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Daniel Guggenheim School of Aerospace Engineering

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search.filters.applied.f.advisor: Seitzman, Jerry M. × End date: 2022 × Start date: 2020 × search.filters.applied.f.resourcesubtype: Dissertation ×

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    Numerical simulations of real-gas flows with phase-equilibrium thermodynamics
    (Georgia Institute of Technology, 2020-07-20) Tudisco, Principio
    Motivated by the complex physics of multi-component mixtures in strongly non-ideal, real-gas (RG) conditions reported in the field of chemical engineering, this work aims to address the behavior of multi-phase thermodynamics from a broader point of view. The focus is to evaluate the differences, as well as the possible sources of errors that would arise in a computational fluid-dynamics (CFD) simulation when conventional single-phase and multi-phase equilibrium RG thermodynamics are employed: an area of research that despite the active interest in many communities (especially CFD), has not been completely understood. Knowledge of the effects that multi-phase RG thermodynamics with the assumption of vapor-liquid equilibrium (VLE) can have on a flow dynamics is important because it establishes the relevance of the fully coupled CFD-VLE solver. In fact, this relevance may go beyond the stand-alone calculation of a multi-phase state, providing important insights about the physics that may not be captured if the single-phase assumption is invoked. This work provides an extensive study of RG mixtures from a physical and numerical point of view. The difficulties associated with their modeling are discussed in detail and solutions are provided accordingly. Emphasis is given to the occurrence and suppression of numerical noise in form of pressure oscillations that can pollute the simulation to the point that it cannot be performed. Extension of existing models to eliminate such problem is achieved by incorporating the effects of VLE thermodynamics in a consistent manner, ultimately forming a new and robust tool to investigate the physics further. The resulting model is applied to non-reacting and reacting flows in canonical setups where emphasis is devoted to the discussion of the differences and sources of errors that would occur if this multi-phase behavior is not taken into account. Results show that the different thermodynamic states reached by this advanced model can have an impact on the flow physics, especially in a non-reacting (or more in general cold) regime. In particular, the strong non-linear coupling between the VLE thermodynamics and the transport properties is identified as a key element of difference with respect to the single-phase model counterpart. These differences manifest into the occurrence of localized changes in the fluid properties (such as density) that affect the flow-field in their vicinity, causing visible discrepancies even when time-averaging is performed. Concurrently, results obtained on the reacting side and carried out (for the first time) with finite-rate kinetics suggest that any VLE formation between the products and the reactants may be considered of minor importance. The latter conclusion is supported by the analysis conducted on the multi-phase field which appears to be largely composed of the vapor solution, as expected, hence limiting the analogous effect observed the non-reacting system where a broader range of phase-separation appears instead.
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    Reduced-order model for prediction of staged-combustor NOx emissions with detailed chemistry and finite-rate mixing
    (Georgia Institute of Technology, 2020-04-24) Goh, Edwin
    The ground power industry is targeting combined cycle plant efficiencies of 65% and above, which can be achieved primarily through higher combustor firing temperatures. Because conventional combustors fail to meet NOx regulations at such temperatures, there is a pressing need for high-temperature, low-emissions combustors. In this regard, the staged combustion architecture is one such concept that shows promise due to its enhanced emissions performance and operational flexibility. The prohibitive cost of building prototypes relegates full-scale testing to the final stages of the product design cycle, while accurate models with turbulence and detailed chemistry cannot be used to efficiently explore the design space. Therefore, an efficient computational model is necessary to study a broad range of architectures. Despite extensive research on staged combustion and the related jet-in-crossflow (JICF) problem, there is little published research regarding the minimum NOx levels achievable by staged combustion architectures. The first contribution of this thesis presents a set of fundamental minimum NOx levels that are obtained by wrapping a constrained optimization routine around a reduced-order staged combustor model. For a firing temperature of 1975 K which corresponds to 65% efficiency, the minimum NO levels are determined to be roughly 1 ppm. Sensitivities of these minimum NOx levels to operational, geometric and computational parameters are identified and discussed. Recognizing that a turbulent flow field affects NOx chemistry primarily through mixing, the second contribution presents a reduced-order Limited Mixing and Entrainment (LiME) reactor model to predict emissions based on mixing and entrainment time scales. Molecular mixing is simulated based on the Interaction by Exchange with the Mean (IEM) model through interacting Lagrangian particles. The consensus is that better JICF mixing leads to lower NOx emissions, but little work has been done to characterize the effects of large-scale entrainment and small-scale mixing on NOx in isolation. The third contribution elucidates the sensitivity of NOx to mixing and entrainment time scales using reduced-order models and demonstrates a potential use case of this model in a constrained design optimization problem to identify minimum NOx levels under fixed entrainment rates. The impact of fuel and air staging on NOx under mixing and entrainment-limited scenarios is elucidated.