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. / Doctor of Philosophy / This work is inspired by the spray combustion in gas turbines where the pressurized liquid fuel jet is injected in the combustion chamber and converted into dilute sprays after undergoing a series of processes. Due to the presence of higher air to fuel ratio for these spray droplets, they become the localized combustion sites with rapid evaporation rates. Understanding the evaporation of these droplets becomes crucial, as it sets the stage for their subsequent combustion. In an attempt to understand this chemically and fluid-dynamically complex phenomenon, abundant experimental studies are available with focus on overall atomization process and velocity field evolution. However, they lack in resolving the small-scale processes which govern the evaporation, therefore combustion.
With the intent to investigate in detail about the combustion aspect, this problem is reduced to analyzing behavior of isolated droplets. Despite the sophisticated measurement technologies particle-scale processes such as temperature and species mass fraction evolution are yet unknown. Moreover, the majority of these studies are performed with simplifying assumptions. assumption has been that the droplet remains spherical throughout its lifetime. However, in practical applications, particularly when exposed to convective and turbulent environments, droplets can undergo significant deformation due to the presence of inherent surface tension of liquid. This deformation can influence their evaporation and burning rates. Additionally, the droplet's shape governs the flow field around it, potentially altering droplet-droplet interactions.
Direct Numerical Simulation (DNS) approach is one of the numerical methods which can resolve both the phases. It offers a promising approach to reveal these small-scale details, such as droplet shape, vapor and temperature field around a droplet, droplet-droplet interaction, droplet motion etc. With the aim to bridge this gap, this dissertation focuses on the study of evaporation and combustion of an isolated deformable droplet under various conditions.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/116071 |
| Date | 21 August 2023 |
| Creators | Setiya, Meha |
| Contributors | Mechanical Engineering, Palmore, John A., Meadows, Joseph, Coutier-Delgosha, Olivier, Tafti, Danesh K. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International, http://creativecommons.org/licenses/by-nc-nd/4.0/ |
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