The stability of stratified horizontal flows of carbon dioxide at supercritical pressures

TL;DR

This study investigates how buoyancy and stratification influence heat transfer and turbulence in supercritical carbon dioxide flows, revealing that flow stability and heat transfer efficiency depend on heating direction and density stratification.

Contribution

It provides experimental insights into the complex dynamics of supercritical CO2 flows, highlighting the effects of stable and unstable stratification on heat transfer and turbulence.

Findings

Unstable stratification enhances vertical mixing and heat transfer.

Stable stratification suppresses vertical motion and deteriorates heat transfer.

Flow behavior varies significantly with heating direction and density stratification.

Abstract

Fluids at supercritical pressures exhibit large variations in density near the pseudo critical line, such that buoyancy plays a crucial role in their fluid dynamics. Here, we experimentally investigate heat transfer and turbulence in horizontal hydrodynamically developed channel flows of carbon dioxide at 88.5 bar and 32.6{\deg}C, heated at either the top or bottom surface to induce a strong vertical density gradient. In order to visualise the flow and evaluate its heat transfer, shadowgraphy is used concurrently with surface temperature measurements. With moderate heating, the flow is found to strongly stratify for both heating configurations, with bulk Richardson numbers Ri reaching up to 100. When the carbon dioxide is heated from the bottom upwards, the resulting unstably stratified flow is found to be dominated by the increasingly prevalent secondary motion of thermal plumes, enhancing vertical mixing and progressively improving heat transfer compared to a neutrally buoyant setting. Conversely, stable stratification, induced by heating from the top, suppresses the vertical motion leading to deteriorated heat transfer that becomes invariant to the Reynolds number. The optical results provide novel insights into the complex dynamics of the directionally dependent heat transfer in the near-pseudo-critical region. These insights contribute to the reliable design of heat exchangers with highly property-variant fluids, which are critical for the decarbonisation of power and industrial heat. However, the results also highlight the need for further progress in the development of experimental techniques to generate reliable reference data for a broader range of non-ideal supercritical conditions.