55 research outputs found

    On Secondary Atomization and blockage of surrogate cough droplets in single and multi-layer face masks

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    By now it is well-understood that the usage of facemasks provides protection from transmission of viral loads through exhalation and inhalation of respiratory droplets. Therefore, during the current Covid-19 pandemic the usage of face masks is strongly recommended by health officials. Although three-layer masks are generally advised for usage, many commonly available or homemade masks contain only single and double layers. In this study, we show through detailed physics based analyses and high speed imaging that high momentum cough droplets on impingement on single- and double-layer masks can lead to significant partial penetration and more importantly atomization into numerous much smaller daughter droplets, thereby increasing the total population of the aerosol, which can remain suspended for a longer time. The possibility of secondary atomization of high momentum cough droplets due to impingement, hydrodynamic focusing and extrusion through the microscale pores in the fibrous network of the mask has not been explored before. However, this unique mode of aerosol generation poses a finite risk of infection as shown in this work. We also demonstrate that in single layer masks close to 70 % of a given droplet volume is atomized and only 30 % is trapped within the fibers. The entrapped volume is close to 90 % for double layer masks which still allows some atomization into smaller droplets. We however found that a triple-layer surgical mask permits negligible penetration and hence should be effective in preventing disease transmission.Comment: 8 pages, 4 figures, 1 tabl

    Precipitation dynamics of surrogate respiratory sessile droplets leading to possible fomites

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    HYPOTHESIS: The droplets ejected from an infected host during expiratory events can get deposited as fomites on everyday use surfaces. Recognizing that these fomites can be a secondary route for disease transmission, exploring the deposition pattern of such sessile respiratory droplets on daily-use substrates thus becomes crucial. EXPERIMENTS: The used surrogate respiratory fluid is composed of a water-based salt-protein solution, and its precipitation dynamics is studied on four different substrates (glass, ceramic, steel, and PET). For tracking the final deposition of viruses in these droplets, 100 nm virus emulating particles (VEP) are used and their distribution in dried-out patterns is identified using fluorescence and SEM imaging techniques. FINDINGS: The final precipitation pattern and VEP deposition strongly depend on the interfacial transport processes, edge evaporation, and crystallization dynamics. A constant contact radius mode of evaporation with a mixture of capillary and Marangoni flows results in spatio-temporally varying edge deposits. Dendritic and cruciform-shaped crystals are majorly seen in all substrates except on steel, where regular cubical crystals are formed. The VEP deposition is higher near the three-phase contact line and crystal surfaces. The results showed the role of interfacial processes in determining the initiation of fomite-type infection pathways in the context of COVID-19

    Pair dispersion of turbulent premixed flame elements

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    Flame particles are mathematical points comoving with a reacting isoscalar surface in a premixed flame. In this Rapid Communication, we investigate mean square pair separation of flame particles as a function of time from their positions tracked in two sets of direct numerical simulation solutions of H-2-air turbulent premixed flames with detailed chemistry. We find that, despite flame particles and fluid particles being very different concepts, a modified Batchelor's scaling of the form <vertical bar Delta(F) (t) -Delta(F) (0)vertical bar(2)> = C-F ( <epsilon >(F)(0) Delta(F)(0))(2/3)t(2) holds for flame particle pair dispersion. The proportionality constant, however, is not universal and depends on the isosurface temperature value on which the flame particles reside. Following this, we attempt to analytically investigate the rationale behind such an observation

    Investigation of Blowoff Mechanism and Forced Response of Bluff Body Stabilized Turbulent Premixed Flames

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    Important problems related to the governing mechanism of flame blowoff in bluff body stabilized turbulent premixed combustion have been investigated in this dissertation. In the first part of the dissertation fundamental aspects of unforced and forced response in premixed and partially premixed fuel stratified flames encountered in low NOx gas turbine engines and afterburners are studied. Combustion in these devices presents complexities and instabilities introduced by thereto-acoustic, entropy wave and convective interactions. In this context, generalized blowoff limits and transfer functions of premixed turbulent flames under controlled acoustic perturbations and mixture gradients have been characterized. ^ The second part of the study concerns the flame dynamics of a bluff body stabilized turbulent premixed flame as it approaches lean blowoff. Experiments were performed in a laboratory scale burner as well as in a prototypical combustor with different geometries (axisymmetric and planar two dimensional), length and velocity scales. High speed chemiluminescence imaging along with simultaneous particle imaging velocimetry and OH planar laser-induced fluorescence were utilized in both these experiments for premixed propane-air flames to determine the sequence of events leading to blowoff and provide a quantitative analysis of the experimental data. It was found that near blowoff, the flame front and shear layer vortices overlap as a result of the reduced flame speed in fuel lean mixtures, to induce high local stretch rates on the flame. The high stretch rates exceeded the extinction stretch rate values instantaneously and in the mean, resulting in local flame extinction along the shear layers. Following partial or whole shear layer extinction, fresh reactants could pass through the non-reacting shear layers to react within the recirculation zone with some or all other parts of the flame extinguished. The flame kernel within the recirculation zone might survive for a few milliseconds and could reignite the shear layers such that the entire flame can be reestablished for a short period. This extinction and reignition event could happen several times before final blowoff event which occurred when the flame kernel failed to reignite the shear layers and ultimately lead to total flame extinguishment. Strikingly similar findings in the two different experimental setups suggest the general validity of the proposed flame blowoff mechanism and its insensitivity to a particular geometry. ^ Finally, recent results from ongoing research on the mechanism of forced blowoff and an experimental study on scalar mixing in an interacting field of two successively generated counter rotating laminar line vortices at the interface of two gas streams are presented.

    Life of flame particles embedded in premixed flames interacting with near isotropic turbulence

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    Flame particles are surface points that always remain embedded on, by comoving with a given iso-scalar surface within a flame. Tracking flame particles allow us to study the fate of propagating surface locations uniquely identified throughout their evolution with time. In this work, using Direct Numerical Simulations we study the finite lifetime of such flame particles residing on iso-temperature surfaces of statistically planar H-2-air flames interacting with near-isotropic turbulence. We find that individual flame particles as well as their ensemble, experience progressively increasing tangential straining rate (K-t) and increasing negative curvature (kappa) near the end of their lifetime to finally get annihilated. By studying two different turbulent flow conditions, flame particle tracking shows that such tendency of local flame surfaces to be strained and cusped towards pinch-off from the main surface is a rather generic feature, independent of initial conditions, locations and ambient turbulence intensity levels. The evolution of the alignments between the flame surface normals and the principal components of the local straining rates are also tracked. We find that the surface normals initially aligned with the most extensive principal strain rate components, rotate near the end of flame particles' lifetime to enable preferential alignment between the surface tangent and the most extensive principal strain rate component. This could explain the persistently increasing tangential strain rate, sharp negative curvature formation and eventual detachment. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved
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