Measurements in Fluid Mechanics 058:180:001 (ME:5180:0001) Time & Location: 2:30P - 3:20P MWF 218 MLH Office Hours: 4:00P – 5:00P MWF 223B-5 HL Instructor: Lichuan Gui

2 Lecture 18. Thermal anemometry

3 Pressure difference methods Measurement of local flow velocity Thermal methods Frequency-shift methods Marker-tracing methods Mechanical methods - examples: Pitot-static tube, multi-hole probes - examples: hot-wire and hot-film anemometers - examples: Laser-Doppler velocimeter, ultralsonic Doppler velocimeter - trace the motion of suitable flow markers, optically or by other means. - take advantage of the forces and moments that a moving stream applies on immersed objects. - utilize analytical relationships between the local velocity and the static and total pressure - compute flow velocity from its relationship to the convective heat transfer from heated elements. - based on the shifting of the frequency of waves scattered by moving particles. - examples: Chronophotography, particle image velocimetry (PIV), pulsed-wire anemometry - examples: vane, cup, and propeller anemometers

4 Hot-wire (HW) sensor Thermal anemometry - platinum or tungsten wires of mm long and  m in diameter - mounted at two ends on thin tapered metallic prongs - Wollaston wire or gold plated wire to avoid non-uniform temperature distribution Hot-film (HF) sensor - usable in clean gas flows  m thick film of platinum or nickel - on wedge shape support or hollow glass tube etc. - usable in both gas and liquid flows

5 Energy balanceElectric resistanceHeat convection coefficient Flow velocity Velocity measurement using HW & HF Thermal anemometry I – electric current R w – electric resistance A w – surface area T w – wire temperature T f – fluid temperature R Ref – reference resistance T Ref – reference temperature v f – fluid velocity

6 HW & HF measuring system Thermal anemometry

- experimentally determined as: 77 Heat-transfer characteristics Thermal anemometry Nusselt number: General relationship: - Pr disregarded in air flows - Gr ignored in most cases - M neglected in incompressible flows (V<100 m/s)- Kn neglected in continum regime Simplified relationship: King’s law: - flow direction perpendicular to HW - A and B determined with calibration HW/HF resistance relationship:

8 Constant-current anemometry (CCA) Thermal anemometry - Disadvantages: difficult to use output decreases with velocity risk of probe burnout - Current through sensor is kept Constant with R S >>R W - Frequency response improved with a compensation circuit - Advantages: simple electric circuit, high frequency response Constant-temperature anemometry (CTA) - Sensor resistance is kept constant through electronic feedback system - Advantages: easy to use high frequency response low noise - Disadvantages: more complex circuit

Velocity orientation effect Thermal anemometry Effective cooling velocity: Modified by considering tangential component: - constant k determined by calibration, typical k 2 values between 0.05 and 0.20 Others Prong interference effects - Prongs and probe body produce interference to heat transfer between flow and HW Heat conduction effects - End conduction adds system error when l/d not large enough Compressibility effects - King’s law not sufficient for HW when M>0.6 Temperature-variation effects - Response deviates from calibration relationship because of temperature variation

10 Cross-wire (X-wire) anemometer Thermal anemometry - Identical sensors inclined exactly  45  - Tow velocity components may be calculated as: - Calibration required for high accuracy Multi-sensor probes - Data reduction more complicated when wires not perpendicular to each other Three or four sensor probes - used to measure three velocity components

11 Homework - Questions and Problems: 1 on page Read textbook 11.1 on page Due on 10/07 Hint: The temperature for tungsten to oxidizes can be found in page 250.