Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session A03: Reacting Flows: Instabilities & General |
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Chair: Reza Nazari, Arizona State University Room: North 121 A |
Sunday, November 21, 2021 8:00AM - 8:13AM |
A03.00001: Aerodynamics of Tsuji burners with augmented fuel injection Brandon Li, Jose Grana, Antonio L Sanchez, Forman A Williams This study addresses the aerodynamics of Tsuji burners, involving a flame developing from the forward stagnation region of a cylindrical porous fuel injector placed in a uniform air stream with moderately large Reynolds number. Attention is focused on conditions under which the fuel-injection velocity is not sufficiently small compared with the outer air velocity for the boundary layer to remain attached to the cylinder surface. In the resulting flow, the flame is embedded in the thin mixing layer that forms at the surface separating the outer air stream from the fuel stream, both having, in general, different densities. The flow on the air side of the mixing layer is potential, while that on the fuel side is rotational because fuel injection generates vorticity through the requirement that fuel emerges normal to the cylinder surface. It is shown that introduction of a suitably density-weighted stream function reduces the problem to that of constant-density flow, with the density-square-root-weighted ratio of injection velocity to free-stream velocity Λ emerging as the only controlling parameter. The numerical solution, involving determination of the vorticity distribution through an iterative scheme, provides the structure of the flow, including the flame location and associated strain-rate distribution. Numerical results are presented for values of Λ ranging from small (Λ<<1) to large (Λ>>1) injection velocities. The inviscid results in the limit of vanishingly small injection velocities, Λ approaching zero, demonstrate that the outer air velocity never approaches the classical solution corresponding to potential flow around a solid cylinder (Λ=0), a result with important implications for the analysis of flames stabilized in Tsuji burners. |
Sunday, November 21, 2021 8:13AM - 8:26AM |
A03.00002: Comprehensive scaling analyses of fire whirls based on experimental data Sriram Bharath Hariharan, Joseph L Dowling, Christina Liveretou, Michael J Gollner The phenomenon of fire whirls has been studied for many decades to understand the hazards they pose during wildland fires. Experimental and numerical simulations have been used to predict their occurrence, although challenges remain in using laboratory-scale data to predict occurrence in the field. In this work, we present comprehensive scaling analyses of experimental work on fire whirls in the literature. Several combinations of characteristic length scales (pool diameter, flame width, flame height, etc.) were used to define nondimensional quantities for burning rate, circulation and flame height. The relationships between these quantities were explored to compare the different normalization methods prevalent in the literature. The most important parameters to predict flame height are the representations of momenta in the axial and azimuthal directions, defined by nondimensional buoyant and circulation momenta, which are in turn estimated using heat-release rate and circulation, respectively. The data for experimental data obtained using gaseous-fuel burner and liquid-fuel pools show distinct differences in scaling behavior, which stems from differences in the flame width. Finally, for experiments with fuel pool diameter, d = 10.5 cm, in the classic four-wall, natural air entrainment configuration, the critical enclosure-wall height (H) required for stable fire whirl formation was found to be 3.5d. |
Sunday, November 21, 2021 8:26AM - 8:39AM |
A03.00003: Flame intermittency and its influence on fire spread across a porous vegetative fuel bed Abhinandan Singh, Reza M Ziazi, Albert Simeoni Recent years have seen a significant rise in the frequency of high-intensity and wind-driven wildfires, encouraging fundamental research to develop reliable prediction models. Intermittency is an inherent fire phenomenon leading to flame breakup. The coupling between intermittency and heat transfer makes the flame spread analysis of spreading flames complex. This study investigates the role of flame intermittency and vortical structures on various aspects of heat transfer and flame spread. Experiments were performed by heterogeneously distributing longleaf pine needles across a testbed of 0.6x1.65m2 placed inside a wind tunnel. The unsteady structures generated due to flame intermittency were acquired using time-resolved measurements captured with a high-speed camera at 500fps. Flame puffing frequency, along with the size of vortical structures, was measured for a range of fuel and crossflow conditions. Additionally, total and radiative heat flux was measured at various locations along the bed, and their correlation against the flame intermittency was analyzed. The mean flame length was observed to increase with intermittency, leading to a higher flame spread rate. Moreover, the flame puffing frequency increased with increasing wind velocity, suggesting a faster flame breakup. |
Sunday, November 21, 2021 8:39AM - 8:52AM Not Participating |
A03.00004: Triple flames in vortex flows Xiao Zhang, Joseph D Chung, Elaine S Oran The blue whirl is a lifted, laminar, whirling, blue flame that burns liquid hydrocarbon fuels with no soot production. It was first created in a laboratory when a relatively large, swirling fire whirl intensified before spontaneously making a transition to a blue whirl. Recent work has shown that vaporized fuel can also create a blue whirl, and that key components of the blue whirl consist of a triple (tribrachial) flame in a vortex flow and a bubble mode of vortex breakdown. To try to understand how we might create the blue whirl in a safer, more controlled way, we focus on the basic interaction between vortex flow and energy release from combustion reactions and how it leads to a flame with the structure of a blue whirl. Here we present the results of three-dimensional numerical simulations of vortex flows with triple flames in gaseous heptane-air mixtures. We show the effect of swirl number on the final flow pattern and flame structure and analyze the effects of vorticity interactions with the triple flames. |
Sunday, November 21, 2021 8:52AM - 9:05AM |
A03.00005: Printing spongy liquid-based structures: Spontaneous emulsification at micellar solution-nanoparticle dispersion interfaces parisa bazazi, Hossein Hejazi All-in-liquid three-dimensional (3D) printed devices have broad potential applicability in processes ranging from energy storage to drug delivery and tissue engineering. Conventionally, they are produced by the jamming of nanoparticle-polymer at the oil-water interface, where one liquid is arrested in a desired non-equilibrium shape in the second liquid phase. Such structures lack the multiscale porosity that presents in equivalent solid hierarchies. Remarkably, we report on printing spongy all-in-liquid materials utilizing direct ink writing techniques, similar to those of solid architectures. Stable liquid columns of nanoparticle dispersions are formed inside micellar solution at Weber numbers three orders of magnitude smaller than that previously reported in the liquid-liquid system, featuring the printing capability at significantly low injection rates. Liquid columns are stabilized due to the rapid formation of a highly viscoelastic microemulsion phase at the nanoparticle dispersion-micellar solution interfaces. The printed aqueous phase turns into an emulsion zone, creating a porous texture in the oil phase. Consequently, a 3D structure with flexible walls consists of layered microemulsions is achieved, counterintuitive with the current liquid-based printed structures. Spongy liquids can revolutionize the current state of art liquid-in-liquid printing techniques and open novel routes to design liquid lab-on-chip devices. |
Sunday, November 21, 2021 9:05AM - 9:18AM |
A03.00006: Interfacial instability induced by an (A+B→C)-type reaction in channel Surya N Maharana, Manoranjan Mishra When a miscible solution of reactant fluid A displaces another viscosity-matched reactant B in a channel, it is found that Kelvin-Helmholtz (K-H) type billows occur at the fluid-fluid interface. A local viscosity contrast created by the high viscous product fluid C of (A+B→C)-type reaction induces these unstable K-H roll-ups at the reactive interface. The dynamics of instability are controlled by various governing parameters such as the log-mobility ratio (R), Damköhler number (Da), Péclet number (Pe), Reynolds number (Re). Hence flow features such as enstrophy and mixing dynamics are analyzed by varying these parameters. Moreover, through the directional field of vorticity, it is shown that a Horse-shoe type instability develops at the wall near to inlet with an increasing log-mobility ratio. Strikingly it is found that, even at a higher reaction rate of reaction, if the log mobility ratio is lower than a critical value (Rcrit), then the unstable K-H roll-ups do not show up. The dependency of the Rcrit on other governing parameters (Da, Pe, and Re) is depicted through the onset time (ton) and the log-mobility ratio (R) space. A manifested proportionate onset dynamic with respect to Pe and reversed dependency of onset on higher Re will be discussed in more detail. |
Sunday, November 21, 2021 9:18AM - 9:31AM |
A03.00007: Laminar Flame Dynamics of Coaxial Methane-Air Jets Under Acoustic Forcing Andres Vargas, Sarina Kiani, Ann R Karagozian The present experiments investigate the response of coaxial methane-air laminar jet diffusion flames exposed to transverse forcing within a cylindrical acoustic waveguide. High speed imaging of the forced flame dynamics was analyzed via proper orthogonal decomposition (POD) to study transitions in the combustion process. With increasing amplitude of forcing, the flame's response transitioned from sustained oscillatory combustion (SOC) to periodic lift-off and reattachment (PLOR) and eventual blow-off (BO). In the SOC regime, the flame oscillated primarily at the applied forcing frequency (fa=332Hz), while during PLOR, the periodic response was dominated by low frequencies corresponding to the much slower cycles of periodic lift-off (fl=11 to 23 Hz), creating differing POD mode coefficient maps. The effect of different outer to inner jet velocity ratios (R=0, 0.3, and 1) was investigated for different wall thicknesses (δ). It was found that higher R values can aid in flame attachment, although larger absolute velocities can cause the flame to detach and blow-off. A thicker inner tube wall also appeared to promote reattachment, suggesting the likelihood of wake-shear layer interactions with different natural stability characteristics. |
Sunday, November 21, 2021 9:31AM - 9:44AM |
A03.00008: Reduced-order modeling of flame dynamics functions with inlet acoustic modulation Zheng Qiao, Yu Lv, Adrian Sescu The study presents a G-equation based cost-efficient reduced-order model for predicting the flame dynamics function (FDF) with inlet acoustic modulation. A novel two-step approach is employed in this model: first, a steady-flame profile, which is obtained via direct numerical simulation, is employed to predict the linear mode-shape of the flow field; and then the simulation based on the G-equation is carried out to capture the dynamic behavior of the flame accurately. The main advantage of this method is that the flame profile in the nontrivial aerodynamic environment can be precisely replicated, and the flame dynamic is predicted under the physically consistent flow modulation mode. In the present work, we demonstrate the efficacy of our model with the consideration of the premixed Bunsen flame and M-shaped flame, and the comparison of our predictions with the DNS simulation results will be discussed in detail. |
Sunday, November 21, 2021 9:44AM - 9:57AM |
A03.00009: Numerical Simulations of Radial Viscous Fingering Induced by an Instantaneous Chemical Reaction Priya Verma, Vandita Sharma, Manoranjan Mishra Convective flows in a porous medium are ubiquitous in various physical phenomena such as enhanced oil recovery, chromatography separation, and contamination in aquifers. When a high viscous fluid is displaced by a less viscous fluid, the interface deforms into finger-like patterns. This phenomenon is known as viscous fingering (VF). We consider radial displacement of miscible and reactive fluids undergoing an instantaneous chemical reaction which induces viscosity variation and hence VF. The flow dynamics are controlled by the forces due to convection, diffusion, and reaction where diffusion stabilizes the flow. We model this flow as a system of coupled non-linear partial differential equations consisting of Darcy’s law and convection-reaction-diffusion equations for mass conservation. For a slow to moderately fast chemical reaction, the existence of some critical viscosity ratio for instability is reported as a consequence of the radial displacement. We gain insight into an infinitely fast chemical reaction by computing the onset of instability for various Péclet number (Pe) and viscosity ratios. It is found that the onset time can be made independent of Pe by a suitable rescaling. |
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