We present a tomographic spectrometer for the measurement of gas phase species parameter distributions within the cross section of a generic exhaust gas test rig built for the investigation of the physico-chemical processes during exhaust gas aftertreatment with selective catalytic reduction (SCR). A urea-water solution (UWS) is injected into the hot gas flow to supply ammonia (NH3) through thermolysis and hydrolysis processes as the reducing agent of nitrous oxides (NOx) within the SCR catalyst. To achieve a full NOx conversion, the pre-catalyst distribution of the major exhaust gas species is of great importance. The injection leads to several desirable and undesired highly complex processes including spray breakup and film formation on the opposing wall. To further optimize the pre-catalyst distribution while maximizing the efficiency of the overall process, the knowledge of mole fraction and temperature distributions are crucial.
The developed biaxial tomographic spectrometer utilizes a combined measurement method - laser absorption tomography by employing tunable diode laser absorption spectroscopy (TDLAS) data with the linear hyperspectral absorption tomography (LHAT) approach to achieve simultaneous multiparameter reconstructions. An image frame rate of 49 Hz was achieved following a phase averaging, and a tomographic signal-to-noise ratio limit was applied. Virtual experiments and measurements in homogeneous samples were performed to ensure the spectromter’s ability for qualitative and quantitative reconstructions. Measurements during water injection cycles into the hot gas flow have shown two main features within the distribution that can be linked to the presence of a vapor boundary layer at the channel bottom and an evaporated spray-induced cone feature. Temperature measurements of a thermocouple downstream of the measurement plane agree well with the reconstructed temperatures.
The spectrometer is intended to further improve the understanding of the pre-catalyst flow in SCR systems and the results can be taken as reference for corresponding numerical calculations.