Entrainment and Mixing of heterogeneous fluids in COnfined COunter-flow Reactor – EMCO2Re

The problem of turbulent mixing of heterogeneous jets -viscosity, density, differential diffusivity- needs to be addressed from a fundamental research perspective to overcome scientific and technical obstacles in order to optimize LLIN processes. The main objectives are focused on the study of the mixture preparation and the time scales characteristic of such applications. Of major importance are the interactions between fluids - across the jet interface - with different dynamics and varying thermochemical properties. The project is approached from three angles: experimental, numerical and analytical, exploring heterogeneous fluid mixtures.

In this program, we will focus on a jet discharging into an ambient. This device may seem far from counterflow technologies, but has nevertheless the advantage of being a simple device, without being simplistic. Moreover, the experimental device has the advantage of facilitating comparison of the results with the literature. In parallel, a counter-current reactor was designed and built. A 3D tomo PIV study was conducted to characterize the interaction between the central jet and the return jet.

In this study, we looked at the effects on the mixing of fluids with varying thermophysical properties. The project was conducted by considering three angles of attack: experimental, numerical and analytical.
The study of the physical jet allowed to show different mixing regimes attributed to density effects. Moreover, the study of the conditioned statistics at the interface has highlighted the effect of variable density at the location where the turbulence is born.
The study on the time jet with mass diffusion and variable viscosity showed the importance of the thermo-physical parameters on the mixture, in particular concerning the terms of scalar dissipation and kinetic energy. These terms are key parameters for the modeling of the mixture.

Setting up of projects

2 study topics with ITV RWTH Aachen University, Germany
- Numerical approaches: DNS and LES
- Experimental approaches : Coupled PIV/ LIF measurements

1 RIN Hybrid, Effect of Differential Diffusion on H2/CO2 mixtures

1. M. Gauding, M. Bode, Y. Brahami, E. Varea, L. Danaila, Self-similarity of turbulent jet flows with internal and external intermittency,, Journal of Fluid Mechanics, 2021.
2. M. Gauding, M. Bode, D. Denker, Y. Brahami, L. Danaila, E. Varea, On the combined effect of internal and external intermittency in turbulent non-premixed jet flames, Proceedings of the Combustion Institute 000, 1–8, 2020
3. L. Danaila, Y. Brahami, E. Varea, M. Gauding, On the scalar-dissipation rate in a temporal jet flow with variable viscosity and mass diffusivity, APS Division of Fluid Dynamics, 2020.
4. M. Gauding, M. Bode, D. Denker, Y. Brahami, L. Danaila, E. Varea, The combined effect of internal and external intermittency in turbulent jet flows, APS Division of Fluid Dynamics, 2020.
5. Y. Brahami, M. Gauding, D. Denker, E. Varea, L. Danaila, Turbulent mixing in variable-density helium-air jet, 17th European Turbulence Conference, 2019.
6. M. Gauding, D. Denker, Y. Brahami, E. Varea, L. Danaila, Internal and external fluctuations in a turbulent non-premixed planar flame, 17th European Turbulence Conference, 2019.
7. Consumption speed determination from spherically expanding flames, E. Varea, B. Renou, Laminar Burning Velocity Workshop, Lisbon, Portugal, 2019.
8. Y. Brahami, J.H. Goebbert, E. Varea, L. Danaila, M. Gauding, Conditional statistics of turbulent flows with variable viscosity, John Von Neumann Institut for Computing Symposium, 2018.
9. E. Varea, H. Larabi, A. Lefebvre, V. Modica, G. Lartigue, V. Moureau, B. Renou, Determination of spatially averaged consumption speed from spherical expanding flame. A new experimental methodology, International Symposium on Combustion, Dublin, Ireland, 2018.
10. Y. Brahami, E. Varea, M. Gauding, L. Voivenel, L. Danaila, On mass entrainment in variable thermophysical property round jets, International Symposium on Applications of Laser Techniques to Fluid Mechanics, Lisbon (Portugal), 2018.
11. Conditional and conventional statistics of variable viscosity jet flows, Y. Brahami, M. Gauding, L. Danaila, E. Varea, 11th Pacific Symposium on Flow Visualization and Image Processing, Japan, 2017.

Electrical power generation is one of the major issues in the context of economic developments and eco-responsible policy. By 2020 for solar, wind and gas turbines (GTs) fuel types by the Californian Independent System Operator (CalISO). The load ramps highlighted by the black arrows in the early morning and in the evening are expected to be of the order of +8.103MW, -6.3.103MW and +13.5.103MW, respectively. These loads would need to be generated or absorbed (GT shut down) within a range of two hours each. According to the US Department of Energy, by 2040, natural gas (for GTs) and renewables would be the two first fuel types for electricity generation. Both natural gas and renewables would see their contribution in electrical power generation increase by 15 and 30%, respectively. The other actual resources (nuclear, coal, liquids) would see their contribution decrease. Therefore, in order to fulfill the electrical power demand, especially when it comes to face with the energy supply fluctuations inherent to renewable energy sources, gas turbines provide a reliable solution. However, a large variability in their operating conditions leads to a high level of pollutant emissions and high risks of damage.

Diluted regime combustion (e.g. MILD - Moderate or Intense Low oxygen Dilution) appears to be an auspicious process for gas turbines to ensure low emission levels over a wide range of operating conditions. In this regime, fuel is mixed with a highly diluted and heated air to create a spatially distributed reaction zone with a reduced peak temperature. COnfined COunterflow Reactors (CO²Res) were design with the aim to provide the most suitable flow features favorable to the MILD combustion. Therefore, they can be viewed as the ideal benchmark to research studies devoted to bring the technological breakthrough needed to: i) improve the efficiency and; ii) reduce pollutants emissions in MILD combustion applications.

To optimize MILD combustion, fast and efficient mixing of reactants with exhaust gases is mandatory. The latter is not only a technical issue, but the lack of theoretical knowledge on turbulent mixing in such flow configurations is clearly pointed out (Kruse et al. 2015). This combustion regime is a priori characterized by a competition between the mixing and chemistry time-scales with a strong influence of the differential diffusion effects (Christo and Dally, 2005, Cavaliere and de Joannon 2004). It thus drastically differs from the conventional jet flames for which mathematical models generally account for fast chemistry and negligible differential diffusion effects.

This project aims at bringing concrete elements of fundamental research to understand, model and predict the turbulent active-scalar mixing, in the context of MILD combustion. Such mixing does couple back on the flow dynamics.
As a consequence, a correct accounting of mixing is even required to describe the flow dynamics. The multi-disciplinary consortium composed of 4 young researchers, 1 post-doc and 1 senior Professor has proven its experimental, numerical and theoretical expertise in the analysis of turbulent mixing.

We therefore aim at performing analytical developments as well as experimental approaches and numerical simulations of a simplified academic based MILD combustor. The project will be divided in three interconnected blocs: i) Turbulent mixing; ii) Turbulent/Non-Turbulent interface and iii) Turbulent modelling. A full description of the phenomenology of the turbulent active-scalar mixing in CO²Res will be provided. Finally, models for CO²Res based on joint experimental, numerical and analytical developments will be proposed in order to perform reliable predictive simulations.