Hydrogen Micro-Mix Fuel Injection: Design & Evaluation

Abstract This article presents a novel micro-mix fuel injection concept designed specifically for hydrogen combustion, “direct hydrogen injection” (DHI). In this arrangement, fuel and air are injected into the combustor as individual discrete jets. By varying the angle of the fuel and air holes, the flow field may be shaped, altering mixing processes and flame stabilization. The DHI concept offers significant advantages for the combustion of hydrogen when compared to conventional fuel injection arrangements. The small size of the injection sites is advantageous as it results in smaller flame volumes, reduced combustor residence times and, as a direct consequence, reduced emissions of pollutants — principally the oxides of nitrogen (NOx). This article presents the first optical measurements of the DHI concept, with a subset of injector configurations tested at larger than engine scale. These clearly demonstrate the ability to control the structure of the flame and flow field by altering air jet angle leading to the formation of substantially changed reaction zones, both a conical flame and a V-flame have been created by changing only the air/fuel injection angle. The influence of the number of fuel injection sites on the flame structure is also investigated, with a significant reduction in flame length achieved by increasing the number of injection sites. Steady-state RANS predictions are also performed and compared to the experimental results, with the CFD found to reasonably recover the overall structure of the flame for most cases. Despite reasonable first order performance, fuel jet penetration is typically over predicted, leading to longer jet flames, whilst the flame thickness is possibly under-predicted, suggesting opportunity for improvement. The combined experimental, numerical study, however, shows significant potential for the direct hydrogen injection concept. Compact, stable flames are generated, without ignition challenges or stability issues. The flame structures are further unchanged by air velocity and/or fuel flow rate.