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Jordanian – German Winter Academy Amman, 4-11/ Feb. 2006 Hot-Wire Anemometry HWA

Jordanian – German Winter Academy Amman, 4-11/ Feb. 2006 Hot-Wire Anemometry HWA. 0/48. Definition. Features. Applications. Operation and Measurement principle. About probes. Operation Modes. Governing Equations. Calibration. Deficiencies and Limitations.

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Jordanian – German Winter Academy Amman, 4-11/ Feb. 2006 Hot-Wire Anemometry HWA

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  1. Jordanian – German Winter Academy Amman, 4-11/ Feb. 2006 Hot-Wire Anemometry HWA 0/48

  2. Definition. Features. Applications. Operation and Measurement principle. About probes. Operation Modes. Governing Equations. Calibration. Deficiencies and Limitations. Measurements in 2 and 3 dimensions. Data acquisition. Steps of a Good HWA. Ending and Discussions. Discussed Topics 1/48

  3. Hot-wire anemometry is the most common method used to measure instantaneous fluid velocity. The technique ( found in the early 70s by King and others) depends on the convective heat loss to the surrounding fluid from an electrically heated sensing element or probe. If only the fluid velocity varies, then the heat loss can be interpreted as a measure of that variable, ( relate heat loss to flow ). 2/48

  4. Features • Measures velocities from few cm/s to supersonic. • High temporal resolution: fluctuations up to several hundred kHz. • High spatial resolution: eddies down to 1 mm or less. • Measures all three velocity components simultaneously, and Provides instantaneous velocity information. 3/48

  5. Applications » Aerospace» Automotives» Bio-medical & bio-technology» Combustion diagnostics» Earth science & environmen» Fundamental fluid dynamics» Hydraulics & hydrodynamics» Mixing processes» Processes & chemical engineering» Wind engineering» Sprays (atomization of liquids) 4/48

  6. Principles of operation • Consider a thin wire mounted to supports and exposed to a velocity U. When a current is passed through wire, heat is generated ( I 2 Rw). In equilibrium, this must be balanced by heat loss (basically convection) to the surroundings. • If the velocity changes, convective heat transfer coefficient will change, so the wire’s temperature will change and eventually reach a new equilibrium. 5/48

  7. Principle of operation 6/48

  8. Measurement Principles • The control circuit for hot-wire anemometry is in the form of a Wheatstone bridge consisting of four electrical resistances, one of which is the sensor. When the required amount of current is passed through the sensor, the sensor is heated to the operating temperature, at which point the bridge is balanced. If the flow is increased, the heat transfer rate from the sensor to the ambient fluid will increase, and the sensor will thereby tend to cool. the accompanying drop in the sensor's electrical resistance will upset the balance of the bridge. This unbalance is sensed by the high gain DC amplifier, which will in turn produce a higher voltage and increase the current through the sensor, thereby restoring the sensor to its previously balanced condition. The DC amplifier provides the necessary negative feedback for the control of the constant temperature anemometer. The bridge or amplifier output voltage is, then an indication of flow velocity. 7/48

  9. Probes 8/48

  10. Probe Types • Hot film , which is used in regions where a hot wire probe would quickly break such as in water flow measurements. 2. Hot wire , This is the type of hot wire that has been used for such measurements as turbulence levels in wind tunnels, flow patterns around models and blade wakes in radial compressors. 9/48

  11. Hot wire sensor 10/48

  12. Hot film sensor 11/48

  13. Probe selection • For optimal frequency response, the probe should have as small a thermal inertia as possible. • Wire length should be as short as possible (spatial resolution; want probe length << eddy size) • Aspect ratio ( L/d ) should be high (to minimize effects of end losses) • Wire should resist oxidation until high temperatures (want to operate wire at high T to get good sensitivity, high signal to noise ratio) • Temperature coefficient of resistance should be high (for high sensitivity, signal to noise ratio and frequency response) • Wires of less than 5 µm diameter cannot be drawn with reliable diameters 12/48

  14. Modes of operation • Constant Current anemometry (CCA) • Constant Temperature anemometry (CTA) 13/48

  15. Constant current anemometer CCA Principle: Current through sensor is kept Constant Advantages: - High frequency response Disadvantages: - Difficult to use - Output decreases with velocity - Risk of probe burnout 14/48

  16. Constant Temperature Anemometer CTA Principle: Sensor resistance is kept constant by Servo amplifier Advantages: -Easy to use -High frequency response -Low noise -Accepted standard Disadvantages: -More complex circuit 15/48

  17. Governing equations I Governing Equation: E = thermal energy stored in wire E = CwTs Cw = heat capacity of wire W = power generated by heating W = I² Rw recall Rw = Rw(Tw) H = heat transferred to surroundings 16/48

  18. Governing equations II • Heattransferred to surroundings ( convection to fluid H = sum off + conduction to supports + radiation to surroundings) Convection Qc = Nu · A · (Tw -Ta) Nu = h ·d/kf = f (Re, Pr, M, Gr,α), Re = ρU/μ Conduction f (Tw , lw , kw, Tsupports) Radiation f (Tw- Tf ) 17/48

  19. Simplified static analysis I • For equilibrium conditions the heat storage is zero: • and the Joule heating W equals the convective heat transfer H • Assumptions : • Radiation losses small • Conduction to wire supports small • Tw uniform over length of sensor • - Velocity impinges normally on wire, and is uniform over its entire length, and also small compared to sonic speed. • Fluid temperature and density constant 18/48

  20. Simplified static analysis II • Static heat transfer : • W = H I ² Rw = hA(Tw -Ta) I²Rw = Nu kf/dA( Tw -Ta) • h = film coefficient of heat transfer • A = heat transfer area • d = wire diameter • kf = heat conductivity of fluid • Nu = dimensionless heat transfer coefficient • Forced convection regime, i.e. Re > Gr^(1/3 ) (0.02 in air) and Re<140 • Nu = A1 + B1 · Re ⁿ= A2+ B2 · U ⁿ • I ² Rw ² = E² = (Tw -Ta)(A + B · U ⁿ)“King’s law” Then the voltage drop is used as a measure of velocity. 19/48

  21. Heat transfer from Probe • Convective heat transfer Q from a wire is a function of the velocity U, the wire over-temperature Tw –T0 and the physical properties of the fluid. The basic relation between Q and U for a wire placed normal to the flow was suggested by L.V. King (1914). In its simplest form it suggests : where Aw is the wire surface area and h the heat transfer coefficient, which are merged into the calibration constants A and B. 20/48

  22. Hot-wire static transfer function Velocity sensitivity (King’s law coeff. A = 1.51, B = 0.811, n = 0.43) Output voltage as fct. of velocity 21/48

  23. HOT-WIRE CALIBRATION • The hot-wire responds according to King’s Law: where E is the voltage across the wire, u is the velocity of the flow normal to the wire. A, B, and n are constants. You may assume n = 0.5, this is common for hot-wire probes. A can be found by measuring the voltage on the hot wire with no flow, i.e. for u = 0, so A = E^2 as we can see. Make sure there is no flow, any small draft is significant. The HWLAB software operating in calibration mode will give you a voltage. Once you know A, you can measure the wire voltage for a known flow velocity and then determine B from King’s law, were B = (E ^2 – A)/ U ⁿ ) 22/48

  24. Calibration curve 23/48

  25. Problem sourcescontamination I • Most common sources: - dust particles - dirt - oil vapors - chemicals • Effects: - Probe Change flow sensitivity of the sensor (DC drift of calibration curve) - Reduce frequency response • What to do: - Clean the sensor - Recalibrate 24/48

  26. Problem SourcesProbe contamination II • Drift due to particle contamination in air 5 m Wire, 70 m Fiber and 1.2 mm Steel Clad Probes (From Jorgensen, 1977) - Wire and fiber exposed to unfiltered air at 40 m/s for 40 hours - Steel Clad probe exposed to outdoor conditions 3 months during winter conditions 25/48

  27. Problem SourcesProbe contamination III • Drift due to particle contamination in water Output voltage decreases with increasing dirt deposits 26/48 (From Morrow and Kline 1971)

  28. Problem SourcesProbe contamination IV - slight effect of dirt on heat transfer were heat transfer may increase ! effect : • low velocity indication, for increased surface vs. insulating effect • High Velocity, - more contact with particles especially in laminar flow, were turbulent flow has a “cleaning effect” • Influence of dirt INCREASES as wire diameter DECREASES • Deposition of chemicals INCREASES as wire temperature INCREASES * FILTER THE FLOW, CLEAN SENSOR AND RECALIBRATE 27/48

  29. Further Problem SourcesBubbles in Liquids I • Drift due to bubbles in water In liquids, dissolved gases form bubbles on sensor, resulting in: - reduced heat transfer - downward calibration drift (From C.G.Rasmussen 1967) 28/48

  30. Bubbles in Liquids II • Effect of bubbling on : portion of typical calibration curve ( noised signal ) • Bubble size depends on : - surface tension - overheating ratio - velocity • Precautions : - Use low overheat - Let liquid stand before use - Don’t allow liquid to mix with air - Clean sensor (From C.G.Rasmussen 1967) 29/48

  31. Stability in Liquid Measurements • Fiber probe operated stable in water - De-ionized water (reduces algae growth) - Filtration ( should be better than 2 m) - Keeping water temperature constant (within 0.1oC) (From Bruun 1996) 30/48

  32. Eddy shedding I • Eddy shedding from cylindrical sensors • Occurs at Re ~50 • * Select small sensor diameters/ Low-pass filter for signal (From Eckelmann 1975) 31/48

  33. Eddy shedding II • Vibrations from prongs and probe supports: • - Probe prongs may vibrate due to there own shedding or due to induced vibrations from the surroundings via the probe support ( effects of resonance and vortices ). • - Prongs have natural frequencies from 8 to 20 kHz • Always use stiff and rigid probe mounts. 32/48

  34. Temperature Variations I • Fluctuating fluid temperature • Heat transfer from the probe is proportional to the temperature difference between fluid and sensor. • E2 = (Tw-Ta)(A + B·Un) • As (Ta ) varies: • - heat transfer changes • - fluid properties change • Air measurements: • - limited effect at high overheating ratio • - changes in fluid properties are small • Liquid measurements effected more, because of: • - lower overheats • - stronger effects of T change on fluid properties 33/48

  35. Temperature Variations II • Anemometer output depends on both velocity and temperature • When ambient temperature increases the velocity is found to be low if not corrected for. (From Joergensen and Morot1998) 34/48

  36. Temperature Variations III Film probe calibrated at different temperatures 35/48

  37. Temperature Variations IV • To deal with temperature variations: • Keep the wire temperature fixed (no overheat adjustment), measure the temperature along and correct anemometer voltage prior to conversion • Keep the overheat constant either manually, or automatically using a second compensating sensor. • Calibrate over the range of expected temperature and monitor simultaneously velocity and temperature fluctuations. 36/48

  38. Measurements in 2D Flows I • X-ARRAY PROBES (measures within ±45o with respect to probe axis): • Velocity decomposition into the (U,V) probe coordinate system • where U1 and U2 in wire coordinate system are found by solving: 37/48

  39. Measurements in 2D Flows II • Directional calibration provides the coefficients k1and k2 (Obtained with Dantec Dynamics’ 55P51 X-probe and 55H01/H02 Calibrator) 38/48

  40. Measurements in 3D Flows I TRIAXIAL PROBES (measures within a 70o cone around axis): 39/48

  41. Measurements in 3D Flows II • Velocity decomposition into the (U,V,W) probe coordinate system • where U1 , U2and U3 in wire coordinate system are found by solving: • left hand sides are effective cooling velocities. Yaw and pitch coefficients are determined by directional calibration. * Measurements taken for previous situation 40/48

  42. Measurements in 3D Flows III • U, V and W measured by a Triaxial probe, when rotated around its axis. Inclination between flow and probe axis is 20o. (Obtained with Dantec Dynamics’ Tri-axial probe 55P91 and 55H01/02 Calibrator) 41/48

  43. 0.5 E = ((T - T )/(T - T )) E corr w ref w acq acq. Measurement at Varying TemperatureTemperature Correction I • Recommended temperature correction: • Keep sensor temperature constant, measure temperature and correct voltages or calibration constants. • I) Output Voltage is corrected before conversion into velocity -This gives under-compensation of approximately 0.4%/ C in velocity. Improved correction: Selecting proper m (m = 0.2 typically for wire probe at a = 0.8) improves compensation to better than ±0.05%/C. 42/48

  44. Measurement at Varying TemperatureTemperature Correction II • Temperature correction in liquids may require correction of power constants A and B: • * In this case the voltage is not corrected 43/48

  45. Data acquisition I • Data acquisition, conversion and reduction: • Requires digital processing based on • Selection of proper A/D board • Signal conditioning • Proper sampling rate and a number of samples 44/48

  46. Data acquisition II A/D boards convert analogue signals into digital information (numbers), They have the following main characteristics: • Resolution: • - Minimum 12 bits (~1-2 mV depending on range) • Sampling rate: • - Minimum100 kHz (allows 3D probes to be sampled with approximately 30 kHz per sensor) • Simultaneous sampling: • - Recommended (if not sampled simultaneously there will be phase lag between sensors of 2 and 3D probes) • External triggering: • Recommended (allows sampling to be started by external event) 45/48

  47. Data acquisition III Sample rates and number of samples : • Time domain statistics (spectra) require sampling 2 times the highest frequency in the flow • Amplitude domain statistics (moments) require uncorrelated samples. Sampling interval minimum 2 times integral time scale. • Number of samples should be sufficient to provide stable statistics (often several thousand samples are required) • Proper choice requires some knowledge about flow’s nature • It is recommended to try to make autocorrelation and power spectra first, as basis for the choice 46/48

  48. CTA AnemometrySteps needed to get good measurements: • Get an idea of the flow (velocity range, dimensions, frequency) • Select right probe and anemometer configuration • Select proper A/D board • Perform set-up (hardware set-up, velocity calibration, directional calibration) • Make a first rough verification of the assumptions about the flow • Define experiment (traverse, sampling frequency and number of samples) • Perform the experiment • Reduce the data (moments, spectra, correlations) • Evaluate results • Recalibrate to make sure that the anemometer/probe has not drifted 47/48

  49. Thank you for listening… 48/48

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