Direct numerical simulation of convective heat transfer of supercritical pressure in a vertical tube with buoyancy and thermal acceleration effects

Abstract

Supercritical pressure fluids are widely used in heat transfer and energy systems. The benefit of high heat transfer performance and the successful avoidance of phase change from the use of supercritical pressure fluids are well-known, but the complex behaviours of such fluids owing to dramatic thermal property variations pose strong challenges to the design of heat transfer applications. In this paper, the turbulent flow and heat transfer of supercritical pressure $\textrm {CO}_2$ in a small vertical tube influenced by coupled effects of buoyancy and thermal acceleration are numerically investigated using direct numerical simulation. Both upward and downward flows with an inlet Reynolds number of 3540 and pressure of 7.75 MPa have been simulated and the results are compared with corresponding experimental data. The flow and heat transfer results reveal that under buoyancy and thermal acceleration, the turbulent flow and heat transfer exhibit four developing periods in which buoyancy and thermal acceleration alternately dominate. The results suggest a way to distinguish the dominant factor of heat transfer in different periods and a criterion for heat transfer degradation under the complex coupling of buoyancy and thermal acceleration. An analysis of the orthogonal decomposition and the generative mechanism of turbulent structures indicates that the flow acceleration induces a stretch-to-disrupt mechanism of coherent turbulent structures. The significant flow acceleration can destroy the three-dimensional flow structure and stretch the vortices resulting in dissipation.

Accelerating turbulence in heated micron tubes at supercritical pressure

“…The code was successfully validated for velocity profiles and temperature profiles of heated and unheated turbulent flows. The details of their numerical methods and validation can be found in Cao et al (2021), He et al (2021) and Seddighi (2011). The good agreement indicates that the turbulence generator can generate reasonable turbulence in the tube.…”

“…A schematic diagram of the heated section for upward pipe flow is shown in figure 4. Before the heated section, a periodic inflow turbulence generator with L * /R * = 10 is applied to generate the time-dependent velocity as inlet boundary condition (Cao et al 2021). The heated section is strongly heated with a constant non-dimensional heat flux, Case Type Flow dir.…”

“…In the micron-scale tubes, flow acceleration and scale effect will induce more complicated flow and heat transfer characteristics, which is not yet fully understood. Numerical simulation has been regarded as a method to help understand the phenomenon and mechanisms of supercritical heat transfer (Bae, Yoo & Mceligot 2008;He, Kim & Bae 2008;Mohseni & Bazargan 2011;Nemati et al 2015;Peeters et al 2016;Jiang, Wang & Xu 2018;Cao et al 2021;He et al 2021). Herein, we would like to give a brief introduction to current knowledge of turbulent dynamics of heated flows at supercritical pressures and to set the scene for the research in this present paper.…”

“…They noted that the thermal expansion had the largest influence. For millimetre-scale channels, Cao et al (2021) conducted a DNS study of convective heat transfer of supercritical pressure CO 2 in a heated vertical tube with d * = 1 mm, with coupling buoyancy and thermal acceleration effects. The authors observed that under the coupled effects, the turbulent flow and heat transfer exhibited four periods of development in which buoyancy and thermal acceleration alternately dominated.…”

“…In this paper, we aim to investigate the mechanisms of turbulence attenuation and heat transfer deterioration in a micron-scale tube at supercritical pressure. Based on our previous study in millimetre-scale tubes (Cao et al 2021), the DNS method has been modified to conduct simulations of supercritical pressure fluid flow in a heated micron tube with an inner diameter of 99.2 µm while considering the pressure variation along the heated section. The thermophysical property variations with pressure as well as temperature are included, and the pressure gradient term in the energy equation is considered.…”