Numerical study on pseudo-boiling heat transfer of supercritical CO2 in horizontal tube

Abstract

Supercritical carbon dioxide sCO₂ is an attractive fluid candidate for power generation systems to achieve higher efficiency. The heat transfer performance of sCO₂ is of significant concern. Here, sCO₂ with pressure ranging from 8−12 MPa heated in 10.0 mm diameter horizontal tube is investigated numerically. The mass flux and heat flux ranges are 300−700 kg/m²s, 70−210 kW/m², respectively. Based on the pseudo-boiling concept, phase distribution of supercritical fluid is obtained, which involves a liquid-like (LL) core and a vapor-like (VL) layer on the tube wall. The thickness of VL layer is the key to the heat transfer of supercritical fluid, which is determined by the balance of evaporation momentum force and inertia force. For high heat flux, large evaporation momentum force induces thick and wavy VL layer, resulting in heat transfer deterioration HTD. Along the flow direction, there is wall temperature overshoot with the maximum value of 114.2 ^οC. What's worse, the thickness of VL layer in the horizontal tube is non-uniform in the circumferential direction. The VL layer at the top of tube is thicker and the wall temperature is much higher. The circumferential wall temperature difference reaches 138.7 ^οC, maximally. As potential safety hazards for heat transfer exchangers, both wall temperature overshoot and non-uniformity need to be suppressed. The results show that rising pressure only reduces wall temperature value but has a weak impact on non-uniformity. Increasing mass flux raises the inertia force to compete against the evaporation momentum force, yielding thin and smooth VL layer, decreasing both wall temperature overshoot and non-uniformity. In brief, this work not only reveals HTD mechanism of supercritical fluid based on pseudo-boiling theory, but also guides the safety design of horizontal heat exchangers such as parabolic trough solar receivers using sCO₂.