Date of Award

Spring 2024

Embargo Period

7-1-2024

Access Type

Thesis - Open Access

Degree Name

Master of Science in Aerospace Engineering

Department

Aerospace Engineering

Committee Chair

Eric Perrell

First Committee Member

William Engblom

Second Committee Member

Scott Martin

Third Committee Member

L.L. Narayanaswami

College Dean

James W. Gregory

Abstract

This study analyzes the feasibility of On-The-Fly Quasi-Steady-State Approximation (OTF-QSSA) application for solving chemical kinetics within Computational Fluid Dynamics (CFD) simulations, aiming to reduce the computational demand of detailed mechanisms. An algorithm that dynamically identifies and designates Quasi-Steady-State (QSS) species at specific grid locations and instances during the simulation was developed. With this information, our method pseudo-delays the advancement of concentrations for these QSS species—effectively setting their rate of concentration change to zero for a set number iteration before updating using the detailed mechanism and thereby omitting the computationally intensive processes typically required for their calculation during those skipped iteration. This strategy intends to demonstrate computational time savings at the cost of minimal accuracy loss. To evaluate the effectiveness of OTF-QSSA, we conducted a series of tests on a 1D channel flow model simulating hydrogen and air combustion, utilizing the Evans & Schexnayder 25 reaction-12 species chemistry model alongside two derived models: an 8 reaction-7 species model commonly used in the community, and a 16 reaction-8 species model. The findings indicate that OTF-QSSA in simple scenarios, such as the 8 reaction model showed poorer performance, most likely due to the overhead of implementing OTF-QSSA outweighing the potential time savings. However, the approach yields significant efficiency improvements in more complex cases such as the 16-reaction and the full 25-reaction model with the 25 reaction model showing a system time reduction of approximately 15.59%. This reduction in computational time was achieved with minimal impact on the accuracy of major species concentrations, though some minor species, specifically the nitrogen based species, did exhibit slight deviations which did not substantially affect the overall simulation outcomes. The implications of these findings suggest a promising avenue for reducing computational demands in modeling detailed chemical reactions, enabling better efficient and practical simulations in combustion and other areas of fluid dynamics.

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