A recently published research article in Light | Science & Applications introduced a new technique for measuring temperature in combustion diagnostics, especially in high-temperature and turbulent conditions.
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The single-shot, single-beam coherent Raman scattering (SS-CRS) thermometry technique uses optically induced air lasing for rapid and accurate temperature measurements in challenging environments. The method enables precise temperature and species concentration diagnostics.
Advancements in Temperature Measurement Technology
Accurate measurement of temperature and species concentration is essential for analyzing physical and chemical processes in reacting flows and plasmas, which are relevant to defense, energy, and space exploration.
Traditional combustion diagnostics often struggle when temperatures change rapidly, and gas composition variations vary, particularly in extreme conditions. Coherent Raman scattering (CRS) spectroscopy provides a nonintrusive method for accurately measuring temperature and species in these environments.
Recent advances in hybrid femtosecond/picosecond CRS techniques show promise but typically require multiple laser beams, increasing system complexity. Air lasers offer a simpler alternative, providing a coherent, well-collimated probe beam to improve single-beam CRS methods.
Developing Single Shot Single Beam CRS Technique
Researchers developed an SS-CRS thermometry system that uses air lasing as a probe for high-speed temperature measurements at 1 kHz. The system employs a single femtosecond laser beam to excite molecular rotational coherence, utilizing the temporal properties of air lasing, including its rapid rise and slow decay. This method provides a simplified setup for precise temperature analysis.
The experimental setup used a commercial titanium (Ti) sapphire laser system producing 40 femtosecond pulses at 800 nm. The laser was focused into a low-pressure nitrogen-filled gas chamber, generating coherent air-lasing emissions at approximately 428 nm. These emissions were the probe for measuring rotational coherent Stokes Raman scattering (RCSRS) signals from oxygen molecules.
The authors conducted experiments in a controlled furnace environment to regulate and measure temperatures accurately. They recorded single-shot RCSRS spectra at various temperatures, ranging from room temperature to high temperatures (up to 773 K). The temperature in the interaction region was calculated by fitting the observed spectra to theoretical models based on the Boltzmann distribution of molecular populations.
The study used advanced data acquisition techniques to ensure accurate and consistent measurements, demonstrating the potential of this method for real-time applications.
Impact of Using Newly Developed Measuring Technique
The outcomes demonstrated the successful implementation of single-shot RCSRS spectra at a 1 kHz acquisition rate. The researchers collected 1,000 consecutive spectra at temperatures of 294 K and 773 K. These spectra showed distinct rotational Raman transitions, clearly resolved due to the narrow spectral bandwidth of the air lasing emission.
Temperatures in the interaction region indicated a strong correlation between the rotational population of oxygen molecules and gas temperature. The authors confirmed the accuracy and reliability of the SS-CRS thermometry technique, with relative errors as low as 3.2 % at room temperature.
At higher temperatures, accuracy decreased to 17.0 % at 773 K, attributed to simplified theoretical assumptions and possible spectral interference from nitrogen signals. Despite this, measurement precision remained consistent, with errors below 2.3 % across the temperature range, highlighting the method's robustness.
The system also showed the ability to monitor temperature changes in real time, capturing rapid fluctuations through consecutive single-shot measurements. This feature is particularly beneficial for applications requiring immediate feedback for process control.
The findings demonstrated the potential of air-lasing-assisted SS-CRS thermometry as a reliable, non-intrusive diagnostic tool for combustion environments. It provides rapid and precise temperature measurements critical for engineering and research applications.
Applications of Novel Techniques in Combustion Diagnostics
SS-CRS thermometry is applicable to combustion diagnostics and environmental monitoring. It enables accurate and rapid temperature measurements in turbulent and reactive flows, facilitating the study of dynamic combustion processes.
The single-beam approach simplifies the setup by eliminating the complexity of traditional multi-beam systems and the need for precise synchronization of multiple lasers. This simplification enhances the stability and reliability of measurements, making the technique ideal for real-time monitoring in harsh environments and high-temperature applications.
The air-lasing-based thermometry method can also be applied to other areas, such as greenhouse gas detection and analyzing reactive flows in industrial processes. By enabling simultaneous measurements of temperature and species concentrations, this technique offers valuable insights into complex chemical processes, advancing the understanding of combustion dynamics and pollutant formation.
Future Directions
The novel technique proved effective for temperature measurement through single-shot, single-beam CRS thermometry based on optically induced air lasing. This method not only simplified the measurement process but also provided high precision and repeatability in dynamic environments.
As the demand for accurate temperature measurements in such environments grows, it has significant potential to advance the understanding of combustion and reactive processes, ultimately contributing to more efficient and environmentally friendly energy production.
Future work should aim to optimize the experimental setup by improving optical components and data analysis techniques to address issues like signal contamination and pulse propagation effects. Combining this technology with other diagnostic methods could facilitate detailed studies of combustion processes, with potential applications in energy production and environmental management.
Journal Reference
Lu, X., et al. (2024). Single-shot single-beam coherent Raman scattering thermometry based on optically induced air lasing. Light Sci Appl. DOI: 10.1038/s41377-024-01598-9, https://www.nature.com/articles/s41377-024-01598-9
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