PhD (M/F). Interactions Between Turbulence and Temperature in Concentrating Solar Power Systems

Job Description

Thesis Subject

Title. Interactions Between Turbulence and Temperature in Concentrating Solar Power Systems
This thesis research aims to improve the understanding and modeling of turbulent flows subjected to strong thermal gradients within solar thermal (T) and photovoltaic-thermal (PVT) collectors used for the conversion of concentrated solar energy. The project will be carried out using theoretical, numerical, and experimental approaches. Unique physical processes occur in these systems. These processes are related to how the heat transfer fluid is heated—either through contact with a high-temperature wall that absorbs concentrated solar radiation or within a volume directly exposed to the radiation. These configurations give rise to interactions between turbulence and thermal gradients, which must be elucidated theoretically and validated experimentally. Several levels of modeling will be developed.
Turbulent flows exhibit a highly complex structure, particularly in the presence of temperature gradients. The various characteristic quantities of the flow exhibit a random nature. The technique generally employed involves adopting a statistical representation of turbulence that provides direct access to characteristic global quantities without relying on the complete realizations of turbulent fields, obtained—for example—through direct numerical simulations. This approach, however, introduces a closure problem, meaning that the number of unknowns exceeds the number of equations. Numerous closure attempts have been proposed, ranging from single-point models to more sophisticated theories based on approaches that describe turbulent fields using statistical correlations at multiple points, such as spectral models of the EDQNM type.
The theoretical and computational objectives of this thesis are to develop an EDQNM model of turbulent flow in the presence of strong temperature gradients caused by asymmetric heating (only one wall is heated). The inclusion of radiation in the EDQNM model for a flow consisting of an absorbing gas (such as CO₂) will also be considered. Through numerical simulations of the developed model, the primary goal will be to calculate the energy spectra for an isothermal case. The strong temperature gradients will alter the slope of the energy spectrum and thus modify energy transfers. Parametric analyses and sensitivity studies will be conducted to deduce the fundamental mechanisms of energy redistribution across turbulence scales. Subsequently, based on the equations and results from the EDQNM modeling, a simplified model (a single point, under the Boussinesq assumption) will be developed. This type of model will be extremely useful for investigating flow in solar collectors.
As for the experimental part, preliminary tests were conducted using a low-power solar furnace to generate a temperature gradient in a shallow volume of nanofluid. This nanofluid is a suspension of graphene nanoparticles in water. It absorbs 85% of solar radiation over a thickness of 1 cm at a graphene concentration of 0.75 g/L. This absorption can be controlled by varying the concentration of graphene particles. The tests were conducted by exposing the surface of the nanofluid to a flash of solar radiation (lasting on the order of a second). The fluid is subjected to radiation on the order of 10 MW/m² over an area 1 cm in diameter. This radiation is absorbed and causes very rapid local heating. Observation using a thermal camera revealed the formation of “thermal plumes” which, as they develop, can generate turbulent flow. These plumes do not appear when temperature gradients are too low, suggesting the existence of a transition between different flow and heat transfer regimes, shifting from a predominantly conductive to a predominantly convective regime.
The experimental objective of this dissertation is to set up an experimental setup capable of characterizing regime transitions and identifying the physical mechanisms underlying these transitions. The doctoral student will be tasked with developing existing experimental components based on thermal and optical imaging to ensure the repeatability and accuracy of measurements, and with conducting the relevant parametric studies. Furthermore, the analysis of the results will involve the application of advanced concepts in heat transfer, characteristic times, and waves, through the development of a stability model. The experiments will be conducted in collaboration with laboratory members who are experimentalists and experts in concentrated solar power.
Finally, a comparison will be made between experimental results and model simulations, with a particular focus on analyzing the physical phenomena underlying these couplings. These two complementary approaches are essential for conducting a detailed analysis of the heat transfer processes occurring within a solar receiver.

Your Work Environment

The PROMES laboratory focuses its research on solar energy and, in particular, its high-temperature applications. Concentrated solar power systems offer a range of solar furnaces with power outputs ranging from 1 kW to 1 MW, capable of producing very high temperatures (3,000°C) thanks to very high concentration ratios (15,000 times the sun's intensity). The principle behind concentrated solar power technologies is to use mirrors to concentrate the sun's rays onto a receiver to generate heat. The key component of this process is the solar receiver, which operates at high temperatures and under high heat flux. Within this solar receiver, the interactions between turbulence and thermal phenomena make the physics particularly complex and fascinating. Mastering highly anisothermal turbulent flows is therefore a key scientific challenge for the development of next-generation solar power plants for heat and electricity generation. In addition to the purely thermal conversion of solar energy, compact hybrid (PVT) systems make it possible to simultaneously generate medium-temperature heat (150–250 °C) and electricity using a single device. This device consists of a solar absorber on which a photovoltaic cell is mounted, operating under non-standard conditions. Several solutions are possible, but all require a better understanding of the interactions between flow and heat transfer, as these govern the system's overall efficiency.
The answers to these preliminary questions will inform the more applied research conducted as part of the SHIP4D project (“Solar Heat in Industrial Processes for Decarbonization”) under the PEPR SPLEEN program. This dissertation is part of that project.

Constraints and risks

Experiments with a 1 kW solar furnace
No particular risks.

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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