Influence of preheating and thermal power on cyclonic burner characteristics under mild combustion

Autoignition and stabilization of distributed combustion regimes have been proved to occur when a sufficient entrainment of hot species in the fresh reactants jets is reached, thus providing simultaneously for the sensible enthalpy to promote the auto-ignition process and the mass to dilute the incoming fresh reactants. The present study investigates the stabilization process along with the performance of the combustion process in a cyclonic burner operated under MILD combustion conditions. The cyclonic flow has been achieved by means of two pairs of oxidant/fuel jets injected using an anti-symmetric configuration in a prismatic combustion chamber thus realizing a centripetal cyclonic flow field directed toward the top-central gas outlet. Propane/air combustion experimental campaigns without external dilution, on a detailed grid of equivalence ratio and preheating temperature values, at different average residence time and nominal thermal power values were made. In each test condition, temperature measurements inside the chamber and gas sampling analyses have been carried out in order to evaluate the operability range of the cyclonic burner and its performances. These tests allowed to demonstrate the feasibility of stable MILD Combustion regimes in a wide range of operating conditions even when feeding the cyclonic burner with undiluted air. The residence time of the streams inside the burner plays an important role for both reactive structure stabilization and combustion performances/emissions. Significantly, fuel-lean conditions correspond, in the considered cases, to simultaneously low CO and NOx emissions. Furthermore, it has been demonstrated that stable combustion can be sustained in absence of any preheating in a considerable thermal load range and that it is possible, in this condition, to achieve a complete fuel conversion, with a remarkably low pollutant emission for thermal loads up to 8 kW.