Advancing combustion technology: A focus on alternative liquid fuels and innovative designs of porous media burners
Increasing demands for energy efficiency and reduced emissions drive ongoing advancements in combustion technology. Traditional combustion methods face challenges related to efficiency, emissions, and fuel flexibility. Therefore, a need exists for novel combustion technologies that innovate fuel utilization and combustion processes. The exploration of alternative liquid fuels, replacing conventional options like ethanol, holds considerable potential in enhancing overall energy consumption and expanding fuel options. Similarly, porous media burners (PMBs) present a transformative approach, offering improved combustion efficiency, leaner flammability limits, and reduced emissions. This thesis ventures to explore and advance these novel combustion technologies, aiming to enhance our understanding of the fundamental thermodynamic mechanisms and provide practical insight into real world applications. First, we present an experimental and computational analysis of the isolated droplet burning of two furanic compounds, 2-methylfuran (MF) and 2,5 dimethylfuran (DMF), noted by the Department of Energy as prospective biomass derived additives into existing petroleum-based fuels. Experiments were performed in a reduced gravity environment, facilitating one-dimensional burning amenable to numerical simulations with detailed chemistry. Further, the oxidative stability of stored samples was investigated. Gas chromatography-mass spectrometry (GC-MS) revealed important considerations when storing DMF due to the polyunsaturated chemical structure and a susceptibility to oxidation and chemical breakdown. Nonetheless, these fuels are considered ”top tier” alternatives because of the well-established production methods from biomass. Second, we studied the effects of dynamic stability in a conventional SiC PMB, motivated by the fluctuating fuel flow and composition in biomass gasification. Experiments involved varying sinusoidal equivalence ratios (Φ) while maintaining constant flow rate to simulate volatility from the gasification process. Tests across different Φ amplitudes and forcing frequencies revealed a nonmonotonic relationship between mixture pore scale Reynolds number (Repore) and dynamic stabilization. Notably, a Φ of 0.2 was sustained under specific initial conditions, corresponding to the widest flammability range on the steady stability map. However, emissions data showed higher CO accumulation during sinusoidal experiments compared to baseline, attributed to the convective lag of fuel during a cycle. Additionally, a transfer function derived from stable data predicted system response under different conditions. This study provides insights into the dynamic thermal response of PMBs and the potential for controlling combustion in bio-derived fuels from fluctuating sources. Next, we employed additive manufacturing (AM) to create and analyze four PMBs with internal morphology inspired by biological systems, such as butterfly wings and mitochondrial membranes. Specifically, we integrated the triply periodic minimal surfaces (TPMS) architectures of diamond (D), gyroid (G), IWP (I), and Schwartz primitive (P) into PMBs. Each burner was designed to a constant porosity of 0.75 and similar pore-size gradation scheme. Experimental analyses cover lean stability, emissions, and temperature distribution. A volume-averaged model utilizing correlations for Nusselt numbers specific to each TPMS was used to predict stability regimes and temperature profiles. Lastly, computational fluid dynamics (CFD) was performed to explore the porescale hydrodynamic and heat transfer mechanisms underlying the experimental observations. We found the I and D burners exhibited the widest stable operation ranges and highest temperatures. Additionally, the volumetric heat transfer coefficient used within the model captured the trends seen experimentally. Furthermore, pore-scale simulations revealed the existence of thermal ”pockets” potentially explaining the enhanced interphase heat exchange unique to the I burner at low flow rates. This study isolates morphologic effects on combustion performance, highlighting interphase heat exchange as a dominant factor. Our results underscore the importance of internal morphology and the influence on combustion physics. Finally, we studied the impact of porosity, morphology, and material composition on the mechanical performance of AM ceramic PMBs. Thermal-structural simulations of five different TPMS-based PMBs were conducted to analyze thermal-stress distributions. Alumina and mullite structures were 3D-printed and tested in a methane-air combustion experiment, revealing superior durability in mullite compared to alumina. TPMS burners with higher specific surface area, tortuosity, and moderate pore diameter, such as D and I, exhibited lower thermal strain and reduced propensity for thermal-structural failure while maintaining the ability to sustain a flame. X-ray imaging confirmed a correlation between predicted stress regions and experimental crack formation. These findings provide a foundation for future work in optimizing PMB performance and longevity through AM techniques.