NUMERICAL AND EXPERIMENTAL STUDY ON A WALL SHEAR STRESS CALIBRATION SETUP FOR HOT-FILM PROBES BY THE USE OF A RADIAL FLOW CELL ASSAY

摘要

In the present work, a calibration setup for flush-mounted hot-film sensor probes in turbulent and transitional flow regimes is evaluated experimentally and numerically. The diverging flow characteristic of a radial flow cell assay (RFC) is used to investigate position dependant wall shear stress distributions for different flow conditions in water. With regard to further quantitative wall shear stress measurements in hygienic designed multistage pumps, the new sensor calibration setup has to be applicable for a wall shear stress range up to 460 Pa. Due to the temperature-sensitive internal resistance shift and fluctuating sensor responses in the examined flow regimes, classical hot-film calibrations are limited to a small range of temperature differences and wall shear stresses. The limitation of the classical calibration technique is demonstrated in this work. As a more appropriate procedure a different calibration method is used. Herein, the sensor is calibrated in dependency of analytical wall shear stress distributions, temperatures as well as voltage responses. Thus, the measurement accuracy is improved by interpolating the measured wall shear stress values solely from measured temperatures and signal voltage responses. Emphasis is placed on the influence of varying fluid temperatures between 14.5°C up to 20°C within the investigated wall shear stress range. Additionally, numerical simulations of the RFC are performed with commercial Computational Fluid Dynamics-Codes (CFD). As turbulence model the k-ω- SST-model with enhanced wall treatment is used. The experimental response is compared with the numerical and calculated results at a fluid temperature of 20°C. The results demonstrate that the new calibration setup reproduces the wall shear stress range very accurately.