Sascha Klett

Numerical and Experimental Analysis of the Momentum and Heat Transfer in Exhaust Gas Sensors

Modern zirconia oxygen sensors are heated internally to achieve an optimal detection of the oxygen concentration in the exhaust gas and fast light off time. The temperature of the gas in the exhaust pipe varies in a wide range. The zirconia sensor is cooled by radiation and forced convection caused by cold exhaust gas. If the zirconia temperature falls, the oxygen detection capability of the sensor decreases. To minimize the cooling effects, protection tubes cover the zirconia sensor. However, this is in conflict with the aim to accelerate the dynamics of the lambda sensor. In this paper, the heat transfer at the surface of a heated planar zirconia sensor with two different double protection tubes of a Bosch oxygen sensor is examined in detail. The geometric configuration of the tubes forces different flow patterns in the inner protection tube around the zirconia sensor. The zirconia sensor is internally electrically heated by a platinum heater layer. At the sensor surface heat transfer caused by radiation takes place. In the inner protection tube radiation is absorbed, emitted and reflected at the surfaces. A fully 3d numerical model for the flow and the heat transfer is developed to predict the heat transfer and flow pattern and the temperature field of fluid and solids. The models for the internal heater and the radiant heat transfer including reflection are implemented in the commercial CFD code Fluent. The gas is modeled as a compressible, ideal gas with temperature dependent fluid properties. An experimental apparatus is designed to measure the temperature distribution in the reference channel of a heated zirconia oxygen sensor with protection tubes, that is mounted in an exhaust pipe. The comparison of the predicted values and the measured data show good agreement. With the numerical model it is possible to predict the temperature of the sensor element and the protection tubes as a function of the flow conditions, the heater power and the aging of the sensor. With the use of the non dimensional reduced temperature it is possible to reduce all measured temperature profiles to one single curve. Therefore the course of the temperature profile is dominated by the structure of the heater meander, the absolute value of the temperature by the boundary conditions of the flow and the heater.

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.