In a thermal anemometer sensor element is used which is heated electrically, and the electrical resistance depends on the temperature. By the flow around a heat transfer takes place in the flow medium, which varies with the flow rate. By measuring the electrical quantities can be closed to the flow velocity.
The sensor elements can be carried out very differently (wire, film, ...) and they are partially applied to a substrate. For the operation of the sensors, a special electronics is needed, which controls the heating current and amplifies the sensor signal. Since the sensor properties and the electronic control and amplifier systems have a significant impact on the measurement signal, usually the entire measurement chain is calibrated.
Areas of application and comparison with other methods
Hot-wire probes offer the possibility to determine the flow velocity with high temporal resolution, as it is necessary, for example for the following examinations:
- Unsteady effects ( flow separation )
- Aero-acoustic effects
- Boundary layer, especially transition from laminar to turbulent flow
- Measuring degree of turbulence
As an alternative to thermal anemometry there for this kind of measurements essentially only two other methods:
- Pressure probes ( for example, Prandtlsonde )
- Laser Doppler anemometry
Pressure sensors provide a cost-effective and robust solution dar. With multi-hole pressure probes, the flow direction can also be determined. As the pressure from the square of the velocity dependent, measurements are below 10 m / s not very accurate in air. In addition, in most of the probes is a problem that the static pressure is required, which can not be accurately determined in many cases. Typically, a frequency resolution of about 1 kHz is also almost impossible to realize.
The great advantage of laser Doppler anemometry is that the flow is not disturbed by a probe. However, the cost of a measuring device are very high, the flow must be " polluted" with particles are ( seeding ), it has an optical accessibility exist and there are protective measures due to the very strong laser beams necessary.
These measurement methods, the hot-wire anemometry is an interesting alternative represents the velocity can be measured very high temporal resolution, typically up in the range of 5 to 10 kHz (with special adjustment of the measuring chain even beyond ). If more wire probes, not only the magnitude but also the direction can be determined. The costs are significantly lower than those of a LDA system and there are no special precautions required. However, the probes are relatively susceptible to contamination, and the signal is dependent on the flow temperature. The influence of temperature, however, can be detected and corrected via calibration.
Further, the hot-wire anemometry for the measurement of small flow rates is very good because it has the greatest sensitivity especially in the lower speed range. For operation with minimal heat output, the probes can also be alternatively used as a very fast-response thermometer. Can hot-wire probes appropriate to the calibration in all speed ranges ( even supersonic ) are used.
In the hot-wire anemometer, a very thin wire is used, which typically has a diameter of 2.5- 10 microns. He should be at least 200 times the diameter long to keep marginal influences low. As the material, platinum, nickel, tungsten and other different alloys are used depending on the requirements of physical properties. The wire thickness is the determining parameters for the dynamics. The thinner the wire is, the higher frequencies can thus be detected, but the greater is its mechanical sensitivity.
The wire is stretched between two much thicker steel spikes to which it is welded. These so-called prongs protruding from a ceramic body, which provides for the mechanical stability and electrical insulation. This combination represents the actual hot-wire sensor that is plugged into a special holder or connected with such proof. On the holder, and the wires connected to the electrical connection to the hot-wire bridge is formed.
From the hot wire speed component is detected in a plane perpendicular to the wire. The component tangential to the wire has only a very small effect and can be neglected in most cases.
With a single- sensor hot-wire probe can be determined at a known flow direction only the amount of a one-dimensional flow. Contrast can be measured with " double hot wire " or " two-sensor hot wire " a two-dimensional flow (u = f (x, z)). In this type of probe is in thermal wake of the hot wire front, with the magnitude of the flow velocity is determined, parallel mounted a short distance towards a so-called hot wire detector. It must be considered that there is no backflow and measurements are only useful with this type of sensor, if still impact of the front wire are proven in the signal of himself at thermal tracking sensor. For determining a speed vector dreidimsionalen u = f (x, y, z ) are the three- or four-sensor hot wire is used. In these three types of probe sensors form a cube measuring about the components of the velocity vector are derived. In the four-sensor hot-wire probe is additionally mounted according to the principle of " dual - hot wire " for one of the three rate-determining sensors in thermal tracking for an additional sensor, which allows detection of the reverse flow.
Different control loops
For the operation of hot-wire sensors, a special electronic control and amplification is necessary. The following section will address the two main operating modes:
Constant-current anemometry (CCA )
The CCA represents the simplest method because it can abandon the costly regulation. The sensor is heated with a constant current. By the flow around the resistance and hence the voltage drop across the sensor, which represents the measuring signal varies. The disadvantages of this simple system consist in the lack of temperature compensation and in a poorer frequency resolution. Furthermore, the notion of long-term measurements is unsuitable because the wire is rapidly aging by temperature fluctuations.
Constant-Temperature Anemometry (CTA )
The CTA methods is attempted by very fast control loops to maintain the sensor at a constant temperature in the middle. The electronic implementation is therefore correspondingly complex and each case must be adapted to the individual sensor including its wiring. Since the sensor temperature can be determined, a theoretical compensation of the temperature influence is possible. Also, this mode of operation on a wide frequency range.
Formula for calculation
The first important work was supported by L.V. King performed in 1914. For the electric power required by the formula named after him:
It is therefore dependent on the temperature difference between the wire and fluid and the flow velocity (actually the mass flow rate, significantly for applications with approximately constant pressure and flow speeds below the speed of sound can be simplistically calculated only with the speed ) and the physical boundary conditions of the execution. For the compensation function there are different approaches. The following approach has worked well in practice:
The temperature sensor can not be determined directly, but must be calculated using the jumper settings. It has been shown that in order to correct the influence of temperature and optimal performance, a slightly lower temperature sensor is to be used than calculated. This corrected temperature sensor must be determined via an appropriate calibration at different temperatures. If this is not possible, a correction can be made the basis of experience, which causes a slightly larger temperature errors.
Calibration is typically performed in small probe wind tunnels. The determination of the rate is made via the back-pressure or above the pressure in the antechamber of the wind tunnel. Due to the quadratic relationship between pressure and velocity are obtained for small velocities the largest uncertainties. Key factors are:
- Pressure sensor
- Atmospheric pressure
- A / D conversion
Typically the uncertainty at low speeds determined by the pressure measurement and at high temperature through the measurement.
Accuracy of the measurement method
The ambient temperature has not only an impact on the calibration, but also later on the hot wire. The resultant error depends on the temperature difference between the sensor and the fluid. This error can typically be reduced to about 1% by optimizing the calibration at different temperatures and thus the possible correction of the sensor temperature.
The resistance of the sensor line is a constant in the sensor system. The resistance changes (eg by plugging and unplugging or using a different line), this leads to a measurement error.
Due to contamination of the heat flow is usually inhibited, which is too low speed is displayed. In this case, the wire needs to be cleaned and calibrated.
In the form of a differential - wire anemometer, that There are two heating wires closely spaced used, eg of platinum, not only temporally stationary currents (wind ), but also sound velocities can be measured.