Influence of test model material on the accuracy of transient heat transfer measurements in impulse facilities

Abstract

The development of suitable heat-resistant materials and appropriate thermal structure design for a hypersonic aircraft requires highly precise heat transfer predictions. Unfortunately, existing techniques for measuring the transient heat flux by thermal sensors in impulse facilities are overly complex; any slight deviation from ideal conditions may lead to inaccuracy. In this study, the influence of different model materials leading to lateral heat conduction between model and sensor on the accuracy of heat flux measurements using type-E coaxial thermocouples was investigated. The behavior of the materials results to a deviation from the assumption of one-dimensional heat conduction more or less. The materials examined were stainless steel, aluminum, carbon steel, and polyamide, which are frequently used as model materials in ground tests. The influence of the model materials was estimated by comparing the heat flux derived from the junction temperature with the actual heat flux loading. Particular attention was paid to aluminum, which is extensively used as wind tunnel model material. An engineering-based approach is also presented to conduct high accuracy measurements. The results show that the difference in the thermal properties between the sensor and the model materials creates complicated lateral heat conduction between them; stainless steel 304 is suggest as the use of model material whenever high-accuracy heat transfer measurements are desired due to its similarity of the thermal properties to type-E sensors. The use of polyamide PA6 material resulted in a larger heat flux due to its smaller thermal effusivity and the aluminum and carbon steel led to lower heat fluxes due to their larger thermal effusivity. The deviation of either material increases over testing time.