Numerical and Experimental Analysis of the 3D Flow-Pattern in Exhaust Gas Sensors

In new exhaust system specifications such as single cylinder balancing, closed coupled catalyst systems, sensor locations close to the engine, turbo applications, fast light off situations and diesel engine applications the dynamic behavior of the lambda sensor becomes more important. This demands a detailed knowledge and modeling of the relevant parameters. In former analysis of exhaust gas sensors the main focus has been the electrochemical processes in the sensor. The influence of flow structure and protection tubes had lower priority. In this paper we present the numerical and experimental analysis of cold air flowing in a pipe including mounted exhaust sensors. Two double-protection tubes from the Robert Bosch GmbH have been examined named (a) and (b). The predicted results have been compared with values measured with Laser Doppler Anemometry (LDA). The flow pattern in the protection tube type (a) depends on the geometric configuration of the sensor element and the tubes. Particles and droplets in the flow reach the surface of the heated zirconia sensor, which sometimes leads to damages of the sensor element. The protection tube type (b) causes a helical flow pattern with negligible influence of the geometric configuration. In addition the centrifugal force deposits particles and droplets at the wall of the inner protection tube. The predicted velocities show a good agreement with the measured values. Based on these results a one-dimensional, analytical model to predict the mass flow through the protection tube type (b) is developed. The analytical data show good agreement with the results of the three-dimensional CFD analysis. A theoretical limitation of the mass flux through the sensor as a function of geometric parameters is found.