Physical Mechanisms of a Millimeter-Scale Combustion System Design
13 Pages Posted: 4 Aug 2022
Date Written: July 29, 2022
Gas-fired burners are designed for a specific gaseous fuel at a designated supply pressure to achieve a select energy output. Variations in any of fuel composition, or fuel supply pressure, or a ratio of fuel to air supplied to the burner can produce variations in energy output. This study relates to the physical mechanisms of a millimeter-scale combustion system design. Computational fluid dynamics simulations are conducted to gain insights into combustion characteristics and performance such as temperatures and flames. The factors affecting combustion characteristics are determined for the millimeter-scale combustion system. The effect of fuel-air equivalence ratio on the combustion characteristics and performance of the millimeter-scale combustion system is investigated. Particular focus is placed on determining essential factors that affect the performance of the millimeter-scale combustion system. The results indicate that for given temperature and pressure conditions, fuel and air mixtures are flammable only within a certain range of equivalence ratios. The fuel-air equivalence ratio has a strong effect on flame stability. There are issues of efficiency loss for fuel-rich cases. The operating range for a millimeter-scale combustion system is finite: equivalence ratio values cannot be less than required to sustain combustion and cannot be greater than allowable by the physical constraints of the millimeter-scale combustion system. The lean blow-out boundary line relates to lean premixing combustion designs, wherein the fuel-air ratio is significantly less than the stoichiometric concentration. Combustion instability is actively controlled by periodically modulating the equivalence ratio for the millimeter-scale combustion system, such that the actual operating point modulates between the preselected reference points. Selection of the reference points is dependent upon the operating parameters of the system and shape of the oscillation region. For millimeter-scale combustion systems having single-sided oscillation regions, multiple fuel injectors are ideally incorporated in the combustion design, such that the periodic equivalence ratio modulation technique is applied to the individual fuel injectors or nozzles to reduce overall combustion instability.
Keywords: Combustion systems; Physical mechanisms; Flame stability; Equivalence ratios; Stoichiometric concentrations; Thermal management
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