Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Mechanical Engineering & Applied Mechanics

First Advisor

Noam Lior

Abstract

The massive global use of combustion and the large exergy destruction in fuel combustion which is typically 20% to 30% of the useful energy highlight the importance of seeking for more exergy-efficient combustion. This dissertation pursued quantitative understanding of the reasons for the irreversibilities, i.e., their cause, location and magnitude, and recommended approaches for reducing them. This was studied by two different approaches: 1) the conventional and most frequently numerical way, the intrinsic analytical method (IAM) based on the solution of the system differential equations, which requires complex computation, and 2) the novel heuristic finite increment method (HFIM) that solves a system of algebraic equations, which is an orders-of-magnitude computationally easier approximation. The results demonstrated that the overall exergy destruction ratio computed by the IAM (22.58%) and the HFIM (19.6% to 22.3% for the 14 paths) agreed well for the studied adiabatic isobaric hydrogen/air combustion. The fractions of the individual contributors (chemical reaction, heat transfer, mass transfer and viscous dissipation) to the overall exergy destruction in the two approaches, however, were quite different. The IAM predicted that the chemical reaction was the dominant contributor (at 80.56%), while the HFIM predicted that the heat transfer was the dominant one (at 47.4 % to 72.1%). The difference was explained, for the first time, and it was because the HFIM assumed the combustion occurring in a prescribed path that made the chemical reaction rate at low temperature higher than reality. This established the connection of the two different approaches. Ways to reduce irreversibilities, by the sensitivity analysis in the IAM, comprised of reducing the excess air coefficient, increasing the inlet temperature, making the combustor walls as close as possible to adiabatic and optimizing the inlet velocity. Methods to decrease irreversibilities, by analyzing 14 hypothetical paths in the HFIM, were applying stoichiometric oxygen combustion and heat recirculation from high-temperature to low-temperature chambers. This dissertation also compared the exergy destruction results for similar combustion conditions computed by the HFIM and the IAM for the first time. The comparison enabled the identification of the hypothetical paths in the HFIM which were the closest to reality.

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