Heat transfer characteristics of tubular heat exchanger using reverse air injection flameless combustion

Abstract

The heat transfer characteristics of a co-axial triple tube reformer using flameless combustion were studied herein. Methane steam reformers for hydrogen production have been developed with a long history, but as hydrogen has recently come into the limelight as a future energy, research on the development of more efficient reformers is being pursued. Flameless combustion has the advantage of being applicable for the high performance reformers, but studies to date have focused on the combustion phenomenon itself, and research on the heat transfer phenomenon is lacking. In this study, RAI (reverse air injection) flameless combustion, which generates high intensity combustion and heat transfer, was applied to the reformer and its heat transfer effect was analyzed in parallel with experiments and CFD analysis. In the experiment, it was confirmed that the RAI flameless combustion increased the heat transfer by up to 58% compared to the conventional flame type combustion, and the case of two air nozzles was found to have a better heat transfer enhancement effect than the case of four. However, the case using four air nozzles showed a more axisymmetric flow pattern. The heat transfer enhancement effect of RAI flameless combustion is caused by the high-speed air jet creating a highly recirculating flue gas to generate uniform temperature distribution and high convective heat transfer on the surface. As RAI flameless combustion is characterized by being mixed with fuel at the rear end of a strong air jet, which is a completely different configuration from these existing combustors, it is expected that the limitations of the EDC model, which has been widely used in previous studies, will become more prominent. The equilibrium-PDF model based on chemical equilibrium and tabulated chemistry gave a closer result to the experimental results.