Numerical study on premixed hydrothermal combustion in tube reactor

Abstract

Premixed hydrothermal combustion of ethanol was numerically studied as a fundamental research to develop an energy conversion system utilising biomass and low quality oil. A simulation code available for oxidation of water solution mixture under sub/super critical conditions was developed. Lee–Kesler equation was selected as the equation of state. Chemical species of ethanol, oxygen, carbon dioxide and water, were considered as reactants and products of complete reaction. Mixing laws by Chung et al. and Wilke et al. were used for transport properties and that of Plöcker et al. was used for thermal properties and Lee-Kesler equation of state. Behaviours of ethanol oxidation under sub/super critical conditions were numerically simulated. For the reaction rate, we selected Schanzenbächer et al.'s expression obtained by the experiment under supercritical condition. Effects of preheat temperature, heat loss and flow velocity were examined by simulation. The differences of preheat temperatures led to the differences of temperature increase Δ T around critical point because of the specific heat peak at the critical point. Larger flow velocity caused slower increase of ethanol conversion and temperature increase. Changes of reaction rate for different flow velocities showed that larger flow velocity caused thicker reaction zone and higher reaction peak than that for smaller one because of the fluid advection. From the calculation with different heat losses (different heat transfer rate coefficients α), two-step temperature decrease appeared for larger heat loss (α = 2.5 × 104 W· m−2· K−1) while monotonous temperature decrease appeared for smaller heat loss (α = 2.5× 103 W· m−2· K−1).