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Browsing by Author "Setiya, Meha"

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    Effect of straining flow and droplet shape on vaporization rate of liquid fuel droplet
    Setiya, Meha; Palmore, John A., Jr. (2020-11-24)
    This study focuses on the effect of planar straining flow on the vaporization of droplets. This work is motivated by spray combustion in gas turbines where the turbulence inside the combustor leads to the presence of both significant flow strain and droplet deformation. While a small amount of literature exists on the effect of droplet deformation on vaporization, there are no systematic investigations of the effect of flow strain on the vaporization of freely-deforming droplets. Recent theoretical studies on ellipsoidal droplets suggest that deformation enhances the vaporization rate. Additionally, our initial studies suggest that flow strain can also impact the vaporization rate. Therefore, a complete understanding of droplet vaporization requires studying the interaction between both droplet deformation and flow strain. This study uses an in-house code for interface-resolved direct numerical simulations of vaporizing multiphase flows. It mimics a freely-deforming droplet falling at its terminal velocity with an imposed strain rate. The influence of the deformation and flow strain on vaporization will be investigated by varying the relevant non-dimensional groups such as the Weber number and the non-dimensional strain rate.
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    Method to study effect of straining flow on droplet vaporization at low Reynolds number
    Setiya, Meha; Palmore, John A., Jr. (2020-03)
    Current trends in gas turbine development requires cleaner and efficient combustion. In order to understand the behavior of spray combustion in detail, the numerical study on combustion can give insights about the complete process including fuel injection, droplet breakup, droplet evaporation and its combustion. We use a numerical framework developed by Palmore and Desjardins to simulate this phenomena [1]. This framework uses NGA which is a Direct Numerical Simulation (DNS) code for simulating low-Mach number Navier-Stokes equations. It uses interface-resolved DNS in which dynamics of flow are solved using first principles i.e. conservation of mass, momentum and energy. Matching conditions at the interface of liquid-gas phase ensures the conservation of mass, momentum and energy across the interface. As a result, the deformation of the evaporating droplet, internal flow, boundary layer growth and its separation from the droplet are captured in detail. Although the framework is capable of studying 3D flows fully, this initial study will use 2D simulations to reduce the computation expense. The results of this study will motivate further detailed investigations in 3D in future. The present work involves the method development for inflow boundary condition for single droplet evaporation problem. In addition to this, the paper studies about the effect of planar straining flow on evaporation rate of the fuel droplet. The fuel used for this study is n-Decane.
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    Numerical Investigation on Shape Impact of Deformable Droplets on Evaporation and Combustion: Method Development and Characterization
    Setiya, Meha (Virginia Tech, 2023-08-21)
    Inspired by the dilute spray regime in spray combustion, this dissertation explores the evaporation and combustion of an isolated droplet. Under a highly convective environment inside a gas combustor, due to imbalance of inertial and surface tension forces, the droplets of larger size in sprays exhibit notable deformations from spherical to non-spherical shapes. Such shape changes are generally observed but not quantified in experimental studies. Therefore, the effect of this deformation on droplet combustion dynamics is unknown yet. To bridge this gap, a comprehensive investigation of an isolated freely deforming droplet can be insightful as it can reveal more about the interaction of droplet shape with its evaporation and combustion. This work attempts to analyze and quantify the impact of such deformations on evaporation and combustion using interface-capturing Direct Numerical Simulation approach. With the focus on small-scale processes involved in evaporation as it is a pre-step for combustion, this dissertation first covers a thorough examination on evaporation of a deformable droplet under both natural and forced convection. A single component jet-fuel surrogate n-decane is chosen. To ensure that the droplet remains stationary throughout its lifetime, a novel numerical method called "gravity update method" is developed and implemented. The results obtained from these two separate studies are validated against experimental results and analytical correlations respectively. The findings from the investigation of droplet evaporation under forced convective flow at moderate Reynolds numbers are noteworthy. The droplet shape under such flow conditions is governed by Weber number (We) which is a ratio of inertial force to surface tension force. The results demonstrated upto 20% en- hancement in total evaporation rate for highly deformed droplets. This improvement is a net results of increased droplet surface area and alteration in the distribution of local evaporation flux ( m'' ). It is found that m'' is proportional to its curvature up to the point of flow separation which agrees with low Re theories on droplet evaporation by Tonini and Cossalli (International Journal of Heat and Mass Transfer 2013), Palmore (Journal of Heat Transfer 2022). Beyond the flow separation point, evaporation flux distribution depends on the boundary layer development and flow evolution downstream of the droplet. For highly deformed droplets, a larger wake region creates favorable fuel vapor gradients and promotes mixing in droplet wake, hence higher evaporation flux. Such positive impact of droplet deformation on total evaporation rate motivated further investigation on droplet combustion under a low Reynolds number convective flow. High pressure and temperature gas flow leads to Damköhler number is higher than 1. This fa- vors the generation of envelope type flame. The results show overall little sensitivity to combustion related parameters despite the droplet shape change and significant (upto 9%) enhancement in total evaporation rate. It is also noted that while burning, droplets do not reach critical deformation conditions and break-up even beyond the critical Weber number, suggesting the suppression of deformation due to faster evaporation rate. The findings presented in these studies provide substantial evidence for the interaction between droplet shape and flow dynamics. Therefore, it demonstrates the potential for enhancing the existing numerical models and analytical correlations by accounting the influence of droplet shape.