Numerical analysis on flow and heat transfer of supercritical CO<sub>2</sub> in horizontal tube

Abstract

In the present study, the three-dimensional steady-state numerical simulation has been performed by using ANSYS Fluent15.0 with SST <i>k-ω</i> low Reynolds turbulence model to study flow and heat transfer characteristics for supercritical CO<sub>2</sub> in the horizontal straight tube with inner diameter <i>d</i><sub>i</sub> = 22.14 mm and heating length <i>L</i><sub>h</sub> = 2440 mm under heating condition. The reliability and accuracy of the numerical model was verified by the experimental data of flow and heat transfer of supercritical CO<sub>2</sub> in horizontal tube. Firstly, flow and heat transfer characteristics of supercritical CO<sub>2</sub> was studied in horizontal tube. Based on the assumption that the supercritical CO<sub>2</sub> will undergoes “phase transition” between liquid-like and vapor-like at pseudocritical temperature <i>T</i><sub>pc</sub>, the differences between top generatrix and bottom generatrix of horizontal tube at flow and heat transfer behaviors were revealed. The results show flow and heat transfer characteristics of supercritical CO<sub>2</sub> in horizontal tube are similar to those under subcritical pressure. Then, the influences of heat flux <i>q</i><sub>w</sub> and mass flux <i>G</i> on flow and heat transfer of supercritical CO<sub>2</sub> were analyzed. The higher heat flux <i>q</i><sub>w</sub> is or the smaller mass flux <i>G</i> is, the higher inner wall temperature <i>T</i><sub>w,i</sub> at top generatrix is. The reasons for difference in the distribution of inner wall temperature <i>T</i><sub>w,i</sub> at top generatrix under different heat flux <i>q</i><sub>w</sub> and mass flux <i>G</i> were explained by capturing detailed information about thermophysical properties distribution including specific heat at constant pressure <i>c</i><sub>p</sub> and thermal conductivity <i>λ</i>, axial velocity distribution and turbulent kinetic energy distribution in the fluid domain. It is observed that vapor-like film thickness <i>δ</i>, vapor-like film property characterized by specific heat at constant pressure <i>c</i><sub>p</sub> and thermal conductivity <i>λ</i>, axial velocity <i>u</i> and turbulent kinetic energy <i>k</i> are the main factors affecting the difference in inner wall temperature distribution at top generatrix. The present work can provide a theoretical guidance for design and safe operation of heat exchanger for supercritical CO<sub>2</sub> Brayton cycle.