Guard-Heated Wall Shear Stress Sensors for Turbulent Flows

Fluids Seminar
October 2, 2013 11:00 pm

Speaker:  Ali Etrati, PhD student, Complex Fluids Lab, UBC, Canada

Location:  LSK 203

Intended Audience:  Public

This talk presents analysis of the performance of multi-element guard-heated hot-film wall shear stress microsensors for turbulent flows. Previous studies of conventional, single-element sensors show that a significant portion of heat generated in the hot-film travels through the substrate before reaching the fluid, causing spectral and phase errors in the wall shear stress signal and drastically reducing the spatial resolution of the sensor. Earlier attempts to reduce these errors have focused on reducing the effective thermal conductivity of the substrate. New guard-heated microsensor designs proposed to overcome the severe deficiencies of the conventional design are investigated. Guard-heaters remove the errors associated with substrate heat conduction, by forcing zero temperature gradient at the edges and bottom face of the hot-film, and hence, block the indirect heat transfer to the flow. Air and water flow over the sensors are studied numerically to investigate design, performance and signal strength of the guard-heated sensors. Our results show, particularly for measurements in low-conductivity fluids such as air, that edge guard-heating needs to be supplemented by a sub-surface guard-heater, to make substrate conduction errors negligible. With this two-plane guard-heating, a strong non-linearity in the standard single-element designs can be corrected, and spectral and phase errors arising from substrate conduction can be eliminated.

Ali Etrati completed his undergraduate degree in University of Tehran, Iran, then moved to UVic for his Master’s degree. He has recently started his PhD in Complex Fluids Lab group under supervision of Professor Frigaard. Ali is going to talk about what he has done in his Master’s in UVic.