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Mechanical Engineering
 

Hot Wire Anemometry

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Hot wire anemometry is a common experimental technique for measuring fluid velocity during flow tests. Velocity is often an important parameter of the flow to characterize. But as with many flow properties, it is challenging to measure without impacting the experiment. Measurement of turbulent flows adds an additional complication, since the velocity can fluctuate rapidly in this regime. Hot wire anemometers are a successful diagnostic because they can be made very small so as to minimize their impact on the measured flow. And they have a fast enough time response to resolve rapid velocity fluctuations in the flow. This video will illustrate how a hot wire anemometer works in the constant temperature configuration. And then show how these devices are calibrated in preparation for their use in flow experiments.

In its simplest form, a hot wire anemometer consists of a short resistive wire suspended between two supports and placed in the flow of interest. The wire is electrically heated to a temperature well above the ambient fluid temperature such that the heat generated counteracts the heat lost from convective cooling. Since the convective cooling rate depends on the velocity of the fluid, a relation between the electrical power into the anemometer and the flow velocity can be defined. Joule heating of the wire is the product of the wire resistance and the square of the electric current. For most materials, the resistance is temperature dependent and is well approximated by a linear relationships, where the constant alpha is specified by the wire material. The convective cooling can be modeled using King's Law. In this equation A and B are constants to be determined during calibration. And the exponent of the velocity N is typically around 0.45. At steady state, heating and cooling are balanced. But determining the instantaneous input power depends on knowing both the resistance and current simultaneously. One strategy known as Constant-Temperature Anemometry is to supply only enough current to keep the temperature and hence the resistance of the wire constant. This is accomplished using a Wheatstone Bridge with the hot wire serving as one leg of the bridge circuit. The bridge can be thought of as two voltage dividers in parallel, and the difference in their outputs is the bridge voltage. If the ratios are one to R2 and RW to R3 are equal, the bridge is balanced and the bridge voltage is zero. Otherwise, the bridge voltage is amplified and used as a feedback signal. Current will then flow through the hot wire, heating it until the bridge is brought into balance. The baseline temperature of the hot wire can be set by adjusting the control resister R1. Ohm's Law can be used to relate the current flowing through the hot wire to the bridge voltage. Now, since our W is held constant, the resistance terms can be absorbed into the calibration constants. And the velocity is specified as a function of the bridge voltage. The calibration constants are determined by placing the anemometer in a reference flow system. Measurements must be taken at several known flow velocities so that a least squares regression can be performed. The vena contracta of the free jet is a useful reference flow because the mean velocity is well characterized by the fluid density and pressure differential. Now that you understand how the hot wire anemometer works, let's see how the device can be calibrated experimentally using the vena contracta of a free jet.

Before you begin setting up, familiarize yourself with the layout and safety procedures of the facility. The flow system consists of a plenum pressurized by a centrifugal fan. A free jet forms from the slit on the side of the plenum and the stack on the top allows the plenum pressure to be adjusted by swapping restricting orifice plates. Now setup the data acquisition system as shown in the diagram in the text. Connect the plenum pressure tab to the positive port of the pressure transducer and leave the negative port open to the atmosphere. Set channel zero on the data acquisition system to the bridge voltage of the anemometer and channel one to the pressure transducer. In the data acquisition software, set the hot wire channel to zero and the pressure transducer channel to one. Finally, set the constant N to 0.45 and the sampling rate to 100 Hertz and the total samples to 1000. Enter the values for the local air density in the transducer calibration constant in the appropriate fields so that the data acquisition system reports the results in meters per second. Use a calibrated spaced to set the slit width to 19.05 millimeters or three quarters of an inch. And then affix the hot wire anemometer in the flow at the position of the vena contracta which is at a distance of 1.5 times the slit width from the opening. Locate the anemometer and the mid plane of the jet by traversing the probe in the cross stream direction until the signal on the oscilloscope shows minimal fluctuations. Now cover the stack completely to establish the maximum jet velocity and then turn on the flow facility. Use the software to record a data point. After the measurement is complete, exchange the orifice plate for one with the smallest opening. Take another measurement with the new orifice plate. Continue exchanging orifice plates and taking measurements until you have collected at least six data points including one with the stack fully unrestricted. Recover the calibration constants from the data acquisition software.

Once you have finished collecting data, you will have voltage measurements from the bridge circuits corresponding to a range of different flow velocities. Plot the voltage squared as a function of velocity raised to the power 0.45. And then perform a least squares linear fit to the data. The slope and intercept from the fit are the calibration constants for this hot wire anemometer. Now that the anemometer has been calibrated it can be used in a different flow setup to measure an unknown velocity.

Hot wire anemometers are frequently used in scientific flow experiments. Hot wire anemometry is extensively used to study boundary layer flows in wind tunnels. Boundary layers are one of the oldest subjects of research in fluid mechanics due to their relevance to technological applications such as aerodynamic design, naval engineering, and power generation among others. To the detriment of all these fields, many effects involving the boundary layer are still incipiently understood. Highly irregular roughness, density and viscosity gradients and compressibility to mention a few. With this in mind, hot wire anemometry is used in laboratory settings to assess boundary layer flows relevant to the above mentioned applications. Using strategies similar to the one demonstrated in the current experiment. Industrial ventilation systems are used to control fumes, particulates, aerosols, combustion products or other contaminants in industrial settings. Commonly, each contaminant will be generated at a different rate. Hence, the exhausting system will require a different flow rate for each contaminant to eliminate them efficiently. This procedure is typically carried out with the help of a hot wire anemometer. Wherein the engineer sets the position of a damper in each line, seeking to make the reading of the anemometer match a pre-estimated velocity value.

You've just watched Jove's introduction to hot wire anemometry. You should now understand how a constant temperature anemometer works, and how to calibrate this device for use in flow experiments. Thanks for watching.

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