PRESENTATION ON MEASUREMENT OF FLUID VELOCITY Submitted by:  Prativa Giri (072bme628)  Sutishna Pokharel(072bme646)

 In physics, a fluid is a substance that continually deforms (flows) under an applied shear stress. Fluids are a subset of the phases of matter.  Fluids are substances that have zero shear modulus, or, in simpler terms, a fluid is a substance which cannot resist any shear force applied to it.  Fluids include: liquids and gases What is fluid?

Fluid Flow Measurement Flow measurement is the quantification of bulk fluid movement. Flow may be measured by measuring the velocity of fluid over a known area. The fluid flow is measured in terms of fluid pressure,flow rates of fluid,fluid velocity,etc. The fluid flow is usually proportional to fluid velocity. Therefore, measurement of flow can be obtained by measuring velocity.

Measurement of fluid velocity For the measurement of the fluid velocity there are various instruments  Velocity map using Pitot tube and Pitot static tube  Hot wire anemometer  Cup and vane anemometers  Ultrasonic flow meter

Pitot tube: The pitot tube measures the velocity by measuring the total and static pressure of a flowing fluid. The difference between total and static pressures gives velocity pressure. The differential pressure (Pt-Ps) is measured using a differential pressure sensor. i.e. t = s+ v= s+ 2/2 r, for incompressible fluids and for compressible fluids Pitot tube: The

fig: pitot tube

Working of pitot tube: The Pitot tube is introduced in the fluid flow area where point pressure details are required (which is an indirect measure of flow rate). The pressure in the outer tube is the static pressure in the line. The total pressure in the inner tube is greater than static pressure. That is, total pressure is the static pressure plus the impact pressure. The differential pressure (Pt-Ps) is measured using a differential pressure sensor. This differential pressure becomes a measure of flow rate at that point where the pitot tube is present in the flowing fluid.

Advantages : Pitot tubes are extensively used in laboratories to measure velocity, pressure and flow rates of fluids. It causes no pressure loss in the flowing fluid. Pitot tube is cheap and very easy to install. Limitations: It is difficult to obtain proper alignment of the pitot tube with flowing direction. It cannot be used in fluids with suspended solids and impurities. The fluid velocity should be high in order to get a measurable pressure difference.

Cup and vane anemometer: Cup and vane anemometers are devices for measuring speed of air movement. The device consists essentially a rotating element whose speed of rotation varies with the velocity of the flow  Cup anemometer : In a cup anemometer cups are attached to radial arms mounted on a shaft. Drag forces are set up in these cups when a flow stream in the plane of rotation approaches the unit from any direction.

Vane type Anemometer: In vane type anemometer,vanes of the wind mill type are mounted in a support so that the fluid flow is parallel to the axis of rotation.The rotor drives a low friction gear train that in turn drives a pointer that indicates the wind speed on a dial.

Common difference between vane and cup Cup type Usually mounted on rigid shaft while readings are taken. Best for relatively low speeds Vane type Usually held in hand while readings are taken. Measures large speeds more accurately.  Experiments indicate that provided the wind speed is not large, the relation between wind speed and angular velocity of cups/vanes is linear.

Hot wire anemometer: Most well known Thermal Anemometer. Basically involves the power that is dissipated by a hot wire to an ambient fluid that is moving past it. Core of anemometer is an exposed hot wire. Heat lost to fluid convection is a function of the fluid velocity. The larger the velocity of the fluid, larger the heat dissipation from the wire – for a fixed wire temperature. Alternately - larger the velocity of the fluid, smaller the wire temperature – for a fixed heat dissipation rate from the wire The velocity information is converted to thermal – either temperature change or change in the heat dissipation rate – information.

Types: i) constant current method ii) constant temperature method i) Constant current method: The hot wire current is held fixed i.e constant voltage is supplied. The resistance of the wire responds to change in the velocity V.

The principle in this method is to maintain the temperature and resistance of the sensing wire at a constant level. Due to the heat transfer takes from the sensing wire to the flowing gas during gas flow the temperature and hence the resistance of the wire tends to change. As a result in order to maintain these values at initial level, the current is increased. The increase in the electric current becomes a measure of flow rate of the gas when calibrated. ii) constant temperature method

Advantages The constant-temperature anemometers are more widely used than constant-current anemometers due to their reduced sensitivity to flow variations. High frequency response, >10kHz (up to 400kHz). Small size of the probe reduces the probe blockage in comparison to similar measuring.Limitations: Its main disadvantage is that it can be used only in clean gas flows. Needs to be recalibrated frequently due to dust accumulation.

An ultrasonic flow meter is a type of flow meter that measures the velocity of a fluid with ultrasound to calculate volume flow. Using ultrasonic transducers, the flow meter can measure the average velocity along the path of an emitted beam of ultrasound, by averaging the difference in measured transit time between the pulses of ultrasound propagating into and against the direction of the flow or by measuring the frequency shift from the Doppler effect. Ultrasonic flow meters are affected by the acoustic properties of the fluid and can be impacted by temperature, density, viscosity and suspended particulates depending on the exact flow meter. Ultrasonic flow meter

Types : There are three different types of ultrasonic flow meters. 1.Transmission flow meters. 2. Doppler-Shift flow meters(reflection flow meter) 3.Open-Channel flow meter.

Advantages of ultrasonic flow meter  Flexible mountings,  Process safety,and  No moving parts to replace or lubricate so maintainence cost is low,  Can be used to measure the flow of slurries and liquids which cause damage to conventional sensors,  Can be clamped in pipe with no penetration,  Can measure high and low pressure.

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