Heat Flux Temporal Variation and Turbulent Structures on Diesel Flame-Impinged Wall by Comparing High-Speed Infrared Thermography and MEMS Sensor Measurements

For further elucidation of the extremely complex mechanism of wall heat transfer during diesel flame impingement, heat flux measurement results based on two different relatively new approaches, high-speed infrared thermography and Micro Electro- Mechanical Systems (MEMS) heat flux sensor, were compared. Both measurements were conducted on the chamber wall impinged by a diesel flame achieved in constant volume combustion vessels under similar experimental conditions. Infrared thermography was conducted using a high-speed infrared camera (TELOPS M3k, 13,000 fps, 128×128 pixels), allowing the capture of time-series temperature and heat flux distributions on the wall surface with a spatial resolution of 70 μm (9 mm / 128 pixels). This high-resolution imaging also enables detailed estimation of near-wall turbulent structures, which are considered to significantly influence the heat flux distributions. The MEMS sensor is composed of closely aligned (520 microns separated) multiple highly sensitive thin-film Resistance Temperature Detectors (RTDs) of 235×235 microns, enabling estimation of near-wall turbulent fluid motion based on a cross-correlation analysis of measured heat flux fluctuations. The comparison between these measurements allows for mutual complementation of limited temporal resolution and quantitative accuracy of high-speed thermography and limited ability for spatial comprehension of the near-wall turbulence structure using the MEMS sensor. The time-series heat flux distribution obtained via high-speed thermography exhibited distinctive radial striped patterns. These patterns initially appeared as fine streaks with a high advection velocity immediately after wall impingement. As time progressed, they gradually increased in scale and exhibited a decrease in advection velocity. This behavior likely reflects the development of a boundary layer on the wall surface during the highly transient diesel flame impingement. Similarly, the heat flux measured by the MEMS sensor showed a comparable trend: the measured oscillation frequency corresponded well with the behavior of the striped patterns observed in the thermography, and the estimated fluid motion velocity was high immediately after wall impingement but gradually decreased over time.