Design considerations for aerodynamically quenching gas sample probes

One method for determining species concentrations in combustion gases is to extract a sample of the gases using a sampling probe and then to transfer the sample to the appropriate gas analysis equipment. During the extraction process, however, chemical reactions occurring within the probe or transfer line may significantly alter species concentrations unless these reactions are quenched. Since an aerodynamic quench provides the most rapid reduction in static temperature and pressure, it is the most promising method for quenching temperature and pressure-dependent chemical reactions. Attempts have been made to utilize an aerodynamic quench but many of these attempts have been unsuccessful due to the lack of understanding of the internal aerodynamics of probes. A one-dimensional model developed previously by the authors has been used for the design and analysis of aerodynamically quenching probes. This paper presents in detail the important aerodynamic and heat transfer equations used in the model, a description of the method of solution, a comparison of the theoretical and experimental pressure distributions obtained from an instrumented probe designed to achieve an aerodynamic quench, and the results of a probe geometry sensitivity study. These calculations demonstrate the limitations and important tradeoffs in design and operating conditions of probes using an aerodynamic quench.