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Stability of thin liquid films flowing over heterogeneous surfaces and gas-solid flow of decomposing particles with applications to biomass pyrolysis

Yenhan Lin, University of Massachusetts Amherst

Abstract

The first part of this dissertation is focused on the dynamics and stability of thin liquid films flowing over heterogeneous surfaces, which are involved in numerous applications ranging from coating processes to microfluidics devices. Because of the small height-to-length ratio of the thin film, interfacial forces dominate the hydrodynamics in such flows. For thin films flowing over a locally heated substrate, the temperature gradient at the free surface induces a surface tension gradient, or Marangoni stress, at the liquid-gas interface. The flow due to this stress redistributes liquid from warmer to cooler regions, creating a pronounced bump in the film profile that is susceptible to a hydrodynamic instability that can cause a degradation in microdevice performance. Since a gradient in surface tension can also be induced by a surfactant concentration gradient, the thin film flowing over a locally heated substrate may behave markedly differently in the presence of surfactant. Utilizing the appropriate simplifications of lubrication theory, evolution equations are derived for the local film thickness and surfactant concentration, and predictions based on nonlinear and linearized equations of state for the surface tension are compared. Despite a localized accumulation of surfactant near the upstream edge of the heater, the predictions of the two models are found to be in good agreement. More significantly, a detailed linear stability analysis reveals that the surfactant stabilizes the film. The physical mechanism for this stabilization is discussed and linked to a qualitative change in the kinematics associated with the elimination of fluid recirculation near the heater. The effect of surfactant is also investigated for films over isothermal, inclined substrates with topographical features and for the dynamics of a film on a heated, horizontal substrate with topographical features. In the absence of surfactant, the effect of inertia on the stability of a liquid film flowing over a locally heated substrate is investigated by incorporating the leading-order term for inertia within the framework of lubrication approximation. The linear stability analysis and the energy analysis reveal that the inertial term destabilizes the film. ^ The second part of this dissertation is focused on two fundamental studies relevant to the pyrolysis of biomass. The heating of a biomass particle produces a temperature gradient within the particle, which creates spatially varying reaction rates. A reaction-transport model is developed to investigate the significance of intra- and extra-particle heat transfer effects on the pyrolysis of nonporous particles of cellulose, a major component of lignocellulosic biomass. Recently measured kinetic parameters are incorporated. The explicit shrinkage of a biomass particle undergoing pyrolysis is related to the overall mass loss as gaseous products are formed. Numerical predictions for measurable properties, including the temporal evolution of the residual mass and the final yield of char, which is an undesired byproduct, are validated through comparison to experimental data for various particle sizes and external temperatures. The particles are found to be sufficiently non-isothermal during typical processing conditions that heat transfer influences the char and product yield. Extensions are made to the pyrolysis of porous biomass particles. ^ In continuous flow processing, the catalytic pyrolysis of biomass typically occurs in a fluidized bed reactor. Solid biomass particles enter the reactor along with a carrier gas through a vertical standpipe, decompose, and produce condensable vapors that can be further processed into biofuel. Due to the coupling of the hydrodynamics to the rate of particle decomposition, the selection of appropriate operating conditions for the feed device can be challenging. For inert gas-solid flows, rich nonlinear dynamics can be observed due to momentum transfer between the phases through the drag force coupled to the compressibility of the gas. For decomposing solids as in pyrolysis, the exchange of mass between the phases further complicates the hydrodynamics. Using the approach of interacting and interpenetrating continua, a one-dimensional, steady-state model is developed for the gravity-driven flow of gas and decomposing particles through a vertical standpipe. The theory yields the particle and gas flow rates, the pressure profile, and the particle size and void fraction distributions. Preferred operating conditions are identified to produce steady flow, avoid a transition to moving bed flow, and provide desired values of the pressure, particle size, and solids flow rate at the pipe exit. Applications are made to the pyrolysis of wood particles. The admissible range of operating conditions is found to increase with the particle decomposition rate, which increases strongly with temperature.^

Subject Area

Chemical engineering

Recommended Citation

Lin, Yenhan, "Stability of thin liquid films flowing over heterogeneous surfaces and gas-solid flow of decomposing particles with applications to biomass pyrolysis" (2013). Doctoral Dissertations Available from Proquest. AAI3589069.
http://scholarworks.umass.edu/dissertations/AAI3589069

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