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Revisiting Hot-Wire AnemometryClose to Solid Walls
KTH, School of Engineering Sciences (SCI), Mechanics.
2017 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

A well-known problem of hot-wire anemometry (HWA), is the “wall effect”, namely theoverestimation of the measured velocity near a wall. The overestimation occurs due toadditional heat loss from the heated wire-sensor to the wall. The extra heat loss dependson parameters such as the heat conductivity of the wall material, the overheat ratio ofthe wire, and the sensor geometry. This problem has been studied for quite some timeand there are several suggestions with regard to the effect of these parameters for meanflow corrections, however the effect on measurements of turbulent fluctuation has notbeen investigated. The present work aims at providing further insight on this topic, byelucidating how these parameters affect measurements of both the mean and fluctuatingvelocity. Furthermore, the present study proposes a theoretical model on the total heattransfer from hot-wire sensor to explain the phenomenon.In the experimental part of the study, the measurements under both no flow and flowconditions are carried out to consider natural convection and forced convection separately.The results showed that the effect of the parameters is consistent with what is agreedwidely: higher wall conductivity, higher overheat ratio, and larger wire exposed area leadto higher output from an anemometer. On the other hand, it is observed that the conduc-tion under natural convection can be scaled with the overheat ratio. Velocity fluctuationsare found to decrease by employing higher overheat ratio and for walls with higher heatconductivity.In the numerical part of the study, a two-dimensional steady calculation using Open-FOAM is performed and the parameter dependency with respect to the overheat ratio andwall heat conductivity is investigated. The results qualitatively agree with the experi-mental results. Moreover, the inner scaling commonly employed in wall-turbulence isfound to be inadequate to resolve the wall effect of HWA when various sensor heights areconcerned.Lastly, a theoretical model on the total heat transfer from the wire close to solid wallsis established based on a superposition of the convection and the conduction contributions.The proposed model with the empirically determined coefficients is found to be capableof capturing the qualitative behaviours found in the experiment and numerical analysis, howewer for more practical use it leaves several issues to be further analysed.

Place, publisher, year, edition, pages
2017. , p. 96
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-203127OAI: oai:DiVA.org:kth-203127DiVA, id: diva2:1081137
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Available from: 2017-03-13 Created: 2017-03-13 Last updated: 2017-03-13Bibliographically approved

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