Experimental and numerical investigation of a novel meso-scale combustor for liquid fuels

Over the past ten years, a significant amount of research has focused on developing meso or small scale combustion systems that can produce power in the 1-1000 W range. The length scales in these meso-scale systems can range from 1 mm to 10 cm. Combustion-based meso-scale power devices can become effective replacement for conventional batteries used in personal electronics. However, high heat loss associated with high surface area to volume ratio in meso-scale devices makes it difficult to sustain the flame at small scales. Further, the need to utilize high energy density liquid fuels introduces problems of effective fuel atomization, fuel-air mixing, combustion efficiency and pollutant emissions in meso-scale combustion systems.;A novel fuel-flexible meso-scale combustion concept utilizing a flow-blurring injector to produce fine fuel droplets, a counter-flow heat exchanger to preheat reactants using product gas enthalpy, and porous inert medium (PIM) to homogenize reactants and stabilize the flame is presented. The overall system is 30 mm long and 17 mm in diameter, with combustor diameter of 10 mm. For atmospheric pressure operation on kerosene fuel, the combustion system achieved heat release rate of up to 460 W, pertaining to energy density of 90 MW/m3 based on the total volume and 230 MW/m3 based on the combustor volume of 2 cm3. A comprehensive Computational Fluid Dynamics (CFD) model incorporating conjugate heat transfer, radiation heat exchange, flow and heat transfer in PIM, and heat release by combustion was developed and validated in this study. The CFD model accurately predicted the thermal performance of the combustion system. A 1D, burner-stabilized flame model incorporating detailed chemical kinetics was used to predict the flame structure. Nearly 94% of the heat released was retained by the products, thus, less than 6% of the heat released was lost to the ambient. The combustor maintains excellent performance over a range of air and fuel flow rates.;It was hypothesized that combustion with flame stabilized in interior of PIM would improve system performance compared to that with PIM surface stabilized flame. Thus, surface mode and interior mode of combustion with PIM were compared. The interior combustion mode extended the lean blow off limit, allowing for significant reduction in both CO and NOx emissions. Fuel-flexibility was demonstrated by operating the combustor on both liquid (kerosene) and gaseous (methane) fuels. On methane, it was possible to operate the combustor at heat release rate of up to 840 W with very low CO and NOx emissions. To the best of our knowledge, the fuel-flexible meso-scale combustor developed in this study surpasses the performance of other small-scale combustion systems reported in literature.