Flow Past Heated Bluff Bodies

Flow measurement is the quantification of bulk movement. Flow can be measured in a variety of ways.

  1. Flow Past Heated Bluff Bodies Of Water

A positive displacement flowmeter of the oval gear type. Fluid forces the meshed gears to rotate; each rotation corresponds to a fixed volume of fluid.

Counting the revolutions totalizes volume, and the rate is proportional to flow.An oval gear meter is a positive displacement meter that uses two or more oblong gears configured to rotate at right angles to one another, forming a T shape. Such a meter has two sides, which can be called A and B. No fluid passes through the center of the meter, where the teeth of the two gears always mesh. On one side of the meter (A), the teeth of the gears close off the fluid flow because the elongated gear on side A is protruding into the measurement chamber, while on the other side of the meter (B), a cavity holds a fixed volume of fluid in a measurement chamber. As the fluid pushes the gears, it rotates them, allowing the fluid in the measurement chamber on side B to be released into the outlet port. Meanwhile, fluid entering the inlet port will be driven into the measurement chamber of side A, which is now open. The teeth on side B will now close off the fluid from entering side B.

This cycle continues as the gears rotate and fluid is metered through alternating measurement chambers. Permanent magnets in the rotating gears can transmit a signal to an electric reed switch or current transducer for flow measurement. Though claims for high performance are made, they are generally not as precise as the sliding vane design. Gear meter Gear meters differ from oval gear meters in that the measurement chambers are made up of the gaps between the teeth of the gears. These openings divide up the fluid stream and as the gears rotate away from the inlet port, the meter's inner wall closes off the chamber to hold the fixed amount of fluid.


The outlet port is located in the area where the gears are coming back together. The fluid is forced out of the meter as the gear teeth mesh and reduce the available pockets to nearly zero volume.Helical gear Helical gear flow meters get their name from the shape of their gears or rotors. These rotors resemble the shape of a helix, which is a spiral-shaped structure.

As the fluid flows through the meter, it enters the compartments in the rotors, causing the rotors to rotate. The length of the rotor is sufficient that the inlet and outlet are always separated from each other thus blocking a free flow of liquid. The mating helical rotors create a progressive cavity which opens to admit fluid, seals itself off and then opens up to the downstream side to release the fluid. This happens in a continuous fashion and the flowrate is calculated from the speed of rotation.Nutating disk meter This is the most commonly used measurement system for measuring water supply in houses.

The fluid, most commonly water, enters in one side of the meter and strikes the disk, which is eccentrically mounted. The disk must then 'wobble' or nutate about the vertical axis, since the bottom and the top of the disk remain in contact with the mounting chamber. A partition separates the inlet and outlet chambers. As the disk nutates, it gives direct indication of the volume of the liquid that has passed through the meter as volumetric flow is indicated by a gearing and register arrangement, which is connected to the disk. It is reliable for flow measurements within 1 percent.

Turbine flow meter The turbine flow meter (better described as an axial turbine) translates the mechanical action of the turbine rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The turbine tends to have all the flow traveling around it.The turbine wheel is set in the path of a fluid stream. The flowing fluid impinges on the turbine blades, imparting a force to the blade surface and setting the rotor in motion. When a steady rotation speed has been reached, the speed is proportional to fluid velocity.Turbine flow meters are used for the measurement of natural gas and liquid flow.

Turbine meters are less accurate than displacement and jet meters at low flow rates, but the measuring element does not occupy or severely restrict the entire path of flow. The flow direction is generally straight through the meter, allowing for higher flow rates and less pressure loss than displacement-type meters.

They are the meter of choice for large commercial users, fire protection, and as master meters for the water distribution system. Strainers are generally required to be installed in front of the meter to protect the measuring element from gravel or other debris that could enter the water distribution system. Turbine meters are generally available for 4 to 30 cm (​ 1 1⁄ 2–12 in) or higher pipe sizes. Turbine meter bodies are commonly made of bronze, cast Iron, or ductile iron. Internal turbine elements can be plastic or non-corrosive metal alloys. They are accurate in normal working conditions but are greatly affected by the flow profile and fluid conditions.Fire meters are a specialized type of turbine meter with approvals for the high flow rates required in fire protection systems. They are often approved by Underwriters Laboratories (UL) or Factory Mutual (FM) or similar authorities for use in fire protection.

Portable turbine meters may be temporarily installed to measure water used from a. The meters are normally made of aluminum to be lightweight, and are usually 7.5 cm (3 in) capacity. Water utilities often require them for measurement of water used in construction, pool filling, or where a permanent meter is not yet installed.Woltman meter The Woltman meter (invented by Reinhard Woltman in the 19th century) comprises a rotor with helical blades inserted axially in the flow, much like a ducted fan; it can be considered a type of turbine flow meter.

They are commonly referred to as helix meters, and arepopular at larger sizes.Single jet meter A single jet meter consists of a simple with radial vanes, impinged upon by a single jet. They are increasing in popularity in the UK at larger sizes and are commonplace in the.Paddle wheel meter Paddle wheel flow meters consist of three primary components: the paddle wheel sensor, the pipe fitting and the display/controller. The paddle wheel sensor consists of a freely rotating wheel/impeller with embedded magnetswhich is perpendicular to the flow and will rotate when inserted in the flowing medium. As the magnets in the blades spin past the sensor, the paddle wheel meter generates a frequency and voltage signal which is proportional to the flow rate. The faster the flow the higher the frequency and the voltage output.The paddle wheel meter is designed to be inserted into a pipe fitting, either ‘in-line’ or insertion style. These are available with wide range of fittings styles, connection methods and materials such as PVDF, polypropylene, and stainless steel.

Similar to turbine meters, the paddle wheel meter require a minimum run of straight pipe before and after the sensor.Flow displays and controllers are used to receive the signal from the paddle wheel meter and convert it into actual flow rate or total flow values. The processed signal can be used to control the process, generate an alarm, send signals to external etc.Paddle wheel flow meters (also known as sensors) offer a relatively low cost, high accuracy option for many flow system applications, typically with water or water like fluids.Multiple jet meter A multiple jet or multijet meter is a velocity type meter which has an impeller which rotates horizontally on a vertical shaft. The impeller element is in a housing in which multiple inlet ports direct the fluid flow at the impeller causing it to rotate in a specific direction in proportion to the flow velocity.

This meter works mechanically much like a single jet meter except that the ports direct the flow at the impeller equally from several points around the circumference of the element, not just one point; this minimizes uneven wear on the impeller and its shaft. Thus these types of meters are recommended to be installed horizontally with its roller index pointing skywards.Pelton wheel The turbine (better described as a ) translates the mechanical action of the Pelton wheel rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The Pelton wheel tends to have all the flow traveling around it with the inlet flow focused on the blades by a jet. The original Pelton wheels were used for the and consisted of a radial flow turbine with 'reaction cups' which not only move with the force of the water on the face but return the flow in opposite direction using this change of fluid direction to further increase the of the.Current meter.

A propeller-type current meter as used for hydroelectric turbine testing.Flow through a large such as used at a plant can be measured by averaging the flow velocity over the entire area. Propeller-type current meters (similar to the purely mechanical, but now with electronic data acquisition) can be traversed over the area of the penstock and velocities averaged to calculate total flow. This may be on the order of hundreds of cubic meters per second. The flow must be kept steady during the traverse of the current meters. Methods for testing hydroelectric turbines are given in standard 41. Such flow measurements are often commercially important when testing the efficiency of large turbines.Pressure-based meters There are several types of flow meter that rely on, either by measuring the differential pressure within a constriction, or by measuring and to derive the.Venturi meter A constricts the flow in some fashion, and measure the differential pressure before and within the constriction.

This method is widely used to measure flow rate in the transmission of gas through, and has been used since times. The of Venturi meter ranges from 0.93 to 0.97. The first large-scale Venturi meters to measure liquid flows were developed by, who used them to measure small and large flows of water and beginning at the very end of the 19th century. Orifice plate An is a plate with a hole through it, placed perpendicular to the flow; it constricts the flow, and measuring the pressure differential across the constriction gives the flow rate. It is basically a crude form of, but with higher energy losses.

There are three type of orifice: concentric, eccentric, and segmental. Dall tube The Dall tube is a shortened version of a Venturi meter, with a lower pressure drop than an orifice plate. As with these flow meters the flow rate in a Dall tube is determined by measuring the pressure drop caused by restriction in the conduit. The pressure differential is typically measured using diaphragm pressure transducers with digital readout. Since these meters have significantly lower permanent pressure losses than orifice meters, Dall tubes are widely used for measuring the flow rate of large pipeworks. Differential pressure produced by a Dall tube is higher than Venturi tube and nozzle, all of them having same throat diameters.Pitot-tube A is used to measure fluid flow velocity.

The tube is pointed into the flow and the difference between the at the tip of the probe and the at its side is measured, yielding the dynamic pressure from which the fluid velocity is calculated using. A volumetric rate of flow may be determined by measuring the velocity at different points in the flow and generating the velocity profile.

Multi-hole pressure probe. See also:Multi-hole pressure probes (also called impact probes) extend the theory of Pitot tube to more than one dimension. A typical impact probe consists of three or more holes (depending on the type of probe) on the measuring tip arranged in a specific pattern. More holes allow the instrument to measure the direction of the flow velocity in addition to its magnitude (after appropriate calibration).

Three holes arranged in a line allow the pressure probes to measure the velocity vector in two dimensions. Introduction of more holes, e.g. Five holes arranged in a 'plus' formation, allow measurement of the three-dimensional velocity vector.Cone meters. 8inch (200mm) V-cone flowmeter shown with ANSI 300# raised faceCone meters are a newer differential pressure metering device first launched in 1985 by McCrometer in Hemet, CA. The cone meter is a generic yet robust differential pressure (DP) meter that has shown to be resistant to effects of asymmetric and swirling flow.

While working with the same basic principles as Venturi and orifice type DP meters, cone meters don’t require the same upstream and downstream piping. The cone acts as a conditioning device as well as a differential pressure producer.

Upstream requirements are between 0–5 diameters compared to up to 44 diameters for an orifice plate or 22 diameters for a Venturi. Because cone meters are generally of welded construction, it is recommended they are always calibrated prior to service. Inevitably heat effects of welding cause distortions and other effects that prevent tabular data on discharge coefficients with respect to line size, beta ratio and operating Reynolds numbers from being collected and published. Calibrated cone meters have an uncertainty up to ±0.5%. Un-calibrated cone meters have an uncertainty of ±5.0%.Linear resistance meters Linear resistance meters, also called laminar flow meters, measure very low flows at which the measured differential pressure is linearly proportional to the flow and to the fluid viscosity. Such flow is called viscous drag flow or laminar flow, as opposed to the turbulent flow measured by orifice plates, Venturis and other meters mentioned in this section, and is characterized by Reynolds numbers below 2000. The primary flow element may consist of a single long capillary tube, a bundle of such tubes, or a long porous plug; such low flows create small pressure differentials but longer flow elements create higher, more easily measured differentials.

These flow meters are particularly sensitive to temperature changes affecting the fluid viscosity and the diameter of the flow element, as can be seen in the governing. Variable-area flow meters. Techfluid-CG34-2500 rotameterA 'variable area meter' measures fluid flow by allowing the cross sectional area of the device to vary in response to the flow, causing some measurable effect that indicates the rate.A is an example of a variable area meter, where a weighted 'float' rises in a tapered tube as the flow rate increases; the float stops rising when area between float and tube is large enough that the weight of the float is balanced by the drag of fluid flow. A kind of rotameter used for medical gases is the. Floats are made in many different shapes, with spheres and spherical ellipses being the most common. Some are designed to spin visibly in the fluid stream to aid the user in determining whether the float is stuck or not. Rotameters are available for a wide range of liquids but are most commonly used with water or air.

They can be made to reliably measure flow down to 1% accuracy.Another type is a variable area orifice, where a spring-loaded tapered plunger is deflected by flow through an orifice. The displacement can be related to the flow rate. Optical flow meters. Sonar flow meter on gas lineflow meters are non-intrusive clamp on devices that measure flow in pipes conveying slurries, corrosive fluids, fluids and flows where insertion type flow meters are not desired. Sonar flow meters have been widely adopted in mining, metals processing, and upstream oil and gas industries where traditional technologies have certain limitations due to their tolerance to various flow regimes and turn down ratios.Sonar flow meters have the capacity of measuring the velocity of liquids or gases non intrusively within the pipe and then leverage this velocity measurement into a flow rate by using the cross sectional area of the pipe and the line pressure and temperature. The principle behind this flow measurement is the use of underwater acoustics.In, to locate an object underwater, sonar uses two knowns:.

The speed of sound propagation through the array (i.e. A magnetic flow meter at the in,.Modern innovations in the measurement of flow rate incorporate electronic devices that can correct for varying pressure and temperature (i.e. Density) conditions, non-linearities, and for the characteristics of the fluid.Magnetic flow meters , often called 'mag meter's or 'electromag's, use a applied to the metering tube, which results in a potential difference proportional to the flow velocity perpendicular to the lines. The potential difference is sensed by electrodes aligned perpendicular to the flow and the applied magnetic field. The physical principle at work is of. The magnetic flow meter requires a conducting fluid and a nonconducting pipe liner.

The electrodes must not corrode in contact with the process fluid; some magnetic flowmeters have auxiliary transducers installed to clean the electrodes in place. The applied magnetic field is pulsed, which allows the flowmeter to cancel out the effect of stray voltage in the piping system.Non-contact electromagnetic flow meters A system is called Lorentz force flowmeter (LFF). A LFF measures the integrated or bulk Lorentz force resulting from the interaction between a in motion and an applied magnetic field.

Flow Past Heated Bluff Bodies Of Water

In this case the characteristic length of the magnetic field is of the same order of magnitude as the dimensions of the channel. It must be addressed that in the case where localized magnetic fields are used, it is possible to perform local velocity measurements and thus the term Lorentz force velocimeter is used.Ultrasonic flow meters (Doppler, transit time) There are two main types of: Doppler and transit time. While they both utilize ultrasound to make measurements and can be non-invasive (measure flow from outside the tube, pipe or vessel), they measure flow by very different methods.

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