Annular flowmeters are designed to measure the flow of gas or liquid within an annular space, typically between two concentric pipes or tubes. These flowmeters utilize various principles to determine the flow rate accurately. Here are some common types of annular flowmeters:

1. Variable Area Annular Flowmeters:

   • Variable area annular flowmeters, also known as annubar flowmeters, utilize the principle of differential pressure to measure flow rate. They consist of a central tube surrounded by an annular space. The fluid flows through the annular space, causing a pressure differential between the upstream and downstream sides.
   • The pressure difference is proportional to the square of the flow rate according to the Bernoulli principle. A differential pressure transmitter measures this pressure difference and converts it into a flow rate signal.

2. Ultrasonic Annular Flowmeters:

   • Ultrasonic annular flowmeters utilize ultrasonic waves to measure the velocity of the fluid flow within the annular space. Transducers are installed on the outer surface of the central tube to transmit and receive ultrasonic pulses.
   • The time taken for the ultrasonic pulses to travel between the transducers is measured and used to calculate the flow velocity. By integrating the velocity profile across the annular space, the total flow rate can be determined.

3. Thermal Annular Flowmeters:

   • Thermal annular flowmeters use the principle of heat transfer to measure flow rate. They consist of one or more thermal sensors installed within the annular space. A known amount of heat is applied to the fluid, and the change in temperature downstream of the heat source is measured.
   • The rate of heat transfer is proportional to the flow rate according to the convective heat transfer equation. By measuring the temperature difference, the flow rate can be calculated.

4. Coriolis Annular Flowmeters:

   • Coriolis annular flowmeters exploit the Coriolis effect to measure flow rate. They consist of a pair of vibrating tubes installed within the annular space. As the fluid flows through the annular space, it causes the tubes to bend due to the Coriolis effect.
   • The degree of bending is proportional to the mass flow rate of the fluid. Sensors measure the phase shift or frequency difference between the vibrating tubes, allowing for accurate determination of flow rate.

5. Vortex Annular Flowmeters:

   • Vortex annular flowmeters use the principle of the von Kármán vortex street to measure flow rate. They consist of a bluff body or shedder bar installed within the annular space. As the fluid flows past the bluff body, vortices are shed alternately on each side.
   • The frequency of vortex shedding is proportional to the flow rate according to the Strouhal number. Sensors detect the frequency of vortex shedding and convert it into a flow rate signal.

Each type of annular flowmeter has its advantages and limitations in terms of accuracy, rangeability, cost, and suitability for different applications. The selection of an annular flowmeter depends on factors such as the type of fluid, flow conditions, required accuracy, and budget constraints.