Temperature Dependent Property Considerations for Biot Number Matching With Composite Materials

Abstract

High-temperature materials such as ceramic matrix composites (CMCs) are promising for advancements in gas turbine engines; however, experimental techniques are necessary to characterize the thermal behavior of high-temperature materials in scaled representative environments. In turbine cooling applications, the results of low-temperature experiments in laboratories must be scaled to the intended operating conditions to predict the thermal state at high-temperature engine conditions where such measurements are generally not feasible. Scaling of turbine cooling experiments with metallic components has been aided by the fortuitous fact that the thermal conductivity of turbine-relevant metal alloys varies with temperature in such a way that the Biot number is relatively insensitive to temperature, at least to the extent that Biot number affects the overall effectiveness. However, unlike traditional metallic alloys, materials such as CMCs have unique anisotropic thermal conductivities which vary with temperature in quite different ways than traditional metals. The through-thickness and in-plane thermal conductivities of CMCs may vary with temperature in different ways, thereby adding further complexity. Since most existing low-temperature experimental methods were developed with isotropic metallic alloys in mind, it is necessary to consider the potential for errors due to the unique characteristics of composite materials. Analyses to determine the expected Biot number errors for various composite materials were conducted at a range of experimental conditions. These are compared to a traditional metallic alloy for context and computational simulations were performed on the various materials at engine and laboratory conditions. Additionally, a novel technique was implemented to isolate and quantify the effects of both Biot number and fluid property mismatches at low temperatures. The results show overall effectiveness experiments conducted at low temperatures using composite materials will likely have a large error and could significantly skew the expected thermal characteristics of a turbine component. Careful experimental design should be practiced to mitigate the effect that a mismatched Biot number can have on experimental results.