Larsson, Satish Kumar Evaporation of liquid droplets is a phenomenon that has received considerable attention yet still presents fundamental modeling questions that remain unresolved. Among the most prominent of these questions concerns the similarities and differences between two principal evaporation models: diffusion-limited and one-sided. These models differ greatly in their formulation and ease of implementation but relatively few works to date have performed a side-by-side evaluation of their predictions. We use lubrication theory and numerical simulations to develop physical understanding by comparing and contrasting predictions of these two models. The time evolution of the droplet radius and contact angle are tracked, and the influence of thermal Marangoni forces on these is considered. Because droplets often contain suspended particles, we also investigate the case where the droplet contains colloidal particles and examine the predicted particle deposition patterns. Our results provide insight into the conditions under which the two evaporation models provide qualitatively different predictions as well as the physical mechanisms responsible for these differences. |
Tuesday, November 22, 2022 9:05AM - 9:18AM |
U31.00006: Anisotropic Heat and Mass Transport Induced by Droplet-Scale Hydrodynamics of Multiphase Flows under Shear Yanxing Wang, Tie Wei, Fangjun Shu The heat and mass transport characteristics of a shear-driven multiphase flow with single and multiple droplets have been numerically studied over a wide range of parameters. The heat and mass exchange between the dispersed droplets and the continuous fluid and heat and mass transport within the mixture have been considered. In the presence of flow shear, interface tension at the droplet surface constrains droplet shearing and generates a microscale recirculating flow inside and outside the droplet. The flow recirculation works as a micro mixer, which enhances the heat and mass transport. When the droplet size is large and surface tension cannot resist the tearing of flow shear stress, the droplets deform greatly and even break up. The deformation and breakup impose a complex transient hydrodynamic effect on heat and mass transfer around the droplets. When the volume fraction of the dispersed droplets is large, the contact and non-contact interactions among the neighboring droplets will affect the behaviors of each droplet and cause disturbances in the continuous fluid. The disturbances also enhance heat and mass transport in the system. This research offers promising potential for the precise control of the heat and mass transfer in multiphase flows. |
Tuesday, November 22, 2022 9:18AM - 9:31AM |
U31.00007: Transient growth stability analysis of evaporating sessile drops comprising binary mixtures Katie Thomson, Adam G Williams, George Karapetsas, Omar K Matar, Yutaku Kita, Khellil Sefiane, Daniel Orejon, Prashant Valluri The evaporation and spreading dynamics of a binary mixture sessile drop are complex due to the interplay of thermal and solutal Marangoni stresses alongside the hydrodynamic transport, evaporation, mass diffusion and capillary stress of the drop. Our quasi-steady linear stability analysis of volatile bicomponent sessile drops comprising ethanol-water mixtures placed on heated substrates demonstrates that evaporation is highly unstable with several competing modes. Whilst the analysis qualitatively agrees with experiments, presence of multiple competing modes indicates that the quasi-steady analysis may not be suitable for volatile bicomponent sessile droplet systems. To understand the roles of these modes better, we perform a transient growth analysis. Here, we apply small disturbances to the binary system base state, giving perturbed stability equations that evolve with time. Perturbations are introduced into the system at an early time instance. These are solved alongside the base state to find the linear stability growth characteristics, in order to compare to the dispersion curves obtained from the quasi-steady state stability analysis. Similar to findings from our quasi-steady analysis, our results show drop interfacial instabilities occur predominantly at the contact line. |
Tuesday, November 22, 2022 9:31AM - 9:44AM |
U31.00008: Pinning-Induced Evaporating Droplet Self-Propulsion Hyeongyun Cha, Moon-Kyung Kim, Ho Chan Chang, Lenan Zhang, Evelyn N Wang, Nenad Miljkovic Droplet formation and removal dynamics are ubiquitous processes found in nature and have significant impact on engineering applications. Droplets are prone to adhere on solid surfaces which contain unavoidable surface defects stemming from chemical and topographical heterogeneity. Although powerful, state-of-the-art techniques to overcome the contact line (CL) pinning are spatially limited due to surface energy gradients or require external energy to initiate droplet motion. Here, we show that intrinsic CL pinning on defects can be utilized to generate spontaneous droplet motion without any external energy input. Through experimental and theoretical analysis, we demonstrated that droplets can harness uniformly distributed surface defects to create CL curvature asymmetry and corresponding contact angle variation, which lead to a rapid motion of droplet for a wide range of conditions. Furthermore, we experimentally and numerically demonstrated that self-propelled droplets can enhance evaporation and carry contaminants inside as an alternate self-cleaning mechanism. In contrast to conventional understanding, the insights gained here reveal opportunities for taking advantage and tailoring CL pinning to achieve high droplet mobility. |
Tuesday, November 22, 2022 9:44AM - 9:57AM |
U31.00009: Evaporation of sessile droplets of aqueous solutions in pure vapor environment Vladimir S Ajaev, James Barrett We develop a lubrication-type model of an axisymmetric evaporating sessile liquid droplet in contact with pure vapor. The liquid is a symmetric electrolyte, a two-component expression for disjoining pressure accounts for both unbalanced London–van der Waals interactions and repulsion of electrical double layers formed near liquid–solid and liquid–vapor interfaces. We consider nonequilibirum effects during evaporation from the liquid surface and an increase of solute concentration as a result of solvent evaporation. The presence of solute leads to reduction of the evaporation rates at initial stages of evolution but the trend is reversed at the later stages, resulting in lower lifetimes of evaporating droplets. The apparent contact angle, defined by the maximum interfacial slope, tends to be lower when the electrostatic effects are more significant. Evaporative cooling is also considered in the framework that accounts for heat conduction in the substrate and shown to increase the droplet lifetime. Extensions of the model describing the more general case of evaporating sessile droplets of complex fluids are discussed. |
Tuesday, November 22, 2022 9:57AM - 10:10AM |
U31.00010: A self-driven motion induced by freezing in free supercooled water drops Claudiu A Stan, Armin Kalita, Thomas F Kaldawi The freezing of liquid droplets can induce counterintuitive dynamics such as spontaneous trampolining on hydrophobic surfaces. Here we investigate how the freezing of free supercooled water drops in vacuum can induce a sudden motion of the drops. We propose that the motion is caused by a transient and asymmetric increase in the vapor pressure across the drop surface, as the drop heats to the melting temperature (recalesces) during freezing. For the limiting case where freezing is initiated at the surface of the drop, the magnitude of the induced drop velocity is maximal, and can be described by an analytical formula that does not depend on the drop size. The observed velocities imparted by freezing in two different experimental studies are somewhat smaller than the maximal velocity predicted by our formula, but comparable to it. We will discuss the reasons for observing motions close to the maximal velocity in the experimental studies, and how this self-driven motion could affect studies of supercooled water drops and of their freezing. |
Tuesday, November 22, 2022 10:10AM - 10:23AM |
U31.00011: Dynamics of a Nitrogen Leidenfrost Droplet on a Water pool Markus Schremb, Marijn Kalter, Srinivas Vanapalli The Leidenfrost effect for a nitrogen droplet on a water pool at its freezing temperature is studied experimentally and theoretically, with attention on the processes taking place both on top and below of the water meniscus. The temporal evolution of the droplet size and the ice formation inside the pool are captured in a top-view and a side-view using a high-speed video system. The amount of ice present in the pool after thermal relaxation is used to measure the total heat actually transferred between the droplet and the pool. |
Tuesday, November 22, 2022 10:23AM - 10:36AM |
U31.00012: Stability of Relative Trajectories of Contaminated Spherical Drops in Combined Gravitational and Thermcapillary Motion Michael A Rother The motion of two interacting, spherical drops in the presence of |
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