Date of Award
8-2022
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Mechanical Engineering
Committee Chair/Advisor
Dr. Richard Miller
Committee Member
Dr. Ethan Kung
Committee Member
Dr. John Wagner
Committee Member
Dr. Xiangchun Xuan
Abstract
The supercritical CO2 power cycle (sCO2 ) is a relatively new technology, which promises to reduce CO2 emissions with potentially higher efficiencies. However due to challenging conditions posed by supercritical pressures, the mixing and ignition phenomena in sCO2 combustion is relatively less understood and studied. The primary objective of the current study is to investigate these fundamental processes using homogeneous ignition calculations (HMI) and direct numerical simulations (DNS). Broadly, the study is divided into two major parts. In the first part supercritical mixing in sCO2 relevant conditions is investigated. To achieve this, DNS of temporally developing, three dimensional, CH4 /CO2, CH4 /O2 and CO2 /O2 mixing layers, are conducted at a supercritical pressure of 300 atm. To effectively model the supercritical regime, the employed formulation includes the compressible form of the governing equations, the cubic Peng-Robinson equation of state and a generalized formulation for heat and mass flux vectors derived from non-equilibrium thermodynamics and fluctuation theory. A linear inviscid stability analysis is also performed for each case, to determine its most unstable wavelength. Flow visualizations reveal the presence of high density gradient magnitude regions for all three mixing layers, with conditional averages indicating increased presence of heavier fluid species within these regions. No significant departures are observed from perfect gas behavior, with compressibility factors very close to unity for all three mixing cases. Applicability of presumed probability density function methods (PDF) is examined for the three supercritical mixing layers. An a priori analysis is also conducted to investigate various simplifying assumptions employed in modeling various subgrid scale (SGS) flux models. Two additional terms are identified in the large eddy simulations (LES) equations, the gradient of SGS contribution of pressure in the momentum equation and the gradient of SGS contribution of heat flux in energy equation, whose magnitudes are similar and comparable with their respective resolved terms. The performance of the scale similarity model to represent these additional terms is investigated. The performance of Smagorinsky, gradient and scale similarity models is also investigated to model the conventional SGS fluxes. In the second part, the ignition process in sCO2 combustion is investigated using homogeneous ignition calculations (HMI) and two-dimensional direct numerical simulations (DNS). For selection of a suitable chemical mechanism, HMI calculations are first employed, to investigate the performance of existing skeletal mechanisms against shock- tube experimental data. The chemical characteristics of ignition are further studied using path-flux and sensitivity analysis, with CH3O2 chemistry exhibiting the largest effect on accelerating the ignition process. Different chemical pathways of fuel breakdown are also discussed to aid in interpretation of subsequent DNS case. In the DNS case, autoignition of a two dimensional mixing layer perturbed with pseudo-turbulence is simulated. The ignition is found to be delayed compared to the HMI case, with the ignition kernels forming in a spotty manner. The two phenomena are primarily attributed to variation of scalar dissipation within the mixing layer. The ignition kernels expand and evolve into a tribrachial edge flame propagating along the stoichiometric isosurface. Further investigation on the structure of edge flame revealed an asymmetrical structure, with CH4 molecules being entirely consumed in the triple point region of the flame along the stoichiometric isosurface, and more stable fuels like CO burning in the non-premixed branch of the edge flame. The edge flame propagation speeds are also calculated, with variations found to be correlated with scalar dissipation and upstream progress variable of the reacting mixture.
Recommended Citation
Ovais, Syed Mohammad, "Direct Numerical Simulation of supercritical CO2 mixing and combustion" (2022). All Dissertations. 3073.
https://open.clemson.edu/all_dissertations/3073
Author ORCID Identifier
0000-0001-7857-0888