Characterization of a Computational Fluid Dynamics Thermocouple Model Subjected to Stochastic Environmental Forcing Using Moment Based Analysis

With increasing requirements for model validation when comparing computational and experimental results, there is a need to incorporate detailed representations of measurement devices within the computational simulations. Thermocouples are the most common temperature measurement transducers in flames and fire environments. Even for the relatively simple thermocouple transducer, the coupling of heat transfer mechanisms particularly under unsteady flow conditions leads to interesting dynamics. As experimentalists are well aware, the experimentally determined thermocouple values are not the same as the local gas temperatures and corrections are often required. From the computational perspective, it is improper then to assume that the predicted gas temperatures should be the same as the temperatures that an experimentalist might measure since the thermal characteristics of the thermocouple influence the indicated temperature. The thermal characteristics of simulated thermocouples in unsteady flame conditions are investigated. Validation exercises are presented to test the underlying thermocouple model. The thermocouple model problem is examined for a quasi-steady problem in which the gas temperature and surrounding walls are assumed to be random and described by probability density functions (PDFs). Differences are noted between the predicted thermocouple response and expected response. These differences are interpreted from the perspective of what modeling artifacts might drive the differences.