Abstract
Hydrogen (H2) has several properties that make it a promising alternative fuel. It is carbon-free, has a high gravimetric energy density, and can be compatible with some existing conversion technologies. However, there are several challenges in its large-scale use, such as a high flammability envelope and fundamentally different combustion properties compared with existing fuels such as natural gas. The use of carbon dioxide (CO2) as a working fluid in place of nitrogen (N2) can help mitigate some of these issues such as the laminar burning velocity (LBV). It can help mitigate the challenges to the use of H2 in an internal combustion engine (ICE), such as the flame backfiring into the intake manifold, pre-ignition from hotspots and rapid pressure rise. It is important to understand how this working fluid substitution affects other properties. One important intrinsic property of a mixture is the propensity to form wrinkles and cellular structures on the surface of spherically expanding flames, which are indicative of intrinsic instabilities. In this study, images from experiments of spherically expanding H2-O2-CO2 flames were obtained experimentally in a constant volume combustion chamber (CVCC); the schlieren images are processed to detect wrinkles on the flame surface and determine their length. The composition of the mixture is varied in terms of the fraction of CO2 in the mixture, as well as the oxy-combustion equivalence ratio. The experiments were conducted at an initial mixture pressure of 1 bar. It is generally observed that while all flames display wrinkles relatively quickly and further develop into cellular structures, the behavior is more pronounced at lower equivalence ratios. Furthermore, the results also indicate that mixtures with higher fractions of CO2 are more prone to instabilities.
