Novel two-phase method for supercritical water flow

TL;DR

This paper introduces a novel two-phase approach using VOF modeling to analyze supercritical water flow and heat transfer deterioration, providing new insights into the forces and phase behavior involved.

Contribution

It presents a new two-phase method for supercritical flow analysis, including a pseudo phase change model and a theoretical expression for two-phase thickness.

Findings

Density variation is the primary cause of HTD.

Buoyancy and inertia forces must be comparable to prevent HTD.

Volume fraction mapping reveals phase thickness jumps near the wall.

Abstract

The current resurgence in the phase diagram study beyond the critical point has questioned the conventional belief of supercritical fluid as a single phase with varying properties. On the same line, a novel two-phase approach has been proposed to study the supercritical flow with heat transfer deterioration (HTD) phenomena. The Volume of Fluid (VOF) multiphase model has been used to analyze the flow, and the simulation result reasonably predicts the wall temperature peaks. Moreover, the velocity and turbulent kinetic energy profiles for different axial locations explain the occurrence of HTD, and it is on par with the pre-existing numerical method. Besides, the quantitative analysis presented in the paper expounds on qualitative understanding. The parametric study of the thermophysical properties revealed that the density variation is the primary cause of HTD in supercritical flows. So banking on this conclusion, the propounded study focuses on the forces generated due to the density variation. This technique equips us with a holistic picture of the supercritical flows. It leads to the inference that for no HTD effects to be present, buoyancy and inertia forces have to be of comparable magnitude throughout the flow. In addition, the pseudo phase change model outclasses the existing research by rendering us the volume fraction data. Mapping of this variable reveals a sudden jump in the lighter phase's thickness near the wall at the site of HTD, which is also reflected as a maximum in the nondimensional two-phase thickness (P) plot. Nevertheless, this observation is only restricted to HTD caused by buoyancy. In the end, a theoretical expression has been conceptualized for computing the two-phase thickness (h) value, which can serve as a fundamental length scale in supercritical flows as it marks the region of highest property gradient near the wall.