conditions. The Phase 1 experimental rig is shown in Figure 6 including the combustion chamber and downstream flow measurement and regulating devices. The rig includes optical access windows for hydrogen flame visualization (Figure 7). All of the necessary health and safety protocols have been implemented to allow hydrogen combustion experiments at high temperature and pressure and the use of laser diagnostic instrumentation for data acquisition. The data being acquired from the experimental campaign are being used to evaluate, validate, and calibrate the CFD models. The Phase 1 study will identify suitable injector geometries for the larger-scale Phase 2 studies. Phase 2: Annular Combustor Segment Performance Assessment and Emissions Measurement Phase 2 will combine experimental and complementary CFD numerical studies to: x demonstrate that the down-selected injector geometry and injector-array configurations (in Phase 1) can be scaled to full-size combustor configurations without compromising NOx emissions and combustion efficiency Figure 3. A hydrogen micromix injector prototype. (Source: Cranfield University; used with permission.) H2/Air Offset Mixing Distance x provide proof of concept that the combustor outlet temperature profile and potential combustion instabilities can be controlled by varying fuel scheduling to different parts of the injector array. h Air Inlet H2 Inlet Aspect Ratio = h/d d H2 Injection Diameter Figure 4. A hydrogen micromix injector design variables. (Source: Cranfield University; used with permission.) Phase 3: Subatmospheric Altitude Relight Capability Test Phase 3 will mainly focus on testing the weak extinction and altitude relight of the selected micromix injector at subatmospheric conditions as well as numerically modeling the process. The aim is to determine the limits on conditions for which it is possible to achieve successful ignition and to demonstrate the NOx Simulations With FGM and Thermal NOx Model Promising Designs Momentum Flex Ratio J * Purely Diffusion * Reduced Flame Interaction by High Jet Penetration Figure 5. The influence of hydrogen micromix injector design variables on NOx emissions. (Source: Cranfield University; used with permission.) 72 IEEE Electrification Magazine / JUNE 2022