Investigation of heat recovery and combustion characteristics in small scale combustion chambers

Abstract

Over the last two decades the development of portable devices such as smartphones, laptops, biomedical diagnostic sensors, and GPS (Global Positioning Systems) receivers has led to a miniaturization of their components and to a higher demand of electricity supply that accumulators and batteries cannot fulfil anymore, although they are still widely used today. The most suitable and promising devices to provide electric power to the above mentioned small and portable applications seem to be Ultra-Micro Gas Turbines (UMGTs). The current research focuses on investigating the causes of efficiency decrease due to heat losses from the hot components and proposes a regenerative combustion chamber to improve the thermal exchanges. The combustion chamber has been designed in a way that the cold reactants flow in a helicoidal channel which surrounds the inner cylinder where combustion stabilized on a porous medium occurs. Combustion products are then recirculated through a second outer helicoidal channel so that the unburnt mixture receives heat from both sides. Comprehensive tests were performed on the combustor to measure thermal performances and pollutant emissions. Finally the feasibility of using vegetable oils as fuel is investigated and experimental results exposed. A numerical study has also been performed on a similar model of combustion chamber to provide a further insight of combustion phenomena and to investigate the effects of including a porous medium as flame holder on heat transfer mechanisms. Several simulations using a detailed skeletal mechanism for methane oxidation were run considering different mass flow rates, equivalence ratios and initial reactants’ temperature. Finally, as a complement of the present research and to provide a better understanding on heat transfer mechanisms in regenerative combustion chambers, another numerical study was performed on a three concentric tubes system with combustion occurring in the inner cylinder. Cold reactants and combustion products were set to flow in the middle and outer channels respectively, in a combined counter-current arrangement. Different mass flow rates were considered and also a parametric study with varying wall thermal conductivities was performed to highlight the influence of axial conduction through walls.