Influenza del Rapporto di Quantita' di Moto Aria/Combustibile sulle Emissioni di NOx in una Camera di Combustione ad Idrogeno"

The main aim of this work is to investigate the influence of the air to fuel jet momentum ratio onto the NOx formation of a hydrogen combustion chamber for aircraft engines, the only significant pollutant product of hydrogen combustion with pure air. Other additional parameters complete the analysis of the results. An already existing combustion chamber has been tested. Due to the high reactivity of the hydrogen and to the high risk of flashback no premix of the reactants has been realised. The fuel is therefore injected directly in the liner where air is also injected through a concentric flow. To create a less rich zone of reaction at the burner and decrease the emissions, a forced internal recirculation flow has been achieved adapting the geometry of the liner. The liner has also been provided with an external cooling system to avoid the increase of local temperatures and in turn the reaction rate of the NOx formation. The injected air has been preheated to aim at a flameless combustion. Besides the air and fuel jet momentum and their ratio, air pressure, excess air to fuel ratio, inlet air temperature, thermal power input have been investigated. Among these parameters the excess air ratio has revealed to be the most prominent, creating a rise of the NOx emissions with its increase. This can be mainly attributed to the substantial changing of the shape of the reaction zone. The air to fuel jet momentum ratio has been investigated by changing the number of fuel inlets and this has lead to an unexpected study on the importance of flow homogeneity to reach lower emissions. It has been observed that the air to fuel jet momentum ratio influences the emissions and the effect of the other parameters onto the NOx production itself. This influence can be strong enough to completely delete the positive effect of the air to fuel jet momentum ratio decrease. The investigation has been completed with a change of the fuel inlets area, keeping their number constant in order to alter the fuel jet momentum maintaining the homogeneity of the flow. Within the analysis of the air to fuel jet momentum ratio influence, promising results for a flameless combustion have been achieved. A section of this work has also been dedicated to repeatability of the experiments and their precision. This study has shown the influence of the calculation method for the emissions referred to general conditions. The used method has reported to be highly sensitive to oxygen concentration and to measurement uncertainties. The exhaust gas analysis has been realised simultaneously with the chemiluminescence of the NOx (CLD 700 EL ht) and Electrochemical sensors (Testo 350 M). The influence of the two different analysers have been also investigated. A future development of this work can lead to the improvement of the burner in order to reduce the emission, eventually with a flameless combustion. Some suggestions have been elaborated in the context of this work.