Thermal Performances of a Small-Scale Regenerative Combustion Chamber for Ultra-Micro Gas Turbine

New manufacturing techniques and technologies have led to the development of a new research topic focusing on fluid dynamics and combustion at small scale, in particular in the last 20 years. One of the most promising technologies is represented by the ultra-micro gas turbines, which were developed with the aim of providing a portable and clean power source for devices, such as unmanned aerial vehicles and drones, global positioning system, exoskeletons for military applications, and backup emergency power supply. Currently, not many prototypes have been built because of the issues posed by scaling down the system. Among these there are excessive fluid dynamic and thermal losses decreasing the overall efficiency, elevated combined thermal and mechanical stress of components, and the necessity of developing high speed bearings. This study focuses on investigating experimentally a possible solution to effectively recover heat contained in the combustion products to preheat the unburned mixture, increasing the overall efficiency. Thus, an 18-mm internal-diameter regenerative combustion chamber surrounded by two sets of helicoidal channels was developed. The combustion chamber included a sintered steel porous medium to allow the flame to stabilize. The combustion products were recirculated in order to maximize the heat transferred to the cold mixture, with benefits in terms of flammability limits and fuel consumption. Mass flow rate and equivalence ratio were varied in the tests and the gas temperatures at different locations within the combustion chamber were measured, along with the composition of the combustion products. The heat released was included in the range 65 W and 343 W. Tests were run in both a non-insulated and insulated configuration and comparisons were made. Resulted showed the achievement of clean combustion of liquefied petroleum gas with combustion efficiency higher than 99% for lean mixtures. Also, a good level of heat recovery was achieved, reaching 45% and 23% of heat transferred to the reactants from the exhaust gases in the insulated and non-insulated combustion chamber, respectively.