Open Channel Flow Measurement

The Engineering Concepts

6 years ago

Open Channel Flow Measurement Principle

Any conduit or channel in which the fluid or liquid is flowing with a free surface open to the atmosphere is referred to as an “open channel”. For instance, tunnels, nonpressurized sewers, partially filled pipes, canals, streams, and rivers, all these are Open channels.

Read Books :

Fox and McDonald’s Introduction to Fluid Mechanics

A Textbook of Fluid Mechanics and Hydraulic Machines

Fluid Mechanics : By Douglas (Author)

Open channel flowmeters perform flow measurement of liquids that are open to the atmosphere at some point in the flow measurement path. The liquid may be entirely open to the atmosphere, or may be contained within a closed pipe that is not full of liquid and only open to the atmosphere at the installation point of the flowmeter itself.

Regardless of which type of open channel flow measurement techniques are used, level measurement must be used in combination with velocity measurement, and a flow computer using the manning equation of open channel flow to calculate the true flow rate of any wetted channel (pipes, streams, etc).

Read Also :

Venturi Flow Meter

Orificemeter

Pitot Tube

The common method of measuring flow through an open channel is to measure the height or HEAD of the liquid as it passes over an obstruction (a flume or weir) in the channel. Using ultrasonic level technology, Open channel flow meters include a non-contacting sensor mounted above the flume or weir. By measuring the time from transmission of an ultrasonic pulse to receipt of an echo, the water level or “Head” is accurately measured.

Animation Credits : greyline

Flumes and weirs are specially designed channel shapes that characterize the flow of water. Common types are Rectangular Weirs, V-Notch Weirs, Parshall flumes and Palmer Bowlus flumes. The choice of flume or weir type depends on the application: flow rate, channel shape and solids content of the water.

Traditional Methods : Flumes and weirs are now a days considered as traditional method to measure flow of open channel .

Modern Methods :

Area Velocity Flow Measurement Methods

diagram of the area-velocity method from the ISCO Open Channel Flow Measurement Handbook

There are two types of Area Velocity (AV) methods for measuring open channel flow. Both types use either Transit time or Doppler ultrasonic technologies. The area velocity method calculates flow rate by multiplying the area of the flow by its average velocity. This is often referred to as the continuity equation, Q=AxV, where Q is flow, Ax is the wetted area of flow times V, the average velocity of the flow stream.

Doppler Ultrasonic Flow Measurement Techniques

Doppler Flow Measurement for open channel flow is designed to record instantaneous velocity components at a single point with a relatively high frequency.Flow measurement is performed by measuring the velocity of particles in a remote sampling volume based upon the Doppler shift effect. The velocity components using this technique are combined with level measurement and an open channel flow computer using the manning equation to calculate true open channel flow measurement.

The Doppler flow meter is placed at the bottom of the channel which often times gets dirty thereby causing inaccurate flow measurement. Since Doppler techniques measure the speed of particles coming towards the sensor, it only measures what it sees. Particles in a stream move at different speeds and it is highly unlikely that it can measure all the particles in the stream, hence not representing the average flow in the stream and causing uncertainty in overall open channel flow measurement.

ImageSource : OpenChannelFlow

Article Source : Smart Measurement ;