In the field of energy production, cogeneration systems based on micro gas turbine cycles appear particularly suitable to reach the goals of improving efficiency and reducing pollutants. Moderate and Intense Low-Oxygen Dilution (MILD) combustion represents a promising technology to increase efficiency and to further reduce the emissions of those systems. The present work aims at describing the behavior of a combustion chamber for a micro gas turbine operating in MILD regime. The performances of the combustion chamber are discussed for two cases: methane and biogas combustion. The combustor performed very well in terms of emissions, especially CO and NO ${}_{\mathrm{x}}$ , for various air inlet temperatures and air-to-fuel ratios, proving the benefits of MILD combustion. The chamber proved to be fuel flexible, since both ignition and stable combustion could be achieved by also burning biogas. Finally, the numerical model used to design the combustor was validated against the experimental data collected. The model performs quite well both for methane and biogas. In particular, for methane the Partially Stirred Reactor (PaSR) combustion model proved to be the best choice to predict both minor species, such as CO, more accurately and cases with lower reactivity that were not possible to model using the Eddy Dissipation Concept (EDC). For the biogas, the most appropriate kinetic mechanism to properly model the behavior of the chamber was selected.

Miniaturized heat exchangers are well known for their superior heat transfer capabilities in comparison to macro-scale devices. While in standard microchannel systems the improved performance is provided by miniaturized distances and very small hydraulic diameters, another approach can also be followed, namely, the generation of local turbulences. Localized turbulence enhances the heat exchanger performance in any channel or tube, but also includes an increased pressure loss. Shifting the critical Reynolds number to a lower value by introducing perturbators controls pressure losses and improves thermal efficiency to a considerable extent. The objective of this paper is to investigate in detail collector performance based on reduced-order modelling and validate the numerical model based on experimental observations of flow maldistribution and pressure losses. Two different types of perturbators, Wire-net and S-shape, were analyzed. For the former, a metallic wire mesh was inserted in the flow passages (hot and cold gas flow) to ensure stiffness and enhance microchannel efficiency. The wire-net perturbators were replaced using an S-shaped perturbator model for a comparative study in the second case mentioned above. An optimum mass flow rate could be found when the thermal efficiency reaches a maximum. Investigation of collectors with different microchannel configurations (s-shaped, wire-net and plane channels) showed that mass flow rate deviation decreases with an increase in microchannel resistance. The recirculation zones in the cylindrical collectors also changed the maldistribution pattern. From experiments, it could be observed that microchannels with S-shaped perturbators shifted the onset of turbulent transition to lower Reynolds number values. Experimental studies on pressure losses showed that the pressure losses obtained from numerical studies were in good agreement with the experiments (<4%).