Lothar Diehl

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.