The Importance of Accurate Material Properties in Thermofluid Modeling for Extreme Temperatures and Pressures

Energy sustainability with low carbon impact has become a global challenge, motivating increases the capacity and efficiency of cleaner energy production globally. In thermal power systems and components including heat exchangers and turbines, much emerging research focuses on operating at extreme temperatures (&gt; 800 ^oC) and pressures (&gt;250 bar) to provide unprecedented cycle thermal efficiencies and energy densities. At these unconventional conditions, thermal properties of working fluids can vary significantly within a process, and traditional analysis techniques can produce significant errors in system performance modeling. Additionally, the size of such thermal systems will become increasingly compact, and the solid structures must be fabricated with advanced manufacturing approaches such as 3D printing and post-processing techniques such as precise heat treatments. Characteristics of these solid materials such as grain structure can significantly change in these processes, thus requiring further characterization. Therefore, advanced modeling and testing methods are increasingly needed to accurately characterize material properties and predict the performance of future thermal systems.<br/>As an example of such work, this talk will focus on a comprehensive thermal performance model for microtube heat exchangers under extreme temperature and pressures using real fluid properties of supercritical CO₂ (sCO₂). The numerical model employs the concept of volume averaging for geometry abstraction and reduction of governing equations and can provide highly accurate heat transfer predictions in the fluid-phases and at the interfaces between the solid and the fluids at low computational cost. Unlike most commercial models, the developed model utilizes an enthalpy-based formulation of the energy equation as opposed to the commonly used specific heat method, thus increasing solution accuracy. An integrated test system with a sub-scale sCO₂ heat exchanger has been designed and built to validate the developed correlations and numerical model against the thermal test data. In addition to evaluation of system performance, new methodologies for measuring solid material properties at high temperatures have been established in this work. Implemented with high-accuracy, transient IR temperature measurement tools, a bench-scale experimental system was developed to obtain thermal diffusivity measurements for high temperature alloy and composite materials. The modified Angstrom’s method and the surrogate-accelerated Bayesian framework are employed to provide rapid material characterization with extremely high accuracy (5% deviation compared to reference values) at extreme temperatures (~877 ^oC). With interdisciplinary insights, the discussed methodologies can help understand and characterize the heat transfer and thermal properties for other emerging materials for unconventional conditions.