Boundary Layer Flashback of Turbulent Premixed Hydrogen/DNG/Air Flames produced by a Bunsen Burner

Abstract

Due to the growing energy demand and global warming renewable energy is currently of interest. To deal with the production intermittency of renewable energy, energy might be stored in hydrogen. Hydrogen can be converted into electricity with a fuel cell or by combusting hydrogen in a gas turbine. Gas turbines are more suitable for large power outputs. Large power outputs require large volumetric flow rate through the gas turbine. The large volumetric flow rate causes high gas velocities and results in a turbulent flow. Using hydrogen instead of natural gas as energy resource changes the stable operating regime and increases flame flashback propensity. Current studies done at the Technical University of Munich and the Technical University of Delft suggest flame confinement, burner wall temperature and velocity fluctuations are vital to the flashback process. Des-pite the recent studies flashback is yet not completely understood. \\ This research focuses on premixed turbulent hydrogen and hydrocarbon combustion to further investigate the possibilities of using hydrogen in a gas turbine. The aim of this research is to gain more insight in the flashback process by varying velocity fluctuations while keeping the bulk velocity constant. The first objective is to gain insight in the relation between bulk velocity at flashback and velocity fluctuations. The second objective is to gain insight in the relation between bulk velocity at flashback and low velocity streaks upstream the flame front. The third research objective is to gain insight in the relation between flame front, boundary layer obstruction and bulk velocity at flashback. \\ All experiments were conducted with a Bunsen burner. A screw, acting as boundary layer obstruction, was placed below the burner rim to locally disturb the flow. The influence of the local disturbance on bulk velocity at flashback was investigated by mapping flashback propensity and by examining non-reacting and reacting pipe flow with planar Particle Image Velocimetry (PIV). The fuel compositions studied in this research are DNG, hydrogen and mixtures containing 20, 40, 60 or 80 volumetric percentage $H_2$. \\ The flashback maps show that bulk velocity at flashback increases with obstruction height for all gas mixtures. The obstruction height is the height from burner wall to screw tip. The results obtained through the non-reacting field experiment confirmed that the flow was fully-developed and turbulent. Furthermore, the results showed that obstruction height locally increased velocity fluctuations. The average velocity downstream the obstruction slightly decreased with increasing obstruction height. The reacting field was examined during flashback and during stable operation to relate flashback propensity and time-averaged flow characteristics with statistical analysis. The results showed flame front axial distance from the burner rim decreases with increasing obstruction height, indicating that a flame produced by a burner with boundary layer obstruction is more prone to flashback. The flow characteristics just upstream the flame front were analyzed because once the conditions upstream the flame front are suitable, flashback occurs. \\ A flashback prediction model developed at the TU Munich was modified to account for the obstruction height. The modelled results indicate that, in an unconfined set-up, flashback might be described with time-averaged flow characteristics and that the turbulent burning velocity is strongly correlated to the bulk velocity at flashback. Histograms of the instantaneous velocity showed that obstruction height has no influence on low velocity streaks and thus it seems low velocity streaks are not dominant in the flashback process of premixed unconfined flames. The absence of low velocity streaks for different obstruction heights indicates that flashback of unconfined flames is a different process than flashback of confined flames.