On Directional Sensitivity of Thermo-Anemometer Split-Fiber Probes

Institute of Themomechanics of the Czech Academy of Sciences, Dolejskova 1402/5, 182 00, Prague, Czech Republic

^* Corresponding author: lepic@it.cas.cz

Abstract

Hot-wire thermo-anemometer probes are extremely fragile and they are used mostly in clean flows with neither debris nor small particles to prevent sensor destruction. Consequently, application of hot-wire probes is limited mostly to clean laboratory environments, their employment in semi-industrial research is extremely rare, and not always successful. Film probes with deposited thin metallic sensors on cylindrical fibers are more rugged and can be successfully employed for research tasks in semi-industrial environment. Surprisingly the potentials of these probes are not yet fully utilized. Detailed investigation of direction characteristics of a split-fiber probe was carried out during the course of this work. Several interesting outcomes resulted from this study. First, it has been shown that the split-fiber probe direction sensitivity rises with the increasing velocity contrary to the decrease of the velocity sensitivity, which is a common hindrance to application of single-sensor thermo-probes to high-speed and transonic flows. Second, the analysis of the acquired data revealed a sudden shift in the effective zero angle offset. It can be speculated that such a shift can related to the transition of the laminar vortex street into the turbulent one. However, the observed shift occurred at Reynolds number values between 900 and 1000, which is markedly higher than the usually reported transitional Reynolds number range between 150 and 300. Finally, the resilience of split-fiber probes to impairment by in-flow debris has been demonstrated proving the probe ability for effective use of these probes in semi-industrial or even industrial research tasks.