Xu Chu

Direct Numerical Simulation of Heated Turbulent Pipe Flow at Supercritical Pressure

For fluids at supercritical pressure, the phase change from liquid to gas does not exist. In the meanwhile, the fluid properties change drastically in a narrow temperature range. With supercritical fluid as working fluid in a heated pipe, heat transfer deterioration and recovery has been observed, which correspond to the turbulent flow relaminarization and recovery. Direct Numerical Simulation (DNS) of supercritical carbon dioxide flow in a heated vertical circular pipe at a pressure of 8 MPa is developed with the open-source code OpenFOAM in the present study. Forced convection cases and mixed convection including upward and downward flow have been considered in the simulation. The change of the mean flow and turbulence statistics has been analysed in detail. In the forced convection, flow turbulence is attenuated due to acceleration from thermal expansion, which leads to a peak of the wall temperature. Buoyancy has a stronger impact to the flow. In the upward flow, the average streamwise velocity distribution turns into an M-shape profile because of the ‘external’ effect of buoyancy. Besides that, negative buoyancy production caused by the density variation reduces the Reynolds shear stress to almost zero, which means that the flow is relaminarized. Further downstream turbulence is recovered. This behaviour of flow turbulence is confirmed by visualization of turbulent streaks and vortex structures. The observation of the flow turbulence of this can help to develop advanced turbulence models for applications in nuclear or conventional energy generation technologies.

Investigation of Convective Heat Transfer to Supercritical Carbon Dioxide with Direct Numerical Simulation

For fluids at supercritical pressure, the phase change from liquid to gas does not exist. In the meanwhile, the fluid properties change drastically in a narrow temperature range. With supercritical fluid as working fluid in a heated pipe, heat transfer deterioration and recovery have been observed, which correspond to the turbulent flow relaminarization and recovery. Direct Numerical Simulation (DNS) of supercritical carbon dioxide flow in a heated vertical circular pipe at a pressure of 8 MPa is developed with the open-source code OpenFOAM in the present study. Forced convection cases and mixed convection including upward and downward flow have been considered in the simulation. The change of the mean flow and turbulence statistics has been analysed in detail. In the forced convection, flow turbulence is attenuated due to acceleration from thermal expansion, which leads to a peak of the wall temperature. Buoyancy has a stronger impact to the flow. In the upward flow, the average streamwise velocity distribution turns into an M-shape profile because of the “external” effect of buoyancy. Besides that, negative buoyancy production caused by the density variation reduces the Reynolds shear stress to almost zero, which means that the flow is relaminarized. Further downstream turbulence is recovered. This behaviour of flow turbulence is confirmed by visualization of turbulent streaks and vortex structures. The observation of the flow turbulence of this can help to develop advanced turbulence models for applications in nuclear or conventional energy generation technologies.

Flow turbulence topology in regular porous media: From macroscopic to microscopic scale with direct numerical simulation

Microscopic analysis of turbulence topology in a regular porous medium is presented with a series of direct numerical simulation. The regular porous media are comprised of square cylinders in a staggered array. Triply periodic boundary conditions enable efficient investigations in a representative elementary volume. Three flow patterns—channel with sudden contraction, impinging surface, and wake—are observed and studied quantitatively in contrast to the qualitative experimental studies reported in the literature. Among these, shear layers in the channel show the highest turbulence intensity due to a favorable pressure gradient and shed due to an adverse pressure gradient downstream. The turbulent energy budget indicates a strong production rate after the flow contraction and a strong dissipation on both shear and impinging walls. Energy spectra and pre-multiplied spectra detect large scale energetic structures in the shear layer and a breakup of scales in the impinging layer. However, these large scale structures break into less energetic small structures at high Reynolds number conditions. This suggests an absence of coherent structures in densely packed porous media at high Reynolds numbers. Anisotropy analysis with a barycentric map shows that the turbulence in porous media is highly isotropic in the macro-scale, which is not the case in the micro-scale. In the end, proper orthogonal decomposition is employed to distinguish the energy-conserving structures. The results support the pore scale prevalence hypothesis. However, energetic coherent structures are observed in the case with sparsely packed porous media.Microscopic analysis of turbulence topology in a regular porous medium is presented with a series of direct numerical simulation. The regular porous media are comprised of square cylinders in a staggered array. Triply periodic boundary conditions enable efficient investigations in a representative elementary volume. Three flow patterns—channel with sudden contraction, impinging surface, and wake—are observed and studied quantitatively in contrast to the qualitative experimental studies reported in the literature. Among these, shear layers in the channel show the highest turbulence intensity due to a favorable pressure gradient and shed due to an adverse pressure gradient downstream. The turbulent energy budget indicates a strong production rate after the flow contraction and a strong dissipation on both shear and impinging walls. Energy spectra and pre-multiplied spectra detect large scale energetic structures in the shear layer and a breakup of scales in the impinging layer. However, these large scale st...

Buoyancy induced turbulence modulation in pipe flow at supercritical pressure under cooling conditions

A numerical investigation of cooling of a fluid at supercritical pressure has been performed by means of direct numerical simulations. The simulations were conducted with a uniform heat flux imposed at the wall at an inlet bulk Reynolds number of 5400. The aim of this work is to understand the role of buoyancy in modulating the turbulence in a flow where properties are spatially varying. Heat transfer deterioration followed by recovery is observed in the downward flow while enhancement occurs in the upward flow as compared to forced convection. The decomposition of the skin friction factor and the Nusselt number was performed. The major effects on the skin friction factor were brought by the non-uniform body force due to the gravity. The turbulent parts equally influence the Nusselt number as well as the skin friction factor in supercritical flows. Quadrant analysis and its weighted joint probability density function were analyzed to understand the role of sweep (Q4) and ejection (Q2) events. During the heat transfer deterioration, sweep and ejection events are decreased greatly, triggering the reduction in turbulence. The recovery in turbulence is brought by the Q1 and Q3 (also known as outward and inward interaction) events, contrary to the conventional belief about turbulence generation. The turbulence anisotropy of the Reynolds stress tensor is investigated showing that the turbulence structure becomes rod-like during the deteriorated heat transfer regime in the downward flow and disc-like for the upward flow.A numerical investigation of cooling of a fluid at supercritical pressure has been performed by means of direct numerical simulations. The simulations were conducted with a uniform heat flux imposed at the wall at an inlet bulk Reynolds number of 5400. The aim of this work is to understand the role of buoyancy in modulating the turbulence in a flow where properties are spatially varying. Heat transfer deterioration followed by recovery is observed in the downward flow while enhancement occurs in the upward flow as compared to forced convection. The decomposition of the skin friction factor and the Nusselt number was performed. The major effects on the skin friction factor were brought by the non-uniform body force due to the gravity. The turbulent parts equally influence the Nusselt number as well as the skin friction factor in supercritical flows. Quadrant analysis and its weighted joint probability density function were analyzed to understand the role of sweep (Q4) and ejection (Q2) events. During the h...

Numerical Analysis of Heat Transfer During Cooling of Supercritical Fluid by Means of Direct Numerical Simulation

Supercritical fluids have a wide spectrum of application, ranging from power generation to enhanced oil extraction. The sensitive nature of thermophysical properties makes the heat transfer complicated. Therefore, in this work, an investigation is made for the vertically-oriented pipe to understand the physics behind the heat transfer deterioration occurring during cooling. For that, carbon dioxide is chosen as working fluid, and direct numerical simulations with the open source finite volume code OpenFOAM have been performed with variation in the strength of body force due to buoyancy. It was found out that body force affects the axial temperature profile. Initial examination of turbulence statistics revealed that turbulence is modulated by buoyancy and deacceleration. Further investigation unveiled that long 1-dimensional structures characterized by streak elongation were present in the downward flow. In the end, Octant analysis indicates the reduction in ejection and sweep events for downward flow caused the decrease in turbulence.

Direct Numerical Simulation of Heated Pipe Flow with Strong Property Variation

Using supercritical fluid as coolant in a power cycle is generally considered as an advanced solution for energy conversion. When the pressure is above the critical point (P c ), thermo-physical properties vary significantly with temperature, which leads to complicated heat transfer phenomena. In the current project, direct numerical simulation (DNS) in a horizontal heated pipe has been developed for supercritical CO2 using the numerical solver based on OpenFOAM. DNS enables us to investigate the detailed turbulence modulation and heat transfer characteristics. The horizontal layout of the pipe leads to a flow stratification, which is not observed in the vertical pipes from the report in the last year. Furthermore, the obtained turbulence data are serving for the development of advanced turbulence models.