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Achieving a high wall temperature and wide flame stability limit is of extreme significance for the maximization of power generation in hydrocarbon fuel-driven micro-thermophotovoltaic and thermoelectric devices. For this, detailed experimental investigations are conducted on variable channel height combustors, with the emphasis on analyzing the flame dynamics and thermal performance. A preliminary understanding of the multiple flame morphologies in the combustors is developed by varying the input working conditions and wall materials. The basic types of unstable flames are clarified. The pinch-off phenomenon of a weak flame in a slit combustor is observed for the first time, and the cell tends to decrease and emerge with an increase in the flow rate. The combustible range is shown to be extended with increasing the channel height, while it exhibits a non-monotonic changing trend with the combustor thermal conductivity which highlights the importance of the balance between heat transfer due to the flame-wall coupling and heat losses. Furthermore, a Kriging-NSGA-II optimization model is also developed to obtain the optimal characteristics in terms of radiation efficiency, standard deviation and volume power density. A Pareto-optimal front solution is determined to identify three distinct regions of thermal conductivities, which is crucially useful in guiding the design of practical micro-power systems based on different working requirements. [Display omitted] • A novel plug-in combustor with variable channels is designed. • The flame morphology and destabilization rule under different wall materials are investigated. • The variation of flame morphology under various wall materials is described. • An energy conversion index to evaluate the output characteristics of the combustor is constructed. • Optimization algorithms are used to provide a basis for the selection of wall materials. [ABSTRACT FROM AUTHOR]

• The impact of Pt catalyst and heat recirculation on combustion performance is evaluated. • The homogenous chemistry can be substantially extended in the presence of a catalyst. • Outer wall temperature and uniformity are enhanced with proposed strategies. • Radiation energy and efficiency exhibit a different trend with the inlet flow rate. In this work, the joint effect of applying Pt catalyst and heat-recirculating structure on the flame stability and thermal performance has been numerically investigated in methane-air-fueled micro-combustors by a detailed three-dimensional computational model. Numerical results indicate that such strategies contribute to improving outer wall temperature by 14.4% and extending the blowout limit by a factor of 0.6 compared to those in the straight non-catalytic combustor. Such enhancements are due to the intensified heat transfer, thus highlighting the contributions of heterogeneous combustion chemistry and heat-recirculating effect. Homogeneous chemistry in the presence of a Pt catalyst is shown to be self-sustained over wide-ranging conditions. Meanwhile, the effectiveness of heat recirculation structure on combustion performance is more significant as compared to that of the catalytic combustion effect, as evidenced by a 4% and 17.6% improvement in the wall mean temperature and blowout limit respectively. Further analysis of the heat recirculation catalytic combustors is conducted to obtain the optimal geometry. There exists a critical flow rate corresponding to the maximum outer wall temperature and radiation energy. In general, this work demonstrates the viability of applying a Pt catalyst and heat-recirculating structure to extend the flame stability limit and energy conversion performance in micro-power systems. [ABSTRACT FROM AUTHOR]

• Pt/Al 2 O 3 /Ni catalysts with different mass fractions are prepared. • A diagram depicting the flame structures with different combustor wall is constructed. • The presence of catalyst contributes to a wider flammable limit. • The effect of the catalyst concentration on the blow-off limit and flame structure is identified. To enhance premixed methane/air combustion that occurs under micro-scale conditions, this work considers applying Pt/Al 2 O 3 -based catalyst in a rectangular-shaped combustor with the nickel foam surface. The effects of the inlet velocity, equivalence ratio and catalyst loading concentrations are experimentally evaluated and analyzed. The results show that with the implementation of such catalysts, flame structure becomes more stable, and the flammable range of the mixture is also widened. Meanwhile, the thermal coupling between the gas phase flame and the solid wall is more intense. In addition, with the increase in the catalyst loading mass fraction, the transition point from u-shaped flame to inclined flame was found near the entrance of the combustion chamber, which can remain a stable state over a wide operating time in the micro-channel. The catalytic combustion in enhancing flame stability becomes more effective with the increase of Pt loading mass fraction. In general, the findings gained from this study shed light on the mechanism of designing a stable combustion system by implementing a suitable catalyst. [Display omitted] Flame type diagram of blank micro-combustor: (a) FREI flame, ϕ =1.0, v =0.31m/s; (b) Weak flame, ϕ =1.0, v =0.34m/s;(c) Change of planar flame with flow velocity, ϕ =1.0; (d) Change of U-flame with flow velocity, ϕ =1.0; (e) Variation of inclined flame with flow velocity, ϕ =1.0. [ABSTRACT FROM AUTHOR]