Flow and heat transfer characteristics of natural convection in hydrothermal reactor with segmented heating

Hydrothermal reactor is very suitable to utilize low-grade heat especially concentrated solar energy. However, the heat flux generated by solar heating is usually non-uniform, which affects the natural convection inside the reactor. In this work, natural convection of near critical water in cylindrical hydrothermal reactor is investigated by numerical simulation. With constant total heat input into the cylinder, the sidewall is divided into two halves with different heat flux density. The effect of heating schemes and physical properties on flow structure, tem-perature distribution, fluid motion and wall heat transfer are analyzed. It is found that natural convection is dominated by a large vortex with high velocity when heat flux ratio is lower than 1, and is dominated by two independent weak vortices when heat flux ratio is higher than 1. In addition, the former case has more uniform vertical temperature distribution, stronger fluid flow and weaker wall heat transfer, while the latter case is in the opposite. When the temperature approaches the pseudocritical point, the flow structure remains qualitatively unchanged, but wall heat transfer is enhanced and fluid motion is generally weakened. The change of heat flux ratio significantly regulates fluid motion but slightly affects wall heat transfer. Besides, the regulating effect of heat flux ratio is weakened in near critical region.

Automated summary

This study investigated the natural convection of near critical water in a cylindrical hydrothermal reactor through numerical simulation. It was found that the flow was dominated by a large vortex when the heat flux ratio was lower than 1, and by two independent weak vortices when the ratio was higher than 1. When approaching the pseudocritical point, the flow structure remained unchanged, but wall heat transfer increased and fluid motion weakened. The change in heat flux ratio significantly regulated fluid motion but had a slight effect on wall heat transfer, with a weakened effect in the near critical region.