Amidst the rise in fossil fuel consumption and the global energy crisis, there is a growing demand for clean, environmentally friendly, and sustainable energy solutions in industrial processes. One promising approach is to substitute conventional working fluids in power cycles with supercritical fluids. Over recent decades, substantial research effort has been made in investigating supercritical fluids. Among these, supercritical carbon dioxide (sCO2) has emerged as a practical and alternative solution. Its low critical pressure and temperature, high thermal efficiency, and operational flexibility have garnered widespread interest across various energy applications, particularly in heat exchangers and gas-powered cycles. However, the complex flow dynamics and heat transfer characteristics of sCO2 near its critical point have become the subject of intense research. When sCO2 flows through a heated channel, strong density gradients can generate dominant buoyancy forces that can significantly affect the supercritical fluid structure, mixing and transport properties. Understanding buoyancy-affected flows is highly crucial as it can lead to flow stratification and heat transfer deterioration or enhancement in the channel.

The role of buoyancy forces in flow stratification is quite substantial and can stabilize or destabilize the stratified structure by inducing or dampening instabilities. While numerous computational simulations have been performed to understand the mechanism of buoyancy-affected stratification in ideal fluids, whilst, a notable research gap exists in the literature on supercritical fluid stratified flow. Therefore, the present study aims to investigate the influence of buoyancy on the sCO2 flow stratification in a channel, considering heating from the top and bottom walls. In this context, a Direct Numerical Simulation (DNS) is conducted using the open-source CFD package ”OpenFOAM” to simulate pressure-driven sCO2 channel flow under constant wall heat flux. A buoyant pimple foam solver was adapted to simulate transient supercritical flow. To gain good accuracy in the thermophysical properties, a custom library was prepared to interpolate supercritical fluid properties at simulation run-time. To assess the influence of buoyancy in the heated channel flow of sCO2, a developing flow profile is initiated at a constant pressure of 80 bar. Stratification is achieved by imposing a heat flux at the wall boundary spanning in the heating range of 5 − 15 kW /m2, resulting in density and temperature variation across the fluid. By varying the heat flux at the wall boundary, we analyze the resulting variation in the flow field (temperature, velocity, and pressure distribution), the dynamics of heat transfer, and the influence of buoyancy by the non-dimensional parameter Richardson number in the stratification of supercritical carbon dioxide. The results show that the effect of heating on the developing boundary layer in sCO2 channel flow is substantial. With increasing heat flux, the flow is accelerated near the heated wall while decelerating in the bulk. A strong non-linear variation in the temperature and density distribution is observed in the wall-normal direction. Moreover, as the heat flux increases, the wall shear stress decreases due to strong property variation, while the Richardson number and Reynolds number increases, and the heat transfer coefficient decreases.

Due to the global energy crisis, there is a growing need to make industrial processes more efficient and clean. One such method is to use effective and renewable working fluids in power and refrigeration cycles. Research in this field during the past decade has placed supercritical CO2 (SCO2) in the forefront due to its non-ideal properties and abundance in nature but, to fully understand their heat transfer properties, insight into boundary layer flow and hydrodynamic instabilities are crucial. Experimental setups such as closed loop wind tunnel contraction facilities are needed which provide a laminar steady flow to take boundary layer measurements. However, using a contraction causes various problems such as formation of secondary flows like boundary layer separation, Gortler vortices, cross-flows and non-uniformity’s. In this study a novel 1D contraction shape with curvature on only one side is pro- posed. This will completely eliminate the risk of Gortler vortices on the bottom plate and make it a suitable candidate for boundary layer experiments. Nevertheless, the risk of the other secondary flows and Gortler vortices on the top wall disturbing this laminar flow remains. Hence, the present research aims to optimise this 1D contraction to reduce the risk of such secondary flows and analyse the growth of Gortler vortices on the top wall using numerical simulations.

Steady state laminar simulations were performed to optimise the design of the 1D wind tunnel contraction. Optimum contraction and settling chamber lengths were found by identifying the risk of flow separation and boundary condition effects respectively. For the optimisation of the contraction wall shape, a family of transformed fifth order polynomial curves with varying inflection point distances were selected. To identify the curve with least risk of boundary layer separation and lowest flow non-uniformity, a multi- objective optimisation procedure was implemented. The procedure found that a curve with inflection point of 101mm downstream of contraction inlet gives the best performance.

To analyse the effect of Gortler vortices on outlet uniformity, unsteady laminar simulations were performed on the contraction by forcing sinusoidal perturbations of different wavenumbers. This study found that perturbations form symmetrical steady steam-wise Gortler vortices in the contraction. The most unstable wavenumber was found to be 𝜆=83.33 𝑚^-1 which formed secondary vortices of high vorticity. However it was also seen that for all wavenumbers, the vortices lose energy as they exit the contraction and do not affect the bottom boundary layer. Further, the effects of a side wall on the formation of Gortler vortices were also investigated. The results showed that the Gortler vortex closest to the wall is absorbed into high vorticity corner vortices, while those close to the centreline develop into steady Gortler vortices. No effect on the centreline velocity profile was seen due to the vortices making the 1D contraction suitable for boundary layer experiments. Finally, it was also seen that the 1D contraction shape produces asymmetrical Gortler vortices due to alternative perturbation methods such as random inlet perturbations.