Determining the topology of hydrogen flame using computed tomography of chemiluminescence

Abstract

With ongoing research towards clean combustion, hydrogen has been identified as a potential alternative to natural gas fuel, for example in power generation sectors utilizing gas turbines because of their inherent nature of being a carbon-free energy carrier. However, it is crucial to clarify that the ultimate goal is not just carbon-free combustion but clean combustion, which entails addressing other post-combustion emissions, such as NOx (nitric oxide) emissions. To meet stringent NOx emissions regulations, gas turbine fuels are combusted under premixed conditions. But in these premixed conditions, it has been observed that flames produced as a result of combustion have a tendency to flashback, which means that the flame travels back into the premixing chamber causing severe structural damages. This is particularly concerning when using hydrogen, which due to its high reactivity and flame speed is more prone to flashback than natural gas. To understand this high propensity of hydrogen to flashback, there is a strong requirement to examine the topology or structure of flame which can be obtained using optical combustion diagnostics technique.

Considering a flame has a three-dimensional structure, in this thesis, an optical combustion diagnostics technique of Computed Tomography of chemiluminescence (CTC) was applied to Bunsen burner flames using six CCD cameras. The cameras were arranged around the flame and a tomographic algorithm was used to reconstruct the three-dimensional structure of the flame. The technique was applied to turbulent 100% by volume fraction Dutch Natural Gas (DNG) flames at various Reynolds numbers representing stable, close to flashback, and flashback case. The reconstructed DNG flame results highlighted the capability of the CTC technique to offer valuable insights into the intricate features of the flame. Furthermore, these results not only indicated the possible location of the origin of the flashback within the structure of flame but also revealed specific features associated with events prior to the flashback. recognizing the potential of this technique, it was subsequently applied to a turbulent flame consisting of 50% hydrogen blended with 50% DNG. The reconstruction results offered insights into the fundamental structural differences between a flame consisting of 50% hydrogen and 50% DNG and a pure 100% DNG flame. The conclusions drawn from the visual assessments of the reconstruction results were further supported by the subsequent statistical analysis and the resulting cone angle values.