Can flashback be avoided with humidification in an original micro Gas Turbine combustor? – 1D predeterminations and LES validation

The high temperatures and reaction rates reached during hydrogen (or hydrogen enriched methane) combustion is well known to lead to flame and combustion instabilities, with increased emissions and potential flashback as result. Air humidification, typically applied to the micro Gas Turbines (mGTs) to increase electric efficiency by recovering additional waste heat from the flue gases, is proven to lead to reduced reaction rates and temperatures in classical mGT combustors fired with classical fossil fuels. However, these were mainly considered before as negative side effects. Using these effects to stabilize hydrogen combustion in mGTs without having to redesign the combustor is a promising, but still to explore route. Hence, this work presents a feasibility study on the use of combustion air dilution using combustion air humidification to reduce temperature and flame speed of hydrogen combustion in a typical mGT combustion chamber (Turbec T100) using a combined 1D-3D approach. First, the 1D laminar flame benchmarking allows a predetermination of the necessary minimal water dilution of the combustion air to avoid flashback for several H2/CH4 blends (25/75, 50/50, 75/25 and 100/0 %) at reduced cost. The predefined operating conditions are then tested and validated using high fidelity 3D Large Eddy Simulations of the actual combustor geometry of the T100 mGT. The 1D simulation results show that the combustion, at nominal operating conditions of the T100 (air mass flow rate of 800g/s, and a fuel consumption of 333kW, fuelled with different H2/CH4 blends) can indeed reach the same level of flame speed as pure methane combustion by humidifying the combustion air (using a water-to-air ratio of 3, 7, 12 and 23 % respectively). Finally, the 3D simulation results show stable combustion for these predetermined levels of humidification. Similar temperature and reaction rate levels to those of pure methane combustion are reached in the combustor. Hence we can conclude that this dilution method allows to stabilize H2 combustion, and the 1D approach provides accurate and low cost predetermination of the operating parameter to avoid flashback apparition.