Supercritical fluid has been applied widely as an effective working fluid in engineering systems due to its unique features. In this study, the flow physics of the abnormal laminarisation and re-transition that happen in heated upward pipe flows of supercritical fluids are investigated using Direct numerical simulations (DNS) with an in-house code CHAPSim. It is important to clarify how different factors trigger and affect the reduction and the following regeneration of turbulence in such flows. DNS of supercritical carbon dioxide with one or more effects artificially isolated or eliminated are carried out so as to better understand the complex phenomena. The axial flow development is found to be important during the laminarisation. The effects of the variations of density and viscosity, and buoyancy are found to be similar, in that all of them cause an overall reduction of pressure gradient following a near-wall deficit of downward force, leading by a response of a rising inertia. Based on these findings, a unified approach has been proposed to describe the effect of spatial acceleration, viscosity variation, buoyancy and inertia making use of the concept of pseudo-body forces. With the apparent Reynolds number (ARN) theory applied, a heated upward flow with these effects can be decomposed into an equivalent-pressure-gradient reference flow and a perturbation flow. The turbulent shear stress and axial velocity predicted using the ARN theory agree well with those produced in DNS, suggesting the proposed unified approach and ARN theory successfully characterise the upward heated flow. A new 'full' laminarisation is identified referring to a region where no new vortical structures are generated. This region is found to be akin to the pre-transition region of a boundary layer bypass transition. The structural (direct) effect of the buoyancy on turbulence is initially weak during the laminarisation, but is dominant in the full laminarisation and re-transition region.
Additionally, an assessment of a fluid-to-fluid scaling method proposed in the literature has been carried out using DNS for the first time. Excellent similarities are achieved between the different supercritical fluid flows tested, suggesting the flow and heat transfer of the upward heated flow can be generally characterised by the similarity parameters. The sensitivities of similarity parameters and inlet conditions are also investigated. The Stanton number is found to be better than the Nusselt number, in terms of characterising the similarity for heat transfer.
Finally, the effect of conjugate heat transfer on supercritical fluid flows is studied. For most numerical studies of such flows in the literature, boundary conditions are normally idealised, with a uniform wall heat flux imposed, while in experiments, the redistribution of heat flux and stabilisation of near-wall enthalpy fluctuations exist due to the solid wall conduction. A conjugate heat transfer solver is implemented in the DNS code, and simulations with and without the solid wall are compared. Although the bulk enthalpies are shifted due to the redistribution of wall heat flux hence influencing the entrance effect, Nusselt number away from this region is not largely affected. It is found that the stabilisation effect is limited to a region close to the wall and diminished further away, but the turbulent kinetic energy is significantly affected by the stabilisation.
