Fundamental characteristics of heat transfer in an economizer under the exhaust gas of carbon-free fuels: A focus on hydrogen and ammonia combustion

抄録

When using carbon-free fuels (H₂/NH₃), achieving total heat recovery via an economizer—specifically complete recovery including latent heat—is desirable to compensate for their low calorific values LHV (kJ/mol). To the best of the authors' knowledge, this is the first numerical study on economizers under H₂/NH₃ combustion exhaust gas conditions. Analysis of heat transfer around the heat transfer tube placed in a combustion exhaust gas flow of H₂/NH₃ yielded the following results: (1) A liquid film formed downstream of the tube, creating a region of locally high heat flux. (2) The heat flux into the tube increased with the gas velocity at the economizer inlet, and the heat flux upstream of the tube wall increased significantly. The contribution of latent heat to the heat transfer downstream of the tube wall exhibited a similar moderately increasing trend along the tube wall starting from the vicinity of the 70-degree position, regardless of the inlet gas velocity. (cf. stagnation point: 0°) (3) The effects of the inlet gas temperature on condensation phenomena in the boundary layer and the recovery rates of sensible and latent heat were clarified. This knowledge will facilitate the preliminary design of a multi-tube structure based on the local gas temperature at each tube row of the economizer.