Jordanian German Winter Academy Amman, 4-11

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

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Definition. Highlights. Applications. Operation and Measurement guideline. About tests. Operation Modes. Administering Equations. Adjustment. Inadequacies and Limitations. Estimations in 2 and 3 measurements. Information procurement. Ventures of a Good HWA. Consummation and Discussions. Talked about Topics 1/48

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Hot-wire anemometry is the most widely recognized technique used to gauge quick liquid speed . The system ( found in the mid 70s by King and others) relies on upon the convective warmth misfortune to the encompassing liquid from an electrically warmed detecting component or test. On the off chance that lone the liquid speed changes, then the warmth misfortune can be deciphered as a measure of that variable, ( relate warm misfortune to stream ). 2/48

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Features • Measures speeds from few cm/s to supersonic. • High worldly determination: changes up to a few hundred kHz. • High spatial determination: vortexes down to 1 mm or less. • Measures each of the three speed parts at the same time, and Provides quick speed data. 3/48

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Applications » Aerospace » Automotives » Bio-therapeutic & bio-innovation » Combustion diagnostics » Earth science & environmen » Fundamental liquid progression » Hydraulics & hydrodynamics » Mixing forms » Processes & concoction building » Wind designing » Sprays (atomization of fluids) 4/48

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Principles of operation Consider a thin wire mounted to backings and presented to a speed U . At the point when a current is gone through wire, warmth is created ( I 2 R w ). In balance, this must be adjusted by warmth misfortune (essentially convection) to the environment. On the off chance that the speed changes, convective warmth exchange coefficient will change, so the wire's temperature will change and in the long run achieve another harmony. 5/48

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Principle of operation 6/48

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Measurement Principles The control circuit for hot-wire anemometry is as a Wheatstone connect comprising of four electrical resistances, one of which is the sensor. At the point when the required measure of current is gone through the sensor, the sensor is warmed to the working temperature, and soon thereafter the scaffold is adjusted. On the off chance that the stream is expanded, the warmth exchange rate from the sensor to the surrounding liquid will increment, and the sensor will along these lines tend to cool. the going with drop in the sensor's electrical resistance will agitate the adjust of the extension. This unbalance is detected by the high pick up DC enhancer, which will thusly deliver a higher voltage and increment the current through the sensor, subsequently reestablishing the sensor to its already adjusted condition. The DC intensifier gives the essential negative input to the control of the consistent temperature anemometer. The extension or enhancer yield voltage is, then a sign of stream speed. 7/48

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Probes 8/48

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Probe Types Hot film , which is utilized as a part of locales where a hot wire test would rapidly break, for example, in water stream estimations. 2 . Hot wire , This is the kind of hot wire that has been utilized for such estimations as turbulence levels in wind burrows, stream designs around models and cutting edge wakes in spiral compressors . 9/48

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Hot wire sensor 10/48

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Hot film sensor 11/48

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Probe choice For ideal recurrence reaction, the test ought to have as little a warm inactivity as could be allowed. Wire length ought to be as short as could reasonably be expected (spatial determination; want test length << swirl estimate) Aspect proportion ( L/d ) ought to be high (to limit impacts of end misfortunes) Wire ought to oppose oxidation until high temperatures (need to operate wire at high T to get great affectability, high flag to clamor ratio) Temperature coefficient of resistance ought to be high (for high sensitivity, flag to commotion proportion and recurrence reaction) Wires of under 5 µm breadth can't be drawn with solid diameters 12/48

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

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Constant current anemometer CCA Principle: Current through sensor is kept Constant Advantages: - High recurrence reaction Disadvantages: - Difficult to utilize - Output diminishes with speed - Risk of test burnout 14/48

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Constant Temperature Anemometer CTA Principle: Sensor resistance is kept consistent by Servo speaker Advantages: - Easy to utilize - High recurrence reaction - Low clamor - Accepted standard Disadvantages: - More mind boggling circuit 15/48

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Governing conditions I Governing Equation: E = warm vitality put away in wire E = CwTs Cw = warm limit of wire W = influence created by warming W = I² Rw review Rw = Rw(Tw) H = warm exchanged to environment 16/48

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Governing conditions II Heat exchanged to environment ( convection to liquid H = total off + conduction to underpins + radiation to environment) Convection Qc = Nu · A · (Tw - Ta) Nu = h ·d/kf = f (Re, Pr, M, Gr, α ), Re = ρ U/μ Conduction f (Tw , lw , kw, Tsupports) Radiation f (Tw-Tf ) 17/48

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Simplified static investigation I For balance conditions the warmth stockpiling is zero: and the Joule warming W measures up to the convective warmth exchange H Assumptions : Radiation misfortunes little Conduction to wire bolsters little Tw uniform over length of sensor - Velocity encroaches typically on wire, and is uniform over its whole length, and furthermore little contrasted with sonic speed. Liquid temperature and thickness consistent 18/48

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Simplified static investigation II Static warmth exchange : W = H I ² Rw = hA(Tw - Ta) I ² Rw = Nu kf/dA( Tw - Ta) h = film coefficient of warmth exchange A = heat exchange range d = wire breadth kf = heat conductivity of liquid Nu = dimensionless warmth exchange coefficient Forced convection administration, 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 = E ² = (Tw - Ta)(A + B · U ⁿ) "Lord's law" Then the voltage drop is utilized as a measure of speed. 19/48

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Heat exchange from Probe Convective warmth exchange Q from a wire is a component of the speed U, the wire over-temperature Tw –T 0 and the physical properties of the liquid. The essential connection amongst Q and U for a wire set typical to the stream was recommended by L.V. Lord (1914). In its least difficult frame it proposes : where Aw is the wire surface range and h the warmth transfer coefficient, which are converged into the calibration constants An and B. 20/48

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Hot-wire static exchange work Velocity affectability (King's law coeff. A = 1.51, B = 0.811 , n = 0.43) Output voltage as fct. of speed 21/48

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HOT-WIRE CALIBRATION The hot-wire reacts as per King's Law: where E is the voltage over the wire, u is the speed of the stream ordinary to the wire. A, B, and n are constants. You may accept n = 0.5, this is basic for hot-wire tests. A can be found by measuring the voltage on the hot wire with no stream, i.e. for u = 0, so A = E^ 2 as should be obvious. Ensure there is no stream, any little draft is huge. The HWLAB programming working in alignment mode will give you a voltage. When you know A , you can gauge the wire voltage for a known stream speed and after that decide B from King's law, were B = (E ^2 – A)/U ⁿ ) 22/48

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Calibration bend 23/48

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Problem sources pollution I Most normal sources: - dust particles - dirt - oil vapors - chemicals Effects: -Probe Change stream affectability of the sensor (DC float of adjustment bend) - Reduce recurrence reaction What to do: - Clean the sensor - Recalibrate 24/48

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Problem Sources Probe tainting II Drift because of molecule defilement in air 5  m Wire, 70  m Fiber and 1.2 mm Steel Clad Probes (From Jorgensen, 1977) - Wire and fiber presented to unfiltered air at 40 m/s for 40 hours - Steel Clad test presented to outside conditions 3 months amid winter conditions 25/48

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Problem Sources Probe sullying III Drift because of molecule tainting in water Output voltage diminishes with expanding soil stores 26/48 (From Morrow and Kline 1971)

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Problem Sources Probe pollution IV - slight impact of earth on warmth exchange were warmth exchange may increment ! impact : low speed sign, for expanded surface versus protecting impact High Velocity, -more contact with particles particularly in laminar stream, were turbulent stream has a "cleaning impact" Influence of soil INCREASES as wire breadth DECREASES Deposition of chemicals INCREASES as wire temperature INCREASES * FILTER THE FLOW, CLEAN SENSOR AND RECALIBRATE 27/48

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Further Problem Sources Bubbles in Liquids I Drift because of rises in water In fluids, disintegrated gasses shape rises on sensor, bringing about: -diminished warmth exchange -descending alignment float (From C.G.Rasmussen 1967) 28/48

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Bubbles in Liquids II Effect of rising on : portion of run of the mill calibration bend ( noised flag ) Bubble estimate relies on upon : - surface pressure - overheating proportion - velocity Precautions : - Use low overheat - Let fluid remain before utilize - Don't permit fluid to blend with air - Clean sensor (From C.G.Rasmussen 1967) 29/48

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Stability in Liquid Measurements Fiber test worked stable in water -De-ionized water (lessens green growth development) -Filtration ( ought to be superior to 2  m) -Keeping water temperature consistent (inside 0.1 o C) (From Bruun 1996) 30/48

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Eddy shedding I Eddy shedding from tube shaped sensors Occ