Periodic Reporting for period 2 - CRITICAL (When Flows Turn Turbulent in the Supercritical Fluid Region)
Reporting period: 2022-03-01 to 2023-08-31
Novel energy conversion systems for power and propulsion will operate at increasingly higher pressures to operate at higher efficiencies. One of the major obstacles in successfully realizing these new technologies is the limited knowledge of hydrodynamic stability, compressible turbulence and turbulence-radiation interactions with fluids at high pressure, in particular in a region where strongly non-ideal gas effects are at play. Based on my own preliminary results (see next page), it appears that at certain conditions in the non-ideal regime, it is hardly possible to make a laminar flow turbulent. It is furthermore unknown how the strong fluid compressibility in the supercritical regime affects turbulent heat transfer. If the pressure and temperature are high enough, another currently ignored complexity emerges. Gases that at ambient conditions can be considered optically thin, become opaque and radiation has the potential to strongly interact with the fluid and possibly dramatically disrupt turbulence. However, laminar-turbulent transition, fluid compressibility and radiation greatly affect drag, heat transfer, flow separation and consequently affect efficiency, productivity and reliability of any flow device.
The goal of my research is to shed light on these critical, yet unexplored, phenomena by performing the first systematic study at the intersection of thermodynamics, fluid mechanics and radiation science.I will use a unique combination of advanced hydrodynamic stability analysis, novel numerical simulation tools designed for GPU accelerated computing facilities, and unprecedented experiments to optically investigate heated supercritical flows with infrared thermography.
The goal of my research is to shed light on these critical, yet unexplored, phenomena by performing the first systematic study at the intersection of thermodynamics, fluid mechanics and radiation science.I will use a unique combination of advanced hydrodynamic stability analysis, novel numerical simulation tools designed for GPU accelerated computing facilities, and unprecedented experiments to optically investigate heated supercritical flows with infrared thermography.
Our research team was able to achieve remarkable progress in our investigation of supercritical CO2 flows. We built a cutting-edge experimental facility that is the first of its kind for optically studying the flow of supercritical CO2. In our initial tests, the observations surpassed all expectations. The large density fluctuations close to the critical point of the fluid have allowed us to perform highly detailed schlieren visualizations, providing unprecedented insights into the laminar and turbulence dynamics of supercritical fluid flows. This knowledge is crucial for designing efficient heat exchangers in thermal energy conversion systems that operate in the trans- or supercritical fluid region, such as power cycles, refrigeration or heat pump cycles for novel renewable energy systems.
In addition, we have developed a high-order Navier-Stokes solver that utilizes non-ideal equations of state and runs on GPGPU platforms using OpenACC statements. This innovative code has enabled us to perform simulations with unparalleled accuracy and efficiency, significantly reducing the time required for our research.We have collaborated with computer scientists, who have provided invaluable support in advancing our computational capabilities.
We also attracted the attention of other researchers to establish collaborations. We will soon accept a researcher from Japan and China to visit through the accompanying travel grant (ERC - JSPS, and ERC - NSFC). We also started a collaboration with researchers from La Sapienza University in Rome, and Maryland University in the US.
We have made decisive progress regarding the origin of the new unstable mode in supercritical fluids. After demonstrating the essential inviscid nature of this instability, we have identified the kinematic viscosity profile of the base flow as the key criterion to predict the appearance of this instability. This brings a considerably new understanding as it isolates which feature of supercritical fluids is responsible for this instability. \
In addition, we have developed a high-order Navier-Stokes solver that utilizes non-ideal equations of state and runs on GPGPU platforms using OpenACC statements. This innovative code has enabled us to perform simulations with unparalleled accuracy and efficiency, significantly reducing the time required for our research.We have collaborated with computer scientists, who have provided invaluable support in advancing our computational capabilities.
We also attracted the attention of other researchers to establish collaborations. We will soon accept a researcher from Japan and China to visit through the accompanying travel grant (ERC - JSPS, and ERC - NSFC). We also started a collaboration with researchers from La Sapienza University in Rome, and Maryland University in the US.
We have made decisive progress regarding the origin of the new unstable mode in supercritical fluids. After demonstrating the essential inviscid nature of this instability, we have identified the kinematic viscosity profile of the base flow as the key criterion to predict the appearance of this instability. This brings a considerably new understanding as it isolates which feature of supercritical fluids is responsible for this instability. \
In the first work package of our project, we have made significant advancements in the field by developing a unique experimental setup for optically investigating the flow of supercritical CO2. Our preliminary findings indicate that even minimal heating rates can substantially alter the flow characteristics. This change is dependent on the fluid stratification—either suppressing or enhancing turbulence. One of the groundbreaking aspects of our study is the exploitation of strong density variations near the thermodynamic critical point to map changes in the fluid's refractive index. This feature allowed us to reconstruct the velocity field solely based on these refractive index changes. This is not only an advancement in observational techniques but also a new avenue for in-depth fluid dynamics study. The experimental observations have been further validated by our novel, highly-efficient GPGPU flow simulation code, which was developed by the second Ph.D. student in this work package.
Our second work package, where we've developed a new scaling law for compressible flows, already points toward filling this gap by enabling unprecedented accuracy in estimating skin friction and heat transfer. By the end of the project, we aim to unravel the mechanisms behind turbulence modulation due to compressibility, beyond what is known from mean changes in density and viscosity due to heat transfer. This will have broad implications, particularly for the design of future energy conversion systems in both power and propulsion sectors, making our research not only academically rigorous but also industrially relevant.
Our second work package, where we've developed a new scaling law for compressible flows, already points toward filling this gap by enabling unprecedented accuracy in estimating skin friction and heat transfer. By the end of the project, we aim to unravel the mechanisms behind turbulence modulation due to compressibility, beyond what is known from mean changes in density and viscosity due to heat transfer. This will have broad implications, particularly for the design of future energy conversion systems in both power and propulsion sectors, making our research not only academically rigorous but also industrially relevant.