Experimental Investigation of the Interaction between Water Droplets and Premixed Methane and Hydrogen Flames

Abstract

The growing demand in the aviation industry after the COVID-19 pandemic and the anthropogenic effects on the climate require the aviation sector to identify suitable solutions to continue operating while reducing its impact on the environment. Among the challenges faced by the aviation sector, the reduction of non-CO2 emissions is of particular interest. The use of alternative carbon-free fuels such as hydrogen poses challenges. Compared to conventional fuels, the increased flame temperature of hydrogen flames leads to higher NOx emissions while the increase in flame speed presents difficulties to stabilize the flame
and prevent flashback.

Numerical investigations have assessed the thermal and chemical effects as a result of the injection of water droplets in a hydrogen flame. The present investigation aims to assess the interaction of water droplets on premixed methane and hydrogen flames to determine if they can impart flame stretch. The effect is novel and would represent a third effect in addition to the thermal and chemical ones.

Identifying off-the-shelf atomizers is challenging and the two available and suitable atomizers at the time of the execution of the thesis are selected. The first atomizer is a pressure swirl nozzle and is integrated in a swirl-stabilized flame set-up. The second atomizer is an ultrasonic atomizer and is integrated in laminar premixed methane and hydrogen flames set-ups. The pressure swirl atomizer is integrated with a pressurization system to impose a pressure differential across the nozzle, where the pressure is converted into kinetic energy. Moreover, a 3D printed holder is manufactured to enclose the ultrasonic atomizer.

A thermodynamic model is developed to determine the equilibrium temperature of the products arising from the combustion of an adiabatic flame mixing with the injected water. The model allows to determine the non-dimensional fraction of fuel to water content for each operating condition.

The selected flames are a turbulent partially premixed swirl-stabilized flame and a set of laminar premixed flames. The laminar premixed flames include bunsen flames, plate-stabilized and V-shaped flames. The operating conditions of the flames are determined to characterize the interaction between water droplets and the considered flames. The selection of laminar bunsen flames allows to assume that the baseline configuration is unstretched, while the plate-stabilized flames allow to approach a configuration where the flame is already strained and finally the V-shaped flame allows to achieve a configuration where the
flame front is more accessible compared to the bunsen flame and the plate-stabilized flames. Moreover, the swirl-stabilized flame allows to verify the injection of water droplets in a confined environment in order to assess if the interaction with the water droplets is sufficient to perform measurement of NOx emissions using an exhaust gas analzyer. Additionally, the effects of flame stretch according to theory are reflected in the test point selection.

OH*chemiluminescence average images on the laminar premixed hydrogen flames show that the fluid dynamic interaction between water droplets and the flame surface is limited due to the relatively small size of the hydrogen flame, which must be stabilized using a smaller burner due to enhanced fuel reactivity. The difference between dry and wet conditions shows a change in signal intensity without affecting the flame position. Plate-stabilized flames allow to reduce the dependency of the interaction on the flame size and therefore are more suitable to perform investigations on the interaction between water droplets and hydrogen flames.
The comparison of OH-PLIF and OH* chemiluminescence images allows to state the former technique allows to identify the region where the water droplets interact with the reaction zone, while the latter allows to identify and isolate the effects on the flame front.