Thermal performance characteristics of a microcombustor for heating and propulsion

Wall heat losses are a major impediment to ignition and flame stabilization in microcombustion. In this context, reactant preheating may improve flame stability in a microcombustor. This work is aimed at studying the flame response to changes in inlet temperature of an annular microcombustor burning hydrogen–air stoichiometric mixture. The proposed configuration uses a hollow inner tube filled with nitrogen to facilitate flame stabilization. A detailed axi-symmetric computational model for reactive flow was developed and tested for this purpose. The model predictions were used to evaluate suitability of the proposed design for gas turbine and other applications in terms of different indicators of thermal performance. The baseline data showed the operational feasibility of the proposed configuration. Reduction in the preheating zone length and improved ignition at the cross-stream sites combined to give an increasingly compact flame as inlet temperatures increased. However, the increased wall heat losses at higher inlet temperatures reduced the overall efficiency of microcombustor. This drawback offsets the gain obtained in the form of improved function of the inner tube due to higher inlet temperatures. As a result, the proposed configuration is suitable for gas turbine applications at low to medium values of the investigated temperature range subject to a critical assessment of increased heat losses and reduced heat reflux. The thermophotovoltaic and thermoelectric applications should be feasible at medium to high inlet temperatures.

Research highlights
► Thermal analyses of the proposed annular microcombustor yielded following important results.
► Hollow inner tube with N2 trapped inside improved thermal performance of the annular microcombustor in comparison to the solid inner tube of high thermal conductivity.
► Robust heat recirculation mechanism of the microcombustor facilitated self-sustained combustion without pre-heating of the fuel-air mixture. This characteristic prevents material failure and also helps in limiting NOX emission.
► Suitability of an inlet temperature for employing the annular microcombustor in micropropulsion or heating is determined by the magnitude of wall heat losses, which influence the performance metrics considered in this study.