In a study published in the journal Sensors, a portable H2O-based tunable diode laser absorption spectroscopy (TDLAS) system was designed and applied to assess high temperature and velocity gas flows. This research offers a new valuable method for analyzing thermal hazards in various environments.
Non-Contact Assessment of Combustion Fields
Evaluating hot gas characteristics in complex surroundings has long been a problem in studying industrial furnaces and aircraft engines.
Recently, spectral imaging methods, including absorption, emission, Raman spectrum, and planar laser-induced fluorescence/laser-induced fluorescence, have been used successfully for the non-invasive evaluation of combustion fields.
Among all these techniques, tunable diode laser absorption spectroscopy (TDLAS) stands out as a viable option since it is fast, cost-effective, and easier to implement.
Advancements in Tunable Diode Laser Absorption Spectroscopy (TDLAS)
Tunable diode laser absorption spectroscopy (TDLAS) technology has developed significantly due to advances in semiconductor lasers. Therefore, this technology is now widely utilized for the non-contact assessment of velocity, temperature, or species concentrations for hazardous chemicals detection, petroleum engineering, and environmental engineering.
Incorporating vertical cavity surface emitting and distributed feedback lasers makes the TDLAS system more portable and easier to implement.
Early studies used the absorption spectra of H2O and O2 to determine the hot gases properties in combustion flow fields with high pressures and temperatures. This technology was subsequently expanded to monitor waste gases such as ammonia (NH3) and Nitrogen dioxide (NO2) for the process management of industrial combustion systems.
The useful wavelength range for this technique was expanded to around 700-3300 nm with the introduction of tunable diode laser sources, enabling the investigation of CO, H2O, CH4, and CO2.
Limitations of Tunable Diode Laser Absorption Spectroscopy (TDLAS) System
Although measuring gas molecule absorption spectra using TDLAS appears to be a proven technology, this approach has significant shortcomings.
- A conventional TDLAS study only offers the average of a given parameter.
- TDLAS calibration is challenging because creating a consistent gas flow field at a given pressure, temperature, and velocity is difficult.
- Ambient water vapor can affect H2O-based TDLAS measurements.
- In high-velocity gas flow fields, the doppler-shift can affect TDLAS velocity readings.
Using H2O-Based TDLAS System for Combustion Analysis
H2O is a practical component for flame measurement in TDLAS. As water is the most abundant byproduct of combustion, it provides a valuable indicator for establishing a local thermodynamic equilibrium during the combustion process.
In this study, researchers designed and applied the H2O-based TDLAS system to assess gas flow and CH4/air flames, including velocity and temperature measurements.
A technique for determining gas properties was developed by combining a high-resolution transmission (HITRAN) database and an absorption spectrum.
A shock tube and a static flow field vessel helped establish a stable gas flow field with constant pressures, temperatures, or velocities to accurately calibrate the TDLAS system.
For verification, static experiments were conducted in environments with pressures up to 2 MPa and field testing of constant gas flow at temperatures and velocities up to 1600 K and 950 m/s.
Finally, the system's efficacy in testing small-scale flames was evaluated by conducting an experimental analysis of CH4/air flames.
Important Findings of the Study
The proposed method effectively analyzes hot gas characteristics using the HITRAN database and the H2O absorption spectra in conjunction with tunable diode laser absorption spectroscopy (TDLAS) system.
The combination of a suitable wavelength calibration between the frequency and time domains, center wavelength and the employment of a specific sampling or scanning frequency enables tunable diode laser absorption spectroscopy (TDLAS) system to evaluate small-scale flames.
The designed dual optical route TDLAS system removes background interference. In addition, due to the high-frequency scanning and dual optical route design, the proposed TDLAS system provided satisfactory flame velocity and temperature data.
The system can deliver precise values for high-velocity and high-temperature gas flows within a 0.8 MPa pressure environment. At pressures more significant than 1 MPa, temperature and concentration measurement errors can exceed 10%.
A shock tube was constructed to deliver regulated and constant gas flows at high temperatures or high velocity, which proved to be an appropriate experimental setting for TDLAS parameter calibration under extreme conditions.
Comparing the TDLAS results to thermocouple observations and numerical simulations verified that the TDLAS approach yielded acceptable flame temperature and velocity measurements.
Tu, R., Gu, J., Zeng, Y., Zhou, X., Yang, K., Jing, J., Miao, Z., & Yang, J. (2022). Development and Validation of a Tunable Diode Laser Absorption Spectroscopy System for Hot Gas Flow and Small-Scale Flame Measurement. Sensors. https://www.mdpi.com/1424-8220/22/17/6707/htm