HotWire Anemometry Cảm biến dây nhiệt Chapter 10: HotWire Anemometry Fundamentals Thermal anemometry is a method for measuring fluid velocities by sensing the changes in heat transfer from a small, electrically heated sensor exposed to the fluid motion. The most common thermal anemometer is the hotwire anemometer. The hotwire sensor is a very fine (and easily broken) cylindrical wire. A typical hotwire diameter is about 4μm Use hotwire anemometry to investigate the characteristics of a turbulent air jet, by measuring The radial velocity profile of a round air jet at various axial locations, The mass flux as a function of increasing distance from the jet exit, and hence the entrainment of fluid into the jet The momentum flux at various axial locations The angle associated with the spread of the jet
MCEN 371 – Mechanical Engineering Laboratories Chapter 10: Chapter 10: Hot-Wire Anemometry Hot-Wire Anemometry Hot Wire Sensors & Anemometry Experiment Hot-Wire Calibration Air Jet Properties MCEN 371 – Mechanical Engineering Laboratories Fundamentals • Thermal anemometry is a method for measuring fluid velocities by sensing the changes in heat transfer from a small, electrically heated sensor exposed to the fluid motion. • The most common thermal anemometer is the hot-wire anemometer. • The hot-wire sensor is a very fine (and easily broken) cylindrical wire. A typical hot-wire diameter is about 4μm. MCEN 371 – Mechanical Engineering Laboratories Chapter 10: Chapter 10: Hot-Wire Anemometry Hot-Wire Anemometry Part 1: Hot-Wire Sensor MCEN 371 – Mechanical Engineering Laboratories Sketch of Hot Wire Sensor 1.0 mm Sensing Length Gold plated stainless steel supports Plating to define sensing length Quartz coated platinum film sensor on glass rod (0.051 mm MCEN 371 – Mechanical Engineering Laboratories MCEN 371 – Mechanical Engineering Laboratories How a Hot wire Sensor Works Current flow through wire The current i flowing through the wire generates heat (i 2 R w ) In equilibrium, this must be balanced by heat lost (primarily convective) to the surroundings. Flow Field MCEN 371 – Mechanical Engineering Laboratories How a Hot wire Sensor Works (cont’d) Flow Field Current flow through wire The rate of which heat is removed from the sensor is directly related to the velocity of the fluid flowing over the sensor The hot wire is electrically heated If velocity changes, convective heat transfer changes, wire temperature will change and eventually reach a new equilibrium. MCEN 371 – Mechanical Engineering Laboratories Principles of Operation • In a constant temperature hot-wire, a feedback control acts to vary the current flowing through the wire so that its temperature remains constant. • The fluid velocity can be determined from the measurement of the amount of current (or voltage) required to maintain the sensor at constant temperature. MCEN 371 – Mechanical Engineering Laboratories Hot Wire Response 0 50 100 150 200 250 300 10 5 0 Bridge Output Linearized Output V (ft/sec) E (volts) MCEN 371 – Mechanical Engineering Laboratories Non-linear response of hot wire sensor n BVAE += 2 Square of measured voltage A, B, N are calibration constants Measured Velocity to be determined empirically [...]...MCEN 371 – Mechanical Engineering Laboratories Chapter 10: Hot-Wire Anemometry Part 2: Anemometry Experiment MCEN 371 – Mechanical Engineering Laboratories Goals • Understand the principle of operations of a hot-wire sensor • Use hot-wire anemometry to investigate the characteristics of a turbulent air jet, – – – – by measuring The radial velocity profile... system Electronics Module x Jet tailpipe y MCEN 371 – Mechanical Engineering Laboratories MCEN 371 – Mechanical Engineering Laboratories MCEN 371 – Mechanical Engineering Laboratories Chapter 10: Hot-Wire Anemometry Part 3: Hot-Wire Calibration MCEN 371 – Mechanical Engineering Laboratories Hot-Wire Sensor Calibration • The measurement of the fluid velocity from the hot-wire response: E2 = A + B V n requires... reading, simply define: Y = ln ( E − E ) 2 2 0 and X = ln ( V ) The plot of Y vs X then has a slope of n and an intercept of ln (B) ! MCEN 371 – Mechanical Engineering Laboratories Chapter 10: Hot-Wire Anemometry Part 4: Air Jet Properties MCEN 371 – Mechanical Engineering Laboratories Air Jet Properties • • • • As an air jet issues out of a tailpipe a shear layer develops The jet entrains ambient air