Hot Film Shear Stress Probe in Vertical Annular Flow at High Mass Fluxes

摘要

When the time varying properties of the wall shear stress need to be measured, indirect measurement techniques based on the analogy between heat transfer and momentum transfer are often used. In this paper, a wall shear stress measuring system using a flush-mounted hot film probe is reported. The in-situ calibration method is described. The temperature drift of the probe is analyzed and compensation coefficients are given. The dynamic response related to the measurement of the fluctuations of the shear stress is also investigated. The experiments were performed on the LOTUS rig at Imperial College London. The test section of the LOTU S rig is a vertical copper pipe with a length of 11.6 m and a 31.8 mm I.d. The air mass flux was varied between 40 and 200 kg/( m~2 s). The measured results of wall shear stress were found to be consistent with the values of the wall shear stress calculated from the measured pressure gradients. The fluctuations in wall shear stress signal were analyzed and compared to infrared obscuration signals. The results show that the fluctuations in wall shear stress signal are mainly caused by the movement of wisps in wispy annular flows.