Article

Tomographic spectrometer for the temporally-resolved 2D reconstruction of gas phase parameters within a generic SCR test rig

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Abstract

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.

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... [2] used a transparent manifold system simulating a real exhaust pipe and employed laser diagnostics and a high-speed camera to determine the optimal injector/mixer distance for urea-water injection and investigate the distribution of droplets. Likewise, investigations on SCR sprays in test rigs with optical access have also been conducted [13,[15][16][17][18][19][20] along with some investigations conducted directly using engine dynos [21][22][23]. However, these test rigs either fell short in the complete investigation of spray development (limited optical access) or did not provide stable cross flow conditions (engine exhaust flow). ...
... However, these test rigs either fell short in the complete investigation of spray development (limited optical access) or did not provide stable cross flow conditions (engine exhaust flow). Moreover, a comprehensive understanding of SCR processes cannot be gained by the synopsis of the studies since the environments under which the experiments were performed, such as geometries and operating conditions differed vastly [15]. An efficient way to conduct a full investigation on spray development is through the usage of a wind tunnel which can provide a stable heated crossflow and a large optically accessible test section to enable the use of measurement equipment like Phase Doppler Anemometry (PDA), backlight imaging etc. ...
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A new subsonic heated open-loop wind tunnel has been designed and manufactured specifically with a 0.25 × 0.25 m cross-section and 1.59 m long optically accessible quartz glass cross-section. The purpose of this wind tunnel was to investigate Selective Catalytic Reduction (SCR) sprays using Phase Doppler Anemometry (PDA). It is capable of reaching a flowrate equating to 50 m/s in the test section when operating at standard room conditions. A 145 kW heater is mounted downstream the blower to achieve air temperatures of up to 400 °C with maximum air mass flow of 1200 kg/h. The diffuser, flow conditioning and contraction cone sections of the wind tunnel have been designed to minimize velocity and temperature gradients within the test section while minimizing flow disturbances. A diffuser with a cone angle of 5o and an area ratio of 3.95 and a contraction cone with a contraction ratio of 4.05 was selected. The test section has been designed to give complete visible access through top and both sides, while the bottom is made of stainless steel. The two sides are made of quartz glass to provide the best signal transmission quality for usage with any optical equipment e.g. PDA. The top of the test section has sections made of Robax glass with detachable windows for easy mounting and dismounting of any equipment subject to be tested in the flow section. Experimental studies using PDA and seeded flow were conducted and verified against Computational Fluid Dynamics (CFD) and analytical solutions to verify the flow parameters inside the test section. The streamwise turbulence intensity in the test section was found to be less than 2% in the core with similar values for the cross-stream turbulence intensities. The complete design and construction of each section of the wind tunnel has been presented and discussed in this paper. Together with the good flow quality, a heated crossflow of up to 400 °C with an air velocity of up to 10 m/s can be delivered upstream the flow section. It can be used to simulate various flow conditions e.g. flow from an engine exhaust, but could also allow to record many transient behaviors related with heated crossflows such as droplet wall film stripping, deposits formations in SCRs etc. Moreover, studies involving chemically composed flow can also be conducted since it is an open-loop wind tunnel and the gas flow is constantly being renewed.
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