Hot-Wire Anemometry

Hot-Wire Anemometry is a measurement method involving an electrically-heated wire or film element, placed in the fluid flow of interest. Any changes in the flow conditions would change the element’s convective heat transfer and thus its electrical resistance, which in turn can be related to, for example the flow velocity. HWA is the principle research tool in most studies of turbulent gas flows, offering cost-efficient measurement of one, two or three components of the velocity vector at high accuracy (i.e. < 1%) and excellent spatial resolution due to the small size of the hot-wire sensor (Bruun, 1995, p. 1). The high frequency response of modern hot-wire anemometers makes them suitable for velocity measurements in flows of low to moderate turbulence intensity (~ 25 %).

The basic HWA instrumentation consists of a hot-wire sensor, a probe tube, including its support and cabling, an anemometer and an analogue-to-digital (A/D) converter connected to a computer, controlling the HWA operation. HWA sensors are typically tungsten wires of 5 μm diameter, welded to the prongs of the probe (see Figure 3.4a and Figure 3.4b for schematic examples). There are two main operating modes of hot-wire anemometers: at constant current (CC), where the electrical current across the wire is kept constant while the wire temperate varies, or at constant temperature (CT), where the wire’s electrical resistance and temperature are kept constant by varying the current. CT anemometers have more complex electrical circuit, but are much simpler to use than CC anemometers, therefore they are usually the

Vent Fuel gas Air Signal output Collector plate Flame

Tubing, connected to probe Ignitor

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preferred anemometer type for velocity measurements (Bruun, 1995, p.37). The operating principle of a CT anemometer is shown in Figure 3.3. The hot-wire is placed in a Wheatstone bridge opposite to a resistor of variable electrical resistance, which defines the operating resistance of the wire and thereby its operating temperature. An increase in the flow velocity would reduce the wire temperature and resistance, causing a change in the voltage output across the bridge. This voltage output is used by a servo amplifier to control the system voltage supply such that the original values of hot-wire temperature and resistance are maintained constant. The amplifier output is thus a function of the flow velocity.

Figure 3.3: Principle diagram of a constant-temperature (CT) anemometer (taken from Dantec Dynamics, 2002)

In this study, constant-temperature HWA with a single-sensor probe was used for two different purposes: 1) to measure the vertical profiles of mean velocity and turbulence intensity of the modelled Atmospheric Boundary Layer (ABL) at sub scale; 2) for one-component velocity measurements of the flow field downstream of a buoyant nozzle jet at 1:200 scale with and without baffles.

A Dantec 55P14 hot-wire sensor (Figure 3.4a), oriented horizontally and at right angles to the flow, was used for the boundary layer measurements, while the velocity measurements of 2) were performed with a 55P13 sensor (Figure 3.4b), oriented vertically. In both cases, the probe was mounted on a three-dimensional traverse in the working section of the 8'×4' Atmospheric Boundary Layer Wind Tunnel (ABLWT), which allowed accurate, computer-controlled movement of the probe to the desired measurement locations.

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(a) (b)

(c)

Figure 3.4: Schematic illustration of (a) Dantec 55P14 miniature hot-wire probe for boundary layer measurements (isometric view, sensor and prongs only); (b) Dantec 55P13 miniature hot-

wire probe (not to scale); (c) Hot-Wire Anemometry (HWA) arrangement for velocity measurements (taken from Dantec Dynamics, 2002)

The measurements were performed at a sampling frequency of 1 kHz and a sampling period of 5 seconds, using a standard CT anemometry arrangement (Figure 3.4c). More details on the operation and data analysis can be found in Dantec Dynamics (2002). The hot-wire probe was connected to a Dantec Dynamics StreamLine CT anemometer with a built-in signal conditioner for amplification and filtering of the CTA

Flow, U Wire sensor Probe body 1.25 mm 2 mm 7.5 mm Connector pins Prongs Prongs Wire sensor 1.25 mm Flow, U 4 mm 1.9 mm

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signal, which was located in the wind tunnel control room. The output signal from the anemometer was passed through an A/D converter and connected to a computer, where the HWA operation was controlled using the StreamWare application software. Temperature in the working section was measured with a Dantec 55P32 Thermistor probe, connected to the CT anemometer. This allowed corrections for temperature variation to be performed in situ during data reduction, within the software.

At the beginning of each test day, the CT anemometer voltage output was calibrated versus freestream velocity by applying polynomial curve fitting. The freestream velocity was measured with a Schiltknecht MiniAir6 Mini vane anemometer approximately at the working section centre line for a range typically between 0.18 and 13 m/s. Calibration tests were repeated during the course of the day, if the temperature in the working section varied considerably (i.e. ≥ 5 ºC) from the temperature at the initial calibration. Calibration errors of the polynomial curve fitting method varied typically between 0 and ± 3%.

Regarding the accuracy of the HWA measurements, it should be noted that the tests at 1:200 scale were performed with an exhaust gas mixture of nitrogen, helium and methane and a freestream of air, to account for the full-scale jet buoyancy and the ambient wind respectively, while the calibration was performed in a freestream of air only. This issue are discussed in more detail in Chapter 7.2.2.