Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session F09: Bubbles: Growth, Heat Transfer and Boiling I |
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Chair: Elias Balaras, George Washington University Room: Georgia World Congress Center B214 |
Monday, November 19, 2018 8:00AM - 8:13AM |
F09.00001: Thermal and hydraulic characteristics of bubbly flow in a metallic-porous-media Daeyeon Kim, Kyung Chun Kim In the past decades, heat transfer and flow behavior in porous medium has attracted many researchers. Geometrical properties of high-porosity open-cell metallic foams possessing extremely high specific surface area and lots of randomly connected path inside the medium body can improve thermal performance of compact heat exchangers, especially in the two-phase bubbly flow. In this study, flow boiling in metal foam inserted channel is experimentally investigated with a focus on the bubbly flow regime. The channel of cross section 5 mm by 50 mm is used to study the effects of vertical-up flow. R245fa is selected as the working fluid and hot water is used as a heating source. Boiling heat transfer coefficient and pressure drop properties across different types of metallic foam structure are measured and discussed with the effects of the inlet vapor quality and the heat flux. In order to observe the bubble behavior through the metallic porous media, a visualization experiment is performed. The results addressed in this paper could be helpful to understand the flow boiling heat transfer mechanism inside the metallic porous media. |
Monday, November 19, 2018 8:13AM - 8:26AM |
F09.00002: Immersed boundary methods for fluid-structure interaction problems in two-phase flows with phase changes Saurabh Chawdhary, Akash Dhruv, Anshu Dubey, Elias Balaras Immersed boundary methods (IBM) have been extensively used to study a variety of flow problems, particularly cases involving complex boundaries undergoing large displacements and/or deformations. However, majority of applications are limited to single phase flows due to numerical challenges in modeling interaction of phase boundaries with the solid surface. Furthermore, existing techniques for multiphase IBM do not account for explicit contact line dynamics important in many physical problems. We will present a numerical framework which accounts for these mechanisms and is applicable to flows over irregular bodies. The mathematical model is implemented in a block-structured grid topology with adaptive mesh refinement (AMR) enabling selective refinement around the areas of interest. The contact line treatment is implemented by satisfying a dynamic contact angle boundary condition for the level-set function which is based on fluid properties and local flow velocity. We demonstrate the accuracy and robustness of the formulation in fluid-structure interaction problems involving phase changes through simulation of benchmark problems involving pool boiling over complex heater geometries. |
Monday, November 19, 2018 8:26AM - 8:39AM |
F09.00003: Long-term Ostwald ripening of gas bubble population in porous media: pore-geometry-driven anti-coarsening effect vs. gravity-driven coarsening effects Qingrong Xiong, Ke Xu In this work, long term evolution of gas bubbles in porous media is studied with a simple pore-network modelling approach, based on analytical models and experimental results we previously published (Xu et al. PRL, 2017). The effect of gravity is emphasized and discussed. The evolution of gas bubbles without gravity matches our experiments well, which further confirms the anti-coarsening effect which is contrary to typical Ostwald ripening and leads to uniformly sized bubbles in a homogeneous medium. Ostwald ripening is shown to be the driving mechanism; however, the relationship between surface curvature and bubble size determined by the pore-throat geometric confinement reverses the ripening direction. Furthermore, the role of gravity is studied and we show that gravity can provide additional driving force that results in gas bubble shrinking at stratum bottom and expending around stratum roof, which lead to gas concentrating at the top. This effect can be considerable when the thickness of porous media is at or above the order of magnitude of 1 m. A field-scale model is then built and validated by this pore-network model, and long-term (decades to centuries) bubble evolution dynamics in porous media is predicted which explains some field CO2 sequestration |
Monday, November 19, 2018 8:39AM - 8:52AM |
F09.00004: Non-intrusive diagnostics of single bubble nucleate pool boiling heat transfer phenomena: Boiling curve under subcooled conditions Surya Narayan L, Tajinder Singh, Suneet Singh, Atul Srivastava Experimental investigation of single vapor bubble formation during nucleate pool boiling of water under sub-cooled conditions has been reported. Measurements have been performed for different values of sub-cooling levels (∆Tsub = 0 to 10oC) in a non-intrusive manner using rainbow schlieren deflectometry technique. The recorded schlieren images are qualitatively as well as quantitatively analyzed to understand the relative influence of various phenomena such as natural convection, bubble growth, evaporative and superheat layers on the overall heat transfer rates during a single bubble cycle. Based on the spatial extent of thermal boundary layer near the heated substrate and the two-dimensional temperature distributions, individual contributions of various heat transfer sub-processes have been estimated. Of all the sub-processes, the contribution of natural convection towards the overall heat transfer rates was found to be relatively dominant at any given subcooled level. Based on the non-intrusive measurements, the classical boiling curve has been extended to subcooled conditions to further classify the nucleate boiling regime into three meta-stable regimes (oscillating, departing and stable vapor bubbles). |
Monday, November 19, 2018 8:52AM - 9:05AM |
F09.00005: Rainbow schlieren-based simultaneous mapping of single vapour bubble dynamics and temperature gradients during nucleate flow boiling in a vertical channel Gulshan Kumar Sinha, Saylee Mahimkar, Atul Srivastava Dynamic parameters of single vapor bubble and the associated thermal gradients in water-based nucleate flow boiling configuration have been simultaneously mapped using rainbow schlieren deflectometry. Flow boiling experiments (upward as well as downward flow) have been conducted in a rectangular cross-section channel for varying Reynolds numbers under subcooled conditions. Experimental results are recorded in the form of real-time schlieren images and interpreted to understand the underlying physics of various phenomena such as bubble growth, bubble departure, bubble lift-off and condensation and their dependence on process parameters. The associated sub-processes such as bubble inception inside superheated layer, microlayer formation, and depletion during departure and scavenging of the superheated layer at the time bubble lift-off have been observed. Quantitative analyses of the images provided useful information in terms of bubble dynamic parameters (equivalent bubble diameter, aspect ratio, contact angle, and bubble trajectory) along with the near wall thermal gradients as a function of varying Reynolds numbers. |
Monday, November 19, 2018 9:05AM - 9:18AM |
F09.00006: Predicting Bubble Size Distribution in Polyurethane Foam through Nucleation and Bubble Growth Modeling Irfan Khan, Valeriy Ginzburg Polyurethane (PU) foams are extensively used in industry due to their ease of processing, insulation efficiency and structural stability. The process of making PU foams involves the reaction of isocyanates with water to make carbon dioxide that generates foam bubbles; called the “blowing” reaction. Additionally the “gelling” reaction, between isocyanate and polyol, is responsible for make polyurethane linkages that provide structural stability. The relative rates of gelling and blowing reactions along with several other parameters such as use of surfactants, catalysts, reactivity of polyols, water content as well as operating conditions determine the final foam properties. Therefore, developing next generation PU insulation material relies on understanding the effect of the material properties and processing conditions on final insulation properties, including bubble size distribution and porosity. A numerical model that simulates nucleation and bubble growth in free-rise PU foams is presented here. Nucleation is based on classical nucleation theory and the influence volume approach has been leveraged to model the bubble growth. By solving the mass, momentum and species balance equations, the model can predict the bubble size distribution and porosity of free-rise PU foams. |
Monday, November 19, 2018 9:18AM - 9:31AM |
F09.00007: Experimental Study of Bubble Dynamics of Direct Contact Condensation of Steam in Subcooled Water Joseph Seo, Sero Yang, Kimoon Lee, Yassin A Hassan A study on the relationship between hydraulic model and heat transfer characteristics of the direct contact condensation in an unstable bubbling condensation oscillation region during vertical upward steam injection into subcooled water pool has been conducted. Experiments were performed by injecting 0.5 g / s to 4.0 g / s of steam through a nozzle having an inner diameter of 10 mm into the water tank containing 70 to 95 C° of DI water. To obtain images of bubble dynamics, a high-speed camera was used. Bubble dynamic parameters such as collapsing frequency, departure frequency, bubble equivalent diameter, rising velocity, and heat transfer coefficient were calculated. The system dimensional analysis was performed, and the result was compared with existing bubble dynamics models developed from the subcooled pool boiling studies. As a result, a similarity was found that the product of the bubble departure frequency and the diameter is proportional to the bubble rising velocity. On the other hand, a difference was seen in the proportionality coefficient due to the phenomenological difference. Furthermore, the correlation between departure frequency in hydraulic dynamics model and heat transfer coefficient was investigated. |
Monday, November 19, 2018 9:31AM - 9:44AM |
F09.00008: Modelling transport of species at bubble interfaces with a combined level set and volume of fluid method Michael Allshouse, Amir Taqieddin, Akram Alshawabkeh Modelling electrochemically gas-evolving systems like alkaline water electrolysis and Hydrogen production requires a multiphase and multiphysics approach. Optimizing the efficiency of these systems demands an understanding of the generation, growth, and dynamics of bubbles as they absorb or release dissolved gas in the fluid. Modelling transport of species between the bubble and fluid requires a precise bubble interface and conservation of mass. The volume of fluid method conserves mass, but inherent numerical diffusion smears the interface and produces spurious velocities. Alternatively, the level set approach does not suffer from numerical diffusion of the interface but does not conserve mass. We use a hybrid solver that combines the level set and volume of fluid approaches to both conserve mass and accurately model the interface. Combining the hybrid solver with a transport of species model in OpenFOAM, we model rising bubbles in reactive and non-reactive flows. The model produces similar results to experiments and accurately captures the stationary growth rate. We also present studies that demonstrate the impact of transport of species on the evolution of rising bubbles. |
Monday, November 19, 2018 9:44AM - 9:57AM |
F09.00009: Ultrasonic Manipulation of Direct Contact Vapor Bubble Condensation Thomas Boziuk, Marc K Smith, Ari Glezer The growth, advection dynamics, condensation rate, and collapse of vapor bubbles injected into a subcooled liquid bath are manipulated using pulsed ultrasound (MHz) actuation. The vapor is injected vertically through a nozzle, forming bubbles with a characteristic diameter larger than that of the acoustic beam. Actuation exploits the mismatch of the acoustic impedance at the liquid-vapor interface, strongly deforming it to form a “spear” of liquid that penetrates the vapor volume, ultimately forming a torus; the acoustic beam focuses within the spear, ejecting atomized droplets that increase the condensation rate. The rapid deformation induces a vortex ring which is advected along the axis of the acoustic beam (absent actuation, a vortex ring forms during bubble collapse and instead advects upwards). High-speed Schlieren imaging and PIV are used to investigate the liquid flow fields around collapsing vapor bubbles in the absence and presence and ultrasonic actuation. It is shown that the impulse of the induced vortex rings increases with increased temperature difference between the vapor and the bulk liquid. The vapor collapse is compared with the acoustically-manipulated collapse of cavitation bubbles having a characteristic diameter much smaller than the acoustic beam. |
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