Supercontinuum Absorption Spectroscopy for Combustion Diagnostics

During recent years, sensors and diagnostic systems have seen an increase in demand, due to stricter legislative regulations for certification, as well as industry trends, such as Internet of Things and Industry 4.0. In addition, recent scientific discoveries (for example gravitational wave detection) are the result of international collaborations in the field of sensors and diagnostics. The ability to measure process-relevant parameters, preferably in situ and disturbance-free, is essential for improving performance of various systems, from chemical plants to internal combustion engines and energy power plants. Only with precise knowledge of the parameters of these processes, an improvement in efficiency and a reduction of pollutant emissions is achievable. Given the process optimizations in the last decade, the conditions under which a diagnostic system has to obtain valid measurements have significantly harshened. Most of the requirements can only be fulfilled with multi-scalar and multi-species measurements. To obtain such measurements, an optical diagnostic system is often unavoidable, especially for in situ measurements. Given its robustness and versatility, absorption spectroscopy offers great possibilities for such measurements. With the recent arrival of Supercontinuum Laser Light Source (SCLs), which offer broad spectral coverage in pulsed form, the concept of Supercontinuum Laser Broadband Absorption Spectroscopy (SCLAS) was developed, relying on a dispersion in time to record optical spectra. Given the broad spectral coverage, it is possible to derive multiple scalars including species concentrations, pressure and temperature purely optical. Furthermore, such broad coverage is essential for measurements in high-pressure environments (i.e. within the cylinder of an internal combustion engine). Based on an extensive discussion of the underlying effects and processes, necessary spectroscopic models and algorithms were developed to process the obtained measurements. Based on these models, several test cases for SCLAS were investigated, including static tests to quantify accuracy and uncertainty, as well as steady-state laminar flames. Based on the knowledge of these experiments, SCLAS was transferred to transient systems including high-pressure cells and was applied for in-cylinder measurements at a transparent engine test bed. In addition, based on the results of the validation and application tests, new spectroscopic models were developed to fully utilize the potential of SCLs in general and SCLAS in particular. These new models were evaluated against standard practices and found to be an improvement with regards to complexity and speed of data-processing. Furthermore, these models, as opposed to standard gas absorption spectroscopy models, allow for modelling of liquids as well as complex non-discrete absorbing species, such as propane and AdBlue (DEF). Overall, the diagnostic technique SCLAS was proven in comparison to established techniques, while advanced approaches to measure in situ in high-pressure high-temperature processes were developed and tested

Data analysis and uncertainty estimation in supercontinuum laser absorption spectroscopy

A set of algorithms is presented that facilitates the evaluation of super continuum laser absorption spectroscopy (SCLAS) measurements with respect to temperature, pressure and species concentration without the need for simultaneous background intensity measurements. For this purpose a non-linear model fitting approach is employed. A detailed discussion of the influences on the instrument function of the spectrometer and a method for the in-situ determination of the instrument function without additional hardware are given. The evaluation procedure is supplemented by a detailed measurement precision assessment by applying an error propagation through the non-linear model fitting approach. While the algorithms are tailored to SCLAS, they can be transferred to other spectroscopic methods, that similarly require an instrument function. The presented methods are validated using gas cell measurements of methane in the near infrared region at pressures up to 8.7 bar

Supercontinuum Absorption Spectroscopy for Combustion Diagnostics

During recent years, sensors and diagnostic systems have seen an increase in demand, due to stricter legislative regulations for certification, as well as industry trends, such as Internet of Things and Industry 4.0. In addition, recent scientific discoveries (for example gravitational wave detection) are the result of international collaborations in the field of sensors and diagnostics. The ability to measure process-relevant parameters, preferably in situ and disturbance-free, is essential for improving performance of various systems, from chemical plants to internal combustion engines and energy power plants. Only with precise knowledge of the parameters of these processes, an improvement in efficiency and a reduction of pollutant emissions is achievable. Given the process optimizations in the last decade, the conditions under which a diagnostic system has to obtain valid measurements have significantly harshened. Most of the requirements can only be fulfilled with multi-scalar and multi-species measurements. To obtain such measurements, an optical diagnostic system is often unavoidable, especially for in situ measurements. Given its robustness and versatility, absorption spectroscopy offers great possibilities for such measurements. With the recent arrival of Supercontinuum Laser Light Source (SCLs), which offer broad spectral coverage in pulsed form, the concept of Supercontinuum Laser Broadband Absorption Spectroscopy (SCLAS) was developed, relying on a dispersion in time to record optical spectra. Given the broad spectral coverage, it is possible to derive multiple scalars including species concentrations, pressure and temperature purely optical. Furthermore, such broad coverage is essential for measurements in high-pressure environments (i.e. within the cylinder of an internal combustion engine). Based on an extensive discussion of the underlying effects and processes, necessary spectroscopic models and algorithms were developed to process the obtained measurements. Based on these models, several test cases for SCLAS were investigated, including static tests to quantify accuracy and uncertainty, as well as steady-state laminar flames. Based on the knowledge of these experiments, SCLAS was transferred to transient systems including high-pressure cells and was applied for in-cylinder measurements at a transparent engine test bed. In addition, based on the results of the validation and application tests, new spectroscopic models were developed to fully utilize the potential of SCLs in general and SCLAS in particular. These new models were evaluated against standard practices and found to be an improvement with regards to complexity and speed of data-processing. Furthermore, these models, as opposed to standard gas absorption spectroscopy models, allow for modelling of liquids as well as complex non-discrete absorbing species, such as propane and AdBlue (DEF). Overall, the diagnostic technique SCLAS was proven in comparison to established techniques, while advanced approaches to measure in situ in high-pressure high-temperature processes were developed and tested