- Title
- Droplet particle interaction in a flowing gas stream
- Creator
- Mitra, Subhasish
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Droplet-particle collision interaction in a flowing gas stream is one of the major phase interaction phenomena in a wide class of multiphase process applications such as spouted bed coating, fluid catalytic cracking unit, fluid coking process for bitumen upgrade process etc. that govern the process performance to a significant extent. Such interactions are manifestations of complex hydrodynamics involving competing interplay of various forces e.g. viscous, capillary, inertial and gravity coupled with simultaneous heat and mass transport process which involves further complexity of phase change. Depending on the size ratio of the droplet-particle pair, relative velocity, physical properties, temperature difference, surface roughness and hydrophobicity; a number of different outcomes are possible which presumably affect the associated transport phenomena to a significant extent. For instance, inefficient contact of atomized feed droplets and hot catalyst particles adversely affects the desired product yield in a fluid catalytic cracking unit. Motivated by a dearth of knowledge in this field, the present research aims at investigating some of these interaction mechanisms with specific focus on the single droplet-particle system to broaden the mechanistic understandings using both non-invasive optical technique (high speed imaging) and numerical modelling wherever applicable. Based on the droplet-particle size ratio (Δ), there different systems were studied: Δ < 1, Δ > 1 and Δ ~ 1. For Δ < 1 system, normal collision behaviour of different fluids namely water, isopropyl alcohol and acetone was studied on a highly thermally conductive stationary brass particle surface in the temperature ranging from normal atmospheric condition (20 °C) to film boiling regime (250 °C - 350 °C) at different impact velocities (0.34 - 1.67 m/s) of droplets using high speed imaging technique. With increasing impact velocity (Weber number), three distinct outcomes were noted – deposition or wetting behaviour at normal atmospheric condition to nucleate boiling regime and then rebound and disintegration in the film boiling regime. In the impact dynamics, broadly two distinct phases were observed – inertia-dominated spreading or advancing phase and surface tension dominated recoiling or receding phase below disintegration limit and only spreading phase above this limit. Specifically in the film boiling regime, a critical Weber number range was determined wherein the transition from rebound to disintegration regime occurs. Two important parameters were quantified – maximum wetted contact area and droplet-particle contact time that governs the collision induced heat transfer process. Using image analysis, maximum spreading parameter was quantified to characterise the wetted area and correlated with impact Weber numbers using a general functional form. Also an analytical expression was suggested to determine this parameter on spherical surface based on an energy balance approach which showed reasonable agreement with experimental measurements. Also determined were a spreading kinetics of two functional forms – power law and recovery type exponential; both of which predicted the spreading trend well. Measured contact times were observed to have inverse dependency on the impact Weber number. Below limit of disintegration, a power law form of contact time as a function of Weber number was obtained which predicted the trend well and also confirmed a general form of contact time in film boiling regime with little dependency on the system characteristics. No such functional form could be established in the disintegration regime wherein contact time appears to be almost independent of Weber number. A 3D computational fluid dynamics (CFD) model based on volume of fluid (VOF) method was developed using the FLUENT platform to simulate the droplet deformation behaviour during impact for Δ < 1. It was shown that the temporal evolution of complex droplet shapes depends critically on the contact angle boundary condition and use of dynamic contact angle improves the prediction compared with static contact angle. It was observed experimentally that in film boiling regime, contact angle hysteresis was minimal due to presence of the insulating vapour film at solid-liquid interface which enhances surface hydrophobicity. Also the effect of contact line velocity and surface temperature on the contact angle variation was found to be insignificant. Based on this observation, it was further shown that in film boiling regime, CFD model could predict the dynamics based on a static contact angle in the limit of super-hydrophobicity and a free slip wall boundary condition to account for the vapour film which reasonably agreed with the experiments both qualitatively and quantitatively. Also studied experimentally for the Δ < 1 system, was in-flight collision interactions between a number of small droplets and a larger particle at different droplet (Weber number ≈ 3.0 – 26) and particle (Reynolds number ≈ 14 – 46) impact velocities where both were in the moving state. The droplets were observed to undergo inelastic collision resulting into complete deposition onto the particle surface in lower Weber number cases and forming a thin film in the higher Weber number cases. The measured film thickness normalized by particle radius was found to be in the range of 0.033 -0.314. Also during collision, significant deflection in the particle trajectory was noted especially at higher droplet velocity. However the angle of deflection was observed to decrease when the relative velocity between droplet and particle was decreased. A force balance model was developed accounting for the impact behaviour during collision to predict the particle trajectory and velocity. The model predicted outcomes were in good agreement with the experimental measurements when the angle of deflection was small however larger deviations were noted when angle of deflections were relatively large. The deviations were attributed to a number of factors such as uncertainty in droplet size due to in-flight coalescence, loss of droplet momentum due to coalescence on particle surface and intricate rotational motion during impact which were not completely accounted in the model. For a Δ > 1 system, collision interactions between a small particle and a larger stationary supported spherical cap droplet were investigated at different particle impact velocities (Weber number ≈ 1.4 - 33). Two outcomes were noted – particle capture or retention at interface and penetration through the droplet interface. A one dimensional model was developed based on force balance approach to predict these collision outcomes. Effect of different competing forces namely gravity, virtual mass, buoyancy, drag, capillary and pressure were analysed. Among others, the capillary force was noted to have dominating effect however effect of the drag force was also observed to be significant when impact velocity was increased. The earlier mentioned CFD model was modified to include the effect of particle motion utilizing a dynamic meshing technique. Using a static contact angle and no-slip wall boundary conditions, the CFD model predicted outcomes were in reasonable agreement with the high speed visualizations and force balance model predictions. Also investigated for the Δ > 1 system was the collision interactions between a small particle and a stationary liquid film confined in a capillary tube using different diameters of particle and impact velocities since the complete penetration behaviour of the impacting particle could not be studied with a supported droplet. Three different outcomes were noted based on the impact Weber number - particle capture/retention at top interface; particle capture or retention at bottom interface and complete penetration through both interfaces. A criterion was developed based on the energy balance approach to predict the collision outcomes. In complete penetration cases, the particle was observed to entrain a certain amount of liquid mass with it which was explained by the end-pinching mechanism of ligament breakup. A model based on the energy balance approach was developed to quantify this liquid mass carryover. Also an empirical correlation was obtained to correlate the liquid mass carryover and the particle Bond number. A sensitivity analysis on the predictions of CFD model was performed using different contact angle boundary conditions – advancing, static and receding contact angle which could only predict a specific instance of the outcome well and not the overall outcome. The simulated particle trajectory and velocity were compared with the experimental measurements. Also the contributions of pressure force and viscous force predicted by the CFD model were analysed to explain the collision outcomes wherein pressure force was found be higher than the corresponding viscous force by at least an order of magnitude. The Δ ~ 1 system was studied experimentally for normal collision between an impacting droplet and a stationary particle where two outcomes were noted – deposition in lower Weber number cases and film formation in higher Weber number cases. Also studied here was the interaction behaviour in higher Weber cases involving heat transfer. The film was observed to rupture when the film reached a limiting thickness due to intense vaporization involving nucleate boiling at the apex point of the particle. This system was also studied computationally using a coupled level-set and volume of fluid (CLSVOF) CFD model using different combinations of droplet-particle size ratio, impact Weber number and impact parameter (collision angle). Three distinct outcomes were noted – deposition, ripping and coating and skirt scattering in the increasing order of impact Weber number which reasonably agreed with the predictions of LBM model previously reported by Gac and Gradon (2014). In both ripping and coating and skirt scattering outcomes, separation of ligaments was explained by the end-pinching mechanism. It was shown that all outcomes are governed by a competition between surface energy and kinetic energy and while higher surface energy favours deposition, higher kinetic energy leads to separation. Force analysis reveals that both pressure force and viscous force increases when impact Weber number is increased. Rise of these forces occur specifically in the early phase of interaction followed by a sharp decay in higher Weber number cases indicating separation of liquid ligament from particle surface. Also investigated was the effect of impact parameter (collision angle) which was shown to be critical for droplet-particle interaction. Increase in impact parameter leads to significant decrease in contact area of liquid-solid interface which consequently results in inefficient momentum transfer. This was evident by the corresponding decrease in the magnitude of both force and strain rate. In continuation with this study, a droplet-particle collision induced heat transfer model was developed applicable for FCC environment. The model computes the heat transfer coefficient based on a conductive heat transfer mechanism involving wetted contact area and droplet-particle contact time based on the interaction mechanism of Δ < 1 system. The model also includes volume fraction of droplets and solid particles to account for the multiphase environment. The developed heat transfer sub-model was incorporated into a lumped parameter vaporization model to compute vaporization times for typical gas-oil feed droplets. Computed vaporization times were compared with the predictions of other heterogeneous vaporization models reported in literature and found to be in reasonable agreement. Finally, a multi-particle system involving injection of an acetone jet in a bubbling fluidized bed of Geldart A-B particles was briefly studied to understand the collision interactions of droplets in a multi-particle environment involving vaporization. Several interesting phenomena like jet breakup into multiple droplets, re-suspension of solid particles due to vapour explosion, droplet shape deformation, coalescence, levitation and nucleate boiling were noted. Also evolution of the vapour concentration profile in freeboard region was visualized using Schlieren imaging technique. Numerically, a two-way coupled Eulerian-Lagrangian CFD model was developed using the FLUENT platform to simulate the jet vaporization process using user defined source terms for heat transfer between the liquid jet and the bed. Simulated vaporization phenomenon qualitatively agreed with experimental observations and captured the key feature of the vaporization process indicating diffusing nature of vapour concentration profile from bed surface to bulk. The CFD model also indicated significant reduction of local bed temperature which indirectly explained the presence of particle agglomerates found in the experiment.
- Subject
- droplet; particle; contact time; high speed imaging; thermal imaging; multiphase interaction; thesis by publication; collision model; deposition; breakup; rebound; collision heat transfer; CFD-VOF; image processing; Schlieren imaging
- Identifier
- http://hdl.handle.net/1959.13/1315709
- Identifier
- uon:22986
- Rights
- Copyright 2016 Subhasish Mitra
- Language
- eng
- Full Text
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