Advancing Renewable Aero Propulsion with Ultrafast Laser Diagnostics
Funding
Personal VIDI grant of the Netherlands Organization for Scientific Research (NWO).
Duration
2017-2022
Team
Dr. Alexis Bohlin is the Principal Investigator of the project. The research is embedded in the program of the Propulsion and Power group headed by Prof. Piero Colonna. The project relies on interdisciplinary connections with five other senior researchers: Dr. Arvind Gangoli Rao (also Propulsion and Power) and Prof. Georg Eitelberg (Flight Performance), Prof. Dirk Roekaerts (Process and Energy), and Prof. Luuk A.M. van der Wielen (Biotechnology).
Summary
The goal of this research is to contribute to the advancement of new clean combustion technology for aero propulsion, with increased fuel efficiency and reduced emissions. We develop transformative ultrafast optical diagnostics to investigate combustors and bioreactors at molecular level. These unique optical tools make available highly accurate in-situ measurements of temperature and key-species which support the search for new chemical and biological insights and for fundamental mechanisms improving their performance. It creates the foundation for building predictive engineering models at designing combustors and bioreactors, where the experiments under tightly controlled boundary conditions have the role of informing and validating the theory and improve the fidelity of numerical simulations. The research plan is to advance knowledge from discovery-based science with expected deliverables leading to the shortest path to utilization in industrial processes. In this project we focus on the following experiments:
- Combustor: Distributed auto-ignition combustion modes with reduced NOx emission
- Bioreactor: Microbial production of biofuels for aero propulsion
Combustor
The combustor experiment deals with the fundamental studies of ignition-kernels and flame propagation in vitiated flows. Vitiated flows are encountered in exhaust-gas-recirculation processes, which are central in ultra-low NOx combustion concepts, e.g. in Flameless Combustion (FC). FC has emerged as one promising option to be investigated for future aero-engine and gas-turbine operation, inspired by the improvements obtained in furnace combustion. It is characterized by fuel oxidation at low peak temperatures and with distributed reaction zones, which make the flames invisible to the naked eye. The FC mode is especially designed to reduce the emissions of NOx, particulates, CO and unburned hydrocarbons, and would arguably be very beneficial for gas turbines because of the uniform temperature profile. The distributed reaction zone leads to a diminished combustor pattern factor, and to reduced perturbation due to thermo-acoustic effects, if compared to conventional combustors. The implementation of the FC mode in gas turbines is not straightforward: the implications of the involved physical scales makes it quite challenging. Fundamental insights, in particular on the effect of vitiation on ignition kernels and on flame propagation are needed in order to define FC and its operational domain. We are commissioning a robust ultrafast CARS system with a new precision benchmark in gas-phase thermometry. This is a unique tool for the investigation of the operation and the stability of the FC mode.
Bioreactor
Clean combustion technology aims at improving fuel efficiency and reducing the emission of pollutants. Non-conventional and especially bio-fuels will play an increasing role in this respect. One prominent way of producing biofuels, is through quantitative microbial cultivation in bioreactors and integrated product recovery. In the case of molecules that form a second liquid phase at fermentation conditions (e.g. sesquiterpenes, alkanes), it is vitally important to understand the secretion mechanism and its role in product droplet stabilization, which is relevant from a recovery point of view. In this experiment, we set the goal of examining in-situ the process by which microorganisms produce the oil and secrete. Such measurement will provide unparalleled insight into the biological process involved in the conversion of biomass into biofuel. This insight is specifically important for further improvement of the microorganism, the fermentation- and the recovery process. Through developing snap-shot hyperspectral CARS wide-field microscopy, with excellent chemical contrast and rapid detection of biomolecules, we will monitor the initial stage of the oil-droplet formation to unravelling the mechanism.
Ultrafast CARS imaging
Coherent anti-Stokes Raman spectroscopy (CARS) is a state-of-the-art technique for laser diagnostics in reacting flows. It is known for its capacity to quantify temperature and species concentration in extreme harsh conditions with the highest levels of accuracy and precision. It is the combination of 1. chemical selectiveness, 2. the ability to probe the entire thermal population distribution over the molecular rotational-vibrational states manifold, and 3. the laser-like properties of the generated signal beam, which makes this technique very powerful. CARS belongs to the family of nonlinear optical four-wave-mixing techniques, which must satisfy two physical conditions. One condition is energy conservation, where three incident photons with energies ωpump, ωStokes, and ωprobe are mixed with the internal energy levels of the probed molecules to generate a fourth photon at energy ωCARS according to ωCARS = ωpump − ωStokes +ωprobe. Another condition is phase-matching (momentum conservation), which constrains how the wave vectors of the incident beams must be arranged to effectively generate the coherently scattered light. The current work utilizes the intrapulse combination – femtosecond/picosecond CARS - which combines highly efficient impulsive excitation in the time domain (femtosecond laser pulse) with high resolution detection in the frequency domain (picosecond laser pulse).
Ultrafast CARS is independent of background species at a wide range of operational temperature and pressure conditions, because the rapid measurement occurs within the collisional dephasing times (~picosecond temporal resolution). In addition, with a time-resolved technique, the suppression of the non-resonant four-wave mixing signal is achieved by simply delaying the probe-pulse relative the coherence preparation pulse. This ultrafast CARS scheme allows for direct imaging over spatial dimensions (1D and 2D) which is a new capability for CARS. It open up for new research avenues to probe any dynamical process that involves spatial effects such as diffusion, mixing, energy transfer and chemical reaction.
